Chap 6_Gas Chromatography

8/13/2019 Chap 6_Gas Chromatography

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Rezaul KarimEnvironmental Science and Technology

Jessore University of Science and Technology

Instrumental Technique for Environmental Analysis

Chapter 6 GC


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Chapter content and

Gas Chromatography (GC)

Principles

Instrumentation

Detectors

Applications of GC

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Reference

1. James W Robinson, 1995. Undergraduateinstrumental analysis, Marcel Dekker, Inc. NY

2. Skoog, Holler & Crouch 2007, InstrumentalAnalysis, Brooks Cole Cengage Learning, USA.

3. Daniel C. Harris , 2010, Quantitative ChemicalAnalysis, 8thedition, W. H. Freeman andCompany 41 Madison Avenue New York.

4. S. Ahuja and N. Jespersen (Eds), 2006,

Comprehensive Analytical Chemistry, Volume47, Elsevier B.V.

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INTRODUCTION

The development of column chromatography did notprogress until the publication of a most valuable paperby Martin and Synge in 1941,

for which they were later awarded the Nobel Prize.

In that paper they introduced liquidliquidchromatography plate theory, a first model thatcould describe column efficiency.

they first suggested the possibility of using gas as themobile phase in a chromatographic system.

Ten years later, James and Martin introduced

the first gas chromatography apparatus- suitableonly for the detection and determination of acids and bases.

The first commercial instrument was delivered by Griffinand George (London) in late 1954

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In 1955 Glueckauf derived the first comprehensiveequation describing the relationship between HETP andparticle size, particle diffusion and film diffusion ion

exchange. The Dutch scientists van Deemter, Zuiderweg and

Klinkenberg developed the rate theory, an alternate tothe plate theory, describing the chromatographicprocess in terms of kinetics and mass transfer.

By the early 1980s, fused-silica capillary columns,

selective and sensitive detectors,

fully automated systems, and

sophisticated techniques such as gas chromatography-mass

spectrometer (GC-MS) and gas chromatography-infrared spectrometer (GC-IR)

gas chromatography by this time was a well-established,well-known analytical technique.


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Gas Chromatography

Gas chromatography has been widely usedin

foods,

petroleum products, pesticide and pesticide residues,

pharmaceutical products,

environmental monitoring,

clinical chemistry and

a number of other fields.

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GC

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Principles

Gas-liquid chromatography (often called gaschromatography) is a powerful tool in analysis.

The basis for gas chromatographic separation is thedistribution of an analyte between two phases.

All forms of chromatography involve a stationaryphaseand a mobile phase.

In all the other chromatography,

the mobile phase is a liquid.

In gas-liquid chromatography, the mobile phase is a gas such as helium

the stationary phase is a high boiling point liquidabsorbed onto a solid.

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Instrumentation

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Instrumentation

A gas chromatographic system iscomposed of four major components:

Carrier gas source,

sample introduction system, column and

Detector

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Carrier gas source The mobilc-phase gas in GC is called the carrier

gas and must be chemically inert. However, a number of restrictions must be placed on

the selection of a carrier gas.

the carrier gas must be inert,not reacting with analytes andwith any components of the GC system.

the carrier gas must be suitable with the detectors. the carrier gas must be of high-purity grade and not

hazardous.

Helium is the most common mobile-phase gas

used, although argon, nitrogen, and hydrogen areused.

In addition, the carrier gas system often contains amolecular sieve to remove impurities and water.

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A two stage regulator is used at the outletof the carrier gas cylinder to set anappropriate cylinder outlet pressure to the GCsystem and to monitor the residual pressure inthe cylinder.

Most GC instruments are equipped with a flow controller flow meter

The flow controller is used to ensure obtaininga constant flow despite changes inpressure and pressure drops through the GC

column. The flow meter is used to set a carrier gas

flow rate to a desirable level and tomonitor the stability of the carrier gas flow


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Sample introduction system

GC samples are generally introduced onto GCcolumn through an injection port,in which thereis a self-sealing septum to ensure no leaking willoccur.

Gaseous samples are best handled by on-columnintroduction, which eliminates band broadeningdue to the dead volume of the injection port.

Liquid samples are usually introduced with asyringe.

Sample injection sizes depend on the concentrations of the sample components being

analysed, the capacity of the column, and the sensitivity of the detector

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The solvent-flush method

1. rinsing the syringe with solvent, completely fillingand expelling the syringe several times;

2. wiping excess solvent from the syringe needle;

3. drawing about IL of solvent into the syringe,followed by about lpL of air, then followed by drawingin excess sample;

4. positioning the syringe plunger for the requiredinjection volume, and wipe excess sample from theneedle;

5.drawing in air until the sample is entirely withinthe syringe barrel; and

6. Inserting the syringe into the injection port,rapidly depressing the plunger, and after a delay ofabout 1 second quickly and smoothly withdrawing thesyringe.

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Thermostatically controlledoven

The gas chromatograph comprises an oven withsufficient volumeto hold one or two columnseasily and which can heat up to more than 400C.

A weak thermal inertia permits a rapid butcontrolled temperature climb (gradient able toattain 100C/ min).

The temperature must be controlled towithin0.1C in order to get reproducible separations inisothermal or temperature programmed

modes. By installation of a cryogenic valve fed with N2

or CO2in the liquid state, the oven can beregulated at low temperature.

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Columns : two general classifications

packed Columns Packed columns are tubes of copper, stainless steel,

glass or other materials formed in any shape that will fitthe GC oven.

Capillary or open tubular columns

The vast majority of analyses use long, narrow opentubular columnsmade of fused silica (SiO2) andcoated with polyimide.

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Packed columns

Common forms are U-shape, W-shape, and coiledtubes).

They are typically 36 mm in diameter and 15 m inlength.

Packed columns are prepared by filling them with finelydivided stationary-phase-coated support.

The solid support is often silica that is silanized toreduce hydrogen bonding to polar solutes.

For tenaciously binding solutes, Teflon is a usefulsupport, but it is limited to 200C.

The support particle size range from 40 to 60 mesh forcoarse particles, and from 100 to 160 mesh for fineparticles.

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Open tubular columns

The vast majority of analyses use long,narrow open tubular columns made offused silica (SiO2) and coated with polyimide

(a plastic capable ofwithstanding 350C

) forsupport and protection from atmosphericmoisture.

Open tubular columns offer higher resolution, shorter analysis time, and

greater sensitivitythan packed columns, but

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the type and form of the coating,capillary column The wall-coated column features a 0.1- to 5 m-

thick film of stationary liquid phase on the inner wallof the column. The most important and widely used type

If it is not specified, the capillary column is generally oftheWCOT type

A support-coated column has solid particlescoated with stationary liquid phase and attached to

the inner wall. In theporous-layer column, solid particles are the

active stationary phase.

With their high surface area, support-coated columns canhandle larger samples than can wall-coated columns.

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Fused-silica wall-coated (FSWC)

Fused-silica capillaries are drawn from specially purifiedsilica that contains minimal amounts of metaloxides.

These capillaries have much thinner walls than glasscolumns.

The tubes arc given added strength by an outsideprotective polyimide coating, which is applied as thecapillary tubing is drawn.

The resulting columns are quite flexible and can bebent into coils with diameters of a few inches.

Silica open tubular columns are available commerciallyand offer several important advantages: flexibility and

inertness.

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Capillary columns are typically coated bya dynamic

In the dynamic method, a dilute coating

solution is passed slowly through the column at

a controlled rate, followed by nitrogen drying.

a static method In the static method, the column is filled with

the coating solution, which is then evaporated

in a laminar fashion using a special oven, leaving

a thin film deposition of the coating on theinternal wall of the column.


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Compares the performances ofcolumns.

Property FSWC WCOT SCOT Packed

Column length, L, m 10-100 10-100 10-100 1-6

Inside diameter, mm 0.1-0.3 0.25-0.75 0.5 2-4

Efficiency,plates/m 2000-4000 1000-4000 600-1200 500-1000

Sample size, ng 10-75 10-1000 10-1000 10-106

Relative pressure low low low high

Relative speed fast fast fast Slow

Flexibility Yes Yes

Chemical ineretness Best poorest

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How separation works on the column:

One of three things might happen

It may condense on the stationaryphase. : A compound with a boiling point higher than the

temperature of the column will obviously tendto condense at the start of the column.

However, some of it will evaporate again inthe same way that water evaporates on a warmday - even though the temperature is well below100C.

The chances are that it will then condenseagain a little further along the column.

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How separation works on the column:

One of three things might happen

It may dissolve in the liquid on the surfaceof the stationary phase.: some compounds will be more soluble in the

liquid than others.

The more soluble ones will spend more of theirtime absorbed into the stationary phase; the lesssoluble ones will spend more of their time in the gas

It may remain in the gas phase: The process where a substance divides itself

between two immiscible solventsbecause itis more soluble in one than the other is knownaspartition.

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Solid Support Materials

The packing, or solid support in a packedcolumn, holds the liquid stationary phase inplaceso that as large a surface area as possibleis exposed to the mobile phase.

The ideal support consists of small, uniform,spherical particles with good mechanicalstrength and a specific surface area of atleast 1m'/g.

In addition, the material should be inertat

elevated temperatures and be uniformlywetted by the liquid phase.

No material is yet available that meets all ofthese criteria perfectly.

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The two ends are fitted with glass wool plugs, toretain packing materials.

Most packed column supports are prepared from

diatomaceous earth, which is composed ofskeletons of diatoms. The diatomite is basically amorphous hydrous

silica.

It consists of the skeletons of thousands of species ofsingle-celled plants that once inhabited ancientlakes and seas.

Such plants received their nutrients and disposed oftheirwastes via molecular diffusion through

their pores. As a result, their remains are well-suited as support

materials because GC is also based on the samekind of molecular diffusion.


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Particle Size of Supports

The efficiency of a gas chromatographic columnincreases rapidly with decreasing particlediameter of the packing.

The pressure difference required to maintain anacceptable flow rate of carrier gas, however, variesinversely as the square of the particlediameter.

the latter relationship has placed lower limits on thesize of particles used in GC because it is not

convenient to use pressure differences thatare greater than about 50 psi. As a result, the usual support particles are 60 to

80 mesh (250 to 170 m) or 80 to 100 mesh(170 to 149 m).

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The Stationary liquid Phase

Desirable properties for the immobilized liquidphase low volatility (ideally. the boiling point of the liquid

should be at least 100C higher than themaximum operating temperature for the

column); thermal stability;

chemical inertness;

solvent characteristics suchthat k and values forthe solutes to be resolved fall within a suitable range

Many liquids have been proposed as stationaryphases in the development of GLC.

The proper choice of stationary phase is oftencrucial to the success of a separation.

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To have a reasonable residence time in the column,an analyte must show some degree ofcompatibility (solubility) with the stationaryphase.

The choice of liquid stationary phase is based onthe rule like dissolves like. Non-polar columns are best for non-polar solutes.

Columns of intermediate polarity are best forintermediate polarity solutes, and strongly polarcolumns are best for strongly polar solutes.

Generally, the polarity of the stationary phase

should match that of the sample components. When the match is good, the order of elution is

determined by the boiling point of the eluents.


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Polar stationary phases

Contain functional groups e.g. -CN, -CO, & -

OH.

polyester phases are highly polar.

Polar analytics include alcohols, acids, and

amines; Non-polar, stationary phases

dialkyl siloxanes and Saturated hydrocarbons

Solutes of medium polarity ethers, ketones, and aldehydes


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Widely Used Stationary Phases

Stationary Phase Common trade

name

Maximum

temp., c

Common Applications

Polydimethyl

siloxane

OY-l, SE-30 350 General-purpose nonpolar phase,

hydrocarbons, polynuclear aromatics,

steroids, PCBs

5% Phenyl-

polydimethyl siloxane

OY-3,SE-52 350 General-purpose nonpolar phase,

hydrocarbons, polynuclear aromatics,

steroids, PCBs

50% Phenyl-polydimethyl siloxane

OY-17 250 Drugs, steroids, pesticides, glycols

50%

Tritluoropropyl-

polydimethyl siloxane

OY-2l0 200 Drugs, steroids, pesticides, glycols

Polyethylene glycol Carbowax20M 250 Free acids, alcohols, ethers,

essential oils, glycols

50% Cyanopropyl

polydimethyl

siloxane

OY-275 - 240 Polyunsaturated fatty acids, rosin

acids, free acids, alcohols

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The current phases correspond in principle to two families:thepolysiloxanes andthepolyethylene glycols


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Polyethyleneglycols (PEG)

The best known representative of this

family is Carbowax. These polar polymers (Mr=1500 to 20 000for the Carbowax 20M) can be used for

deposition, impregnation or as bondedphases (40


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The column temperature

Column temperature is an important variable that must becontrolled to a few tenths of a degree for precise work.

Thus, the column is ordinarily housed in a thermo-stated oven.

The optimal column temperature depends on the boiling pointof the sample and the degree of separation required.

Roughly, a temperature equal to or slightly above the averageboiling point of a sample results in a reasonable elution time (2to 30 min).

For samples with a broad boiling range, it is often desirable toemploy temperature programming, in which the columntemperature is increased either continuously or in steps as the

separation proceeds.

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Figure 2.5 shows the elution of C5 to C15 linear alkanes from a 3-m-long column.


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At a constant temperature of 30C (not shown),heavier alkanes take so long to be eluted andemerge over such a long time that they would notbe detected.

The three traces in Figure 2.5 show what happenswhen column temperature is raised from 30 to150C at rates of (a) 20C/min, (b) 40C/min, and(c) 60C/min.

Broad, late-eluting peaks can be sharpened and

eluted in less time with temperature programming. To maintain adequate resolution of earlier eluting

peaks, programs often include a period of time atconstant, low temperature prior to raising thetemperature

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The detector

The GC detector, producing an electrical signalthat is proportional in intensity to theconcentration or the mass of the eluted analyte.

Since the introductions of gas chromatography, over

40 detectors have been developed. Some are designed to respond to most compounds

in general,while others are designed to be selectivefor particular types of substances.

There are several different types of detector in use. the flame-ionisation (FID),

the thermal conductivity (TCD)

the electron capture detectors (ECD).

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Characteristics of the Ideal

Detector

Adequate sensitivity. Just what constitutes adequatesensitivity cannot be described in quantitative terms.

For example, the sensitivities of the detectors are described inthis section vary by a factor of 107.

Good stability and reproducibility.

A linear response to solutes that extends over several ordersof magnitude.

A temperature range from room temperature to at least400C.

A short response time independent of flow rate.

High reliability and ease of use. The detector should befoolproof in the hands of inexperienced operators, if possible.

Similarity in response toward all solutes

The detector should be non-destructive.

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A flame ionization detector

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A flame ionization detector

The flame ionisationdetector has become themost popular detector forGC over the last 40 years,and nothing suggests that

this position will ever bechallenged.

The reasons for that comefrom its simplicity,reliability, relatively highsensitivity for widevariety of organiccompounds, andexcellent linearity (as highas 10 s).

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The whole detector is enclosed in its own ovenwhich is hotter than the column temperature. Thatstops anything condensing in the detector.

The FID utilises a flame produced by thecombustion of hydrogen and air. Very little ionsare formed because of the combustion of thehydrogen and air.

Large increase in ions will occur when orgamccompounds are introducedinto the flamethrough the FID jet.

The ions will be attracted by the FID collector onwhich a polarising voltage is applied, and produce a

current, which is proportional to thequantity of analyte in the flame. For optimal FID operation, the carrier,

hydrogen and air flow must be properly set andadjusted.


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Advantages

The FID exhibits a high sensitivity (~ 10-13g/s),

large linear response range (~107), and

low noise.

It is generally rugged and easy to use. Disadvantagesof the flame ionization

detector are that

it destroys the sample during the combustionstep and

requires additional gases and controllers.


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Application of GC

To evaluate the importance of GC, we mustdistinguish between the two roles themethod plays.

performing separations.

In this role, GC methods are unsurpassed whenapplied to complex organic, metal-organic,and biochemical systems made up of volatilespecies or species that can be derivatized to yieldvolatile substances.

the completion of an analysis. In this role, retention times or volumes are used for

qualitative identification, and peak heightsor peak areas provide quantitative information.

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Application of GC

For qualitative purposes, GC is much morelimited than most of the spectroscopic

methods considered in earlier chapters.

Thus, an important trend in the field hasbeen in the direction of combining theremarkable separation capabilities of GC

with the superior identification properties

of such instruments as mass, IR, and nuclear

magnetic resonance spectrometers.

Chap 6_Gas Chromatography

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