# Silica, Amorphous (7501)

​SILICA, AMORPHOUS MW: 60.08
SiO2 METHOD: 7501, Issue 3
OSHA : NIOSH: ACGIH:
CAS: Table 1
7501
RTECS: Table 1
EVALUATION: PARTIAL
PROPERTIES:
Table 1 Table 1 Table 1
Issue 1: 15 May 1985 Issue 3: 15 March 2003 solid; d 2.2 g/mL; MP >1600 °C
SYNONYMS: fumed amorphous silica (Aerosil, silica aerogel, silicic anhydride); fused amorphous silica (Cab-o-sil, colloidal silica, xerogel, diatomaceous earth); hydrated amorphous silica (Hi-sil). SAMPLING SAMPLER:
FLOW RATE:
FILTER (Total) or CYCLONE + FILTER (Respirable) (10-mm cyclone, nylon or HigginsDewell + 5-µm PVC membrane) Total: 1 to 3 L/min Nylon cyclone: 1.7 L/min HD cyclone: 2.2 L/min
VOL-MIN: -MAX:
50 L 400 L @ 5 mg/m3 (limit to # 2 mg dust on filter).
SHIPMENT:
Routine
SAMPLE STABILITY:
Stable indefinitely
BLANKS:
2 to 10 field blanks per set
BULK SAMPLE:
High volume area air sample required
MEASUREMENT TECHNIQUE:
X-RAY POWDER DIFFRACTION
ANALYTE:
Cristobalite
ASH:
Llow temperature plasma; acid wash; deposit on PVC filter
XRD ANALYSIS:
XRD for quartz, cristobalite and tridymite before and after heating
HEAT:
1500 °C, 2 h (fumed amorphous silica) or 1100 °C, 6 h (other amorphous silica); redeposit on Ag filter
CALIBRATION:
Standard suspensions of amorphous silica in 2-propanol, converted to cristobalite as for samples
RANGE:
0.2 to 2 mg per sample
ESTIMATED LOD:
0.005 mg per sample
PRECISION ( þ r ):
0.10 @ 0.4 to 5 mg; 0.33 @ 0.2 mg [1]
ACCURACY RANGE STUDIED:
Not studied
BIAS:
Not determined
OVERALL PRECISION (Ö r T ):
Not determined
ACCURACY:
Not determined
APPLICABILITY: This method is specific for amorphous silica in crystalline (e.g., quartz) matrices in the range 1 to 10 mg/m3 for a 200-L air sample. Amorphous silica usually contains some crystalline silica [2]. See also discussion of crystalline silica in Chapter R of this volume [3]. INTERFERENCES: Albite, ammonium phosphate and tridymite interfere with the largest peak of cristobalite. Quartz and cristobalite are quantitated prior to heat treatment and subtracted from the final quantity of cristobalite. Alkali and alkaline earth oxide prevent 100% conversion; these are removed by acid-wash prior to heat treatment. XRD signal intensity is proportional to particle size; similar particle size of samples and standards is desirable.
OTHER METHODS: This method makes improvements in sample handling and quality assurance to P&CAM 316, which it replaces [4,5]. The XRD analysis steps are similar to those in Method 7500 (Silica, crystalline).
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition ​SILICA, AMORPHOUS: METHOD 7501, Issue 3, dated 15 March 2003 - Page 2 of 8 EQUIPMENT:
REAGENTS: 1. Am orphous silica standard appropriate to the air sample, i.e., one of the following: a. fumed amorphous silica, grade CAB-OSIL® M-5, ava ilable from Cabot C orp.; www.cabot-corp.com; b. gelled amorphous silica, grade Britesorb® D300 xerogel, available from The PQ Corp., PO Box 840, Valley Forge, PA 19482-0840; www.pqcorp.com; c. diatom ace ous earth amorphous silica, grade Celite 521, available from Fisher Scientific; www.fishersci.com; d. precipitated amorphous silica, grade Zeothix® 265, available from J.M. Huber Corp.; www.hubermaterials.com. NOT E 1: Som e gelled or precipitated amorphous silicas contain up to 7% moisture. Dry all amorphous sam ples overnight at 115 °C [5]. NOT E 2: XRD signal intensity is proportional to particle size. Sieve the standards (10-µm sieve ), if necess ary, to m atc h the particle size expecte d in the samples 2. 2-Propanol, reagent grade. 3. W ater, deionized. 4. Sodium chloride, reagent grade. 5. Hydrochloric acid, 2% (v/v). Dilute 20 mL conc. H Cl to 1L with deionized water. 6. De siccant.
1. Sam pler: a. Total dust: 37-mm diameter, 5.0-µm pore size, polyvinyl chloride filter (MSA, Ge lman G LA-500 0, or equiva lent) supported with backup pad in a twopiece, 37-mm cassette filter holder (preferably, conductive) held together by tape or cellulose shrink band. NOTE: Gelman VM-1 and Millipore BS filters are unaccepta ble because of high ash or am orph ous silica co nten t. b. Respirable dust: PVC filter (1.a. above) plus cyclone, 10-mm nylon or HigginsDe well (H D), w ith sam pling head holde r. Holder must keep the cassette, cyclone and coupler togethe r rigidly so that air ente rs on ly at the cyclone inlet. c. Area sample: PVC filter (1.a. above) at 3 L/min. 2. Personal sampling pumps 1 to 3 L/min (total or area); 1 .7 L/m in (nylon c yclone ) or 2.2 L/m in (HD cyclone). 3. Filters, PVC, 25-mm diameter, 0.45-µm pore size. 4. Filters, silver mem brane, 25-m m diame ter, 0.45-µm pore size, available from Sterlitech Corp., 22027 70 th Ave S, Kent, W A 980321911; www.sterlitech.com. 5. X-ray powder diffractometer with copper target X-ray tube and scintillation detector. 6. Glue or tape for securing Ag filters to XRD holders. 7. Reference specimen (m ica, Arkansas stone or oth er sta ble sta nda rd) for data normalization. 8. Low temperature plasma asher (LTA) or muffle furnace. 9. Filtration apparatus with side-arm vacuum flask and 25- and 37-mm filter holders. 10. Sieve, 10-µm pore size, for wet sieving. 11. Furnace capable of maintaining 1100 °C for 6 h or 1500 °C for 2 h and of tem perature programm ing to 500 °C at 50 °C/min. 12. Crucibles, platinum, with covers. 13. Analytical balance (0.01 m g). 14. Stirrer, magnetic, with thermally-insulated top. 15. Ultrasonic bath or probe. 16. Pipets, 2-to 25-mL.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition ​SILICA, AMORPHOUS: METHOD 7501, Issue 3, dated 15 March 2003 - Page 3 of 8 EQ UIP M EN T - c on t.: 17. 18. 19. 20. 21. 22. 23. 24.
Volumetric flasks, 1-L. Desicca tor. Bottle, 1-L, with ground glass s topper. Drying oven. W ash bottle, polyethylene. Mortar and pestle, agate. Rubber policeman. Beakers, 100-mL, with watchglass covers.
SPECIAL PRECAUTIONS: 2-Propanol is flamm able. Hydrochloric acid is corrosive and should be handled in a fume hood.
SAMPLING: 1. 2.
Calibrate each personal sampling pump with a representative sampler in line. Sam ple at 1 to 3 L/min (total and area), 1.7 L/min (nylon cyclone) or 2.2 L/min (HD) for a total sam ple size of 50 to 400 L. NOT E 1: Do not allow the cyclone to be inverted at any time. Turning the cyclone to anything other than a horizontal orientation may deposit over-sized material from the cyclone body onto the filter. NOT E 2: A single sa m pler/flow rate s hou ld be u sed for a g iven applica tion. Sa m pling fo r both crystalline silica and coal mine dust should be done in accordance with the ISO /CE N/A CG IH/A ST M resp irable a eros ol sam pling convention . Flow rates of 1.7 L/min for the Dorr-Oliver nylon cyclone and 2.2 L/min for the Higgins-Dewell cyclone have been found to be optimal for this purpose. Outside of coal mine dust sampling, the reg ulatory agencies currently use these flow rates with the Do rr-Oliver cyclone in the United States and the Higgins-Dewell sampler in the United Kingdom. Though the sampling recomm endations presented in a NIOSH Criteria Document have been form ally acce pted by MS HA for coal m ine du st sa m pling, the Dorr-O liver cyclone a t 2.0 L/min with 1.38 conversion factor is currently used in the United States for the purpose of m atch ing an earlier s am pling convention [6]. In any case, a single sam pler/flow rate should be used in any given application so as to eliminate bias introduced by differenc es b etwe en s am pler types an d sa m pler convention s [3].
SAMPLE PREPARATION: 3. 4.
5.
Ash the sam ple filter in a 100 -m L be aker in the LT A ac cording to m anu facturer's instru ctions . After ashing, carefully (so as to avoid loss of sam ple) add 50 m L 2% HC l to each beake r. Cover the beaker with a watchglass. Agitate in an ultrasonic bath for at least 3 min (until the agglom erated particles are broken up). W ash the und erside of the watchglass with distilled water, collecting the was hings in the sam e beak er. Place a 25-m m PVC filter in the filtration apparatus. Attach the funnel secu rely over the entire filter circumference. W ith no vacuum, pour 2 to 3 mL deionized water onto the filter. Pour the sample suspension from the beaker into the funnel, and apply vacuum. After the transfer, rinse the beaker several times with deionized water and add rinsings to funnel for a total volume of 20 mL. Allow the suspension to settle for a few minutes prior to filtration. Do not wash the chimney walls or add deionized water to the chimney after filtration has begun. Rinsing the chimney can disturb the thin layer depo sition. W hen filtration is com plete, k eep vacuum on until filter is dry. Rem ove the filter with forceps and attach it to the sample holder for XRD analysis.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition ​SILICA, AMORPHOUS: METHOD 7501, Issue 3, dated 15 March 2003 - Page 4 of 8 CALIBRATION AND QUALITY CONTRO L: 6. 7.
8.
9.
10.
11.
W eigh ca. 100 m g dried am orph ous silica to the ne ares t 0.01 m g. Quan titatively transfer to a 1-L bottle using 1.00 L 2-propanol. The resulting concentration is 100 µg/mL. Su spend the pow der in 2 -propanol using an ultrasonic probe or b ath for 20 m in. Im m ediate ly move the 1-L bottle to a magnetic stirrer with thermally-insulated top and add a stirring bar to the suspension. Allow the solution to return to room tem perature before withdrawing aliquots. Prepare a series of standard filters, in triplicate, over the range 0.005 to 2 mg per sam ple. a. Mount a 37-mm PVC filter on the filtration apparatus. Place several mL 2-propanol on the filter. Turn off the stirrer and shake the bottle vigorously by hand. Imm ediately remove the stopper and withdraw an aliquot (e.g., 2 to 25 mL) by pipet from the center at half-height of the suspe nsion. Do not adjust the volum e in the pipet by expelling part of the suspension. If m ore than the desired aliquot is withdraw n, disc ard th e aliquot in a beak er, rinse an d dry the pipet, and take a new aliquot. Transfer the aliquot from the pipet to the filter, keeping the tip of the pipet near the surface but not submerged in the delivered suspension. b. Rinse the pipet with several mL 2-propanol, draining the rinse into the funnel. Repeat the rinse several more times. c. Allow the suspension to settle for a few minutes prior to applying vacuum. Apply vacuum and rapidly filter the suspension. Do not w ash dow n the sides of the fu nnel after the deposition is in place sinc e this w ill rearrange the m aterial on the filter. Leave va cuu m on until the filter is dry. W hen thoroughly dry, mount the filter in the XRD sample holder. Prepare and analyze standard filters exactly like samples. a. Ash in LT A; redepos it on 25-mm PVC filters (steps 3 through 5). b. Analyze by XRD (s tep 13). c. Heat to conve rt amo rphous s ilica to cristobalite; redeposit on Ag filters (steps 14 throu gh 16). d. Analyze by XRD (s tep 17). Prepare c alibration graph (Î ox vs. µg of standard). NOTE: Poor repea tability (i.e., S r > 0.1) at any given level indicates that new stan dards should be m ade . The da ta should lie along a straight line. A weighted least s qua res (1/S 2 weighing) is preferable. Curvature can be eliminated with absorption corrections (step 20) [7 ]. Se lect six silver m em brane filters to be analyze d as m edia blanks . Mak e the selectio n ra ndom ly from the same box of filters used for redepositing samples. Mount each media blank on the filtration ap para tus and a pply vac uum to draw 5 to 10 m L of 2 -propan ol throu gh the filter. Rem ove, let dry and mount on XRD holders. Determine net normalized intensity for the silver pea k, î Ag, for ea ch m edia blank . Obtain an average value fro the six m edia blank s, Î Aog. NOTE: The analyst is a critical part of this analytical procedure [3]. A high level of analyst expertise is req uired in order to optim ize instrum ent param ete rs and correct for m atrix interferences either during the sample preparation phase or the data analysis and interpretation phas e. XRD analysts should have som e training (university or short course) in mineralogy or crystallography in order to have a background in crystal structure, diffraction patterns and m ineral transform ation. In addition, an intensive sho rt cou rse in the fu nda m enta ls of X -ray diffra ction c an b e us eful.
MEASUREMENT: 12.
Ob tain a qua litative X-ray diffraction sc an (e.g., 10 to 80 °22) of the bulk (high-volume respirable) sample to determine the presence of free silica polymorphs and interferences. The diffraction peaks are: Mineral Quartz Cristobalite Tridymite Silver
Prima ry 26.66 21.93 21.62 38.12
Peak (2-T heta Deg rees) Secon dary 20.85 36.11 20.50 44.28
Tertiary 50.16 31.46 23.28 77.47
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition ​SILICA, AMORPHOUS: METHOD 7501, Issue 3, dated 15 March 2003 - Page 5 of 8 NOTE:
13.
14.
15.
16. 17.
For quantitative determination of amorphous silica in bulk samples, wet-sieve a portion of the bulk sam ple thro ugh a 10-µm sieve with 2-prop ano l in an ultrason ic bath . Evaporate the alcohol and dry in an oven 2 h. W eigh out, in triplicate, 2-mg portions of the sieved dust and transfer to beakers. Add 10 mL 2-propanol, deposit on a PVC filter (step 5) and continue with steps 13 through 17. Perform the following for each sam ple, working standard an d blank filter. a. Mo unt the referen ce s pec imen. D eterm ine the net intensity, I r, of the reference specimen before and after each filter is scanned. Use a diffraction peak of high intensity that can be rapidly but reproducibly (Sr <0.01) measured. b. Mount the sample, working standard, or blank filter. Measure the diffraction peak area for each silica polymorph. Scan times m ust be long, e.g., 15 min (longer scan times will lower the lim it of detection). c. Measure the background on each side of the peak for one-half the time used for peak scanning. The sum of these two counts is the average background. Determine the position of the background for each sample. d. Ca lculate the net inten sity, Ix, (the difference between the peak integrated count and the total back ground c ount). e. Ca lculate and reco rd the norm alized intensity, î x, for each peak:
NOTE: Select a c onvenient norm alization s cale facto r, N, which is app roximately equivalen t to the net count fo r the re ference spe cim en p eak, and use this value of N fo r all analyses. Normalizing to the reference specimen intensity compensates for long-term drift in Xray tube intensity. If intensity measurem ents are stable, the reference specimen m ay be run less frequently and the net intensities should be normalized to the most recently-m easured re ference inten sity. Rem ove the PVC filter from the XRD holder, fold the filter carefully and place it in a platinum crucible in the furnac e. Ra ise the tem pera ture of the fu rnac e slow ly (ca. 50 °C/m in) to 50 0 °C to ash the filter. W hen ashing is com plete (ca. 0.5 h), rais e the tem perature to 1500 °C and m aintain at 1500 °C for 2 h (for fumed am orphous silica) or to 1100 °C and maintain at 1100 °C for 6 h (for other amorphous silicas). Turn the furnace off and let the crucibles cool overnight in the furnace. Place ca. 10 mg NaC l in the crucible and mix with the ash in the crucible. Transfer contents of the crucible to an agate mortar and grind to a fine powder using an agate pestle. Add distilled water from a polyethylene squeeze bottle to the mortar. Use a rubber policeman to stir. W hen the NaCl is dissolved, transfer the solution to a 100-mL beaker (hold the rubber policeman at the edge of the mortar to guide the flow into the beaker). Rinse mortar, pestle, crucible, and rubber policeman, collecting rinsings in the beaker. Cover the beaker with a watchglass and place in an ultrasonic bath for 2 to 3 min. NOT E 1: Use extrem e care to avoid air c urrents when working with the dry s am ple, as it is easily lost in th e fo rm of a n aerosol. NOT E 2: Use uniform grinding techniques to produce similar particle size in standards and samples. W ash the underside of the watchglass and collect rinsings in the beaker. Place a silver m em brane filter in the filtration appa ratus and filter the contents of the beake r (step 5). Mount the silver mem brane filters in the XRD instrument and: a. Analyze for the three silica polymorphs (step 13); and b. De term ine the norm alized coun t, îAg, of an interference-free silver peak on the sample filter following the sam e procedure. Use a short scan tim e fo r the silver pe ak (e.g., 5 % of s can tim e for analyte peaks) throughout the method.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition ​SILICA, AMORPHOUS: METHOD 7501, Issue 3, dated 15 March 2003 - Page 6 of 8 CALCULATIONS: 18.
Ca lculate the conc entra tion of am orph ous silica, C (m g/m 3), as the diffe renc e in cristoba lite conce ntrations me asured b efore and after hea t treatment in the air volume sa m pled, V (L):
î’x îx
b m f(t) R T îAg ÎAog
NOTE:
normalized intensity for cristobalite sample peak on the Ag filter (step 17.a) normalized intensity for cristobalite sample peak on the PVC filter before heating (step 13) intercept of calibration graph (Î ox vs. :g) slope of calibration graph, counts/:g !R ln T/(1 ! T R) = absorption correction factor (Table 2) sin (1 Ag)/sin (1 x) îAg/(average ÎAog) = transm ittan ce of s am ple normalized silver peak intensity from sample (step 17.b) normalized silver peak intensity from m edia blanks (average of six values) Com pute the applicable OSHA standard using the formulae in Table 1 and the % crystalline silica found in the air sam ple be fore hea t treatm ent.
EVALUATION OF METHOD: This m eth od is based on NIOSH P& CAM 316 [4,5] w hich was furthe r evaluate d with field sam ples in July, 1982 [1]. The relative standard deviation was determined to be related to the type of amorphous silica; gelled, fumed, and precipitated amorphous silica yielded 4.4%, 8.2%, and 4.7%, respectively, over the range of 0.5 to 5 mg. The m ethod was further evaluated using 11 different types of gelled, precipitated, and fumed am orphous silicas and diatomaceous earth [8], with the following conclusions: 1. Not all fumed silicas converted to cristobalite at 1100 °C. A higher temperature (1500 °C) was needed to convert all the fumed silicas to cristobalite. 2. The m oisture content of the gelled and precipitated silicas was ca. 7%; of diatomaceous earth, ca. 4%; and of fu m ed , fro m 0.5 to 3% . 3. The calibration curves from the four different types of amorphous silicas indicated very similar slopes (S r = ± 6.6% ). 4. Com paring the four slopes to a slope of a pure respirable cristobalite material, they were running approximately 30% lower in slope value. Therefore, field samples of amorphous silicas must be compared only to standards prepared from am orphous silicas. 5. Precision studies at 0.2-, 1-, and 2.5-mg levels (six samples per level) of gelled, precipitated, diatom ace ous earth , and fum ed s ilicas indicated a pooled prec ision, þ r, of 8.8, 10.5, 5.6, and 21.5%, respectively, for the above silicas. 6. Recovery studies of the same silicas and concentration levels indicated average recoveries of 82, 115, 95, and 111%, respectively, with pooled þ r equal to 18.2, 13.1, 12.9, and 15.3%, respectively, for the gelled, precipitated, and fumed silicas, and diatomaceous earth.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition ​SILICA, AMORPHOUS: METHOD 7501, Issue 3, dated 15 March 2003 - Page 7 of 8 REFERENCES: [1] [2]
[3]
[4]
[5] [6] [7] [8]
Palassis J [19 82]. N IOS H Interna l Rep ort. Cincinn ati, OH : National Ins titute for O ccu pation al Safety and H ealth, (unpublished, August). NIOSH [1981]. NIOSH /OSH A O ccupatio nal He alth Guidelines for Ch em ical Ha zards. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health. DHHS (NIOSH) Publication No. 81123. Available as GPO Stock #017-033-00337-8 from Superintendent of Documents, W ashington, DC 20402. Key-Schwartz RJ, Baron PA, Bartley DL, Rice FL, Schlecht PC [2003]. Chapter R, Determination of airborne c rystalline silica. In: NIO SH Ma nua l of Analytical Metho ds, 4 th ed., 3 rd Suppl. Cin cinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 2003154. NIOSH [1980]. Amorphous Silica: Method P&CAM 316. In: Taylor DG, ed., NIOSH M anual of Analytical Methods, 2nd ed., Vol. 6. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 80-125. Lange BA, Haartz JC, Hornung RW [1981]. Determination of synthetic amorphous silica on industrial air samples, Anal Chem 53(9):1479-84. Inhaled Particles and Vapours [1961]. Pergamon Press, Oxford, U.K. Leroux, J, Powers C [1969]. Direct X-ray diffraction quantitative analysis of quartz in industrial dust films de pos ited on silver m em bran e filters, Luft 29(5):26. Palassis, J [1984]. Amorphous silica analysis by X-ray powder diffraction. Paper presented to the Am Ind Hyg Con ference. De troit, MI (May 1984).
METHOD REVISED BY: Rosa Key-Schwartz, Ph.D., NIOSH/DART
T ABL E 1 . C AS Nu m be rs , R T EC S N um bers, and Exposure Lim its for Am orphorous Silica Forms Amorphous Forms
CAS#
RTECS
OSHA PEL (mg/m3)
NIOSH PEL (mg/m3)
ACGIH TLV (mg/m3)
diatomaceous earth, <1% crystalline SiO2
61790-53-2
HL8600000
80/% SiO2
6
10 (total)
precipitated, and gel
7699-41-4, 112926-00-8
VV8850000
80/% SiO2
6
10 (total)
fumed
69012-64-2 112945-52-5
VV310000
80/% SiO2
2 (respir)
fused
60676-86-0
VV7328000
0.1 (respir)
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition ​SILICA, AMORPHOUS: METHOD 7501, Issue 3, dated 15 March 2003 - Page 8 of 8 Table 2. MATRIX ABSORPTION CORRECTION FACTORS FOR CRISTOBALITE (HEATCONVERT ED AMOR PHOUS SILICA) AND SILVER PEAKS. Degrees 2-1 Cristoba lite Silver
21.93 38.12
21.93 44.28
21.93 38.12
21.93 44.28
T
f(T)
f(T)
T
f(T)
f(T)
1.00
1.0000
1.0000
0.74
1.2806
1.3278
0.99
1.0087
1.0100
0.73
1.2944
1.3440
0.98
1.0174
1.0201
0.72
1.3084
1.3605
0.97
1.0264
1.0305
0.71
1.3226
1.3774
0.96
1.0355
1.0410
0.70
1.3372
1.3946
0.95
1.0447
1.0517
0.69
1.3521
1.4122
0.94
1.0541
1.0625
0.68
1.3673
1.4303
0.93
1.0636
1.0736
0.67
1.3829
1.4487
0.92
1.0733
1.0849
0.66
1.3987
1.4675
0.91
1.0831
1.0963
0.65
1.4150
1.4868
0.90
1.0932
1.1080
0.64
1.4316
1.5064
0.89
1.1034
1.1199
0.63
1.4485
1.5266
0.88
1.1137
1.1320
0.62
1.4659
1.5472
0.87
1.1243
1.1443
0.61
1.4836
1.5684
0.86
1.1350
1.1568
0.60
1.5018
1.5900
0.85
1.1460
1.1696
0.59
1.5204
1.6122
0.84
1.1571
1.1827
0.58
1.5394
1.6349
0.83
1.1685
1.1959
0.57
1.5590
1.6582
0.82
1.1800
1.2095
0.56
1.5790
1.6820
0.81
1.1918
1.2232
0.55
1.5995
1.7065
0.80
1.2038
1.2373
0.54
1.6205
1.7317
0.79
1.2160
1.2516
0.53
1.6421
1.7575
0.78
1.2284
1.2663
0.52
1.6642
1.7840
0.77
1.2411
1.2812
0.51
1.6870
1.8112
0.76
1.2540
1.2964
0.50
1.7103
1.8391
0.75
1.2672
1.3199
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition