QUARTZ ANALYTICAL METHOD (P-7)

1 PRINCIPLE OF THE METHOD

1.1 Airborne respirable dust samples are collected on membrane filters (in capsules pre-weighed to one hundredth of a milligram, or one thousandth of a milligram) using MSHA/NIOSH approved personal respirable dust samplers as described in 30 CFR Part 74.

1.2 After collection, the filter capsules are reweighed to one hundredth of a milligram in order to determine the net sample mass.

1.3 The sample filters are ashed in a low-temperature radio-frequency asher to destroy the organic matrix (coal dust and collection filter).

1.4 Ashed samples are deposited onto a 0.64 cm² circular area (9 mm diameter) of a DM-450 vinyl/acrylic copolymer filter. (The use of smaller deposit areas may increase the sensitivity of the analysis. Redeposited samples to be analyzed for quartz must be of the same deposit diameter as calibration samples.)

1.5 The redeposited, ashed dust samples are scanned by infrared spectrometry between frequencies of 1,000 and 700 cm^-1 to determine the quartz and kaolinite content.

1.6 The mass of quartz in the deposit is determined (after correcting for the interference by kaolinite) using calibration data from standard quartz samples and standard kaolinite samples.

1.7 The percentage of quartz in the samples is calculated using the quartz mass determined from the analysis and the and the samples mass of dust.

2 RANGE AND LIMIT OF DETECTION

2.1 The range of this method is 25 to 250 micrograms of quartz. The mass required is 0.45 milligram of respirable coal mine dust if filters are preweighed to 0.01 milligram and 0.100 milligram of dust if preweighed to 0.001 milligram.

2.2 The detection limit of this method (3 time the standard deviation of the analysis of 10 blank filters) is 3 micrograms quartz.

3 INTERFERENCES

3.1 Cristobalite, tridymite, kaolinite, and amorphous silica have absorbance peaks at 800 cm^-1. Of these, however, only kaolinite has been detected in coal mine dust.

3.2 Coal mine dust frequently contains kaolinite. Since kaolinite has an absorption band at the same frequency as quartz (800 cm^-1), its presence causes an overestimation of the quartz mass. However, kaolinite also has an infrared absorbance band at 917 cm^-1. (The kaolinite absorbance at 917 cm^-1 is approximately eight times that at 800 cm^-1.) The kaolinite content in the sample is determined from the 917 cm^-1 band and the kaolinite contribution to the absorbance at 800 cm^-1 is determined from the absorbance of kaolinite at 917 cm^-1 using the calibration data. The quartz absorbance is the remainder of the absorbance at 800 cm^-1 after subtracting the kaolinite contribution from the total absorbance.

4 PRECISION AND ACCURACY

4.1 The error of the low-temperature asher, infrared method for the determination of quartz mass is between 5 and 10 percent relative standard deviation (RSD in the 100 to 500 microgram quartz range.⁽¹⁾

4.2 The accuracy of the method may depend on the particle size distribution and purity of standard materials used in preparation of instrument calibration.

5 ADVANTAGES AND DISADVANTAGES OF THE METHOD

5.1 Advantages

5.1.1 The quartz analysis can be performed on a single sample having a weight gain as low as 0.100 milligram if the sample is pre- and post-weighed to a thousandth of a milligram, or a weight gain as low as 0.45 milligram if the sample is weighed to a hundredth of a milligram.

5.1.2 The detection limit is lower than either the X-ray diffraction method (Anderson 1983) or the Talvitie method (Talvitie 1951, 1958).

5.1.3 Samples prepared for analysis can also be analyzed using X-ray diffraction without any additional preparation.

5.1.4 The analytical equipment is considerably less expensive than that required for X-ray diffraction.

5.2 Disadvantages

The method requires the low-temperature ashing and redeposition of all samples prior to analysis.

6 APPARATUS

6.1 Fourier transform infrared spectrophotometer; PE Model 1750, 1725x or equivalent with optional sample shuttle accessory.

6.2 Polystyrene film standard (0.05 millimeter thickness); PE No. 186-2082 or equivalent.

6.3 Low-temperature radio-frequency (RF) asher and vacuum pump; LFE Model LTA-504 or equivalent.

6.4 Oxygen tank and two-stage regulator for low temperature asher, second stage of regulator capable of being set at 2 to 10 psi.

6.5 Microbalance capable of weighing to 0.001 or 0.01 milligrams; Mettler M-3 or equivalent.

6.6 Filters for sample redeposit; Gelman Instrument Company, DM-450, vinyl/acrylic copolymer membrane, 47 millimeter diameter, 0.45 micrometer pore size.

6.7 Filters for supporting collection filter during redepositing; Gelman Type A/E glass fiber filters, 25 millimeter diameter.

6.8 Filtration apparatus consisting of: 3-place manifold, fritted supports and clamps, rubber stoppers, a 4,000 ml side-arm filtering flask and vacuum tubing. (See Figure 1 for illustration of this apparatus.)

6.9 Vacuum source (water aspirator or vacuum pump) with trap, (2,000 ml side-arm flask). (See Figure 1 for illustration of this apparatus.)

6.10 Small, stainless steel forceps for opening filter cassettes and for handling filters.

6.11 Petrislides for 47 millimeter diameter filters; Millipore No. PD15-047-00 or equivalent.

6.12 Ultrasonic bath, 200 watt input.

6.13 Wash bottles, polyethylene, 250 milliliter.

6.14 Beakers, Pyrex, 50 ml, one per sample to be analyzed (Beakers are numbered in sequence and grouped in lettered sets to prevent mixing of samples, i.e., A1, A2,...A28; B1, B2,...B28; etc.)

6.15 Diamond Marking Pencil (used to mark numbers on beakers in Section 6.14).

6.16 Aluminum filtering funnels (These are specially fabricated funnels, similar to the Millipore No. XX10-025-40, and are shown on Figure 2. All funnels used for calibration and analysis must have the same bore diameter, such as the 9 millimeters illustrated.)

6.17 Desiccator for storing the quartz and kaolinite standard materials.

6.18 Sample holders for infrared instrument (These are specially-made steel plates with a center hole having a diameter the same as, or slightly smaller than, that of the sample deposit (9 mm). Small ring magnets are used to hold the filters in position on the sample holder.)

6.19 Slide warmer (for drying filters after filtering).

6.20 Volumetric pipettes, Class A, Pyrex-type or borosilicate glass, in the following sizes: 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, and 30 milliliters (for preparing calibration standards and QA samples).

6.21 Pipette filler, rubber or neoprene.

6.22 Flasks, volumetric, Pyrex-type, with stoppers, 500 and 1,000 milliliter sizes.

6.23 Spatula, micro, stainless steel, (for weighing out quartz and kaolinite standard materials).

6.24 Weighing boats, aluminum, 12 millimeter; Cahn Instruments No. 1187.

7 MATERIALS

7.1 Isopropanol (IPA), technical and ACS reagent grade.

7.2 Quartz for calibration samples; Standard Reference Material(SRM)1878, "Respirable Alpha Quartz," U.S. Department of Commerce, National Institute for Standards and Technology, Washington, D.C. or -5 micrometer MINUSIL, Pennsylvania Glass and Sand Company, Berkeley Springs, West Virginia.

7.3 Kaolinite for calibration samples; Georgia Kaolin, Hydrite UF or equivalent.

8 PROCEDURE

8.1 Assemble the numbered beakers, the respirable coal mine dust samples to be analyzed, and the sample handling data sheet that is obtained from the computer. (See Sample Handling Procedure-Quartz Lab, April 22, 1993.)

8.2 Using the forceps, remove the collection filter and stainless steel wheel from the cassette capsule and place them, first the wheel and then the filter with the dust side down, into the bottom of a numbered beaker.

8.3 Put the beakers containing the samples into the low-temperature asher. With each run of filters ashed (up to 25), also include three quality assurance (QA) samples. (See Internal Quality Assessment Program for Quartz Analysis, October 1992 for an explanation of the QA samples.)

8.4 Ash samples until the filters are completely ashed, approximately 1 hour, using 300 watts RF power and an oxygen flow rate of approximately 200-300 cc/minute. (See Standard Operating Procedures Low-Temperature Ashers.) Carefully remove the glass beakers from the low-temperature asher. As each beaker is removed, gently rinse the sides with about 10 ml of technical grade IPA using a wash bottle. Do not squirt the IPA directly at the bottom of the beaker as this will blow the ash out of the beaker.

8.5 Set-up the filtering apparatus (Section 6.8) as illustrated in Figure 1. Place two Type A/E glass fiber filters on top of each fritted support.

8.6 For each sample to be filtered, cut a 47 mm DM-450 filter into quarters. Place one quarter, glossy side down, on top of the Type A/E glass fiber filter.

8.7 Position the filter funnel on top of the DM-450 filter and clamp securely in place.

8.8 Add several milliliters of technical grade isopropanol (IPA) to the funnel with a wash bottle. Apply the vacuum and check for leakage at the base of the funnel. If leakage occurs, unclamp and reposition funnel, then recheck for leakage. Turn off vacuum.

8.9 Place the beakers containing the dust/alcohol slurry into the ultrasonic bath for 1 to 2 minutes to disperse the dust.

8.10 Remove the beakers from the bath, wipe any excess water from the outside, then carefully pour the slurry into the filter funnel, being careful not to lose any drops of alcohol. Apply the vacuum.

8.11 Rinse the inside of the beaker with IPA from the wash bottle and add the rinsing to the funnel. Repeat this rinsing a second time.

8.12 When the depth of the liquid in the funnel reaches about 2 centimeters above the filter, gently rinse the inside of the funnel with IPA and filter until all alcohol is removed. When the alcohol is below the 'lip' on the funnel, carefully rinse the 'lip' on the inside of the funnel so as not to disturb the ash deposit.

8.13 When filtration is complete, remove the clamp and lift off funnel while taking care not to scrape off or disturb the deposit on the DM-450 filter. Release the vacuum.

8.14 Remove each filter from the filter base. Number the corner of the filter with the beaker number and place the filter in a petrislide which is numbered and lettered the same as the beaker so that the samples are not mixed up.

8.15 Prepare a filter blank by filtering IPA through a DM-450 filter quarter as in Sections 8.5 through 8.8. Place the blank filter in a petrislide marked "blank".

8.16 Set the petrislides containing the filters on the slide warmer with the lids partially ajar to permit evaporation. Several holes, approximately 1 mm in diameter drilled through the top of the petrislides ensure thorough evaporation of the alcohol. Allow the filters to dry for about 30 minutes. Remove the petrislides from the slide warmer and close the lids.

Note: Drying samples in this manner is necessary due to the presence of an interfering peak, created from the combination of the alcohol, polystyrene (petrislide) and the heat from the hotplate, which causes an error in the baseline position and the quartz peak height measurement.

8.17 Set the instrument parameters on the FTIR as follows:

Resolution:	8 cm-1
Acquire mode:	Interleaved (requires sample shuttle accessory)
Apodization:	Normal
Number of scans:	1

Note: When using the FTIR, the setting of these parameters and the following sections, 8.18 through 8.20 are performed through computer programs.

8.18 Prior to scanning any set of samples or standards, check the operation of the infrared spectrophotometer using a polystyrene film standard. Scan the polystyrene standard between 1,000 and 850 cm^-1. An absorption band should occur at 908 cm^-1. The intensity (peak height) of the absorption band from a baseline drawn between the shoulders on each side of the peak should be within the limits established for the film on the particular instrument being used.

Note: this intensity and its limits vary from instrument to instrument and between one standard film and another, and must be established by direct observation over a period of time. If either or both of these conditions are not met, then it may indicate that there is a problem with the spectrophotometer. No samples should be analyzed until the problem is brought to the supervisor's attention and until corrective action is taken as dictated by the operator's manual or repair personnel.

The polystyrene film is used as a standard and must be cared for accordingly. Keep polystyrene films in suitable containers to prevent them from becoming dirty or scratched.

8.19 Scan all samples between frequencies of 1,000 and 700 cm^-1, beginning with the three QA samples. If quartz is present in a sample, a doublet will be present in the scanned range, the higher frequency peak at 800 cm^-1. Kaolinite in the sample is recognizable as an absorption band at 917 cm^-1, usually on the shoulder of a broad, more intense band. (If both quartz and kaolinite are present in a sample, the kaolinite band in the 800 cm^-1 region is obscured by the quartz doublet.) The infrared scan for a typical field sample containing both quartz and kaolinite is shown in Figure 3.

8.19.1 Remove one filter at a time, so as not to mix samples, from its petrislide holder and place it onto an infrared sample holder (Section 6.18). Use a light source to center the deposit over the hole in the holder, then secure the filter with a ring magnet.

8.19.2 Insert the holder with the filter sample into the sample position of the sample shuttle.

8.19.3 Place the dried blank filter onto a holder, secure with a ring magnet, and insert into the reference position of the sample shuttle.

8.20 Determine the results for all samples, beginning with the three QA samples. The QA samples are a test of the total system operation: Low-temperature ashing, the filtering process, and infrared scanning. If the QA sample results are not within established limits, notify the laboratory supervisor. If the QA sample results are within the QA program limits, then determine the quartz content of all samples ashed. (See Internal Quality Assessment Program for Quartz Analysis, October 20, 1992 for an explanation of the QA samples.)

8.20.1 When not automatically performed through computer programs, determine the peak heights for the absorption bands for each sample according to the following steps.

8.20.1.1 Draw a baseline for the quartz/kaolinite absorbance band between 815 and
770 cm^-1.

8.20.1.2 Draw a baseline for the kaolinite absorbance band between 950 and 895 cm^-1 (see Figure 3).

8.20.1.3 For each peak, draw a straight line from the vertex of the peak at either 917 or 800 cm^-1 down through the baseline drawn in Sections 8.20.1.1 and 8.20.1.2.

8.20.1.4 Determine the height of each peak in absorbance units, from the peak vertices to the point where the vertical lines intersect the baselines.

8.20.2 Calculate the corrected absorbance for quartz at 800 cm^-1:

8.20.2.1 Determine the kaolinite contribution to the quartz/kaolinite peak at 800 cm^-1 from the calibration data (Section 9):

kaolinite abs.@ 800 cm-1 =

kaolinite abs.@ 917 cm-1 X mean abs.@800

abs.@917

8.20.2.2 Subtract the kaolinite absorbance contribution to the 800 cm^-1 quartz/kaolinite peak from the total absorbance:

corrected abs. quartz =

abs. quartz + kaolinite @800 cm^-1 - abs. kaol. @ 800 cm^-1

8.20.3 Calculate the mass of quartz present in the samples using the calibration information (Section 9):

mass of quartz (µg) = corrected absorbance

mean abs./microgram

8.21 To determine the percent quartz, divide the mass of quartz (micrograms) by the sample mass (converted to micrograms) and multiply by one hundred. The precision of the percent determination is dependent on the precision of the sample mass determination.

9 CALIBRATION

9.1 Kaolinite

9.1.1 Prepare a suspension of kaolinite in IPA, 100 micrograms of kaolinite per milliliter of alcohol.

9.1.1.1 Weigh out 25.000 ±0.001 milligrams of dried kaolinite.

9.1.1.2 Quantitatively transfer the kaolinite to a 500 milliliter volumetric flask, adjust the temperature of the suspension to 20°C using either hot or cold water baths as necessary, and bring to volume with reagent grade IPA.

9.1.1.3 Disperse the kaolinite by placing the flask in an ultrasonic bath for 30 minutes.

9.1.1.4 Cool the flask in a cold water bath until the temperature of the suspension returns to 20°C.

9.1.2 Prepare kaolinite standards using the same filtering apparatus described in Sections 6.8, 6.9, and 6.16 and illustrated in Figure 1. Position two Type A/E glass fiber filters and one quarter of a DM-450 filter on each fritted support as described in Sections 8.5 and 8.6. Position and clamp the filtering funnel in place and visually check for leaks as indicated in Sections 8.7 and 8.8 using reagent grade IPA.

9.1.3 Prepare at least five DM-450 filters containing the following quantities of the kaolinite standard material: 50, 100, 300, and 500 and 750 micrograms. These are obtained by pipetting 1, 2, 6, 10, and 15 milliliters, respectively, of the standard suspension onto the filters.

9.1.3.1 Vigorously shake the flask with the kaolinite suspension about 15 times.

9.1.3.2 Withdraw an aliquot of the suspension from the center of the flask with the appropriate pipette, then allow this to drain back into the flask to condition the pipette. Repeat.

9.1.3.3 Draw the liquid up to the mark on the pipette, adjusting as necessary.

9.1.3.4 Drain the suspension into the filter funnel. Do not blow out the pipette.

9.1.3.5 Apply vacuum to the funnel and complete the preparation of the kaolinite standard as described in Sections 8.13 to 8.16.

9.1.3.6 Complete preparation of the remaining kaolinite standards as described above. Shake the flask three or four times before each aliquot is withdrawn.

9.1.4 When dry, scan the standards on the infrared spectrophotometer between the frequencies of 1,000 and 700 cm^-1, following Sections 8.17 to 8.19.

9.1.5 Calculate the absorbance of kaolinite at 917 cm^-1 and at 800 cm^-1, as described in Section 8.20.1.1 through 8.20.1.4, for each of the kaolinite standards.

9.1.6 Determine the ratio of the net absorbance at 917 cm^-1 to the net absorbance at 800 cm^-1 for each sample.

9.1.7 Determine the mean ratio for the five samples. This ratio is used to determine the kaolinite contribution to the 800 cm^-1 quartz peak.

9.1.8 Calculate the absorbance per microgram of kaolinite for each of the standards.

ex.:

absorbance per microgram = absorbance @ 917 cm-1

mass, µg

9.1.9 Calculate the mean absorbance per microgram for the five standards. This value is used to determine the mass of kaolinite in a sample.

9.2 Quartz

9.2.1 Prepare a suspension of -5 micrometer MINUSIL quartz in isopropyl IPA, 10 micrograms of MINUSIL per milliliter of alcohol.

9.2.1.1 Weigh out 10.00 ±0.01 milligrams of dried -5 micrometer MINUSIL.

9.2.1.2 Quantitatively transfer the MINUSIL to a 1,000 ml volumetric flask, adjust the temperature of the suspension to 20C using either hot or cold water baths as necessary, and bring to volume with reagent grade IPA.

9.2.1.3 Disperse the MINUSIL by placing the flask in an ultrasonic bath for 30 minutes.

9.2.1.4 Cool the flask in a cold water bath until the temperature of the suspension returns to 20C.

9.2.2 Prepare the quartz standards using the same filtering apparatus described in Sections 6.8, 6.9, and 6.16, and Figure 1. Position two Type A/E glass fiber filters and one quarter of a DM-450 filter on each fritted support as described in Sections 8.6 and 8.7.

9.2.3 Prepare at least five DM-450 filters containing the following quantities of the quartz standard material: 20, 50, 100, 200, and 300 micrograms. These are obtained by pipetting 2, 5, 10, 20, and 30 milliliters, respectively, of the standard suspension onto the filters. Complete preparation of the standards following sections 9.1.3.1 through 9.1.4.

9.2.4 Calculate the absorbance of quartz at 800 cm^-1 for each standard, as in Section 8.20.1.1, 8.20.1.3 and 8.20.1.4.

9.2.5 Calculate the absorbance per microgram of quartz for each of the quartz standards.

9.2.6 Calculate the mean absorbance per microgram for the five quartz standards. This value is used to determine the mass of quartz in the sample.

BIBLIOGRAPHY

Anderson, C. C. Collaborative Tests of Two Methods for Determining Free Silica in Airborne Dust. DHHS (NIOSH) Publication No. 83-124, Contract No. 210-79-0059, February 1983.

Freedman, R. W., S. Z. Toma, and H. W. Lang. On Filter Analysis of Quartz in Respirable Coal Dust by Infrared Absorption and X-ray Diffraction. Am. Ind. Hyg. Assoc. J., v. 35, 1974, p. 411.

Talvitie, N. A. Determination of Quartz in Presence of Silicates Using Phosphoric Acid. Analytical Chemistry, v. 23, No. 4, April 1951, pp. 623-626.

Talvitie, N. A., and F. Hyslop. Colorimetric Determination of Siliceous Atmospheric Contaminants. Am. Ind. Hyg. Assoc. J., v. 19, No. 1, February 1958, pp. 54-58.

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