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
