CHROMIUM and compounds, as Cr: METHOD 7024, Issue 2, dated 15 August 1994 - Page 3 of 4 CALIBRATION AND QUALITY CONTROL: 9.
10. 11. 12. 13. 14.
Calibrate with at least six working standards. Add known amounts of calibration stock solution, covering the range 0 to 1000 µg Cr (0 to 200 µg Cr per sample) to 100-mL volumetric flasks and dilute to volume with 5% HNO3. Analyze the working standards together with the blanks and samples (steps 15 and 16). Prepare a calibration graph of absorbance vs. solution concentration (µg/mL). Aspirate a standard for every 10 samples to check instrument drift. Check analytical recoveries with at least one spiked media blank per 10 samples. Use method of standard additions occasionally to check for interferences.
MEASUREMENT: 15.
16.
Set spectrophotometer as recommended by the manufacturer and to conditions on page 7024-1. NOTE: Air-acetylene flame may also be used. In a fuel-lean air-C2H2 or reducing (fuel-rich) N2O-C2H2 flame, interference by Fe or Ni is minimized or eliminated, but sensitivity for Cr is reduced. A reducing air-C2H2 flame provides the best sensitivity, but the greatest susceptibility to interference [5]. Aspirate standards and samples. Record absorbance readings NOTE: If the absorbance values for the samples are above the linear range of the standards, dilute the solutions with 5% HNO3, reanalyze, and apply the appropriate dilution factor in the calculations.
CALCULATIONS: 17. 18.
Using the measured absorbances, calculate the corresponding solution concentrations (µg/mL) of chromium in the sample, Cs, and average media blank, Cb, from the calibration graph. Using the solution volumes (mL) of the sample, Vs, and media blanks, Vb, calculate the concentration of chromium, C (mg/m3), in the air volume sampled, V (L):
EVALUATION OF METHOD: Lab testing with spiked filters and generated atmospheres of soluble chromium (potassium dichromate) was done at one-half, one and two times the OSHA standard for chromium compounds other than Cr(VI) of 0.5 mg/m3. The range studied was 0.28 to 0.95 mg/m3. The precision was 0.082 and a bias was not observed [1,2]. Lab testing with spiked filters and generated atmospheres of insoluble chromium, (produced from thermal decomposition of chromium hexacarbonyl) was done at one-half, one and two times the OSHA standard for chromium metal of 1.0 mg/m3 [1,3]. Collection efficiency was 1.00 and analytical recoveries averaged 98% in the range 45 to 190 µg Cr per sample.
REFERENCES: [1] Documentation of the NIOSH Validation Tests, U.S. Department of Health, Education, and Welfare, Publ. (NIOSH) 77-185 (1977). [2] NIOSH Manual of Analytical Methods, 2nd ed., V. 3, S323, U.S. Department of Health, Education, and Welfare, Publ. (NIOSH) 77-157-C (1977). NIOSH Manual of Analytical Methods (NMAM), Fourth Edition, 8/15/94
