Page:NIOSH Manual of Analytical Methods - 7501.pdf/1

 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