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METHOD TO-1 Revision 1.0April, 1984
METHOD FOR THE DETERMINATION OF VOLATILE ORGANIC COMPOUNDSIN
AMBIENT AIR USING TENAX® ADSORPTION ANDGAS CHROMATOGRAPHY/MASS
SPECTROMETRY (GC/MS)
1. Scope
1.1 The document describes a generalized protocol forcollection
and determination of certain volatile organiccompounds which can be
captured on Tenax® GC (poly(2,6-Diphenyl phenylene oxide)) and
determined by thermaldesorption GC/MS techniques. Specific
approaches usingthese techniques are described in the literature
(1-3).
1.2 This protocol is designed to allow some flexibility inorder
to accommodate procedures currently in use.However, such
flexibility also results in placement ofconsiderable responsibility
with the user to documentthat such procedures give acceptable
results (i.e.,documentation of method performance within
eachlaboratory situation is required). Types ofdocumentation
required are described elsewhere in thismethod.
1.3 Compounds which can be determined by this method arenonpolar
organics having boiling points in the range ofapproximately 80E -
200EC. However, not all compoundsfalling into this category can be
determined. Table 1gives a listing of compounds for which the
method hasbeen used. Other compounds may yield satisfactoryresults
but validation by the individual user isrequired.
2. Applicable Documents
2.1 ASTM Standards:
D1356 Definitions of Terms Related to AtmosphericSampling and
Analysis.
E355 Recommended Practice for Gas ChromatographyTerms and
Relationships.
2.2 Other documents:
Existing procedures (1-3).
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U. S. EPA Technical Assistance Document (4).
3. Summary of Protocol
3.1 Ambient air is drawn through a cartridge containing
-1-2grams of Tenax and certain volatile organic compounds
aretrapped on the resin while highly volatile organiccompounds and
most inorganic atmospheric constituentspass through the cartridge.
The cartridge is thentransferred to the laboratory and
analyzed.
3.2 For analysis the cartridge is placed in a heated chamberand
purged with an inert gas. The inert gas transfersthe volatile
organic compounds from the cartridge onto acold trap and
subsequently onto the front of the GCcolumn which is held at low
temperature (e.g., -70EC).the GC column temperature is then
increased (temperatureprogrammed) and the components eluting from
the columnare identified and quantified by mass
spectrometry.Component identification is normally accomplished,
usinga library search routine, on the basis of the GCretention time
and mass spectral characteristics. Lesssophisticated detectors
(e.g., electron capture or flameionization) may be used for certain
applications buttheir suitability for a given application must
beverified by the user.
3.3 Due to the complexity of ambient air samples only
highresolution (i.e., capillary) GC techniques are consideredto be
acceptable in this protocol.
4. Significance
4.1 Volatile organic compounds are emitted into theatmosphere
from a variety of sources including industrialand commercial
facilities, hazardous waste storagefacilities, etc. Many of these
compounds are toxic;hence knowledge of the levels of such materials
in theambient atmosphere is required in order to determinehuman
health impacts.
4.2 Conventional air monitoring methods (e.g., for
workspacemonitoring) have relied on carbon adsorption
approacheswith subsequent solvent desorption. Such techniquesallow
subsequent injection of only a small portion,typically 1-5% of the
sample onto the GC system.However, typical ambient air
concentrations of thesecompounds require a more sensitive approach.
The thermal
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desorption process, wherein the entire sample isintroduced into
the analytical (GC/MS) system fulfillsthis need for enhanced
sensitivity.
5. Definitions
Definitions used in this document and any user prepared
SOPsshould be consistent with ASTM D1356(6). All abbreviationsand
symbols are defined with this document at the point ofuse.
6. Interferences
6.1 Only compounds having a similar mass spectrum and
GCretention time compared to the compound of interest willinterface
in the method. The most commonly encounteredinterferences are
structural isomers.
6.2 Contamination of the Tenax cartridge with the compound(s)of
interest is a commonly encountered problem in themethod. The user
must be extremely careful in thepreparation, storage, and handling
of the cartridgesthroughout the entire sampling and analysis
process tominimize this problem.
7. Apparatus
7.1 Gas Chromatograph/Mass Spectrometry system - should
becapable of subambient temperature programming. Unit
massresolution or better up to 800 amu. Capable of scanning30-400
amu region every 0.5-1 second. Equipped with datasystem for
instrument control as well as dataacquisition, processing and
storage.
7.2 Thermal Desorption Unit - Designed to accommodate
Tenaxcartridges in use. See Figure 2a or b.
7.3 Sampling System - Capable of accurately and preciselydrawing
an air flow of 10-500 ml/minute through the Tenaxcartridge. (See
Figure 3a or b.)
7.4 Vacuum oven - connected to water aspirator vacuum
supply.
7.5 Stopwatch.
7.6 Pyrex disks - for drying Tenax.
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7.7 Glass jar - Capped with Teflon-lined screw cap. Forstorage
of purified Tenax.
7.8 Powder funnel - for delivery of Tenax into cartridges.
7.9 Culture tubes - to hold individual glass
Tenaxcartridges.
7.10 Friction top can (paint can) - to hold clean
Tenaxcartridges.
7.11 Filter holder - stainless steel or aluminum (toaccommodate
1 inch diameter filter). Other sizes may beused if desired.
(optional)
7.12 Thermometer - to record ambient temperature.
7.13 Barometer (optional).
7.14 Dilution bottle - Two-liter with septum cap for
standardspreparation.
7.15 Teflon stirbar - 1 inch long.
7.16 Gas-tight glass syringes with stainless steel needles
-10-500 µ1 for standard injection onto GC/MS system.
7.17 Liquid microliter syringes - 5.50 µL for injecting
neatliquid standards into dilution bottle.
7.18 Oven - 60 + 5EC for equilibrating dilution flasks.
7.19 Magnetic stirrer.
7.20 Heating mantel.
7.21 Variac
7.22 Soxhlet extraction apparatus and glass thimbles -
forpurifying Tenax.
7.23 Infrared lamp - for drying Tenax.
7.24 GC column - SE-30 or alternative coating, glass capillaryor
fused silica.
7.25 Psychrometer - to determine ambient relative
humidity.(optional)
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8. Reagents and Materials
8.1 Empty Tenax cartridges - glass or stainless steel (seeFigure
1a or b).
8.2 Tenax 60/80 mesh (2,6-diphenylphenylene oxide polymer).
8.3 Glasswool - silanized.
8.4 Acetone - Pesticide quality or equivalent.
8.5 Methanol - Pesticide quality or equivalent.
8.6 Pentane - Pesticide quality or equivalent.
8.7 Helium - Ultra pure, compressed gas. (99.9999%)
8.8 Nitrogen - Ultra pure, compressed gas. (99.9999%)
8.9 Liquid nitrogen.
8.10 Polyester gloves - for handling glass Tenax cartridges.
8.11 Glass Fiber Filter - one inch diameter, to fit in
filterholder. (optional)
8.12 Perfluorotributylamine (FC-43).
8.13 Chemical Standards - Neat compounds of interest.
Highestpurity available.
8.14 Granular activated charcoal - for preventingcontamination
of Tenax cartridges during storage.
9. Cartridge Construction and Preparation
9.1 Cartridge Design
9.1.1 Several cartridge designs have been reportedin the
literature (1-3). The most common (1)is shown in Figure 1a. This
design minimizescontact of the sample with metal surfaces,which can
lead to decomposition in certaincases. However, a disadvantage of
this designis the need to rigorously avoid contaminationof the
outside portion of the cartridge sincethe entire surface is
subjected to the purgegas stream during the desorption
process.Clean polyester gloves must be worn at all
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times when handling such cartridges andexposure of the open
cartridge to ambient airmust be minimized.
9.1.2 A second common type of design (3) is shown inFigure 1b.
While this design uses a metal(stainless steel) construction, it
eliminatesthe need to avoid direct contact with theexterior surface
since only the interior ofthe cartridge is purged.
9.1.3 The thermal desorption module and samplingsystem must be
selected to be compatible withthe particular cartridge design
chosen.Typical module designs are shown in Figure 2aand b. These
designs are suitable for thecartridge designs shown in Figures 1a
and b,respectively.
9.2 Tenax Purification
9.2.1 Prior to use the Tenax resin is subjected to aseries of
solvent extraction and thermaltreatment steps. The operation should
beconducted in an area where levels of volatileorganic compounds
(other than the extractionsolvents used) are minimized.
9.2.2 All glassware used in Tenax purification aswell as
cartridge materials should bethoroughly cleaned by water rinsing
followedby an acetone rinse and dried in an oven at250EC.
9.2.3 Bulk Tenax is placed in a glass extractionthimble and held
in place with a plug of cleanglasswool. The resin is then placed in
thesoxhlet extraction apparatus and extractedsequentially with
methanol and then pentanefor 16-24 hours (each solvent)
atapproximately 6 cycles/hour. Glasswool forcartridge preparation
should be cleaned in thesame manner as Tenax.
9.2.4 The extracted Tenax is immediately placed inan open glass
dish and heated under aninfrared lamp for two hours in a hood.
Caremust be exercised to avoid over heating of theTenax by the
infrared lamp. The Tenax is thenplaced in a vacuum oven (evacuated
using a
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water aspirator) without heating for one hour.An inert gas
(helium or nitrogen) purge of 2-3ml/minute is used to aid in the
removal ofsolvent vapors. The oven temperature is thenincreased to
110EC, maintaining inert gas flowand held for one hour. The oven
temperaturecontrol is then shut off and the oven isallowed to cool
to room temperature. Prior toopening the oven, the oven is
slightlypressurized with nitrogen to preventcontamination with
ambient air. The Tenax isremoved from the oven and sieved through
a40/60 mesh sieve (acetone rinsed and ovendried) into a clean glass
vessel. If theTenax is not to be used immediately forcartridge
preparation it should be stored in aclean glass jar having a
Teflon-lined screwcap and placed in a desiccator.
9.3 Cartridge Preparation and Pretreatment
9.3.1 All cartridge materials are pre-cleaned asdescribed in
Section 9.2.2. If the glasscartridge design shown in Figure 1a
isemployed all handling should be conductedwearing polyester
gloves.
9.3.2 The cartridge is packed by placing a 0.5-lcmglasswool plug
in the base of the cartridgeand then filling the cartridge to
withinapproximately 1 cm of the top. A 0.5-1cmglasswool plug is
placed in the top of thecartridge.
9.3.3 The cartridges are then thermally conditionedby heating
for four hours at 270EC under aninert gas (helium) purge (100 - 200
ml/min).
9.3.4 After the four hour heating period thecartridges are
allowed to cool. Cartridges ofthe type shown in Figure 1a are
immediatelyplaced (without cooling) in clean culturetubes having
Teflon-lined screw caps with aglasswool cushion at both the top and
thebottom. Each tube should be shaken to ensurethat the cartridge
is held firmly in place.Cartridges of the type shown in Figure 1b
areallowed to cool to room temperature underinert gas purge and are
then closed withstainless steel plugs.
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VMAX'VbxW
1.5
9.3.5 The cartridges are labeled and placed in atightly sealed
metal can (e.g., paint can orsimilar friction top container).
Forcartridges of the type shown in Figure 1a theculture tube, not
the cartridge, is labeled.
9.3.6 Cartridges should be used for sampling within2 weeks after
preparation and analyzed withintwo weeks after sampling. If
possible thecartridges should be stored at -20EC in aclean freezer
(i.e., no solvent extracts orother sources of volatile organics
containedin the freezer).
10. Sampling
10.1 Flow Rate and Total Volume Selection
10.1.1 Each compound has a characteristic retentionvolume
(liters of air per gram of adsorbent)which must not be exceeded.
Since theretention volume is a function of temperature,and possibly
other sampling variables, onemust include an adequate margin of
safety toensure good collection efficiency. Someconsiderations and
guidance in this regardareprovided in a recent report
(5).Approximate breakthrough volumes at 38EC(100EF) in liters/gram
of Tenax are providedin Table 1. These retention volume data
aresupplied only as rough guidance and aresubject to considerable
variability, dependingon cartridge design as well as
samplingparameters and atmospheric conditions.
10.1.2 To calculate the maximum total volume of airwhich can be
sampled use the followingequation:
where
V is the calculated maximum total volume inMAXliters.
V is the breakthrough volume for the leastbretained compound of
interest (Table 1)in liters per gram of Tenax.
W is the weight of Tenax in the cartridge,in grams.
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QMAX'VMAXt
x1000
B'QMAXBr 2
1.5 is a dimensionless safety factor to allowfor variability in
atmospheric conditions.This factor is appropriate for temperatures
inthe range of 25-30EC. If higher temperaturesare encountered the
factor should be increased(i.e. maximum total volume
decreased).
10.1.3 To calculate maximum flow rate use thefollowing
equation:
where
Q is the calculated maximum flow rate inMAXmilliliters per
minute.
t is the desired sampling time in minutes.Times greater than 24
hours (1440minutes) generally are unsuitable becausethe flow rate
required is too low to beaccurately maintained.
10.1.4 The maximum flow rate Q should yield aMAXlinear flow
velocity of 50-500 cm/minute.Calculate the linear velocity
corresponding tothe maximum flow rate using the
followingequation:
where
B is the calculated linear flow velocity incentimeters per
minute.
r is the internal radius of the cartridgein centimeters.
If B is greater then 500 centimeters perminute either the total
sample flow rate (V )MAXshould be reduced or the sample flow rate(Q
) should be reduced by increasing theMAXcollection time. If B is
less then 50centimeters per minute the sampling rate (Q )MAXshould
be increased by reducing the samplingtime. The total sample value
(V ) cannot beMAXincreased due to component breakthrough.
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10.1.5 The flow rate calculated as described abovedefines the
maximum flow rate allowed. Ingeneral, one should collect additional
samplesin parallel, for the same time period but atlower flow
rates. This practice yields ameasure of quality control and is
furtherdiscussed in the literature (5). In general,flow rates 2 to
4 fold lower than the maximumflow rate should be employed for the
parallelsamples. In all cases a constant flow rateshould be
achieved for each cartridge sinceaccurate integration of the
analyteconcentration requires that the flow beconstant over the
sampling period.
10.2 Sample Collection
10.2.1 Collection of an accurately known volume ofair is
critical to the accuracy of theresults. For this reason the use of
mass flowcontrollers, rather than conventional needlevalves or
orifices is highly recommended,especially at low flow velocities
(e.g., lessthan 100 milliliters/minute). Figure 3aillustrates a
sampling system utilizing massflow controllers. This system readily
allowsfor collection of parallel samples. Figure 3bshows a
commercially available system based onneedle valve flow
controllers.
10.2.2 Prior to sample collection insure that thesampling flow
rate has been calibrated over arange including the rate to be used
forsampling, with a "dummy" Tenax cartridge inplace. Generally
calibration is accomplishedusing a soap bubble flow meter or
calibratedwet test meter. The flow calibration deviceis connected
to the flow exit, assuming theentire flow system is sealed. ASTM
MethodD3686 describes an appropriate calibrationscheme, not
requiring a sealed flow systemdownstream of the pump.
10.2.3 The flow rate should be checked before andafter each
sample collection. If the samplinginterval exceeds four hours the
flow rateshould be checked at an intermediate pointduring sampling
as well. In general, arotameter should be included, as shown
inFigure 3b, to allow observation of thesampling flow rate without
disrupting thesampling process.
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10.2.4 To collect an air sample the cartridges areremoved from
the sealed container just priorto initiation of the collection
process. Ifglass cartridges (Figure 1a) are employed theymust be
handled only with polyester gloves andshould not contact any other
surfaces.
10.2.5 A particulate filter and holder are placed onthe inlet to
the cartridges and the exit endof the cartridge is connected to the
samplingapparatus. In many sampling situations theuse of a filter
is not necessary if only thetotal concentration of a component is
desired.Glass cartridges of the type shown in Figure1a are
connected using teflon ferrules andSwagelok (stainless steel or
teflon) fittings.Start the pump and record the followingparameters
on an appropriate data sheet(Figure 4): data, sampling location,
time,ambient temperature, barometric pressure,relative humidity,
dry gas meter reading (ifapplicable), flow rate, rotameter reading
(ifapplicable), cartridge number and dry gasmeter serial
number.
10.2.6 Allow the sampler to operate for the desiredtime,
periodically recording the variableslisted above. Check flow rate
at the midpointof the sampling interval if longer than fourhours.
At the end of the sampling periodrecord the parameters listed in
10.2.5 andcheck the flow rate and record the value. Ifthe flows at
the beginning and end of thesampling period differ by more than 10%
thecartridge should be marked as suspect.
10.2.7 Remove the cartridges (one at a time) andplace in the
original container (use glovesfor glass cartridges). Seal the
cartridges orculture tubes in the friction-top cancontaining a
layer of charcoal and package forimmediate shipment to the
laboratory foranalysis. Store cartridges at reducedtemperature
(e.g., -20EC) before analysis ifpossible to maximize storage
stability.
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QA'Q1%Q
2%...QNN
Vm'T×QA1000
Vs'Vm×PA760
×298
273%tA
10.2.8 Calculate and record the average sample ratefor each
cartridge according to the followingequation:
where
Q = Average flow rate in ml/minute.AQ , Q , ....Q = Flow rates
determined at1 2 Nbeginning, end, and intermediate points
duringsampling.N = Number of points averaged.
10.2.9 Calculate and record the total volumetric flowfor each
cartridge using the followingequation:
where
V = Total volume sampled in liters atmmeasured temperature and
pressure.
T = Stop time.2T = Start time.1T = Sampling time = T = T ,
minutes2 1
10.2.10 The total volume (V ) at standard conditions,s25EC and
760 mmHg, is calculated from thefollowing equation:
where
P = Average barometric pressure, mmHgAt = Average ambient
temperature, EC.A
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11. GC/MS Analysis
11.1 Instrument Set-up
11.1.1 Considerable variation from one laboratory toanother is
expected in terms of instrumentconfiguration. Therefore each
laboratory mustbe responsible for verifying that theirparticular
system yields satisfactory results.Section 14 discusses specific
performancecriteria which should be met.
11.1.2 A block diagram of the typical GC/MS systemrequired for
analysis of Tenax cartridges isdepicted in Figure 5. The operation
of suchdevices is described in 11.2.4. The thermaldesorption module
must be designed toaccommodate the particular
cartridgeconfiguration. Exposure of the sample tometal surfaces
should be minimized and onlystainless steel, or nickel metal
surfacesshould be employed. The volume of tubing andfittings
leading from the cartridge to the GCcolumn must be minimized and
all areas must bewell-swept by helium carrier gas.
11.1.3 The GC column inlet should be capable of beingcooled to
-70EC and subsequently increasedrapidly to approximately 30EC. This
can bemost readily accomplished using a GC equippedwith subambient
cooling capability (liquidnitrogen) although other approaches such
asmanually cooling the inlet of the column inliquid nitrogen may be
acceptable.
11.1.4 The specific GC column and temperature programemployed
will be dependent on the specificcompounds of interest. Appropriate
conditionsare described in the literature (1-3). Ingeneral a
nonpolar stationary phase (e.g., SE-30, OV-1) temperature
programmed from 30EC to200EC at 8E/minute will be suitable.
Fusedsilica bonded phase columns are preferable toglass columns
since they are more rugged andcan be inserted directly into the MS
ionsource, thereby eliminating the need for aGC/MS transfer
line.
11.1.5 Capillary column dimensions of 0.3 mm ID and50 meters
long are generally appropriatealthough shorter lengths may be
sufficient inmany cases.
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11.1.6 Prior to instrument calibration or sampleanalysis the
GC/MS system is assembled asshown in Figure 5. Helium purge
flows(through the cartridge) and carrier flow areset at
approximately 10 ml/minute and 1-2ml/minute respectively. If
applicable, theinjector sweep flow is set at 2-4 ml/minute.
11.1.7 Once the column and other system componentsare assembled
and the various flowsestablished the column temperature isincreased
to 250EC for approximately fourhours (or overnight if desired) to
conditionthe column.
11.1.8 The MS and data system are set according tothe
manufacturer's instructions. Electronimpact ionization (70eV) and
an electronmultiplier gain of approximately 5 x 10 should4
be employed. Once the entire GC/MS system hasbeen setup the
system is calibrated asdescribed in Section 11.2. The user
shouldprepare a detailed standard operatingprocedure (SOP)
describing this process forthe particular instrument being
used.
11.2 Instrument Calibration
11.2.1 Tuning and mass standardization of the MSsystem is
performed according tomanufacturer's instructions and
relevantinformation from the user prepared
SOP.Perfluorotributylamine should generally beemployed for this
purpose. The material isintroduced directly into the ion source
thougha molecular leak. The instrumental parameters(e.g., lens
voltages, resolution, etc.) shouldbe adjusted to give the relative
ionabundances shown in Table 2 as well asacceptable resolution and
peak shape. Ifthese approximate relative abundances cannotbe
achieved, the ion source may requirecleaning according to
manufacturer'sinstructions. In the event that the user'sinstrument
cannot achieve these relative ionabundances, but is otherwise
operatingproperly, the user may adopt another set ofrelative
abundances as performance criteria.However, these alternate values
must berepeatable on a day-to-day basis.
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11.2.2 After the mass standardization and tuningprocess has been
completed and the appropriatevalues entered into the data system
the usershould then calibrate the entire system byintroducing known
quantities of the standardcomponents of interest into the system.
Threealternate procedures may be employed for thecalibration
process including 1) directsyringe injection of dilute vapor
phasestandards, prepared in a dilution bottle, ontothe GC column,
2) injection of dilute vaporphase standards into a carrier gas
streamdirected through the Tenax cartridge, and 3)introduction of
permeation or diffusion tubestandards onto a Tenax cartridge.
Thestandards preparation procedures for each ofthese approaches are
described in Section 13.The following paragraphs describe
theinstrument calibration process for each ofthese approaches.
11.2.3 If the instrument is to be calibrated bydirect injection
of a gaseous standard, astandard is prepared in a dilution bottle
asdescribed in Section 13.1. The GC column iscooled to -70EC (or,
alternately, a portion ofthe column inlet is manually cooled
withliquid nitrogen). The MS and data system isset up for
acquisition as described in therelevant user SOP. The ionization
filamentshould be turned off during the initial 2-3minutes of the
run to allow oxygen and otherhighly volatile components to elute.
Anappropriate volume (less than 1 ml) of thegaseous standard is
injected onto the GCsystem using an accurately calibrated gastight
syringe. The system clock is startedand the column is maintained at
-70EC (orliquid nitrogen inlet cooling) for 2 minutes.The column
temperature is rapidly increased tothe desired initial temperature
(e.g., 30EC).The temperature program is started at aconsistent time
(e.g., four minutes) afterinjection. Simultaneously the
ionizationfilament is turned on and data acquisition isinitiated.
After the last component ofinterest has eluted acquisition is
terminatedand the data is processed as described inSection 11.2.5.
The standard injectionprocess is repeated using different
standardvolumes as desired.
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11.2.4 If the system is to be calibrated by analysisof spiked
Tenax cartridges a set of cartridgesis prepared as described in
Sections 13.2 or13.3. Prior to analysis the cartridges arestored as
described in Section 9.3. If glasscartridges (Figure 1a) are
employed care mustbe taken to avoid direct contact, as
describedearlier. The GC column is cooled to -70EC,the collection
loop is immersed in liquidnitrogen and the desorption module
ismaintained at 250EC. The inlet valve isplaced in the desorb mode
and the standardcartridge is placed in the desorption module,making
certain that no leakage or purge gasoccurs. The cartridge is purged
for 10minutes and then the inlet valve is placed inthe inject mode
and the liquid nitrogen sourceremoved from the collection trap. The
GCcolumn is maintained at -70EC for two minutesand subsequent steps
are as described in11.2.3. After the process is complete
thecartridge is removed from the desorptionmodule and stored for
subsequent use asdescribed in Section 9.3.
11.2.5 Data processing for instrument calibrationinvolves
determining retention times, andintegrated characteristic ion
intensities foreach of the compounds of interest. Inaddition, for
at least one chromatographicrun, the individual mass spectra should
beinspected and compared to reference spectra toensure proper
instrumental performance. Sincethe steps involved in data
processing arehighly instrument specific, the user shouldprepare a
SOP describing the process forindividual use. Overall performance
criteriafor instrument calibration are provided inSection 14. If
these criteria are notachieved the user should refine
theinstrumental parameters and/or operatingprocedures to meet these
criteria.
11.3 Sample Analysis
11.3.1 The sample analysis process is identical tothat described
in Section 11.2.4 for theanalysis of standard Tenax cartridges.
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11.3.2 Data processing for sample data generallyinvolves 1)
qualitatively determining thepresence or absence of each component
ofinterest on the basis of a set ofcharacteristic ions and the
retention timeusing a reverse-search software routine,
2)quantification of each identified component byintegrating the
intensity of a characteristicion and comparing the value to that of
thecalibration standard, and 3) tentativeidentification of other
components observedusing a forward (library) search
softwareroutine. As for other user specificprocesses, a SOP should
be prepared describingthe specific operations for each
individuallaboratory.
12. Calculations
12.1 Calibration Response Factors
12.1.1 Data from calibration standards is used tocalculate a
response factor for each componentof interest. Ideally the process
involvesanalysis of at least three calibration levelsof each
component during a given day anddetermination of the response
factor(area/nanogram injected) from the linear leastsquares fit of
a plot of nanograms injectedversus area (for the characteristic
ion). Ingeneral quantities of component greater than1000 nanograms
should not be injected becauseof column overloading and/or MS
responsenonlinearity.
12.1.2 In practice the daily routine may not alwaysallow
analysis of three such calibrationstandards. In this situation
calibration datafrom consecutive days may be pooled to yield
aresponse factor, provided that analysis ofreplicate standards of
the same concentrationare shown to agree within 20% on
theconsecutive days. One standard concentration,near the midpoint
of the analytical range ofinterest, should be chosen for injection
everyday to determine day-to-day responsereproducibility.
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Y'A%BX%CX 2
YA'A%BXA%CXA
CA'XAVS
12.1.3 If substantial nonlinearity is present in thecalibration
curve a nonlinear least squaresfit (e.g., quadratic) should be
employed.This process involves fitting the data to thefollowing
equation:
where
Y = peak areaX = quantity of component, nanogramsA, B, and C are
coefficients in the equation
12.2 Analyte Concentrations
12.2.1 Analyte quantities on a sample cartridge arecalculated
from the following equation:
where
Y is the area of the analyte characteristic ionAfor the sample
cartridge.
X is the calculated quantity of analyte on theAsample cartridge,
in nanograms.
A, B, and C are the coefficients calculated fromthe calibration
curve described in Section 12.1.3.
12.2.2 If instrumental response is essentially linearover the
concentration range of interest alinear equation (C=O in the
equation above)can be employed.
12.2.3 Concentration of analyte in the original airsample is
calculated from the followingequation:
where
C is the calculated concentration of analyte inAnanograms per
liter.
V and X are as previously defined in SectionS A10.2.10 and
12.2.1, respectively.
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WT'WIVI×VB
VT'WTd
13. Standard Preparation
13.1 Direct Injection
13.1.1 This process involves preparation of adilution bottle
containing the desiredconcentrations of compounds of interest
fordirect injection onto the GC/MS system.
13.1.2 Fifteen three-millimeter diameter glass beadsand a
one-inch Teflon stirbar are placed in aclean two-liter glass septum
capped bottle andthe exact volume is determined by weighing
thebottle before and after filling with deionizedwater. The bottle
is then rinsed with acetoneand dried at 200EC.
13.1.3 The amount of each standard to be injectedinto the vessel
is calculated from the desiredinjection quantity and volume using
thefollowing equation:
where
W is the total quantity of analyte to beTinjected into the
bottle in milligrams
W is the desired weight of analyte to beIinjected onto the GC/MS
system or spikedcartridge in nanograms
V is the desired GC/MS or cartridge injectionIvolume (should not
exceed 500) in microliters.
V is total volume of dilution bottle determinedBin 13.1.1, in
liters.
13.1.4 The volume of the neat standard to be injectedinto the
dilution bottle is determined usingthe following equation:
where
V is the total volume of neat liquid to beTinjected in
microliters.
d is the density of the neat standard in gramsper
milliliter.
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13.1.6 The bottle is placed in a 60EC oven for atleast 30
minutes prior to removal of a vaporphase standard.
13.1.7 To withdraw a standard for GC/MS injection thebottle is
removed from the oven and stirredfor 10-15 seconds. A suitable
gas-tightmicrober syringe, warmed to 60EC, is insertedthrough the
septum cap and pumped three timesslowly. The appropriate volume of
sample(approximately 25% larger than the desiredinjection volume)
is drawn into the syringeand the volume is adjusted to the exact
valuedesired and then immediately injected over a5-10 seconds
period onto the GC/MS system asdescribed in Section 11.2.3.
13.2 Preparation of Spiked Cartridges by Vapor Phase
Injection
13.2.1 This process involves preparation of adilution bottle
containing the desiredconcentrations of the compound(s) of
interestas described in 13.1 and injecting the desiredvolume of
vapor into a flowing inert gasstream directed through a clean
Tenaxcartridge.
13.2.2 A helium purge system is assembled wherein thehelium flow
20-30 mL/minute is passed througha stainless steel Tee fitted with
a septuminjector. The clean Tenax cartridge isconnected downstream
of the tee usingappropriate Swagelok fittings. Once thecartridge is
placed in the flowing gas streamthe appropriate volume vapor
standard, in thedilution bottle, is injected through theseptum as
described in 13.1.6. The syringe isflushed several times by
alternately fillingthe syringe with carrier gas and displacingthe
contents into the flow stream, withoutremoving the syringe from the
septum. Carrierflow is maintained through the cartridge
forapproximately 5 minutes after injection.
13.3 Preparation of Spiked Traps Using Permeation or
DiffusionTubes
13.3.1 A flowing stream of inert gas containing knownamounts of
each compound of interest isgenerated according to ASTM Method
D3609(6).
-
Note that a method of accuracy maintainingtemperature within +
0.1EC is required and thesystem generally must be equilibrated for
atleast 48 hours before use.
13.3.2 An accurately known volume of the standard gasstream
(usually 0.1-1 liter) is drawn througha clean Tenax cartridge using
the samplingsystem described in Section 10.2.1, or asimilar system.
However, if mass flowcontrollers are employed they must
becalibrated for the carrier gas used in Section
13.3.1 (usually nitrogen). Use of air as the carriergas for
permeation systems is not recommended,unless the compounds of
interest are known tobe highly stable in air.
13.3.3 The spiked cartridges are then stored orimmediately
analyzed as in Section 11.2.4.
14. Performance Criteria and Quality Assurance
This section summarizes quality assurance (QA) measures
andprovides guidance concerning performance criteria which shouldbe
achieved within each laboratory. In many cases thespecific QA
procedures have been described within theappropriate section
describing the particular activity (e.g.,parallel sampling).
14.1 Standard Operating Procedures (SOPs)
14.1.1 Each user should generate SOPs describing thefollowing
activities as they are performed intheir laboratory:1) assembly,
calibration, and operation of
the sampling system,2) preparation, handling and storage of
Tenax cartridges,3) assembly and operation of GC/MS system
including the thermal desorptionapparatus and data system,
and
4) all aspects of data recording andprocessing.
14.1.2 SOPs should provide specific stepwiseinstructions and
should be readily availableto, and understood by, the
laboratorypersonnel conducting the work.
-
14.2 Tenax Cartridges Preparation
14.2.1 Each batch of Tenax cartridges prepared (asdescribed in
Section 9) should be checked forcontamination by analyzing one
cartridgeimmediately after preparation. While analysiscan be
accomplished by GC/MS, manylaboratories may choose to use GC/FID
due tologistical and cost considerations.
14.2.2. Analysis by GC/FID is accomplished asdescribed for GC/MS
(Section 11) except foruse of FID detection.
14.2.3 While acceptance criteria can vary dependingon the
components of interest, at a minimumthe clean cartridge should be
demonstrated tocontain less than one fourth of the minimumlevel of
interest for each component. Formost compounds the blank level
should be lessthan 10 nanograms per cartridge in order to
beacceptable. More rigid criteria may beadopted, if necessary,
within a specificlaboratory. If a cartridge does not meetthese
acceptance criteria the entire lotshould be rejected.
14.3 Sample Collection
14.3.1 During each sampling event at least one cleancartridge
will accompany the samples to thefield and back to the laboratory,
withoutbeing used for sampling, to serve as a fieldblank. The
average amount of material foundon the field blank cartridge may be
subtractedfrom the amount found on the actual samples.However, if
the blank level is greater than25% of the sample amount, data for
thatcomponent must be identified as suspect.
14.3.2 During each sampling event at least one set ofparallel
samples (two or more samplescollected simultaneously) will be
collected,preferably at different flow rates asdescribed in Section
10.1. If agreementbetween parallel samples is not generallywithin +
25% the user should collect parallelsamples on a much more frequent
basis (perhapsfor all sampling points). If a trend of lowerapparent
concentrations with increasing flow
-
rate is observed for a set of parallel samplesone should
consider using a reduced flow rateand longer sampling interval if
possible. Ifthis practice does not improve thereproducibility
further evaluation of themethod performance for the compound
ofinterest may be required.
14.3.3 Backup cartridges (two cartridges in series)should be
collected with each sampling event.Backup cartridges should contain
less than 20%of the amount of components of interest foundin the
front cartridges, or be equivalent tothe blank cartridge level,
whichever isgreater. The frequency of use of backupcartridges
should be increased if increasedflow rate is shown to yield reduced
componentlevels for parallel sampling. This practicewill help to
identify problems arising frombreakthrough of the component of
interestduring sampling.
14.4 GC/MS Analysis
14.4.1 Performance criteria for MS tuning and masscalibration
have been discussed in Section11.2 and Table 2. Additional criteria
may beused by the laboratory if desired. Thefollowing sections
provide performanceguidance and suggested criteria fordetermining
the acceptability of the GC/MSsystem.
14.4.2 Chromatographic efficiency should be evaluatedusing
spiked Tenax cartridges since thispractice tests the entire system.
In generala reference compound such as perfluorotolueneshould be
spiked onto a cartridge at the 100nanogram level as described in
Section 13.2 or13.3. The cartridge is then analyzed by GC/MSas
described in Section 11.4. Theperfluorotoluene (or other reference
compound)peak is then plotted on an expanded time scaleso that its
width at 10% of the peak can becalculated, as shown in Figure 6.
The widthof the peak at 10% height should not exceed 10seconds.
More stringent criteria may berequired for certain applications.
Theasymmetry factor (see Figure 6) should bebetween 0.8 and 2.0.
The asymmetry factor for
-
DL'A%3.3S
any polar or reactive compounds should bedetermined using the
process described above.If peaks are observed that exceed the
peakwidth or asymmetry factor criteria above, oneshould inspect the
entire system to determineif unswept zones or cold spots are
present inany of the fittings and are necessary. Somelaboratories
may choose to evaluate columnperformance separately by direct
injection ofa test mixture onto the GC column. Suitableschemes for
column evaluation have beenreported in the literature (7). Such
schemescannot be conducted by placing the substancesonto Tenax
because many of the compounds(e.g., acids, bases, alcohols)
contained inthe test mix are not retained, or degrade, onTenax.
14.4.3 The system detection limit for each componentis
calculated from the data obtained forcalibration standards. The
detection limit isdefined as
where
DL is the calculated detection limit innanograms injected.
A is the intercept calculated in Section12.1.1 or 12.1.3.
S is the standard deviation of replicatedeterminations of the
lowest levelstandard (at least three suchdeterminations are
required).
In general the detection limit should be 20nanograms or less and
for many applicationsdetection limits of 1-5 nanograms may
berequired. The lowest level standard shouldyield a signal to noise
ratio, from the totalion current response, of approximately 5.
14.4.4 The relative standard deviation for replicateanalyses of
cartridges spiked at approximately10 times the detection limit
should be 20% orless. Day to day relative standard deviationshould
be 25% or less.
-
14.4.5 A useful performance evaluation step is theuse of an
internal standard to track systemperformance. This is accomplished
by spikingeach cartridge, including blank, sample, andcalibration
cartridges with approximately 100nanograms of a compound not
generally presentin ambient air (e.g., perfluorotoluene).
Theintegrated ion intensity for this compoundhelps to identify
problems with a specificsample. In general the user should
calculatethe standard deviation of the internalstandard response
for a given set of samplesanalyzed under identical tuning
andcalibration conditions. Any sample giving avalue greater than +
2 standard deviationsfrom the mean (calculated excluding
thatparticular sample) should be identified assuspect. Any marked
change in internalstandard response may indicate a need
forinstrument recalibration.
-
REFERENCES
1. Krost, K. J., Pellizzari, E. D., Walburn, S. G., and
Hubbard,S. A., "Collection and Analysis of Hazardous
OrganicEmissions", Analytical Chemistry, 54, 810-817, 1982.
2. Pellizzari, E. O. and Bunch, J. E., "Ambient Air
CarcinogenicVapors-Improved Sampling and Analytical Techniques and
FieldStudies", EPA-600/2-79-081, U.S. Environmental
ProtectionAgency, Research Triangle Park, North Carolina, 1979.
3. Kebbekus, B. B. and Bozzelli, J. W., "Collection and
Analysisof Selected Volatile Organic Compounds in Ambient Air",
Proc.Air Pollution Control Assoc., Paper No. 82-65.2. Air
Poll.Control Assoc., Pittsburgh, Pennsylvania, 1982.
4. Riggin, R. M., "Technical Assistance Document for Sampling
andAnalysis of Toxic Organic Compounds in Ambient Air",
EPA-600/4-83-027, U.S. Environmental Protection Agency,
ResearchTriangle Park, North Carolina, 1983.
5. Walling, J. F., Berkley, R. E., Swanson, D. H., and Toth,
F.J. "Sampling Air for Gaseous Organic Chemical-Applications
toTenax", EPA-600/7-54-82-059, U.S. Environmental ProtectionAgency,
Research Triangle Park, North Carolina, 1982.
6. Annual Book of ASTM Standards, Part 11.03,
"AtmosphericAnalysis", American Society for Testing and
Materials,Philadelphia, Pennsylvania.
7. Grob, K., Jr., Grob, G., and Grob, K.,
"ComprehensiveStandardized Quality Test for Glass Capillary
Columns", J.Chromatog., 156, 1-20, 1978.
-
TABLE 1. RETENTION VOLUME ESTIMATES FOR COMPOUNDS ON TENAX
ESTIMATED RETENTION VOLUME ATCOMPOUND 100EF
(38EC)-LITERS/GRAM
Benzene 19Toluene 97Ethyl Benzene 200Xylene(s) -200Cumene
440n-Heptane 20l-Heptene 40
Chloroform 8Carbon Tetrachloride 81,2-Dichloroethane
101,1,1-Trichloroethane 6Tetrachloroethylene 80Trichloroethylene
201,2-Dichloropropane 301,3-Dichloropropane 90Chlorobenzene
150Bromoform 100Ethylene Dibromide 60Bromobenzene 300
-
TABLE 2. SUGGESTED PERFORMANCE CRITERIA FOR RELATIVE ION
ABUNDANCES FROM FC-43 MASS CALIBRATION
% RELATIVEM/E ABUNDANCE
51 1.8 + 0.5 69 100100 12.0 + 1.5119 12.0 + 1.5131 35.0 + 3.5169
3.0 + 0.4219 24.0 + 2.5264 3.7 + 0.4314 0.25 + 0.1
-
Tenax
~ 1.5 Grams (6 cm Bed Depth)
Glass Wool Plugs
(0.5 cm Long)
(a) Glass Cartridge
Glass Cartridge
(13.5 mm ODx
100 mm Long)
1/2" to
1/8"
Reducing
UnionGlass Wool
Plugs
(0.5 cm Long)
1/8" End Cap
Metal Cartridge
(12.7 mm OD x
100 mm Long)
Tenax
~ 1.5 Grams (7 cm Bed Depth)
(b) Metal Cartridge
1/2"
Swagelok
Fitting
FIGURE 1. TENAX CARTRIDGE DESIGNS
-
Teflon
Compression
Seal
Purge
Gas
Cavity for
Tenax
Cartridge
Latch for
Compression
Seal
Effluent to
6-Port Valve
To GC/MS
Vent
Freeze-Out
Loop
Liquid
Nitrogen
Coolant
Carrier
Gas
(a) Glass Cartridges (Compression Fit)
Purge
Gas
Swagelok
End Fittings
Heated Block
Tenax
Trap
Effluent to
6-Port Valve
To GC/MS
Vent
Liquid
Nitrogen
Coolant
Carrier
Gas
(b) Metal Cartridges (Swagelok Fittings)
Figure 2. Tenax Cartridge Desorption Modules
FIGURE 2. TENAX CARTRIDGE DESORPTION MODULES
-
Couplings
to Connect
Tenax
Cartridges
Mass Flow
ControllersOilless
Pump
Vent(a) Mass Flow Control
Vent
Dry
Test
Meter
Rotameter
Needle
Valve
Pump
Coupling to
Connect Tenax
Cartridge(b) Needle Valve ControlFigure 3. Typical Sampling
System Configurations
FIGURE 3. TYPICAL SAMPLING SYSTEM CONFIGURATIONS
-
SAMPLING DATA SHEET(One Sample Per Data Sheet)
PROJECT: DATE(S) SAMPLED:
SITE: TIME PERIOD SAMPLED:
LOCATION: OPERATOR:
INSTRUMENT MODEL NO: CALIBRATED BY:
PUMP SERIAL NO:
SAMPLING DATA
Sample Number: Start Time: Stop Time: *Dry Gas* * Flow
*Ambient*Barometric* * * Meter *Rotameter*Rate,*Q* Temp. *Pressure,
*Relative * Time*Reading* Reading *ml/Min * EC * mmHg *Humidity,%*
Comments))))))3)))))))3)))))))))3)))))))3)))))))3))))))))))3))))))))))3))))))))))1.
* * * * * *
*))))))3)))))))3)))))))))3)))))))3)))))))3))))))))))3))))))))))3))))))))))2.
* * * * * *
*))))))3)))))))3)))))))))3)))))))3)))))))3))))))))))3))))))))))3))))))))))3.
* * * * * *
*))))))3)))))))3)))))))))3)))))))3)))))))3))))))))))3))))))))))3))))))))))4.
* * * * * *
*))))))3)))))))3)))))))))3)))))))3)))))))3))))))))))3))))))))))3))))))))))N.
* * * * * *
*))))))2)))))))2)))))))))2)))))))2)))))))2))))))))))2))))))))))2))))))))))
Total Volume Data**
V = (Final - Initial) Dry Gas Meter Reading, or = Litersm
= Q + Q + Q ...Q x 1 = Liters1 2 3 N N 1000 x (Sampling Time in
Minutes)
* Flowrate from rotameter or soap bubble calibrator (specify
which).** Use data from dry gas meter if available.
FIGURE 4. EXAMPLE SAMPLING DATA SHEET
-
Purge
Gas
Thermal
Desorption
Chamber
6-Port High-Temperature
Valve
Heated
Blocks
Carrier
Gas Liquid
Nitrogen
Coolant
Freeze-Out Loop
Vent
Capillary
Gas
Chromatograph
Mass
Spectrometer
Data
System
Figure 5. Block Diagram of Analytical System
FIGURE 5. BLOCK DIAGRAM OF ANALYTICAL SYSTEM
-
E
A B C
D
Asymmetry Factor =
BC
AB
Example Calculation:
Peak Height = DE = 100 mm
10% Peak Height = BD = 10 mm
Peak Width at 10% Peak Height = AC = 23 mm
Therefore: Asymmetry Factor = 12
11
= 1.1
AB = 11 mm
BC = 12 mm
FIGURE 6. PEAK ASYMMETRY CALCULATION
-
METHOD TO-2Revision 1.0April, 1984
METHOD FOR THE DETERMINATION OF VOLATILE ORGANIC COMPOUNDS
INAMBIENT AIR BY CARBON MOLECULAR SIEVE ADSORPTION AND
GASCHROMATOGRAPHY/MASS SPECTROMETRY (GC/MS)
1. Scope
1.1 This document describes a procedure for collection
anddetermination of selected volatile organic compoundswhich can be
captured on carbon molecular sieve (CMS)adsorbents and determined
by thermal desorption GC/MStechniques.
1.2 Compounds which can be determined by this method arenonpolar
and nonreactive organics having boiling pointsin the range -15 to
+120EC. However, not all compoundsmeeting these criteria can be
determined. Compounds forwhich the performance of the method has
been documentedare listed in Table 1. The method may be extended
toother compounds but additional validation by the user isrequired.
This method has been extensively used in asingle laboratory.
Consequently, its generalapplicability has not been thoroughly
documented.
2. Applicable Documents
2.1 ASTM StandardsD 1356 Definitions of Terms Related to
AtmosphericSampling and Analysis.E 355 Recommended Practice for Gas
Chromatography Termsand Relationships.
2.2 Other DocumentsAmbient Air Studies (1,2).U.S. EPA Technical
AssistanceDocument (3).
3. Summary of Method
3.1 Ambient air is drawn through a cartridge containing -0.4of a
carbon molecular sieve (CMS) adsorbent. Volatileorganic compounds
are captured on the adsorbent whilemajor inorganic atmospheric
constituents pass through (orare only partially retained). After
sampling, thecartridge is returned to the laboratory for
analysis.
-
3.2 Prior to analysis the cartridge is purged with 2-3 litersof
pure, dry air (in the same direction as sample flow)to remove
adsorbed moisture.
3.3 For analysis the cartridge is heated to 350E-400EC,
underhelium purge and the desorbed organic compounds arecollected
in a specially designed cryogenic trap. Thecollected organics are
then flash evaporated onto acapillary column GC/MS system (held at
-70EC). Theindividual components are identified and
quantifiedduring a temperature programmed chromatographic run.
3.4 Due to the complexity of ambient air samples, only
highresolution (capillary column) GC techniques areacceptable for
most applications of the method.
4. Significance
4.1 Volatile organic compounds are emitted into theatmosphere
from a variety of sources including industrialand commercial
facilities, hazardous waste storage andtreatment facilities, etc.
Many of these compounds aretoxic; hence knowledge of the
concentration of suchmaterials in the ambient atmosphere is
required in orderto determine human health impacts.
4.2 Traditionally air monitoring methods for volatile
organiccompounds have relied on carbon adsorption followed
bysolvent desorption and GC analysis. Unfortunately, suchmethods
are not sufficiently sensitive for ambient airmonitoring, in most
cases, because only a small portionof the sample is injected onto
the GC system. Recentlyon-line thermal desorption methods, using
organicpolymeric adsorbents such as Tenax® GC, have been usedfor
ambient air monitoring. The current method uses CMSadsorbents (e.g.
Spherocarb®) to capture highly volatileorganics (e.g., vinyl
chloride) which are not collectedon Tenax®. The use of on-line
thermal desorption GS/MSyields a sensitive, specific analysis
procedure.
5. Definitions
Definitions used in this document and any user prepared
SOPsshould be consistent with ASTM D1356 (4). All abbreviationsand
symbols are defined with this document at the point ofuse.
-
6. Interferences
6.1 Only compounds having a mass spectrum and GC retentiontime
similar to the compound of interest will interferein the method.
The most commonly encounteredinterferences are structural
isomers.
6.2 Contamination of the CMS cartridge with the compound(s)of
interest can be a problem in the method. The usermust be careful in
the preparation, storage, and handlingof the cartridges through the
entire process to minimizecontamination.
7. Apparatus
7.1 Gas Chromatograph/Mass Spectrometry system - must becapable
of subambient temperature programming. Unit massresolution to 800
amu. Capable of scanning 30-300 amuregion every 0.5-0.8 seconds.
Equipped with data systemfor instrument control as well as data
acquisition,processing and storage.
7.2 Thermal Desorption Injection Unit - Designed toaccommodate
CMS cartridges in use (See Figure 3) andincluding cryogenic trap
(Figure 5) and injection valve(Carle Model 5621 or equivalent).
7.3 Sampling System - Capable of accurately and preciselydrawing
an air flow of 10-500 ml/minute through the CMScartridge. (See
Figure 2a or b.)
7.4 Dewar flasks - 500 mL and 5 liter.
7.5 Stopwatches.
7.6 Various pressure regulators and valves - for
connectingcompressed gas cylinders to GC/MS system.
7.7 Calibration gas - In aluminum cylinder. Prepared by useror
vendor. For GC/MS calibration.
7.8 High pressure apparatus for preparing calibration
gascylinders (if conducted by user). Alternatively, customprepared
gas mixtures can be purchased from gas supplyvendors.
7.9 Friction top can (e.g. one-gallon paint-can) - With layerof
activated charcoal to hold clean CMS cartridges.
7.10 Thermometer - to record ambient temperature.
-
7.11 Barometer (optional).
7.12 Dilution bottle - Two-liter with septum cap for
standardpreparation.
7.13 Teflon stirbar - 1 inch long.
7.14 Gas tight syringes - 10-500 Fl for standard injectiononto
GC/MS system and CMS cartridges.
7.15 Liquid microliter syringes - 5-50 FL for injecting
neatliquid standards into dilution bottle.
7.16 Oven - 60 + 5EC for equilibrating dilution bottle.
7.17 Magnetic stirrer.
7.18 Variable voltage transformers - (120 V and 1000 VA)
andelectrical connectors (or temperature controllers) toheat
cartridge and cryogenic loop.
7.19 Digital pyrometer - 30 to 500EC range.
7.20 Soap bubble flow meter - 1, 10 and 100 mL
calibrationpoints.
7.21 Copper tubing (1/8 inch) and fittings for gas
inletlines.
7.22 GC column - SE-30 or alternative coating, glass capillaryor
fused silica.
7.23 Psychrometer (optional).
7.24 Filter holder - stainless steel or aluminum (toaccommodate
1 inch diameter filter). Other sizes may beused if desired.
(optional)
8. Reagents and Materials
8.1 Empty CMS cartridges - Nickel or stainless steel (SeeFigure
1).
8.2 CMS Adsorbent, 60/80 mesh-Spherocarb® from Analabs Inc.,or
equivalent.
8.3 Glasswool - silanized.
8.4 Methylene chloride - pesticide quality, or equivalent.
-
8.5 Gas purifier cartridge for purge and GC carrier
gascontaining charcoal, molecular sieves, and a dryingagent.
Available from various chromatography supplyhouses.
8.6 Helium - Ultra pure, (99.9999%) compressed gas.
8.7 Nitrogen - Ultra pure, (99.9999%) compressed gas.
8.8 Liquid nitrogen or argon (50 liter dewar).
8.9 Compressed air, if required - for operation of GC
ovendoor.
8.10 Perfluorotributylamine (FC-43) for GC/MS calibration.
8.11 Chemical Standards - Neat compounds of interest.
Highestpurity available.
9. Cartridge Construction and Preparation
9.1 A suitable cartridge design is shown in Figure 1.Alternate
designs have been reported (1) and areacceptable, provided the user
documents theirperformance. The design shown in Figure 1 has a
built-inheater assembly. Many users may choose to replace
thisheater design with a suitable separate heating block oroven to
simplify the cartridge design.
9.2 The cartridge is assembled as shown in Figure 1
usingstandard 0.25 inch O.D. tubing (stainless steel ornickel), 1/4
inch to 1/8 inch reducing unions, 1/8 inchnuts, ferrules, and
endcaps. These parts are rinsed withmethylene chloride and heated
at 250EC for 1 hour priorto assembly.
9.3 The thermocouple bead is fixed to the cartridge body,
andinsulated with a layer of Teflon tape. The heater
wire(constructed from a length of thermocouple wire) is woundaround
the length of the cartridge and wrapped withTeflon tape to secure
the wire in place. The cartridgeis then wrapped with woven silica
fiber insulation (Zetexor equivalent). Finally the entire assembly
is wrappedwith fiber glass tape.
9.4 After assembly one end of the cartridge is marked with
aserial number to designate the cartridge inlet duringsample
collection.
-
9.5 The cartridges are then packed with -0.4 grams of
CMSadsorbent. Glass wool plugs (-0.5 inches long) areplaced at each
end of the cartridge to hold the adsorbentfirmly in place. Care
must be taken to insure that nostrands of glasswool extend outside
the tubing, thuscausing leakage in the compression endfittings.
Afterloading the endfittings (reducing unions and end caps)are
tightened onto the cartridge.
9.6 The cartridges are conditioned for initial use by heatingat
400EC overnight (at least 16 hours) with a 100mL/minute purge of
pure nitrogen. Reused cartridges needonly to be heated for 4 hours
and should be reanalyzedbefore use to ensure complete desorption of
impurities.
9.7 For cartridge conditioning ultra-pure nitrogen gas ispassed
through a gas purifier to remove oxygen, moistureand organic
contaminants. The nitrogen supply isconnected to the unmarked end
of the cartridge and theflow adjusted to -50 mL/minute using a
needle valve. Thegas flow from the inlet (marked) end of the
cartridge isvented to the atmosphere.
9.8 The cartridge thermocouple lead is connected to apyrometer
and the heater lead is connected to a variablevoltage transformer
(Variac) set at 0 V. The voltage onthe Variac is increased to -15 V
and adjusted over a 3-4minute period to stabilize the cartridge
temperature at380-400EC.
9.9 After 10-16 hours of heating (for new cartridges) theVariac
is turned off and the cartridge is allowed to coolto -30EC, under
continuing nitrogen flow.
9.10 The exit end of the cartridge is capped and then theentire
cartridge is removed from the flow line and theother endcap
immediately installed. The cartridges arethen placed in a metal
friction top (paint) cancontaining -2 inches of granulated
activated charcoal (toprevent contamination of the cartridges
during storage)in the bottom, beneath a retaining screen. Clean
papertissues (e.g., Kimwipes) are placed in can to avoiddamage to
the cartridges during shipment.
9.11 Cartridges are stored in the metal can at all timesexcept
when in use. Adhesives initially present in thecartridge insulating
materials are "burnt off" duringinitial conditioning. Therefore,
unconditionedcartridges should not be placed in the metal can
sincethey may contaminate the other cartridges.
-
QMAX'VMAXt
×1000
9.12 Cartridges are conditioned within two weeks of use. Ablank
from each set of cartridges is analyzed prior touse in field
sampling. If an acceptable blank level isachieved, that batch of
cartridges (including thecartridge serving as the blank) can be
used for fieldsampling.
10. Sampling
10.1 Flow Rate and Total Volume Selection
10.1.1 Each compound has a characteristic retentionvolume
(liters of air per unit weight ofadsorbent). However, all of the
compoundslisted in Table 1 have retention volumes (at37EC) in
excess of 100 liters/cartridge (0.4gram CMS cartridge) except vinyl
chloride forwhich the value is -30 liters/cartridge.Consequently,
if vinyl chloride or similarlyvolatile compounds are of concern the
maximumallowable sampling volume is approximately 20liters. If such
highly volatile compounds arenot of concern, samples as large as
100 literscan be collected.
10.1.2 To calculate the maximum allowable samplingflow rate the
following equation can be used:
whereQ is the calculated maximum sampling rateMAX
in mL/minute.t is the desired sampling time in minutes.V is the
maximum allowable total volumeMAX
based on the discussion in 10.1.1.
10.1.3 For the cartridge design shown in Figure 1 QMAXshould be
between 20 and 500 mL/minute. IfQ lies outside this range the
sampling timeMAXor total sampling volume must be adjusted sothat
this criterion is achieved.
10.1.4 The flow rate calculated in 10.1.3 defines themaximum
allowable flow rate. In general, theuser should collect additional
samples inparallel, at successive 2- to 4-fold lowerflow rates.
This practice serves as a quality
-
control procedure to check on componentbreakthrough and related
sampling andadsorption problems, and is further discussedin the
literature (5).
10.2 Sample Collection
10.2.1 Collection of an accurately known volume ofair is
critical to the accuracy of theresults. For this reason the use of
mass flowcontrollers, rather than conventional needlevalves or
orifices is highly recommended,especially at low flow rates (e.g.,
less than100 milliliters/minute). Figure 2aillustrates a sampling
system based on massflow controllers which readily allows
forcollection of parallel samples. Figure 2bshows a commercially
available sampling systembased on needle valve flow
controllers.
10.2.2 Prior to sample collection the sampling flowrate is
calibrated near the value used forsampling, with a "dummy" CMS
cartridge inplace. Generally calibration is accomplishedusing a
soap bubble flow meter or calibratedwet test meter connected to the
flow exit,assuming the entire flow system is sealed.ASTM Method D
3686 (4) describes anappropriate calibration scheme, not requiringa
sealed flow system downstream of the pump.
10.2.3 The flow rate should be checked before andafter each
sample collection. Ideally, arotometer or mass flow meter should
beincluded in the sampling system to allowperiodic observation of
the flow rate withoutdisrupting the sampling process.
10.2.4 To collect an air sample the cartridges areremoved from
the sealed container just priorto initiation of the collection
process.
10.2.5 The exit (unmarked) end of the cartridge isconnected to
the sampling apparatus. Theendcap is left on the sample inlet and
theentire system is leak checked by activatingthe sampling pump and
observing that no flowis obtained over a 1 minute period.
Thesampling pump is then shut off.
-
QA'Q1%Q
2%...QNN
10.2.6 The endcap is removed from the cartridge, aparticulate
filter and holder are placed onthe inlet end of the cartridge, and
thesampling pump is started. In many situationsa particulate filter
is not necessary sincethe compounds of interest are in the
vaporstate. However, if large amounts ofparticulate matter are
encountered, the filtermay be useful to prevent contamination of
thecartridge. The following parameters arerecorded on an
appropriate data sheet (Figure4): date, sampling location, time,
ambienttemperature, barometric pressure, relativehumidity, dry gas
meter reading (ifapplicable), flow rate, rotometer reading
(ifapplicable), cartridge number, pump, and drygas meter serial
number.
10.2.7 The samples are collected for the desiredtime,
periodically recording the variableslisted above. At the end of the
samplingperiod the parameters listed in 10.2.6 arerecorded and the
flow rate is checked. If theflows at the beginning and end of the
samplingperiod differ by more than 10%, the cartridgeshould be
marked as suspect.
10.2.8 The cartridges are removed (one at a time),the endcaps
are replaced, and the cartridgesare placed into the original
container. Thefriction top can is sealed and packaged forimmediate
shipment to the analyticallaboratory.
10.2.9 The average sample rate is calculated andrecorded for
each cartridge according to thefollowing equation:
whereQ = Average flow rate is ml/minuteAQ , Q ....Q = Flow rates
determined at1 2 Nbeginning, end, and immediate points
duringsampling.
N = Number of points averaged.
-
Vm'T×QA1000
Vs'Vm×Pa760
×298
273%ta
10.2.10 The total volumetric flow is obtained directlyfrom the
dry gas meter or calculated andrecorded for each cartridge using
thefollowing equation:
whereV = Total volume sampled in liters atm
measured temperature and pressure.T = Sampling time = T -T ,
minutes.2 1
10.2.11 The total volume sampled (V ) at standardsconditions,
760 mm Hg and 25EC, is calculatedfrom the following equation:
wherePa = Average barometric pressure, mm Hgta = Average ambient
temperature, EC.
11. Sample Analysis
11.1 Sample Purging
11.1.1 Prior to analysis all samples are purged atroom
temperature with pure, dry air ornitrogen to remove water vapor.
Purging isaccomplished as described in 9.7 except thatthe gas flow
is in the same direction assample flow (i.e. marked end of
cartridge isconnected to the flow system).
11.1.2 The sample is purged at 500 mL/minute for 5minutes. After
purging the endcaps areimmediately replaced. The cartridges
arereturned to the metal can or analyzedimmediately.
11.1.3 If very humid air is being sampled the purgetime may be
increased to more efficientlyremove water vapor. However, the sum
ofsample volume and purge volume must be lessthan 75% of the
retention volume for the mostvolatile component of interest.
-
11.2 GC/MS Setup
11.2.1 Considerable variation from one laboratory toanother is
expected in terms of instrumentconfiguration. Therefore, each
laboratorymust be responsible for verifying that theirparticular
system yields satisfactory results.Section 14 discusses specific
performancecriteria which should be met.
11.2.2 A block diagram of the analytical systemrequired for
analysis of CMS cartridges isdepicted in Figure 3. The thermal
desorptionsystem must be designed to accommodate theparticular
cartridge configuration. For theCMS cartridge design shown in
Figure 1, thecartridge heating is accomplished as describedin 9.8.
The use of a desorption oven, inconjunction with a simpler
cartridge design isalso acceptable. Exposure of the sample tometal
surfaces should be minimized and onlystainless steel or nickel
should be employed.The volume of tubing leading from thecartridge
to the GC column must be minimizedand all areas must be well-swept
by heliumcarrier gas.
11.2.3 The GC column oven must be capable of beingcooled to
-70EC and subsequently temperatureprogrammed to 150EC.
11.2.4 The specific GC column and temperature programemployed
will be dependent on the compounds ofinterest. Appropriate
conditions aredescribed in the literature (2). In general,a
nonpolar stationary phase (e.g., SE-30, OV-1) temperature
programmed from -70 to 150EC at8E/minute will be suitable. Fused
silica,bonded-phase columns are preferable to glasscolumns since
they are more rugged and can beinserted directly into the MS ion
source,thereby eliminating the need for a GC/MStransfer line. Fused
silica columns are alsomore readily connected to the GC
injectionvalve (Figure 3). A drawback of fused silica,bonded-phase
columns is the lower capacitycompared to coated, glass capillary
columns.In most cases the column capacity will be lessthan 1
microgram injected for fused silicacolumns.
-
11.2.5 Capillary column dimensions of 0.3mm ID and 50meters long
are generally appropriate althoughshorter lengths may be sufficient
in manycases.
11.2.6 Prior to instrument calibration or sampleanalysis the
GC/MS system is assembled asshown in Figure 3. Helium purge flow
(throughthe cartridge) and carrier flow are set atapproximately 50
mL/minute and 2-3 mL/minuterespectively. When a cartridge is not
inplace a union is placed in the helium purgeline to ensure a
continuous inert gas flowthrough the injection loop.
11.2.7 Once the column and other system componentsare assembled
and the various flowsestablished the column temperature isincreased
to 250EC for approximately fourhours (or overnight if desired) to
conditionthe column.
11.2.8 The MS and data system are set up according tothe
manufacturer's instructions. Electronimpact ionization (70eV) and
an electronmultiplier gain of approximately 5 x 10 should4
be employed. Once the entire GC/MS system hasbeen setup the
system is calibrated asdescribed in Section 11.3. The user
shouldprepare a detailed standard operatingprocedure (SOP)
describing this process forthe particular instrument being
used.
11.3 GC/MS Calibration
11.3.1 Tuning and mass standardization of the MSsystem is
performed according tomanufacturer's instructions and relevant
userprepared SOPs. Perfluorotributylamine (FC-43)should generally
be employed as the referencecompound. The material is introduced
directlyinto the ion source through a molecular leak.The
instrumental parameters (e.g., lensvoltages, resolution, etc.)
should be adjustedto give the relative ion abundances shown inTable
2, as well as acceptable resolution andpeak shape. If these
approximate relativeabundances cannot be achieved, the ion
sourcemay require cleaning according tomanufacturer's instructions.
In the event
-
that the user's instrument cannot achievethese relative ion
abundances, but isotherwise operating properly, the user mayadopt
another set of relative abundances asperformance criteria. However,
thesealternate values must be repeatable on a day-to-day basis.
11.3.2 After the mass standardization and tuningprocess has been
completed and the appropriatevalues entered into the data system,
the usershould then calibrate the entire GC/MS systemby introducing
known quantities of thecomponents of interest into the system.
Threealternate procedures may be employed for thecalibration
process including 1) directinjection of dilute vapor phase
standards,prepared in a dilution bottle or compressedgas cylinder,
onto the GC column, 2) injectionor dilute vapor phase standards
into a flowinginert gas stream directed onto a CMScartridge, and 3)
introduction of permeationor diffusion tube standards onto a
CMScartridge. Direct injection of a compressedgas cylinder
(aluminum) standard containingtrace levels of the compounds of
interest hasbeen found to be the most convenient practicesince such
standards are stable over a severalmonth period. The standards
preparationprocesses for the various approaches aredescribed in
Section 13. The followingparagraphs describe the instrument
calibrationprocess for these approaches.
11.3.3 If the system is to be calibrated by directinjection of a
vapor phase standard, thestandard, in either a compressed gas
cylinderor dilution flask, is obtained as described inSection 13.
The MS and data system are setupfor acquisition, but the ionizer
filament isshut off. The GC column oven is cooled to-70EC, the
injection valve is placed in theload mode, and the cryogenic loop
is immersedin liquid nitrogen or liquid argon. Liquidargon is
required for standards prepared innitrogen or air, but not for
standardsprepared in helium. A known volume of thestandard (10-1000
mL) is injected through thecryogenic loop at a rate of 10-100
mL/minute.
-
11.3.4 Immediately after loading the vapor phasestandard, the
injection valve is placed in theinject mode, the GC program and
system clockare started, and the cryogenic loop is heatedto 60EC by
applying voltage (15-20 volts) tothe thermocouple wire heater
surrounding theloop. The voltage is adjusted to maintain aloop
temperature of 60EC. An automatictemperature controller can be used
in place ofthe manual control system. After elution ofunretained
components (-3 minutes afterinjection) the ionizer filament is
turned onand data acquisition is initiated. The heliumpurge line
(set at 50 mL/minute) is connectedto the injection valve and the
valve isreturned to the load mode. The looptemperature is increased
to 150EC, with heliumpurge, and held at this temperature until
thenext sample is to be loaded.
11.3.5 After the last component of interest haseluted,
acquisition is terminated and the datais processed as described in
Section 11.3.8.The standard injection process is repeatedusing
different standard concentrations and/orvolumes to cover the
analytical range ofinterest.
11.3.6 If the system is to be calibrated by analysisof standard
CMS cartridges, a series ofcartridges is prepared as described
inSections 13.2 or 13.3. Prior to analysis thecartridges are stored
(no longer than 48hours) as described in Section 9.10. Foranalysis
the injection valve is placed in theload mode and the cryogenic
loop is immersedin liquid nitrogen (or liquid argon ifdesired). The
CMS cartridge is installed inthe helium purge line (set at 50
mL/minute) sothat the helium flow through the cartridge isopposite
to the direction of sample flow andthe purge gas is directed
through thecryogenic loop and vented to the atmosphere.The CMS
cartridge is heated to 370-400EC andmaintained at this temperature
for 10 minutes(using the temperature control processdescribed in
Section 9.8). During thedesorption period, the GC column oven
iscooled to -70EC and the MS and data system aresetup for
acquisition, but the ionizerfilament is turned off.
-
11.3.7 At the end of the 10 minute desorption period,the
analytical process described in Sections11.3.4 and 11.3.5 is
conducted. During theGC/MS analysis heating of the CMS cartridge
isdiscontinued. Helium flow is maintainedthrough the CMS cartridge
and cryogenic loopuntil the cartridge has cooled to
roomtemperature. At that time, the cryogenic loopis allowed to cool
to room temperature and thesystem is ready for further
cartridgeanalysis. Helium flow is maintained throughthe cryogenic
loop at all times, except duringthe installation or removal of a
CMScartridge, to minimize contamination of theloop.
11.3.8 Data processing for instrument calibrationinvolves
determining retention times, andintegrated characteristic ion
intensities foreach of the compounds of interest. Inaddition, for
at least one chromatographicrun, the individual mass spectra should
beinspected and compared to reference spectra toensure proper
instrumental performance. Sincethe steps involved in data
processing arehighly instrument specific, the user shouldprepare a
SOP describing the process forindividual use. Overall performance
criteriafor instrument calibration are provided inSection 14. If
these criteria are notachieved, the user should refine
theinstrumental parameters and/or operatingprocedures to meet these
criteria.
11.4 Sample Analysis
11.4.1 The sample analysis is identical to thatdescribed in
Sections 11.3.6 and 11.3.7 forthe analysis of standard CMS
cartridges.
11.4.2 Data processing for sample data generallyinvolves 1)
qualitatively determining thepresence or absence of each component
ofinterest on the basis of a set ofcharacteristic ions and the
retention timeusing a reversed-search software routine,
2)quantification of each identified component byintegrating the
intensity of a characteristicion and comparing the value to that of
thecalibration standard, and 3) tentative
-
Y'A%BX%CX 2
identification of other components observedusing a forward
(library) search softwareroutine. As for other user
specificprocesses, a SOP should be prepared describingthe specific
operations for each individuallaboratory.
12. Calculations
12.1 Calibration Response Factors
12.1.1 Data from calibration standards is used tocalculate a
response factor for each componentof interest. Ideally the process
involvesanalysis of at least three calibration levelsof each
component during a given day anddetermination of the response
factor(area/nanogram injected) from the linear leastsquares fit of
a plot of nanograms injectedversus area (for the characteristic
ion). Ingeneral, quantities of components greater than1,000
nanograms should not be injected becauseof column overloading
and/or MS responsenonlinearity.
12.1.2 In practice the daily routine may not alwaysallow
analysis of three such calibrationstandards. In this situation
calibration datafrom consecutive days may be pooled to yield
aresponse factor, provided that analysis ofreplicate standards of
the same concentrationare shown to agree within 20% on
theconsecutive days. In all cases one givenstandard concentration,
near the midpoint ofthe analytical range of interest, should
beinjected at least once each day to determineday-to-day precision
of response factors.
12.1.3 Since substantial nonlinearity may be presentin the
calibration curve, a nonlinear leastsquares fit (e.g. quadratic)
should beemployed. This process involves fitting thedata to the
following equation:
whereY = peak areaX = quantity of component injected nanogramsA,
B, and C are coefficients in the equation.
-
YA'A%BXA%CX2
CA'XAVs
12.2 Analyte Concentrations
12.2.1 Analyte quantities on a sample cartridge arecalculated
from the following equation:
whereY is the area of the analyteA
characteristics ion for the samplecartridge.
X is the calculated quantity of analyte onAthe sample cartridge,
in nanograms.
A, B, and C are the coefficients calculatedfrom the calibration
curve described inSection 12.1.3.
12.2.2 If instrumental response is essentially linearover the
concentration range of interest, alinear equation (C=O in the
equation above)can be employed.
12.2.3 Concentration of analyte in the original airsample is
calculated from the followingequation:
whereC is the calculated concentration of analyteAin ng/L.V and
X are as previously defined in Sections A10.2.11 and 12.2.1,
respectively.
13. Standard Preparation
13.1 Standards for Direct Injection
13.1.1 Standards for direct injection can be preparedin
compressed gas cylinders or in dilutionvessels. The dilution flask
protocol has beendescribed in detail in another method and isnot
repeated here (6). For the CMS methodwhere only volatile compounds
(boiling point
-
CT'VI×d
VC×
14.7PC%14.7
×24.4×1000
compressed gas cylinders has been found to bemost convenient.
These standards aregenerally stable over at least a 3-4 monthperiod
and in some cases can be purchased fromcommercial suppliers on a
custom preparedbasis.
13.1.2 Preparation of compressed gas cylindersrequires working
with high pressure tubing andfittings, thus requiring a user
prepared SOPwhich ensures that adequate safety precautionsare
taken. Basically, the preparation processinvolves injecting a
predetermined amount ofneat liquid or gas into an empty high
pressurecylinder of known volume, using gas flow intothe cylinder
to complete the transfer. Thecylinder is then pressurized to a
given value(500-1000 psi). The final cylinder pressuremust be
determined using a high precisiongauge after the cylinder has
thermallyequilibrated for a 1-2 hour period afterfilling.
13.1.3 The concentration of components in thecylinger standard
should be determined bycomparison with National Bureau of
Standardsreference standards (e.g., SRM 1805-benzene innitrogen)
when available.
13.1.4 The theoretical concentration (at 25EC and 760mm
pressure) for preparation of cylinderstandards can be calculated
using thefollowing equation:
whereC is the component concentration, in ng/mLT
at 25EC and 760 mm Hg pressure.V is the volume of neat liquid
componentI
injected in FL.V is the internal volume of the cylinder,c
in L.d is the density of the neat liquid
component, in g/mL.P is the final pressure of the cylinderc
standards, in pounds per square inchgauge (psig).
-
13.2 Preparation of Spiked Traps by Vapor Phase Injection
This process involves preparation of dilution flask orcompressed
gas cylinder containing the desiredconcentrations of the
compound(s) of interest andinjecting the desired volume of vapor
into a flowing gasstream which is directed onto a clean CMS
cartridge. Theprocedure is described in detail in another method
withinthe Compendium (6) and will not be repeated here.
13.3 Preparation of Spiked Traps Using Permeation or
DiffusionTubes
13.3.1 A flowing stream of inert gas containing knownamounts of
each compound of interest isgenerated according to ASTM Method
D3609 (4).Note that a method of accurately maintainingtemperature
within + 0.1EC is required and thesystem generally must be
equilibrated for atleast 48 hours before use.
13.3.2 An accurately known volume of the standard gasstream
(usually 0.1-1 liters) is drawn througha clean CMS cartridge using
the samplingsystem described in Section 10.2.1, or asimilar system.
However, if mass flowcontrollers are employed, they must
becalibrated for the carrier gas used in Section13.3.1 (usually
nitrogen). Use of air as thecarrier gas for permeation systems is
notrecommended, unless the compounds of interestare known to be
highly stable in air.
13.3.3 The spiked traps are then stored orimmediately analyzed
as in Sections 11.3.6 and11.3.7.
14. Performance Criteria and Quality Assurance
This section summarizes the quality assurance (QA) measuresand
provides guidance concerning performance criteria whichshould be
achieved within each laboratory. In many cases thespecific QA
procedures have been described within theappropriate section
describing the particular activity (e.g.parallel sampling).
-
14.1 Standard Operating Procedures (SOPs)
14.1.1 Each user should generate SOPs describing thefollowing
activities as accomplished in theirlaboratory: 1) assembly,
calibration andoperation of the sampling system, (2)preparation,
handling and storage of CMScartridges, 3) assembly and operation of
GC/MSsystem including the thermal desorptionapparatus and data
system, and 4) all aspectsof data recording and processing.
14.1.2 SOPs should provide specific stepwiseinstructions and
should be readily availableto, and understood by the laboratory
personnelconducting the work.
14.2 CMS Cartridge Preparation
14.2.1 Each batch of CMS cartridges, prepared asdescribed in
Section 9, should be checked forcontamination by analyzing one
cartridge,immediately after preparation. While analysiscan be
accomplished by GC/MS, manylaboratories may choose to use GC/FID
due tologistical and cost considerations.
14.2.2 Analysis by GC/FID is accomplished asdescribed for GC/MS
(Section 11) except foruse of FID detection.
14.2.3 While acceptance criteria can vary dependingon the
components of interest, at a minimumthe clean cartridge should be
demonstrated tocontain less than one-fourth of the minimumlevel of
interest for each component. Formost compounds the blank level
should be lessthen 10 nanograms per cartridge in order to
beacceptable. More rigid criteria may beadopted, if necessary,
within a specificlaboratory. If a cartridge does not meetthese
acceptance criteria, the entire lotshould be rejected.
14.3 Sample Collection
14.3.1 During each sampling event at least one cleancartridge
will accompany the samples to thefield and back to the laboratory,
having beenplaced in the sampler but without sampling
-
air, to serve as field blank. The averageamount of material
found on the field blankcartridges may be subtracted from the
amountfound on the actual samples. However, if theblank level is
greater than 25% of the sampleamount, data for that component must
beidentified as suspect.
14.3.2 During each sampling event at least one set ofparallel
samples (two or more samplescollected simultaneously) should be
collected,preferably at different flow rates asdescribed in Section
10.1.4. If agreementbetween parallel samples is not generallywithin
+25% the user should collect parallelsamples on a much more
frequent basis (perhapsfor all sampling points). If a trend of
lowerapparent concentrations with increasing flowrate is observed
for a set of parallel samplesone should consider using a reduced
samplingrate and longer sampling interval, ifpossible. If this
practice does not improvethe reproducibility further evaluation of
themethod performance for the compound ofinterest might be
required.
14.3.3 Backup cartridges (two cartridges in series)should be
collected with each sampling event.Backup cartridges should contain
less than 10%of the amount of components of interest foundin the
front cartridges, or be equivalent tothe blank cartridge level,
whichever isgreater.
14.4 GC/MS Analysis
14.4.1 Performance criteria for MS tuning and
massstandardization have been discussed in Section11.2 and Table 2.
Additional criteria can beused by the laboratory, if desired.
Thefollowing sections provide performanceguidance and suggested
criteria fordetermining the acceptability of the GC/MSsystem.
14.4.2 Chromatographic efficiency should be evaluateddaily by
the injection of calibrationstandards. A reference compound(s)
should bechosen from the calibration standard andplotted on an
expanded time scale so that its
-
DL'A%3.3S
width at 10% of the peak height can becalculated, as shown in
Figure 6. The widthof the peak at 10% height should not exceed
10seconds. More stringent criteria may berequired for certain
applications. Theasymmetry factor (see Figure 6) should bebetween
0.8 and 2.0. The user should alsoevaluate chromatographic
performance for anypolar or reactive compounds of interest,
usingthe process described above. If peaks areobserved that exceed
the peak width orasymmetry factor criteria above, one shouldinspect
the entire system to determine ifunswept zones or cold spots are
present in anyof the fittings or tubing and/or ifreplacement of the
GC column is required.Some laboratories may choose to
evaluatecolumn performance separate by directinjection of a test
mixture onto the GCcolumn. Suitable schemes for columnevaluation
have been reported in theliterature (7).
14.4.3 The detection limit for each component iscalculated from
the data obtained forcalibration standards. The detection limit
isdefined as
whereDL is the calculated detection limit in
nanograms injected.A is the intercept calculated in Section
12.1.3.S is the standard deviation of replicate
determinations of the lowest levelstandard (at least three
suchdeterminations are required). The lowestlevel standard should
yield a signal tonoise ratio (from the total ion currentresponse)
of approximately 5.
14.4.4 The relative standard deviation for replicateanalyses of
cartridges spiked at approximately10 times the detection limit
should be 20% orless. Day to day relative standard deviationfor
replicate cartridges should be 25% orless.
-
14.4.5 A useful performance evaluation step is theuse of an
internal standard to track systemperformance. This is accomplished
by spikingeach cartridge, including blank, sample, andcalibration
cartridges with approximately 100nanograms of a compound not
generally presentin ambient air (e.g., perfluorotoluene).Spiking is
readily accomplished using theprocedure outlined in Section 13.2,
using acompressed gas standard. The integrated ionintensity for
this compound helps to identifyproblems with a specific sample. In
generalthe user should calculate the standarddeviation of the
internal standard responsefor a given set of samples analyzed
underidentical tuning and calibration conditions.Any sample giving
a value greater than + 2standard deviations from the mean
(calculatedexcluding that particular sample) should beidentified as
suspect. Any marked change ininternal standard response may
indicate a needfor instrument recalibration.
14.5 Method Precision and Recovery
14.5.1 Recovery and precision data for selectedvolatile organic
compounds are presented inTable 1. These data were obtained
usingambient air, spiked with known amounts of thecompounds in a
dynamic mixing system(2).
14.5.2 The data in Table 1 indicate that in generalrecoveries
better than 75% and precision(relative standard deviations) of
15-20% canbe obtained. However, selected compounds(e.g. carbon
tetrachloride and benzene) willhave poorer precision and/or
recovery. Theuser must check recovery and precision for
anycompounds for which quantitative data areneeded.
-
References
1. Kebbekus, B. B. and J. W. Bozzelli. Collection and Analysisof
Selected Volatile Organic Compounds in Ambient Air.Proceedings of
Air Pollution Control Association, Paper No.82-65.2, Air Pollution
Control Association, Pittsburgh,Pennsylvania, 1982.
2. Riggin, R. M. and L. E. Slivon. Determination of
VolatileOrganic Compounds in Ambient Air Using Carbon Molecular
SieveAdsorbents, Special Report on Contract 68-02-3745 (WA-7),
U.S.Environmental Protection Age