NIOSH Manual of Analytical Methods (NMAM), Fifth Edition

 

SILICA, Respirable Crystalline, by IR (KBr pellet) 7602

SiO2 MW: 60.08 CAS: 14808-60-7 RTECS: VV7330000 (quartz)

 14464-46-1 VV7325000 (cristobalite)

 15468-32-3 VV335000 (tridymite)

METHOD: 7602, Issue 4 EVALUATION: FULL Issue 1: 15 February 1984

Issue 4: 25 July 2017

OSHA: 0.05 mg/m3

NIOSH: 0.05 mg/m3; carcinogens

OTHER OELS: [1,2]

PROPERTIES: solid; d 2.65 g/cm3; crystalline transformation: 

α-quartz to β-quartz @ 573 °C; quartz to tridymite @ 867 °C; 

tridymite to cristobalite @ 1470 °C

SYNONYMS: free crystalline silica; silicon dioxide

SAMPLING

SAMPLER: CYCLONE + PRE-WEIGHED FILTER (PVC), 

with conductive filter holder

FLOW RATE: ~2 to ~4 L/min, specific for the cyclone 

used

VOL-MIN: 400 L

 -MAX: 1000 L (total dust < 4 mg)

SHIPMENT: Routine

SAMPLE

STABILITY: Stable

BLANKS: 3 field blanks minimum per sample set

ACCURACY

RANGE 

STUDIED: 5 to >160 μg

BIAS: Varies depending on matrix effects

OVERALL

PRECISION (SS�rrrr): 0.18 [3]

ACCURACY: ±19% at ≈40-160 μg per sample; ±36% at

≈15 μg per sample [4]

MEASUREMENT

TECHNIQUE: INFRARED SPECTROMETRY

ANALYTE: Quartz / Cristobalite

ASHING: Muffle furnace or RF plasma asher

PELLET: Mix residue with KBr; press 13-mm pellet

IR DETECTION: Scan absorbance between 400 cm-1 and 

1000 cm -1 

CALIBRATION: Certified reference material (CRM) 

respirable crystalline silica (RCS) quartz & 

cristobalite; e.g., NIST SRM 1878b quartz, 

NIST 1879a cristobalite, and/or other RCS

CRMs; diluted in KBr and pressed into 13-

mm pellet

RANGE: 10 to >160 μg 

ESTIMATED LOD: 5 μg 

PRECISION (SS�rr): < 0.15 @ 30 μg per sample

APPLICABILITY: The working range is 0.025 to 0.4 mg/m3 for a 400-L air sample or 0.02 to 0.25 mg/m3 for a 1000-L air sample. 

Quartz and cristobalite have a major IR absorbance band at ≈800 cm-1 and can be measured together if present in samples; 

tridymite can also be measured by IR, but its presence is rare in workplaces [5-9]. 

INTERFERENCES: Amorphous silica, kaolinite, muscovite, talc, albite, vermiculite and other minerals that absorb IR radiation in 

the 800 cm-1 region are potential interferences to RCS measurement [5,10] [Table 1]. Potentially interfering silicates can be 

removed using a phosphoric acid cleanup procedure [11]. Calcite at >20% dust loading can negatively interfere by reacting with 

quartz during muffle ashing [11, 12]. The sample preparation procedures described in this method enable minimization of

interferences.

OTHER METHODS: NIOSH Method 7603 describes IR measurement of respirable crystalline silica (RCS) after redeposition onto a 

filter for analysis (vs. use of a KBr pellet) [13]. X-ray diffraction (e.g., NIOSH Method 7500 [13]) is a complementary method for RCS 

measurement. An international voluntary consensus standard describing the measurement of RCS by IR spectrometry, ASTM 

D7439, has been promulgated [14]. 
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NIOSH Manual of Analytical Methods (NMAM), Fifth Edition

REAGENTS:

1. Respirable crystalline silica (RCS) primary 

certified reference materials (CRMs); e.g., 

NIST SRM 1878b (quartz), NIST SRM 1879a 

(cristobalite) [15].

2. Potassium bromide (KBr), IR quality.

3. Kaolinite (kaolin) CRM. 

4. Deionized water, ≥18 MΩ-cm resistivity.

5. 2-Propanol, reagent grade.

6. Ethanol, 95%, for cleaning of sample 

handling equipment.*

7. Hydrochloric acid (HCl), 9% w/w.* Add 25 mL 

conc. HCl (37% w/w) to 50 mL of deionized 

water, cool, and bring to 100 mL with 

deionized water.

8. Calibration stock standard, 0.5% w/w.

Accurately weigh and thoroughly mix 5 g 

KBr (dried overnight at 110  ̊C) with 25 mg 

quartz. Store in a bottle in a desiccator. 

*See SPECIAL PRECAUTIONS.

EQUIPMENT:

1. Sampler:

a. Filter, 37-mm dia., 5-μm pore size, polyvinyl 

chloride (PVC), supported with backup pad

or screen in 2-piece, conductive / staticdissipative filter holder.

b. Cyclone, respirable, designed to operate at

a flow rate of ≈2 or ≈4 L/min, specific for 

each sampling device. 

c. Sampling head holder, for keeping the 

cassette, cyclone and coupler tightly 

together so that air enters only through

cyclone inlet.

2. Personal sampling pumps, for RCS sample 

collection at ≈2 or ≈4 L/min.

3. Infrared spectrometer, double-beam 

dispersive or Fourier transform device, with 4 

cm-1 resolution or better.

4. Laboratory press (10 tons pressure), for 

preparing KBr pellets, with evacuable 13-mm 

KBr pellet die.

5. Vacuum pump, for connecting to laboratory 

press.

6. Low-temperature (RF plasma) asher and 

aluminum weighing pan, or muffle furnace 

and porcelain crucibles.

7. Analytical balance, capable of weighing to 

the nearest 0.001 mg.

8. Mortar and pestle, 50-mm agate or mullite

9. Metal microspatula

10. Non-serrate, non-magnetic forceps

11. Desiccator

12. Camel’s hair brush

13. Glassine paper / weighing boats

14. Membrane filtration apparatus (47-mm)

15. Laboratory oven 

SPECIAL PRECAUTIONS: Wear appropriate personal protection during sampling activities, sample 

preparation and analysis. It is essential that suitable gloves, eye protection, laboratory coat, etc., be used, 

especially when working with concentrated acids. Note that KBr is incompatible with strong acids. Ethanol 

and 2-propanol are flammable. Avoid exposure by inhalation of silica dust or acid fumes. Perform sample 

preparation and analysis in a clean, well-ventilated area that is well removed from any possible

contamination. Any skin affected by exposure to acids must be immediately washed with plenty of water. 
RESPIRABLE CRYSTALLINE SILICA BY IR (KBr Pellet): METHOD 7602, Issue 4, dated 25 July 2017 - Page 3 of 7 

NIOSH Manual of Analytical Methods (NMAM), Fifth Edition

SAMPLING:

1. Calibrate each personal sampling pump with a representative sampler in line. Use a conductive filter 

holder.

2. Sample at ≈2 L/min [16] or ≈4 L/min [17] using a pump set at a specific flow rate for the selected 

cyclone, designed to collect the respirable aerosol fraction (in accordance with ISO 7708[18]), for a 

total sample size of 400 to 1000 L. Take care not to overload the filter.

NOTE 1: Do not allow the sampler assembly to be inverted at any time when using a cyclone. Turning 

the cyclone to anything more than a horizontal orientation may deposit oversized material 

from the cyclone body onto the filter.

NOTE 2: Higher sampling flow rates may be suitable for certain sampler designs [19].

3. After collection of samples, remove the filter holders from the cyclones. Seal the filter holders 

(cassettes) and transport them to the laboratory in a container designed to prevent damage in transit.

SAMPLE PREPARATION:

4. Use one of the following methods to prepare samples and blanks:

a. Low-temperature (RF plasma) asher: Remove the filters from filter holders and place them in clean, 

labelled aluminum dishes. Place the dishes in the low-temperature asher so that sample exposure 

to the plasma is optimized. Ash the samples according to manufacturer’s instructions. Carefully 

bring the asher to atmospheric pressure and remove the dishes.

b. Muffle furnace: For samples expected to contain a significant amount of calcite (>20% of the total 

dust loading), wash filters with 9% w/w HCl per procedure in 4.b.i. For other samples, proceed to 

step 4.b.ii. 

i. Place a 0.5-μm pore size, 47-mm dia. PVC filter in the filtration apparatus. Remove the 37-mm 

dia. sample filter from the filter holder and center atop the 47-mm PVC filter. Clamp a funnel 

over the frit so that the dust deposit on the filter is completely exposed to rinsates. Add 10 mL 

of 9% HCl and 5 mL of 2-propanol; allow to stand for 5 min. Apply vacuum and slowly aspirate 

the acid and alcohol in the funnel. Wash with three successive 10-mL portions of deionized 

water. Release vacuum.

ii. Place 37-mm filter samples and blanks in porcelain crucibles, loosely cover, and ash in muffle 

furnace for 2 h at 600  ̊C (800  ̊C if graphite present). Allow to cool to room temperature.

5. Add approximately 300 mg KBr, weighed to the nearest 0.1 mg and dried overnight at 110  ̊C, directly 

to each sample. Mix the sample residue (remaining from the above ashing procedure) with KBr using a 

pestle and grind / homogenize the mixture with mortar and pestle. Quantitatively transfer the mixture 

to a 13-mm evacuable pellet die using glassine paper and camel’s hair brush. Place the die into the 

laboratory press and apply requisite pressure to prepare the sample KBr pellet. Weigh the finished 

pellet to the nearest 0.1 mg and calculate the ratio of the weight of the finished pellet to the weight of 

KBr initially added. Clean sample handling equipment with ethanol between samples.

CALIBRATION AND QUALITY CONTROL:

6. Prepare at least 5 working standard KBr pellets. Calibration standards shall be prepared from primary

RCS CRMs. 

NOTE: Suitable RCS standards include primary reference materials such as NIST SRMs 1878b (quartz) 

and 1879a (cristobalite) [15]; the use of secondary reference materials as calibration standards is 

not appropriate for IR measurements [12, 20].

a. Weigh, to the nearest 0.001 mg, portions of primary CRMs containing 10 to 200 μg RCS of material.

b. Add an accurately weighed (to the nearest 0.001 mg) 300 mg portion of KBr. Prepare KBr pellet 

samples as in Step 5. Compute the ratio of the weight of finished pellet / weight of solids added. 

c. Measure the absorbance at 800 cm-1 for each KBr pellet calibration standard following the 

analytical procedure below (Step 9). Plot the absorbance vs. mass of SiO2 (μg).
RESPIRABLE CRYSTALLINE SILICA BY IR (KBr Pellet): METHOD 7602, Issue 4, dated 25 July 2017 - Page 4 of 7 

NIOSH Manual of Analytical Methods (NMAM), Fifth Edition

7. If samples were low-temperature ashed (Step 4.a.) and kaolinite is known to be present in the matrix of 

the collected samples, prepare at least five different KBr pellets containing 100 to 600 μg of kaolinite. 

Measure the absorbance at 800 cm-1 (RCS) and at 915 cm-1 (kaolinite). Use the relationship between 

the RCS and kaolinite peak to correct the absorbance value at 800 cm-1 samples containing kaolinite

[21]. 

8. Carry media blanks and filters spiked with known amounts of RCS (e.g., quartz) through the sample 

preparation procedures (Steps 4 and 5) to monitor for contamination and losses. 

NOTE: Knowledge of and training in geochemistry and mineralogy is strongly recommended for users 

of this method. Although many analytical chemists are familiar with IR spectroscopy (e.g., as 

applied to organic analysis), mineralogical samples such as those containing RCS require 

additional knowledge of geochemistry and mineralogy to correctly interpret IR spectra and 

account for matrix interferences and mineral transformations.

MEASUREMENT:

9. Set the infrared spectrometer to absorbance mode and to the appropriate settings for quantitative 

analysis. For each IR scan, place a KBr pellet in the sample holder and obtain a spectrum at 4 cm-1

resolution or better from 400 cm-1 to 1000 cm-1. Rotate the KBr pellet 45 ̊ and scan again. Repeat twice 

more until 4 scans have been obtained. Draw an appropriate baseline under the absorbance band at 

800 cm-1 from approximately 820 to 670 cm-1. Measure the absorbance from the 800 cm-1 peak 

maximum to baseline in absorbance units. Average the four absorbance values for each KBr pellet

sample. 

NOTE: If the peak at 800 cm-1 is small, expand the ordinate to enhance the peak height. 

10. If the sample was ashed at low temperature (Step 4.a.), the presence of kaolinite will be indicated by an 

absorption band with maximum at 915 cm-1. Draw an appropriate baseline under the absorbance 

band at 915 cm-1 from approximately 960 to 860 cm-1. Measure the absorbance from the 915 cm-1 peak 

maximum to baseline.

CALCULATIONS:

11. To correct for the presence of kaolinite (if necessary), use the measured absorbance at 915 cm-1 (Step 

10) and refer to the kaolinite curve (Step 6) to establish the corrected absorbance for RCS at 800 cm-1. 

Use this corrected RCS absorbance value in Step 13. 

12. If correction for kaolinite is not required, use the measured (uncorrected) absorbance at 800 cm-1 to 

determine the weight (Ws, μg) of RCS (e.g., quartz) from the calibration graph. 

13. From the weight of RCS (Ws, μg), calculate its concentration, C (mg/m3

), in the volume of air sampled, V 

(L):

C = [Ws / V], mg/m3

14. If percent RCS (%Q) is of interest, divide the weight of RCS, Ws (μg), by the total sample weight, Wt (μg): 

%Q = [Ws / Wt] × 100

EVALUATION OF METHOD:

From various experimental studies, analytical figures of merit for the KBr pellet IR method for RCS have 

been summarized in Ref. [14]. Filter collection was carried out using respirable cyclone samplers with flow 

rate ≈2 L/min, with sampling volumes ranging from 300 to 1000 L. Following sample ashing with a plasma 

asher or muffle furnace, the remaining material was homogenized with KBr and pressed into a KBr pellet. 

IR spectrometric measurements were then carried out on the as-prepared KBr discs. By comparison of 

sample IR response to the signal from similarly-prepared CRM quartz standards, and with interference 
RESPIRABLE CRYSTALLINE SILICA BY IR (KBr Pellet): METHOD 7602, Issue 4, dated 25 July 2017 - Page 5 of 7 

NIOSH Manual of Analytical Methods (NMAM), Fifth Edition

correction [21], the RCS content was measured. Applicable analytical ranges were from 3 to 900 μg per 

sample, with estimated method detection limits (MDLs) of 5 μg or less. Reported precision estimates, in 

terms of relative standard deviation, were 0.15 or below at RCS mass per sample of 30 μg. To further 

optimize the method, matching CRM particle size distribution to that of RCS samples [20] has been applied 

to minimize the effect of particle size on the IR response. The KBr pellet IR method has been used to 

measure the crystalline silica content of respirable dusts collected at various workplaces, including coal 

mines [7,8,22], construction sites [23], granite quarries [9] and gold mines [24]. Laboratories using the KBr 

disc method have performed successfully in interlaboratory proficiency analytical testing programs 

[3,25,26].

REFERENCES:

[1] ACGIH [2017] TLVs and BEIs based on the documentation of the Threshold Limit Values for chemical 

substances and physical agents and Biological Exposure Indices. Cincinnati, OH: American 

Conference of Governmental Industrial Hygienists.

[2] Institute for Occupational Safety and Health of the German Social Accident Insurance (IFA) [2017]. 

GESTIS international limit values, http://limitvalue.ifa.dguv.de/. 

[3] Harper M, Sarkisian K, Andrew M [2014]. Assessment of respirable crystalline silica using Proficiency 

Analytical Testing results from 2003-2013. J Occup Environ Hyg 11:D157-D163. 

[4] NIOSH [2016]. Development and evaluation of methods (Chapter ME). By Kennedy ER, Fischbach TJ, 

Song R, Eller PM, Hull RD. In: NIOSH manual of analytical methods, 5th ed.; Ashley K, O’Connor PF, eds. 

Cincinnati, OH: U.S. Department of Health and Human Services, Centers for Disease Control, National 

Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2014-151. 

[5] Ojima J [2003]. Determining crystalline silica in respirable samples by infrared spectrophotometry in 

the presence of interferences. J Occup Health 45:94-103. 

[6] NIOSH [1977]. Method P&CAM 100 - Quartz in coal dust by infrared spectroscopy. In: Taylor DG, ed. 

NIOSH manual of analytical methods, 2nd ed. Cincinnati, OH: U.S. Department of Health, Education 

and Welfare, Center for Disease Control, National Institute for Occupational Safety and Health, 

DHEW (NIOSH) Publication No. 77-157.

[7] Larsen DJ, von Loenhoff LJ, Crable JV [1972]. The quantitative determination of quartz in coal dust 

by infrared spectroscopy. Am Ind Hyg Assoc J 33:367-372. 

[8] Dodgson J, Whittaker W [1973]. The determination of quartz in respirable dust samples by infrared 

spectrophotometry – 1: The potassium bromide disc method. Ann Occup Hyg 16:373-387. 

[9] Cares JW, Goldin AS, Lynch JJ, Burgess WA [1973]. The determination of quartz in airborne respirable 

granite dust by infrared spectrophotometry. Am Ind Hyg Assoc J 34:298-305. 

[10] Taylor DG, Nenadic CM, Crable JV [1970]. Infrared spectra for mineral identification. Am Ind Hyg 

Assoc J 31:100-108. 

[11] Talvitie NA [1951]. Determination of quartz in the presence of silicates using phosphoric acid. Anal 

Chem 23:623-626. 
RESPIRABLE CRYSTALLINE SILICA BY IR (KBr Pellet): METHOD 7602, Issue 4, dated 25 July 2017 - Page 6 of 7 

NIOSH Manual of Analytical Methods (NMAM), Fifth Edition

[12] NIOSH [2003]. Determination of airborne respirable silica (Chapter R). By Key-Schwartz RJ, Baron PA, 

Bartley DL, Rice FL, Schlecht PC. In: NIOSH manual of analytical methods, 4th ed.; Schlecht PC, 

O’Connor PF, eds. Cincinnati, OH: U.S. Department of Health and Human Services, Centers for 

Disease Control, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication 

No. 94-113. 

[13] NIOSH [2003]. Methods 7603 and 7500. In: NIOSH manual of analytical methods, 4th ed.; Schlecht PC, 

O’Connor PF, eds. Cincinnati, OH: U.S. Department of Health and Human Services, Centers for 

Disease Control, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication 

No. 94-113.

[14] ASTM International [2014]. ASTM D7948, Standard test method for measurement of respirable 

crystalline silica in workplace air by infrared spectrometry. West Conshohocken, PA: ASTM 

International. 

[15] NIST [no date]. Standard reference materials (SRMs). Gaithersburg, MD: U.S. Department of 

Commerce, National Institute of Standards and Technology, https://www.nist.gov. Date accessed: 

July 25, 2017. Technology (NIST), Standard Reference Materials (SRMs); www.nist.gov (Accessed 

August 2016). 

[16] Stacey P, Mecchia M, Verpaele S, Pretorius C, Key-Schwartz R, Mattenklott M [2014]. Differences 

between samplers for respirable dust and the analysis of quartz – An international study. In: Silica 

and Associated Respirable Mineral Particles (ASTM STP 1565); Harper M, Lee T, Eds. W. 

Conshohocken, PA: ASTM International; pp. 73-102. 

[17] Lee T, Lee EG, Kim SW, Chisholm W, Kashon M, Harper M [2012]. Quartz measurement in coal dust 

with high-flow rate samplers – Laboratory study. Ann Occup Hyg 56:413-425. 

[18] ISO [1995]. Air quality – Particle size fraction definitions for health-related sampling. Geneva, 

Switzerland: International Organization for Standardization.

[19] Verpaele S, Jouret J [2013]. A comparison of the performance of samplers for respirable dust in 

workplaces and laboratory analysis for respirable quartz. Ann Occup Hyg 57:54–62. 

[20] Stacey P, Kauffer EM, Moulut J-C, Dion C, Beauparlant M, Fernandez P, Key-Schwartz RJ, Friede B, 

Wake D [2009]. An international comparison of the crystallinity of calibration materials for the 

analysis of respirable α-quartz using x-ray diffraction and a comparison with results from the infrared 

KBr disc method. Ann Occup Hyg 53:639-649. 

[21] Lee T, Chisholm WP, Kashon M, Key-Schwartz RJ, Harper M [2013]. Consideration of kaolinite 

interference correction for quartz measurements in coal mine dust. J Occup Environ Hyg 10:425-434. 

[22] Bandyopadhyay AK [2010]. Determination of quartz content for Indian coals using an FTIR 

technique. Int J Coal Geol 81:73-87. 

[23] Virji MV, Bello D, Woskie SR, Liu XM, Kalil AJ [2002]. Analysis of quartz by FTIR in air samples of 

construction Appl Occup Environ Hyg 17:165-175. 

[24] Verma DK, Sebestyen A, Julian JA, Muir DCF [1992]. A comparison of two methods of sampling and 

analysis for free crystalline silica α-quartz. Appl Occup Environ Hyg 7:758-763. 

[25] Shulman SA, Groff JH, Abell MT [1992]. Performance of laboratories measuring silica in the 

Proficiency Analytical Testing program. Am Ind Hyg Assoc J 53:49-56. 
RESPIRABLE CRYSTALLINE SILICA BY IR (KBr Pellet): METHOD 7602, Issue 4, dated 25 July 2017 - Page 7 of 7 

NIOSH Manual of Analytical Methods (NMAM), Fifth Edition

[26] Stacey P, Tylee B, Bard D, Atkinson R [2003]. The performance of laboratories analyzing α-quartz in 

the Workplace Analysis Scheme for Proficiency. Ann Occup Hyg 47:269-277. 

METHOD REVISED BY:

Kevin Ashley, Ph.D., Rosa J. Key-Schwartz, Ph.D., and Martin Harper, Ph.D., CIH, CDC/NIOSH. 

Disclaimer: Mention of any company or product does not constitute endorsement by NIOSH. In addition, 

citations to websites external to NIOSH do not constitute NIOSH endorsement of the sponsoring 

organizations or their programs or products. Furthermore, NIOSH is not responsible for the content of 

these websites. All web addresses referenced in this document were accessible as of the publication date.

Table 1. Minerals potentially encountered and their characteristic IR bands (450-1000 cm-1) 

Mineral Major/Interfering Peaks, cm-1 Identifying Peaks, cm-1

Quartz 800, 780 694, 512, 467

Cristobalite 798 623, 490

Tridymite 789 617, 476

Amorphous silica 800 464

Kaolinite 795, 754 915, 547, 474

Muscovite 800, 750 535, 481

Mullite 837, 748 556, 468

Pyrophyllite 830, 814 948, 477, 457

Albite 788, 746 726, 652, 598, 470

Montmorillonite 797 918, 668, 526, 470

Daphnite 798, 771 667, 610, 539, 467

Anorthite 760, 730 577, 538, 481

Orthoclase 765, 745, 730 645, 593, 540

Talc 797, 778 668, 641, 620

Vermiculite 810, 755 685, 510