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
