# Silica, Crystalline by IR (KBr pellet) (7602)

​Esta página en Español SILICA, CRYSTALLINE by IR (KBr pellet) SiO2
MW: 60.08
CAS:14808-60-7 14464-46-1 15468-32-3
METHOD: 7602, Issue 3
OSHA : NIOSH: ACGIH:
RTECS: VV7330000 (quartz) VV7325000 (cristobalite) VV335000 (tridymite)
EVALUATION: PARTIAL
quartz (respirable): 10 mg/ m3 / (% SiO2 + 2); cristobalite and tridymite (respirable): 1/2 the above 0.05 mg/m3 ; carcinogens quartz (respirable) 0.1 mg/m3 cristobalite and tridymite (respirable) 0.05 mg/m3
7602
PROPERTIES:
Issue 1: 15 February 1984 Issue 3: 15 March 2003 solid; d 2.65 g/cm3 ; crystalline transformation: quartz to tridymite @ 867 °C; tridymite to cristobalite @ 1470 °C; "quartz to $-quartz @ 573 °C
SYNONYMS: free crystalline silica; silicon dioxide SAMPLING
MEASUREMENT
CYCLONE + FILTER (10-mm nylon or Higgins-Dewell (HD), and PVC filter, 37-mm, 5-µm)
TECHNIQUE:
INFRARED ABSORPTION SPECTROPHOTOMETRY
ANALYTE:
Quartz
FLOW RATE:
Nylon cyclone: 1.7L/min HD cyclone: 2.2 L/min
ASH:
Muffle furnace of RF plasma asher
PELLET:
Mix residue with KBr; press 13-mm pellet
VOL-MIN: -MAX:
400 L 800 L
IR:
Scan absorbance from 1000 to 600 cm-1
SHIPMENT:
Routine
CALIBRATION:
SAMPLE STABILITY:
NIST SRM 1878a quartz, NIST SRM 1879a cristobalite, USGS 210-75-0043 tridymite diluted in KBr
Stable RANGE:
10 to 160 µg quartz
BLANKS:
2 to 10 field blanks per set
SAMPLER:
ESTIMATED LOD: 5 µg quartz BULK SAMPLE: High volume or settled dust to identify interferences
PRECISION ( þ r ):
< 0.15 @ 30 µg quartz per sample, in coal dust [1]
ACCURACY RANGE STUDIED:
Not studied
BIAS:
Not determined
OVERALL PRECISION (Ö r T ): Not determined ACCURACY:
Not determined
APPLICABILITY: The working range is 0.025 to 0.4 mg/m3 for a 400-L air sample. Cristobalite and tridymite also have major absorbance peaks at 800 cm-1 which can be used for their determination [1-6]. IR methods can quantify quartz, cristobalite and tridymite if amorphous silica and silicates are not present in large amounts. However, sensitivity is reduced if multiple polymorphs are present and secondary peaks must be used. A potential for bias exists when correcting for matrix absorption effects, with an increasing risk of bias at lower quartz concentrations. See also discussion of crystalline silica in Chapter R of this volume [7]. INTERFERENCES: Amorphous silica, calcite, cristobalite, kaolinite and tridymite interfere; see APPENDIX. OTHER METHODS: This is P&CAM 110 in a revised format [1]. It is similar to Method 7603, except for sample preparation (KBr pellet vs. redeposition). XRD (method 7500) can distinguish the three silica polymorphs and silica interferences can be eliminiated by phosphoric acid treatment. Crystalline silica can also be determined by visible absorption spectrophotometry (e.g., method 7601), but polymorphs cannot be distinguished. Visible absorption methods also have larger laboratory-tolaboratory variability than XRD and IR methods and therefore are recommended for research use only [8].
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition ​Silica, Crystalline by IR: Method 7602, Issue 3, dated 15 March 2003 - Page 2 of 6 EQUIPMENT:
REAGENTS: 1. Silica Standards. a. Quartz* (SRMs 1878a, 2950, 2951, 2958) and Cristobalite* (SRMs 1879a, 2960, 2957 ), available from Standard Reference Materials Program, Rm 204, Bldg. 202, National Institute of Standards and Technology, Gaithersburg, MD 20899; ww w.nist.gov. b. Tridymite* (210-75-0043), available from U.S. Geological Survey, PO Box 25046, MS 973, Denver, CO 80225. 2. Po tas sium bro m ide (KBr), infra red quality. 3. Ethanol, 95% for cleaning sample handling equ ipm ent.* 4. Hydrochloric acid, 9% w/w.* Add 25 mL conc. HC l (37% w/w) to 70 m L with deionized water, cool, and bring to 100mL with deionized water. 5. Ca libration s tock standard , 0.5% w/w. Accurately weigh and thoroughly mix 5 g KBr (dried overnight at 110 °C) w ith 25 m g qu artz. Store in a bottle in a desiccator.
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Sam pler: a. Filter: 37-m m diame ter, 5.0-µm po re size, polyvinyl chloride filter supported with backup pad in a two-piece, 37-m m cass ette filter ho lder (preferably, conductive) held together by tape or cellulose shrink band. b. Cyclone: 10-m m nylon or Higgins-Dew ell (HD ). c. Sam pling head holder: Holder must keep the cassette, cyclone and coupler together rigidly so that air enters only at the cyclone inlet. Area air sampler: PVC m embrane filter, 37mm , 5-µm pore size in two piece filter cassette. Sample closed-faced at 3 L/min. Sam pling pumps for HD cyclone, 2.2 L/min; nylon cyclone, 1.7 L/ m in; and area sam pler, 3 L/min. Infrared spectroph otom eter; laboratory press for preparing KBr pellets; 13-mm KBr pellet die (evacua ble). Low-temperature (RF Plasma) asher and aluminum weighing pan or muffle furnace and porcelain crucibles. Mo rtar an d pe stle, 50 -m m aga te or m ullite metal microspatula; non-serrate, nonm agnetic forceps; desiccator, ca m el’s hair brush, glassine pap er. Analytical balance (0.001 mg) for preparing standards. Mem brane filtration apparatus, 37-m m .
SPE CIAL PR ECAU TIO NS: Avoid inhaling silica dust [9]. Ethanol is flamm able. Keep away from flames. Use personal protective equipment to prevent contact of acids with the skin. Concentrated acids are corrosive. SAMPLING: 1. Calibrate each personal sampling pump with a representative sampler in line. 2. Sa m ple at 1.7 ± 5% L/m in with nylon cyclone, 2.2 ± 5% with H D c yclone , or 2.5 ± 5% with Al cyclone for a total sample size of 400 to 800 L. Do not exce ed 2 m g total dust loading on the filter. NOT E 1: Do not allow the sampler assembly to be inverted at any time when using a cyc lone. Turning the cyclone to anything more than a horizontal orientation may deposit oversized m aterial from the cyclone body onto the filter. NOT E 2: A single sampler/flow rate should be used for a give n ap plication . Sampling for both crystalline silica and c oal mine du st should be don e in accordance with the ISO/CEN/ACGIH/ASTM respirable aerosol sam pling conventio n. F low rates of 1 .7 L/m in for the Dorr-Oliver nylon cyclone and 2.2 L/min for the Higgins-Dewell cyclone have been found optimal for this purpose. Outside of co al mine dus t sam pling, the regulatory agencies currently use these flow rates with the Dorr-Oliver cyclone in the United States and the Higgins-Dew ell sampler in the United Kingdom. Though the sampling recom m endations presented in a NIOSH Criteria Document have been formally accepted by MSHA for coal mine dust sampling, the Dorr-Oliver cyclone at 2.0 L/min with 1.38 conversion factor is NIOSH Manual of Analytical Methods (NMAM), Fourth Edition ​Silica, Crystalline by IR: Method 7602, Issue 3, dated 15 March 2003 - Page 3 of 6 currently used in the United States for the purpose of matching an earlier sampling convention [10]. In any case, a single sampler/flow rate should be used in any given application so as to eliminate bias introduced by differences between s am pler types and sam pler convention s [7]. SAMPLE PREPARATION: 3. Use one of the following methods to ash samples and blanks: a. Low temperature (RF Plasma) asher: Place the filters in lab elled alum inum dishes (previously rinsed with distilled water, followed by ethanol, and air dried). Place the dis he s in the low tem perature ash er so that sam ple ex pos ure to the plasm a is op timized. As h ac cording to m anufacturer's instructions. Carefully bring asher to atmospheric pressure and remove the dishes. b. Muffle furnace: For sa m ples containing a significant amou nt of calcite (> 20% of the tota l dust loading), wa sh filters with 9% w/w hydrochloric aicd. Othe r sam ples proceed to step 3.b.ii. i. Place a 0.5-µm , 47-mm PVC filter in the filtration apparatus. Remove sample filter from cas sette and center on top of first filter. Clam p fu nnel over the frit so that dust d eposit is com pletely exposed. Add 10 mL 9% w/w HCl and 5 mL 2 -propanol; allow to stand 5 min. Ap ply vacuum and slowly aspirate the acid and alcohol in the funnel. W ash with three successive 10-mL portions of distilled water. Release vacuum. ii. Place filter samples and blanks in porcelain crucibles, loosely cover, and ash in muffle furnace for 2 h at 600 °C (80 0 °C if graphite is present). 4. Add approximately 300 m g KB r, weighed to 0.1 mg and dried overnight at 110 °C, directly to each sample. Mix the sample ash and KBr thoroughly with a pestle. Transfer to mortar to complete mixing if necessary. Transfer the mixture to a 13-mm evacuable pellet die using glassine paper and cam el's hair brush. Press a pellet using standard technique. Weigh the finished pellet to 0.1 m g. Ca lculate the ratio (weight of finish ed p ellet/weight KBr initially added); it is us ually abo ut 0.98. C lean sam ple handling equipment with ethanol between samples. NOTE: A low relative humidity environm ent will facilitate sam ple handling when us ing KBr.
CALIBRATION AND QUALITY CONTRO L: 5. Prepare at least 5 working standard pellets: NOT E 1: Calibration standards are limited to NIST and USG S certified standards of known purity, particle size, and sample-to-sample homogeneity. At least 12 materials, including 5 µm Min-U-Sil, previously used by laboratories throughout the United States and C anada, have been evaluated, and none has been found to be an acceptable alternative to the certified sta ndards cited within this m eth od [7]. Sta ndard re ference m ate rials should be corrected for ph ase purity. NOTE 2: Establishing traceability of secondary calibration standards to the specified NIST and USGS primary standards req uires the use of m easurem ent m ethods with better precision and accuracy than the XRD, IR and visible absorption spectrophotometry methods com m only used in the industrial hygiene field can provide. In addition, particle size distribution m eas urem ents have considerable error. Therefore, the use of secondary calibration standards that are traceable to NIST and USG S certified standards is not appropriate. a. W eigh, to 0.001 mg, portions of NIST "-quartz SRM 1878a, NIST cristobalite SRM 1879a, USGS tridymite 210-75-00473 containing 10 to 200 µg standard. b. Add an accurately-weighed 300 mg KBr portion. Proceed as in step 4. c. Calculate the ratio (weight of finished pellet/weight of solids added); usually about 0.98. d. Determine the absorbance at 800 cm ! 1 for each stand ard pellet following the analytical procedure (step 8). Plot absorbances vs. µ g SiO 2. 6. If sam ples were low temperature ashed (step 3.a.) and kaolinite is present, prepare pellets containing 100 to 600 µg kaolinite. Determine the absorbance at 800 cm ! 1 and at 915 cm ! 1 as absc issa. Prepare at least five differe nt ka olinite concentrations. Use this curve to correct the absorbance value at 800 cm ! 1 for any sample containing kaolinite. 7. Carry m edia blanks an d filters spik ed w ith known am oun ts of q uartz through the sample preparation procedures (steps 3 and 4) to monitor for contamination and losses.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition ​Silica, Crystalline by IR: Method 7602, Issue 3, dated 15 March 2003 - Page 4 of 6 NOTE: S om e training (university or short course) in geology or mineralogy can be useful for the analyst and/or laboratory management. Although most analytical che m ists are fam iliar with the IR technique as applied to org anic analyses, mineralogical samples require additional knowledge of geology and mineralogy to correctly interpret crystal structure, matrix interferences and mineral transformation for the lab orato ry client.
MEASUREMENT: 8. Set the infrared spectrophotometer to absorbance mode and to the appropriate settings for quantitative analysis. Scan the pellet from 1000 cm ! 1 to 600 cm ! 1. Rotate the pellet 45° and scan this diam eter. Repeat twice m ore u ntil 4 scans have be en o btaine d. If the peak at 800 cm ! 1 is sm all, use 5X ordinate expansion to enhance the peak height. Draw an appropriate baseline under the absorbance band at 800 cm ! 1 from approximately 820 to 670 cm ! 1. Measu re the absorbance at 800 cm ! 1 from m aximum to baseline in absorbance units. Average the four values for each sample. 9. If the sample was ashed at low temperature (step 3.a), the presence of kaolinite will be indicated by an absorption band with a maximum at 915 cm ! 1. Draw an appropriate baseline under this band from approxim ate ly 960 to 860 cm ! 1 and measure the absorbance at 915 cm ! 1 from this baseline to the m axim um .
CALCULATIONS: 10. 11. 12.
13.
To correct for kaolinite, determine the sample absorbance at 915 cm ! 1 (step 9) and refer to the kaolinite curve (step 6) to find the absorbance at 800 cm ! 1. Use this corrected value in step 11. If correction for kaolinite is not required, use the absorbance at 800 cm ! 1 to find the we ight of qua rtz, W q (µg), from the calibration graph. Ca lculate the conc entra tion of silica, C (m g/m 3), in the volum e of air sam pled, V (L):
If percent quartz, % Q , is required, divide the weight of qu artz, W q (µg), by the total sam ple weight, W s (µg):
EVALUATION OF METHOD: This m eth od is based on the well-ch arac terized infrare d m etho d [1]. T his m etho d is pa rticle size d epe nde nt. A m ismatch of standards and sample particle sizes will result in an uncorrectable bias. Only NIST-certified standard reference materials (SRM) 1878a and 1879a and USGS 210-75-0043 have been found to have sufficient purity, sam ple-to-sam ple hom oge neity in crystalline silica c onte nt and pa rticle size n ece ssa ry to ensure that results are similar to results obtained by other laboratories using other methods. A thorough study of the range of biases in IR m ethods has not been done.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition ​Silica, Crystalline by IR: Method 7602, Issue 3, dated 15 March 2003 - Page 5 of 6 REFERENCES: [1]
[2] [3] [4] [5] [6] [7]
[8] [9] [10]
NIOSH [1977]. Quartz in coal dust by infrared spectroscopy: Method P&CAM 110. In: Taylor DG , ed., NIOSH Manual of Analytical Methods, 2nd. ed., Vol. 1. Cincinnati, OH: U.S. Department of He alth, Education, and W elfare, Publ. (NIOSH) 77-157-A. Talvitie NA [1951]. Determination of quartz in the presence of silicates using phosphoric acid. Anal C hem 23:623-626. 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. Dodgson J, W hittaker W [1973]. The determination of quartz in respirable dust samples by infrared spectrophotometry-1: The potassium brom ide disc method. Ann Occup Hyg 16:373-387. Ca res JW , Go ldin AS , Lynch JJ, B urge ss W A [19 73]. Th e dete rm ination of q uartz in airborne res pirable granite dust by infrared spectrophotometry. Am Ind Hyg Assoc J 34:298-305. Taylor DG , Nenad ic CM , Crable JV [1970]. Infrared spectra for mineral identification. Am Ind Hyg Assoc J 31:100-108. Key-Sc hwa rtz RJ, Baron PA, Bartley DL, Rice FL, Schlecht PC [2003]. Chapter R, Determ ination of airborne crystalline silica. In: NIOSH M anual of Analytical M etho ds, 4 th ed., 3 rd Suppl. Cincinnati, OH: U.S. Department of Health and Hum an S ervices, P ublic H ealth Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2003-154. Eller PM , Fen g HA, Song RS, Key-Schwartz RJ, Esc he C A, G roff J H [1999 ]. Proficiency analytical testing (PAT) silica variability, 1990-1998. Am Ind Hyg Assoc J 60(4):533-539. NIOSH [1974]. Criteria for a Recom m end ed S tandard. Oc cup ationa l Exposu re to Crystalline Silica. U.S. Department of Health, Education, and W elfare, Publ. (NIOSH) 75-120. Inhaled Particles and Vapours [1961]. Pergamon Press, Oxford, U.K.
METHOD REVISED BY: Peter Eller, Ph.D., Paul Schlecht, Rosa Key-Schwartz, Ph.D., NIOSH/DART
APPE ND IX A: INTERFERENCES Qu artz and cristobalite can be determ ined in ea ch other's presence by the use of less sensitive bands at 695 cm ! 1 (quartz) and 625 cm ! 1 (cristobalite). Tridymite can only be determined in the absence of the other two polymorphs; it is ra rely enc oun tered in industrial hygiene s am ples. Interfering silicates can be removed using a ph osp horic acid c leanup p rocedu re [2]. Cristoba lite and tridymite interfere positively at the 800 cm ! 1 peak, although they are rarely present in industrial hygiene sam ples. Kao linite, a com m on com ponent of co al, can interfere whe n RF plasm a as hing is use d to rem ove the collection filter, if it is present in sufficient quantity. A correction procedure is outlined in the method (steps 6 and 10). Calcite, at greater than 20% of the total dust loadings, can interfere by reacting with the quartz during muffle ashing. A procedure for its removal is given (step 3.b). Amorphous silica may interfere if present in large amounts. This interference can be minimized by accounting for its broad absorbance band when drawing the baseline. Qu artz is a co m m on c om pon ent of soil, rocks, sand, mortar, cement, fluxes, abrasives, glass, porcelain, paints and brick. Cristobalite is less comm on and found in volcanic rocks and soils, and can be formed in high tem perature processes such as foundry processes, calcining diatom aceous earth, brick m anufacturing, ceram ic manufacturing and silicon carbide production. Tridymite is rarely encountered, but is present in some volca nic rock s an d so ils.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition ​Silica, Crystalline by IR: Method 7602, Issue 3, dated 15 March 2003 - Page 6 of 6 APPENDIX B: DETERM INATION OF QUART Z IN BULK SAMPLES Additional equipment needed:
Drying oven W ilks Mini-Press, or other pellet press Min-U-Sil 30
1. Ho m oge nize the sam ple if necessary (e.g., by grinding using a large m ortar and pes tle) and dry it (e.g., in a vacuum oven at 110 °C for 2-4 hours). 2. Using a small aluminum weighing boat and a six-place analytical balance, weigh out 0.1 to 1 mg of the sample. The amount weighed out should contain a quantity of quartz which will produce an absorbance which is measurable, but which is not greater than about 0.9. NOTE: The weighed amount of sample and the amount of KBr (step 3) are specified for the W ilks Minipress, which pro duces pellets a pproxim ate ly 8 m m in diam ete r. Adju st th e am ounts appropriately if another pellet press is used. 3. Quantita tively transfer the weighed sample to a small mortar and grind it thoroughly. Add between 75 and 100 mg dry KBr and mix it thoroughly with the ground sample. 4. Quantita tively transfer the mixture to the pellet press. Press the pellet according to the manufacturer’s instructions. NOTE: W eighing paper and a cam el’s hair brush are helpful in m ak ing the transfer. 5. Scan the pellet from 1000 cm -1 to 600 cm -1, using a blank KBr pellet as background. Rotate the pellet 90 degrees and obtain another spectrum. Measure and record the mean peak absorbance at 798 cm -1 for the sample. 6. Calibration and Q uality Control. a. Prepare calibration standards, using dry Min-U-Sil 30, in exactly the same m anner as samples (steps 2-5) and plot peak a bsorbance vs. m icrogram s S iO 2. b. Analyze bulk sample of known quartz content and prepare a control chart of recovery for us e with subsequent analyses.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition