7. ANALYTICAL METHODS - Agency for Toxic Substances … · 7. ANALYTICAL METHODS ... diameter of 3.5 µm; however, NIOSH Method 0600 lists a diameter size of 4 µm (NIOSH 1998; OSHA

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7. ANALYTICAL METHODS

The purpose of this chapter is to describe the analytical methods that are available for detecting,

measuring, and/or monitoring silica, its metabolites, and other biomarkers of exposure and effect to silica.

The intent is not to provide an exhaustive list of analytical methods. Rather, the intention is to identify

well-established methods that are used as the standard methods of analysis. Many of the analytical

methods used for environmental samples are the methods approved by federal agencies and organizations

such as EPA and the National Institute for Occupational Safety and Health (NIOSH). Other methods

presented in this chapter are those that are approved by groups such as the Association of Official

Analytical Chemists (AOAC) and the American Public Health Association (APHA). Additionally,

analytical methods are included that modify previously used methods to obtain lower detection limits

and/or to improve accuracy and precision.

7.1 BIOLOGICAL MATERIALS

Limited analytical methods reported the analysis of c-silica or a-silica in biological materials. All forms

of silica are considered to be poorly soluble particles. Inhaled silica particles, not cleared by mucociliary

escalators or coughing, are embedded and remain in the lung (Cox 2011).

OSHA method PV2121 characterizes the term ‘respirable dust’ as dust particle sizes with a median

diameter of 3.5 µm; however, NIOSH Method 0600 lists a diameter size of 4 µm (NIOSH 1998; OSHA

2015). The European standard PN-EN 481:1998 and international standard PN-ISO 7708:2001 describe

the term ‘respirable dust’ as a cumulated log-normal distribution, with the median diameter of 4.25 µm

and geometric standard deviation of 1.5 (Maciejewska 2008).

NIOSH (2003b) Analytical Method 7601 is a standardized method used to determine the concentration of

c-silica by x-ray diffraction (XRD) with filter redeposition in respirable or total dust, settled dust, and

biological samples, although studies describing the use of this method with biological samples were not

identified.

7.2 ENVIRONMENTAL SAMPLES

Table 7-1 lists the methods used for determining silica in environmental samples. Silica is a common

material in the environment with many distinct forms. Determination of the form of silica present and

concentration of each form of silica in a sample may be achieved through several different analytical

***DRAFT FOR PUBLIC COMMENT***

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Table 7-1. Analytical Methods for Determining Silica in Environmental Samples

Sample matrix Preparation method

Analytical method

Sample detection limit

Percent recovery Reference

Air (dust) Collection with 10-mm nylon cyclone and 5-µm PVC membrane

Method 7500; XRD

0.005 mg SiO2

per sample 0.08

±18% (accuracy)

NIOSH 2003a

Air (dust) Collection with PVC membrane filter

P-2; XRD 20 µg quartz ±20% (accuracy)

MHSA 2013b

Air (dust) Collection with 25 mm, 5-µm PVC membrane filter or Ag filter (XRD), according to MDHS 14/3

MDHS 101; FT-IR or XRD

FT-IR: 20 µg XRD: 10 µg RSD 0.087

±20% (accuracy 0.5–2.0 limit value)

MHSA 2005

Air (dust) Collection with 10-mm nylon cyclone and 37 mm, 5-µm PVC membrane

Method 7602; IR

0.005 mg SiO2

per sample RSD <0.15

Not reported

NIOSH 2003d

Air (dust) Collection with 10-mm nylon cyclone and 0.8-µm MCE or 5-µm PVC membrane

Method 7501; XRD

5 µg quartz Not reported

NIOSH 2003c

Air (dust) 37 mm, 5-µm PVC membrane

ID-142; XRD

20 µg quartz RSD 0.11

±26% (error)

OSHA 1996

Air (dust) Collection with 10-mm nylon cyclone and 37 mm, 5-µm PVC membrane

Method 7603; IR, FT-IR

10 µg quartz RSD 0.098

±25.6 to 43.4% (accuracy)

NIOSH 2003e

Air (dust) Collection with membrane filter

P-7; IR 20 µg quartz ±13% (accuracy)

MHSA 2013

Air (dust) Collection with 37 mm, 5-µm PVC membrane

MDHS 37; IR

Varies with particle size

Not reported

NIOSH 2002


MDHS 38; IR

Varies with particle size

Not reported

NIOSH 2002

Air (dust) Collection with membrane filter

IR 5–27 µg quartz 6–16 µg cristobalite

Not reported

Foster and Walker 1984

Air (dust) Collection with 10-mm nylon cyclone and 0.8-µm MCE or 5-µm PVC membrane

Method 7601; VIS

10 µg SiO2 per sample RSD 0.09

Not reported

NIOSH 2003b

Air (dust) and soil

48% HBF4 at 70°C, filter, dissolve in 1:1 KHCO3

and KCl, boiling water, add 0.1 mL 10%

Spectrophotometer

8 µg quartz 99.8% Stopford 1994

ammonium molybdate, adjust pH to 2.1


LIBS 0.16 µg/cm2 10% (relative errors)

Stipe et al. 2012


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Table 7-1. Analytical Methods for Determining Silica in Environmental Samples

Sample Analytical Sample Percent matrix Preparation method method detection limit recovery Reference Water Filter sample with Spectro 2 mg silica/L Not NPDES 1978 (dissolved silica)

0.45-µm membrane filter; add molybdate ion in

photometer/ EPA 370.1

reported

acidic solution to filtrate; the color complex is measured

Water Add molybdate ion in Spectro 0.1 mg silica/L Not USGS 1989 (dissolved ascorbic acid solution to photometer/ reported silica) filtrate; the color complex USGS

is measured NWQL I-2700-85

Water Filtered, acid preserved ICP-AES/ 26 µg/L 96–104% EPA 1994b (dissolved sample is mixed with 1% EPA 200.7 (estimated) (in tap silica) v/v HNO3 water) Water Samples are solubilized ICP-AES/ 0.017 mg/L Not EPA 2000 (dissolved or digested prior to EPA 6010c (estimated) reported silica) analysis Water Sample undergoes a ICP-AES/ 0.01 mg/L Not USGS 1987 (dissolved direct-reading, no USGS (estimated) reported silica) preparation NWQL

I-1472-87 Water Acidify sample with HNO3 AVICP 0.01 mg silica/L Not EPA 2003 (dissolved to pH <2 AES/EPA reported silica) 200.5 Water Acidify sample with HNO3 ICP-OES/ Not reported Not USGS 1998 (dissolved to pH <2 USGS reported silica) NWQL

I-4471-97 Soil LiCO3 /H3BO3 flux and AAS Not reported 99.7% Barredo and Diez

stabilization with fluoride 1980 Soil Sample weighed Thermal <1% Not Sheffield 1994

analysis reported

AAS = atomic absorption spectroscopy; AVICP-AES = axially viewed inductively coupled plasma-atomic emission spectrometry; EPA = Environmental Protection Agency; FT-IR = Fourier transform infrared spectrometry; ICP-AES = inductively coupled plasma-atomic emission spectrometry; ICP-OES = inductively coupled plasma-optical emission spectrometry; IR = infrared spectrometry; MCE = mixed cellulose ester; MSHA = Mine Safety and Health Administration; NIOSH = National Institute for Occupational Safety and Health; NPDES = National Pollutant Discharge Elimination System; NWQL = National Water Quality Laboratory; OSHA = Occupational Safety and Health Administration; PVC = polyvinyl chloride; RSD = relative standard deviation; USGS = U.S. Geological Survey; VIS = visible absorption spectrophotometry; XRD = x-ray diffraction


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techniques. The use of XRD and infrared spectroscopy (IR) allows for the separate determination of

quartz, cristobalite, and tridymite (Maciejewska 2008). The total content of all crystalline forms of silica

is obtained with visible absorption spectrophotometry. Optical microscopy, electron microscopy, thermal

analysis, selective dissolution, and density separation may also be used to analyze silica. Several

analytical methods have been reported by regulatory agencies including NIOSH, OSHA, USGS, and

MSHA.

Mineral interferences may be reduced prior to analysis with sample preparation techniques; for example,

a phosphoric acid digestion is used if there is a presence of a-silica (Talvitie 1951). a-Silica and some

smaller c-silica particles dissolve in phosphoric acid (Eller 1999). c-Silica particles <3 µm dissolve in hot

phosphoric acid; therefore, the amount of free silica may be underestimated when using this method

(Yabuta and Ohta 2003). Hydrochloric acid is used to remove calcite, magnetite, and hematite. Air

samples collected with filters are ashed or dissolved in tetrahydrofuran (NIOSH 2003f). The ashed

sample is suspended in a solvent and deposited onto an analytical filter. Another preparation method used

to obtain the free silica in respirable dust samples uses pyrophosphoric acid and a closed vessel

dissolution technique with microwave heating (Shinohara 1993).

Particle-size distribution of silica samples is measured by laser scattering or air-jet screening (Florke et al.

2008). Cyclone air samples, filter cassette, and filter media are used to retain the respirable dust fraction

without non-respirable particles. Criteria for collecting particles of the appropriate size with cyclone air

samplers are established by the International Organization for Standardization (ISO), the European

Committee for Standardization (CEN), and ACGIH (NIOSH 2003f). The XRD, IR, and colorimetric

analytical methods are subject to different particle size effects, and each cyclone exhibits its own unique

particle collection characteristics (NIOSH 2003f).

XRD patterns are able to distinguish c-silica polymorphs from each other and from other a-silica forms.

XRD patterns are produced specific to the c-silica crystalline structure (IARC 1997). The polymorph

α-quartz has a primary diffraction line at 26.66 °2θ (3.343 Å). NIOSH (2003a) Analytical Method 7500

is a standardized method used to determine the concentration of c-silica by XRD with filter redeposition

in dust. NIOSH (2003c) Analytical Method 7501 is a standardized method used specifically for a-silica

in crystalline (e.g., quartz) matrices with XRD analysis. The Department of Labor’s MSHA has method

MDHS 101 (replaces MDHS 51/2), an XRD method for the determination of quartz in dust.


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c-Silica polymorphs have distinct infrared absorption patterns. α-Quartz has a doublet at 798–790 and

779–780 cm-1, with secondary peaks at 694, 512, 460, 397, and 370 cm-1 (NIOSH 2003f). Cristobalite

peaks are found at 798, 623, 490, 385, 297, and 274 cm-1 and tridymite peaks are found at 793, 617, and

476 cm-1 . NIOSH (2003d) Analytical Method 7602 is a standardized method used to determine the

concentration of c-silica by IR analysis in air dust samples. NIOSH (2003e) Analytical Method 7603 is a

standardized method used to determine the concentration of c-silica in coal dust by Fourier transform

infrared spectroscopy (FT-IR) or IR analysis. The Department of Labor’s MSHA also has an IR method

for the determination of quartz in respirable coal mine dust (MSHA 2013). MDHS 37 and MDHS 38 are

other standardized methods used to determine the silica content of a sample by IR (NIOSH 2003f).

The combination of XRD and IR has been used to quantify the a-silica and c-silica content of samples to

obtain the c-silica and total silica content, respectively (Bye et al. 1980). A direct differential scanning IR

method has been described to determine the c-silica content in respirable atmospheric dust samples

(Foster and Walker 1984). A difference spectrum method may be used to correct for interfering spectra

when determining the c-silica content of dust samples with IR (Ojima 2003). Kaolinite is a commonly

found mineral in coal mine dust that interferes with IR analysis of quartz, and corrections with a standard

reference material have been suggested (Lee et al. 2013; NIOSH 2003f). Field-portable IR spectrometers

are used to provide more timely estimates of silica exposure (Miller et al. 2012). A direct-on-filter

method using partial least squares regression to the infrared transmission spectra of samples deposited on

porous polymeric filters was developed to allow for the use of field-portable infrared spectrometers

(Weakley et al. 2014).

NIOSH (2003b) Analytical Method 7601 is a standardized method used to determine the concentration of

c-silica by visible absorption spectrophotometry (VIS) in respirable or total dust, settled dust, and

biological samples. Colorimetric methods require preparation steps and color development methods

(citric acid and tartaric acid); absorbance is measured at 785 nm (Stopford 1994). Colorimetric methods

for c-silica are less precise than XRD or IR (NIOSH 2003f).

Laser-induced breakdown spectroscopy quantifies quartz in coal dust samples collected on filter media

with extremely low (0.16 µg/cm2) limit of detection levels for silica (Stipe et al. 2012).

Dissolved silica concentrations are used to determine the silicon content of water (USGS 1998).

Molybdate ion in acidic solution, when added to a water sample containing dissolved silica, forms a

greenish-yellow color complex proportional to the dissolved silica and is measured spectrophoto


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metrically. Dissolved silica in drinking, surface, and saline waters and domestic and industrial wastes is

measured using EPA Method 370.1 or USGS NWQL I-2700-85 with a spectrophotometer (NPDES 1978;

USGS 1989). Silica in solution is also measureable using inductively coupled plasma-atomic emission

spectrometry (ICP-AES) according to EPA Method 200.7 or 6010.C or USGS NWQL I-1472-87 (EPA

1994b, 2000; USGS 1987), axially viewed inductively coupled plasma-atomic emission spectrometry

(AVICP-AES) by EPA Method 200.5 (EPA 2003) and inductively coupled plasma/optical emission

spectrometry by USGS NWQL I-4471-97 (USGS 1998). Samples prepared by EPA Method 200.2 are

acidified, mixed, and held at a pH <2 (EPA 1994a).

The silica content of rock samples may be obtained by decomposing the sample in LiCO3/H3BO3 flux

followed by stabilization in fluoride (Barredo and Diez 1980). An atomic absorption spectrophotometer

(AAS) and EEL lamp are used to detect the major elements of the rock samples. Thermal analysis of

c-silica, to measure the energy associated with phase changes based on changes in temperatures of

samples, has also been developed (Sheffield 1994). An analytical procedure detects the a-silica content of

a sample by converting to cristobalite with heating (Lange et al. 1981).

Scanning electron microscopy (SEM) identifies minerals by energy dispersive techniques and sometimes

by morphology, but does not enable differentiation between the polymorphs of c-silica, a-silica, glasses,

and opal (Miles 1999). The transmission electron microscope (TEM), when combined with energy

dispersive X-ray spectroscopy and electron diffraction, is used to distinguish grains of c-silica.

Differentiation of the forms of a-silica involves investigation into the chemical composition, physical

properties, and characteristics of the particles (Waddell 2006). The amount of silica, percentage of

associated water, total solids content of nonoxidizable materials, presence of stabilizers and carbon

content, level of soluble salts, metal impurities, and silanol group density are important chemical

composition information. The pH, density and tamped density, viscosity, turbidity, refractive index, and

light-scattering properties are important physical characteristics. Specific surface area, average particle

size and size distribution, sieve residue, porosity (including average pore diameter and pore volume),

degree of aggregation, and oil absorption information is used to characterize the silica particles. Although

analytical techniques exist to distinguish between a-silica polymorphs, most are too sophisticated for

routine measurements (IARC 1997). Therefore, environmental exposures are typically reported for

a-silica, rather than for specific polymorphs.


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Particle-size measurement is important in silica-gel characterization. Granular gel standardized sieve

screening uses method ASTM D 4513. Static light scattering and conductivity methods are preferred for

particle size analysis of particles roughly 1–1,000 mm. Dynamic light scattering, electron microscopy,

and small-angle X-ray scattering are used for particle sizes of roughly 10–1,000 nm (Florke et al. 2008).

Porosity of silica gels is described by pore diameter as microporous (<2 nm), mesoporous (approximately

2–50 nm), or macroporous (>50 nm) (Florke et al. 2008). Thermogravimetric analysis, vibrational

spectroscopy, and nuclear magnetic resonance are used to study hydroxyl concentration, hydrogen bond

interaction between hydroxyl groups, and distribution of silica-oxygen species on the surface of silica gel

(Florke et al. 2008).

Density separation uses heavy liquids to separate particles and is based on differences in density of the

forms of silica and silicates (Miles 1999), although it is usually impossible to fully liberate c-silica from

other phases. This technique works best with (mono-mineral) mineral grains ≥0.1 mm.

7.3 ADEQUACY OF THE DATABASE

Section 104(i)(5) of CERCLA, as amended, directs the Administrator of ATSDR (in consultation with the

Administrator of EPA and agencies and programs of the Public Health Service) to assess whether

adequate information on the health effects of silica is available. Where adequate information is not

available, ATSDR, in conjunction with NTP, is required to assure the initiation of a program of research

designed to determine the health effects (and techniques for developing methods to determine such health

effects) of silica.

The following categories of possible data needs have been identified by a joint team of scientists from

ATSDR, NTP, and EPA. They are defined as substance-specific informational needs that if met would

reduce the uncertainties of human health assessment. This definition should not be interpreted to mean

that all data needs discussed in this section must be filled. In the future, the identified data needs will be

evaluated and prioritized, and a substance-specific research agenda will be proposed.


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7.3.1 Identification of Data Needs

Methods for Determining Biomarkers of Exposure and Effect.

Exposure. As discussed in Section 3.12.2, no biomarkers of exposure have been identified for silica.

c-Silica has been detected in the urine of ceramic factory workers (Ibrahim et al. 2011) and development

of sensitive analytical methods may be useful in the assessment of whether urinary silica could be used as

a biomarker of exposure.

Effect. Sensitive biomarkers of effect have not been identified.

Methods for Determining Parent Compounds and Degradation Products in Environmental Media. Silica is ubiquitous in the environment and does not degrade. It is found in air, water, soil,

sediments, and food. Analytical methods exist for the analysis of silica in all of these environmental

media, and these methods have the sensitivity to measure background levels and detect elevated

concentrations due to anthropogenic sources (NIOSH 2002). Additional research to reduce chemical and

matrix interferences is needed to improve the speed and accuracy of the analyses.

7.3.2 Ongoing Studies

No ongoing studies regarding analytical methods for measuring silica in biological materials or

environmental media were located.


7. ANALYTICAL METHODS - Agency for Toxic Substances … · 7. ANALYTICAL METHODS ... diameter of 3.5 µm; however, NIOSH Method 0600 lists a diameter size of 4 µm (NIOSH 1998; OSHA

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