SILICA, CRYSTALLINE, by XRD (filter redeposition) SiO2
MW: 60.08
7500
CAS: 14808-60-7 (quartz) RTECS: VV7330000 (quartz) 14464-46-1 (cristobalite) VV7325000 (cristobalite) 15468-32-3 (tridymite) VV7335000 (tridymite)
METHOD: 7500, Issue 4
EVALUATION: FULL
quartz (respirable) 10 mg/m3 /(%SiO2 +2); cristobalite and tridymite (respirable) ½ the above NIOSH: 0.05 mg/m3 ; carcinogen ACGIH: quartz (respirable) 0.1 mg/m3 cristobalite (respirable) 0.05 mg/m3 tridymite (respirable) 0.05 mg/m3
OSHA :
PROPERTIES:
Issue 1: 15 August 1990 Issue 4: 15 March 2003 solid; d 2.65 g/cm3 @ 0 °C; crystalline transformations: quartz to tridymite @ 867 °C; tridymite to cristobalite @ 1470 °C; "-quartz to ß-quartz @ 573 °C
SYNONYMS: free crystalline silica; silicon dioxide SAMPLING SAMPLER:
FLOW RATE:
CYCLONE + FILTER (10-mm nylon cyclone, HigginsDewell (HD) cyclone, or aluminum cyclone + 5-:m PVC membrane)
- see sampling section
Nylon cyclone: 1.7 L/min; HD cyclone: 2.2 L/min; aluminum cyclone: 2.5 L/min
VOL-MIN: -MAX:
400 L 1000 L
SHIPMENT:
Routine
SAMPLE STABILITY:
Stable
BLANKS:
2 to 10 per set (see step 13.g.)
BULK SAMPLE:
High-volume or settled dust; to identify interferences
MEASUREMENT TECHNIQUE:
X-RAY POWDER DIFFRACTION
ANALYTE:
Crystalline SiO2
ASH:
Muffle furnace or RF plasma asher or dissolve in tetrahydrofuran
REDEPOSIT:
On 0.45-:m Ag membrane filter
XRD:
Cu target X-ray tube, graphite monochromator Optimize for intensity; 1° slit Slow step scan, 0.02°/10 sec Integrated intensity with background subtraction
CALIBRATION:
- IST SRM 1878a quartz, NIST SRM
N 1879a cristobalite, USGS 210-75-0043 tridymite suspensions in 2-propanol.
RANGE:
0.02 to 2 mg SiO2 per sample [2]
ESTIMATED LOD: 0.005 mg SiO2 per sample [2] ACCURACY PRECISION ( þ r ): RANGE STUDIED:
25 to 2500 :g/m3 [1] (800-L sample)
BIAS:
None known
OVERALL PRECISION (Ö rT ):
0.09 (50 to 200 :g) [1]
ACCURACY:
± 18%
0.08 @ 0.05 to 0.2 mg per sample [1]
APPLICABILITY: The working range is 0.025 to 2.5 mg/m3 for an 800-L air sample. INTERFERENCES: Micas, potash, feldspars, zircon, graphite, and aluminosilicates. See APPENDIX. OTHER METHODS: This is similar to the method in the Criteria Document [3] and P&CAM 259 [4] which has been collaboratively tested [1]. This method is similar, except for sample collection, to S315 [5,6]. Method P&CAM 109 [7,8,9], which incorporates an internal standard, has been dropped. XRD can distinguish the three silica polymorphs and silica interferences can be eliminated by phosphoric acid treatment. IR (methods 7602 and 7603) can also quantify quartz, cristobalite and tridymite if amorphorous silica and silicates are not present in large amounts. However sensitivity is reduced if multiple polymorphs are present and secondary peaks must be used. Crystalline silica can also be determined by visible absorption spectrophotometry (e.g., Method 7601), but polymorphs can not be distinguished. Visible absorption methods also have larger laboratory-to-laboratory variabilty than XRD and IR methods and therefore are recommended for research use only [10].
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition
