CH RO MIUM , HEX AVA LEN T: Me thod 7605, Issue 1 dated 15 M arch 200 3 - Page 3 o f 5 3. Filters can be left in the cass ettes for shipping to the lab, b ut to m inim ize sample contamination during shipping, rem ove the filter from the cassette within one hour of completion of sampling and place it in a vial to be shipped to the laboratory. Handle the filter only with gloved hands and nonmetallic forceps. Discard the backup pad. As a precaution, it is recomm ended to ship the samples with bag ged refrige rant.
SAMPLE PREPARATION: 4. Don a clean pair of disposable plastic gloves (to pre vent s am ple contamination). Using forceps, transfer the PVC filter to a 50-mL beaker, and add 5.0 mL filter extraction solution, 2% NaOH/3% Na 2CO 3. Start m edia blank s at this point. NOTE 1: If significant amounts of Cr[III] are expected to be present in the samples, either (a) degas the sodium hydroxide/sod ium carb ona te extraction so lution by bubb ling nitrog en throug h it for 5 m in. before p roceed ing, or (b) us e a precipitation re age nt [1]. NOT E 2: If only soluble chromates are o f interest, use am m onium sulfate buffer in p lace of ca rbon ate extra ction s olution [9, 10]. 5. Cover the beaker with a watchglass and heat it to near the boiling point (100°C to 115°C) in an oven with occasional swirling for 45 min. Do not boil the solution. Longer heating times (up to 90 minutes) may be necessary for some samples (e.g., paint spray). Do not allow the solution to evaporate to d ryness because hexavalent chromium may be lost due to reaction with the PVC filter and/or co-collected aerosol constituents. An indication that hexavalent chromium has been lost in this manner is a brown-colored PVC filter. NOTE: A ho t plate, heate r block, or ultrasonic bath can also b e us ed fo r this ste p [9, 11 ]. a. Cool the solution and transfer it qu antitatively with distilled water rinses to a 25-mL volumetric flask. Bring to volume with distilled water. NOTE: If the solution is cloudy, filter an aliquot through a PTFE luer lock filter attached to a syringe. b. Transfer an aliquot of the solution to the appropriate vial for the chromatograph’s autosampler and analyze (steps 9 through 13).
CALIBRATION AND QUALITY CONTRO L: 6. Calibrate daily with at least six working standards. Transfer 5 mL of extraction solution to each of a series of 25-mL volumetric flasks. Pipet known volumes (0 to 5 m L) of calibra tion sto ck solution (1.0 :g/m L) into the volumetric flasks. For higher standards, pipet 10 - 20 :L of the 1000 :g/mL concentrated stock and bring the volume to 25 mL with distilled water. These working standards contain 0 to 20 :g Cr(VI) per sample. 7. Analyze the working standard s together with blanks and sam ples (steps 9 through 1 3). 8. Prepare a calibration g raph [instrum ent re spo nse vs. :g Cr(VI)].
MEASUREMENT: 9. Set wavelength on the detector to 540 nm. 10. Set the liquid chromatograph to manufacturer's recomm endations and parameters given on page 7605-1. W ith a mobile phase flow rate of 1.0 mL/m in., a post-colum n reagent flow rate of 0.7 mL/min., and a 2.2-m post-column tube, the derivative retention time should be approximately 3.7 - 4.7 minutes. NOTE: If the ins trum ent re spo nse for the sam ples is higher tha n the standard s, dilute using a 1:5 dilution of extraction solution:water to maintain a constant ionic strength; repeat the analysis; and m ultiply the measured concentration by the app ropriate dilution facto r. Alternatively, inject a sm aller volum e and m ultiply by the appropriate factor. 11. After the analysis is complete, flush the entire system with ASTM Type II water for at least one hour at 1.0 mL/m in. with all columns on line. Rem ove the columns and continue flushing for an additional two hours. Flush the autosampler with several injections of water. Leaving the columns in line while the system is idle is not recomm ended.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition
