Page:NIOSH Manual of Analytical Methods - 7703.pdf/4

 CHROMIUM, HEXAVALENT by portable VIS: METHOD 7703, Issue 1, dated 15 March 2003 - page 4 of 5 CALIBRATION AND QUALITY CONTRO L: 12. Calibrate daily with at least 6 working standards over the range of 0 to 2 µg/mL of Cr[VI] per standard. a. To 10-mL volumetric flasks containing ~5 mL of elution solution (strong buffer), pipet known volumes (20 to 300 µL) of Cr[VI] calibration stock solution (100 µg/mL) to produce concentrations of 0.1, 0.2, 0.5, 1.0, and 2.0 µg/mL. Add 100 µL of HCl and 2 mL of diphenylcarbazide (DPC) com plexation solution to each. Dilute to th e m ark w ith elution solution and m ix thoroughly. NOTE: A minimum of two of the concentration levels (e.g., 0.1 and 1.0 µg/mL) should be run at least in triplicate. b. Prepare a blank by pipetting 100 µ L of HC l and 2 m L of DP C com plexation solution into 10-m L volum etric flas k containing ~5 m L of the elution so lution (elution buffer); dilute to the m ark with elution solution and m ix thoroughly. c. Analyze the calibration solutions and the blank (steps 15 to 20). 13. Analyze at least two field blan ks, one field blan k per twe nty sam ples (step s 10, 11 and 1 5 to 20). Also analyze at least three of the calibration solutions in triplicate. 14. Prepare a calibration graph of absorbance vs, Cr[VI] concentration. NO TE : As a n altern ative to steps 12 to 14, the stand ard a ddition app roac h ca n be use d [11].

MEASUREMENT: 15. Turn on the spectrophotometer, and allow for an appropriate warm-up period. 16. Set the spectrophotometer to 540 nm. Set portable spectrophotometer parameters according to the manufacturers instructions and the conditions on page 7703-1. 17. Rinse the quartz cuvette three times with distilled or deionized water, then rinse with blank solution. 18. Measure the blank. Adjust the spectrometer to zero absorbance. 19. Uncap the scintillation vial containing the sample solution to be analyzed. a. Condition the cuvette by filling with the solution to be analyzed, and discard the solution. b. Refill the cuvette with the sample solution to be analyzed. c. Place the cuvette in the spectroph otom eter. NOTE: W ipe any extra moisture or liquid off the sides of the cuvette with a dry laboratory wipe, and take care to handle the cuvette only by the frosted sides. 20. Analyze samples, standards, and blanks. Record the absorbance. NOTE: If the absorbance value is greater than 2 absorbance units, dilute the solution to be analyzed with elution solution (strong buffer) and reanalyze.

CALCULATIONS: 21. From the calibration graph, determine the mass of Cr[VI] in each sample, W (µg), and in the average field blank, B (µg ). NO TE : If standard addition method was used, make appropriate adjustments from the calibration grap h ob tained [11]. 22. Ca lculate the conc entra tion, C (m g/m 3), of Cr[VI] in the air volume sa m pled, V (L):

NOTE:

If samples were diluted during sample preparation, be sure to account for the dilution factor in the calculation.

NIOSH Manual of Analytical Methods (NMAM), Fourth Edition