National Institute for Occupational Safety and Health

“ Add known amounts, covering the range 0 to 100 μg Zn per sample, of calibration stock solution to 100-mL volumetric flasks and dilute to volume with 1% HNO3. Analyze the working standards together with the samples and blanks (steps 16 and 17) . Prepare a calibration graph of absorbance vs. solution concentration (μg/mL) . Aspirate a standard for every 10 samples to check instrument drift. Check recoveries with at least one spiked media blank per 10 samples. Use method of standard additions occasionally to check for interferences. ”

“ Record absorbance readings. NOTE: If the absorbance values for the samples are outside of the range of the standards, dilute the solutions with 1% HNO3, reanalyze, and apply the appropriate dilution factor in the calculations. ”

“ Heat on hotplate (140 °C) until sample dissolves and a slightly yellow solution is produced. Add additional acid as needed to completely destroy organic material. 6. When the sample solution is clear, remove watchglass and rinse into the beaker with 1% HNO3. 7. Place the beakers on the hotplate and allow to go to near dryness (ca. 0.5 mL liquid remaining) . 8. Rinse walls of beaker with 10 mL 10% HNO3. Reheat 5 min to dissolve the residue, then allow to air cool. 9. Transfer the solution quantitatively to a 100-mL volumetric flask and dilute to volume with distilled or deionized water. ”