# Chromium, Hexavalent, by Field-Portable Spectrophotometry (7703)

​CHROMIUM, HEXAVALENT, by Field-Portable Spectrophotometry Cr[VI]
MW: 52.00 (Cr) 99.99 (CrO3 )
METHOD: 7703, Issue 1
CAS: 18540-29-9
EVALUATION: FULL
OSHA : C 0.1 mg/m3 (as CrO3 ) NIOSH: 0.001 mg/m3 /10 h (carcinogen) ACGIH: 0.050 mg/m3 (water-soluble compounds); 0.010 mg/m3 (insoluble compounds)
PROPERTIES:
7703
RTECS: GB6262000 Issue 1: 15 March 2003
oxidizing agent
SYNONYMS: vary depending on the compound; chromate commonly used SAMPLING SAMPLER:
FILTER (5.0-µm PVC membrane [1,2]; 0.8-µm MCE or 1.0-µm PTFE acceptable for field analysis [3]).
FLOW RATE:
1 to 4 L/min
VOL-MIN: -MAX:
10 L (2 L/min for 5 min) 1200 L (2 L/min for 600 min)
SHIPMENT:
refrigerant pack at 4 ± 2 o C (optional)
SAMPLE STABILITY:
BLANKS:
MEASUREMENT TECHNIQUE:
FIELD-PORTABLE VISIBLE SPECTROPHOTOMETRY
ANALYTE:
Cr[VI] - diphenylcarbazone complex
EXTRACTION:
10 mL 0.05 M (NH4 )2 SO4 / 0.05 M NH4 OH (pH = 8 + 0.5), ultrasonic extraction 30 min
Cr[VI] ISOLATION:
Strong anion exchange solid phase extraction
analyze within 24 hours; if applicable, keep samples at 4 ± 2 o C.
ELUTION SOLUTION:
0.5 M (NH4 )2 SO4 / 0.1 M NH4 OH
One per twenty field samples, minimum of 2 per set.
WAVELENGTH:
540 nm, 1-cm path length
CALIBRATION:
standard solutions of K2 CrO4 in 0.5 M (NH4 )2 SO4 / 0.1 M NH4 OH
RANGE:
1 to 400 µg per sample
ACCURACY RANGE STUDIED:
0.045 to 1146 µg/m3 (~20 to ~200-L samples) [3, 4]
BIAS:
-1.00% [3]
ESTIMATED LOD: 0.08 µg Cr[VI] per sample [3] PRECISION ( ÿ r ): OVERALL PRECISION ( Ö r T ):
0.080
ACCURACY:
+ 15.7%
0.035 @ 3 to 400 µg per sample [3]
APPLICABILITY: The working range is (at least) 0.05 to 1000 µg/m3 for a 200 to 500-L air sample. This method may be used for the determination of soluble forms of Cr[VI]. Insoluble Cr[VI] requires modification of the method using ultrasonic extraction with carbonate buffer. INTERFERENCES: Interferences from reducing agents such as Fe2 + are minimized to the extent possible by the alkaline ultrasonic and solid phase extraction procedures. Interferences from other metal cations are eliminated by solid phase extraction [5]. Some reduction can occur on the filter during sampling, and is usually due to the presence of Fe2 + , organic material, and/ or acidic conditions [6]. Reduction of Cr[VI] can occur over time on any filter type, and is especially problematic on MCE filters [7]. However, the use of MCE and PTFE filters has been found to be acceptable for field use, where performance has been found to be equivalent to that of PVC filters [3]. During ultrasonic extraction, oxidation of Cr[III] in solution to Cr[VI] is prevented by the use of an ammonium buffer [8]. OTHER METHODS: This method is designed to be used in the field, but can also be utilized in the fixed-site laboratory. It is an alternative to laboratory methods such as NIOSH method 7605 or OSHA method ID-215 (hot plate digestion and ion chromatography). NIOSH method 7600 is a similar procedure, but no separation step is used. A field method not involving a Cr[VI] isolation step, MDHS method 61, has been promulgated by the British Health and Safety Executive [9].
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition ​CHROMIUM, HEXAVALENT by portable VIS: METHOD 7703, Issue 1, dated 15 March 2003 - page 2 of 5 EQUIPMENT:
REAGENTS: 1. 2. 3. 4. 5. 6. 7. 8.
Am monium sulfate, reagent grade. Am monium hydroxide, reagent grade. W ater, distilled or deionized. Hydrochloric acid (37%), reagent grade. Aceton itrile, reage nt gra de.* 1,5-Diphenylcarbizide (DPC), reagent grade. Methanol, reagent grade. Extraction solution (extraction buffer): 0.05 M (NH 4) 2SO 4 / 0.05 M NH4OH, 1 L, aqueous in distilled or deionized water. NOTE: Modification of method by using carbonate buffer (e.g., sodium carbonate) is required for extraction of insoluble Cr[VI]. 9. Elution solution (elution buffer): 0.5 M (NH 4) 2SO 4 / 0.1 M NH 4OH, 250 m L, in distilled or deionized water. 10. Cr(VI) stand ard (as p otas sium chro m ate),* 1000 µg/mL. 11. Ca libration s tock solution, 100 µg /m L: Dilute 100 0 µg /m L Cr(VI) s tand ard 1 :10 w ith extraction buffer. (Solution is stable for a m onth .) 12. Diphenylcarbazide complexation solution (20 mM): Measure 0.48 g DPC powder and place in a 100-mL volumetric flask. Add ~80 m L of a ceto nitrile and dissolve the DPC . Bring up to the mark with additional aceto nitrile and m ix thoroughly.
1. Sam plers: 5-µm pore size polyvinylchloride (PVC), 0.8-µm pore size mixed cellulose ester (MCE), or 1.0-µm polytetrafluoroethylene (PTFE) filters, 37-m m diam ete r, with backu p pads, in polystyrene cassette filter holder, 2- or 3-piece. NOTE: MCE filters, and some PVC filters, promote reduction of Cr[VI] on a timescale of a few days. However, either filter type is acceptable for field use if the samples are to be analyzed within 24 h of co llection. 2. Personal sa m pling pum p, 1 to 4 L/m in, with flexible connecting tubing. 3. Ultrasonic bath (sonicator), 100 W minimum power. 4. Solid phase e xtrac tion m anifold, 12- o r 24-port. 5. Portable vacuum pum p with pressure metering valve. 6. Portable visible spec troph otom eter, s am ple pa th length 1 cm with Quartz cuvette(s). 7. Strong anion exchange solid phase extraction (SP E) cartridges, 10-m L, disp osa ble; loaded with 500 or 1000 mg quaternary amine bonded silica, capacity ~1 meq/g. 8. Pipettors, m ech anical, ass orted volum es (e .g., 1 to 10 mL) with disposable tips. 9. Micropipettors, mechanical, assorted volumes (e.g., 10 to 100 µL) with disposable tips. 10. Centrifuge tubes, plastic, 15-mL, with screw caps. 11. Scintillation vials, 20-mL, glass, with PTFE-lined screw caps. 12. Assorted beakers (and possibly Erlenmeyer flasks), various volumes. 13. Volumetric flasks, 25-, 100-, 250-, and 1000-mL. 14. Forceps, PTFE-coated. 15. Glass or plastic rods. 16. Disposable gloves, plastic or latex. 17. Laboratory wipes. 18. Portable power ge nerator (if necessary). NOTE: If no power supply is available at the field site, electric power can be provided by means of a portable, gasoline (or othe r) genera tor.
SPECIAL PRECAUTIONS: Hexavalent chro m ium is a hum an re spiratory carcinogen [10]. Efforts m ust be m ade to pre vent aerosolizin g chrom ate -contain ing com pounds and solution s. A ll sam ple preparation should be carried out in a well-ventilated area (vacuum hood preferable); forced ventilation should be used if n o hood is available. Ac eto nitrile solution s are flam m able m ust be handled carefully, i.e., wearing of im perm eable gloves, a nd avoidance of vapors. T o the exte nt possible, solution s should be prepared in the laboratory before taking them to the field.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition ​CHROMIUM, HEXAVALENT by portable VIS: METHOD 7703, Issue 1, dated 15 March 2003 - page 3 of 5 SAMPLING: 1. Calibrate each personal sampling pump with a representative sampler in line. 2. Sam ple at a n ac curately kn own flow ra te in the rang e of 1 to 4 L/m in for a sam ple size of 10 0 to 1000 L. Do not exceed 2 mg of particulate loading on the filter. Label the filter cassette. 3. Don a fresh pair of disposable plastic or latex gloves (to prevent sam ple contam ination). 4. W ith P TFE -coated forceps, rem ove filters from cassette s after of com pletion of s am pling, and place in separate plastic 15-mL centrifuge tubes for subsequent sample preparation. Discard cellulose backup pads and gloves.
SAMPLE PREPARATION: 5. Add 10 mL of extraction solution (weak buffer) to each 15-mL centrifuge tube containing the filter sample. Ensure that the filter is covered by the extraction solution. If necessary, push the filter down with a clean glass or plastic rod to imm erse the entire filter. Cap and label the tubes. 6. Place sample tubes in the ultrasonic bath (sonicator). The water level in the bath should be higher than the liquid level in the centrifuge tube. Sonicate for 30 minutes. NOTE: Num erous centrifuge tubes containing sample filters can be subjected to sonication at one time, depen ding upo n the size of the ultrasonic ba th. Ensure that the ba th is wa rm (but < 40 oC). 7. Set up the solid phase extraction manifold. a. Pla ce disposable solid phase extractio n (S PE ) cartridges in each port, and place scintillation vials beneath the cartridges. Label the cartridges. b. Attach the vacuum pum p to the SPE m anifold. c. To condition SPE cartridges, pipet 3 mL of methanol into each cartridge, and evacuate. Then pipet 3 m L of e xtrac tion so lution into eac h ca rtridge, and evacua te. Re pea t. 8. Extract Cr[VI] from sam ple solution. a. Pipet 3 to 5 mL of each ultrasonicated sample solution from the centrifuge tubes into the disposa ble SPE cartridge. D ispose o f the pipet tip. b. Adjust the vacuum to obtain an extraction rate of about one drop per second (approximately 8" Hg; no more than 10" Hg). Manually tighten cartridges by twisting, if necessary, to slow down the rate of liquid dripping. NOT E 1: For sam ples in the which the expected Cr[VI] concentration is high, sma ller aliquots (1 to 2 mL) should be dispensed into the SPE cartridges to prevent breakthrough. High con cen tration o f Cr[VI] can be ass ess ed visually by its ora nge color. NOT E 2: For samples having low Cr[VI] concentration, additional 3 to 5 mL aliquots of ultrasonicated sam ple solution can be loaded onto SP E c artridges (ste p 8.a.). In th is m ann er, the cartridge c an b e us ed to prec onc entra te Cr[VI]. c. W hen it appe ars the solution has pas sed throu gh a ll the cartridges, increas e the vacuum to ensure that all solution passes through the cartridges. This step selectively binds Cr[VI] to the stationary phase of each cartridge. d. To remove residue of Cr[III] and other potential interferences, turn the vacuum down (by turning counterclockwise) to 0" Hg. Add 1 mL distilled or deionized water to each cartridge, adjust vacuum to 1 drop p er se con d (~8 " Hg), then redu ce to 0" wh en c om pleted . e. Rem ove the scintillation vials beneath the cartridges and discard. NOTE: Th is solution conta ins un wan ted fra ctions that shou ld con tain no Cr[V I]. 9. Place clean, labeled scintillation vials beneath correct cartridges in the SPE manifold. a. Add 9 mL of the elution solution (elution buffer) to each cartridge to elute Cr[VI], and repeat steps 8.b. through 8.d. b. Rem ove the scintillation vials, and cap them. Dispose of the used SPE cartridges. NOTE: The scintillation vials now contain extracted and isolated Cr[VI], which is ready for subsequent analysis. 10. Uncap each scintillation vial conta ining extracted and isolated Cr[VI], an d add 100 µL H cl. 11. Add 2 m L DPC com plexation s olution, reca p vials, a nd m ix thoroug hly. Allow to s tand for at lea st 5 m in for com plete c olor developm ent.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition ​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 ​CHROMIUM, HEXAVALENT by portable VIS: METHOD 7703, Issue 1, dated 15 March 2003 - page 5 of 5 EVALUATION OF METHOD: This method was evaluated in the laboratory with spiked filters [3-5] and a certified reference material containing a known loading of C r(V I) [4]. This certifie d re ference m ate rial (CRM) is European Comm ission, Institute for R eferenc e M aterials and Me asu rem ents (EC /IRM M) CR M 5 45, Cr(VI) and Cr(total) in welding dust loaded on a glass fibe r filter [12]. T he m eth od has also been evaluate d in the field, where samples collected during aircraft maintenance operations were analyzed on-site [3, 4]. The accuracy was estimated using the proto col sum m arized in a NIOS H techn ical rep ort [13]. Alternative filter types can also b e us ed, e.g., PTFE, binder-free glass fiber filters, or quartz fiber filters. Filter m ate rials should be tested be fore use to ensure Cr[VI] sta bility. Filters can be pretreated with ba se to m inim ize Cr[VI] red uction during sa m pling in high-iron or ac idic environm ents [6].
REFERENCES: [1]
[2]
[3] [4]
[5]
[6]
[7] [8]
[9]
[10] [11] [12]
[13]
NIOSH [1994] Chromium, Hexavalent: Method 7600. Eller PM, Cassinelli ME, eds. NIOSH Manual of Analytical M etho ds (N MA M), 4 thed. Cincinnati, OH: National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 94-113. NIOSH [1994] Chromium, Hexavalent: Method 7604. Eller PM, Cassinelli ME, eds. NIOSH Manual of Analytical M etho ds (N MA M), 4 thed. Cincinnati, OH: National Institute for Occupational Safety and Health, DHHS (NIOSH) Publication No. 94-113. Marlow D, W ang J, Wise T J, Ashley K [2000]. Field Test of a Portable Method for the Determination of Hex avalent Chrom ium in W orkplace A ir. Am . Lab. 32 (15): 26-28. W ang J, Ashley K, Marlow D, England E C, and Carlton, G. [1999]. Field Method for the Determination of Hexavalent Chromium by Ultrasonication and Strong Anion Exchange Solid Phase Extraction. Anal. Chem . 71: 1027-1032. W ang J, As hley K, K ennedy E R, Ne um eister C [1997]. Dete rm ination of H exavalent C hrom ium in Industrial Hygiene Sam ples Using Ultrasonic Extraction and Flow Injection Analysis. Analyst 122: 1307-1312. Foster, R D, Howe A M, Cottrell S J, and Northage C [1996]. An Investigation of the Exposure of W orkers to Hexavalent Chromium in the Air of Chrome Plating Works (Project Report). Health and Safety Laboratory, Health and Safety Executive: Sheffield, England. Molina D, and Abell M T [1987]. An Ion Chromatographic Method for Insoluble Chromium in Paint Ae rosol. Am . Ind. Hyg. Assoc. J. 48: 830-835. Ndung’u K, Djane, N.-K, Malcus F, and Mathiasson L [1999]. Ultrasonic Extraction of Hexavalent Chromium in Solid Samples Followed by Automated Analysis Using a Combination of Supported Liquid Mem brane Extraction and UV Detection in a Flow System. Analyst 124: 1367-1372. Health and Safety Laboratory [1998]. Methods for the Determination of Hazardous Substances, MDHS M eth od No . 61.Tota l He xavalent Chrom ium Co m pounds in Air. Health and Safety Laboratory, Health and Safety Executive: Sheffield, England. Hayes R B [1982]. Biological and Environmental Aspects of Chromium. Elsevier: Am sterdam. Sk oog D A , W est D M, and H oller F J [1988]. Funda m enta ls of A nalytical C hem istry, 5 th ed. Saunders: Philadelphia. Co m m ission of the Europe an C om m unities, Institute for R eferenc e M aterials and Me asu rem ents . Ce rtificate o f Analysis, C RM 545 : Cr(V I) and total leac hab le Cr in welding du st load ed o n a filter. EC/IRM M: Bruss els (1997). Kenne dy E R, Fischbac h T J, So ng R, Eller P M, and S hulm an S A [199 5]. Guidelines for Air Sam pling and Analytical Method Development and Evaluation (DHHS [NIOSH] Publication No. 95117). National Institute for Occupational Safety and Health: Cincinnati, OH.
METHOD WRITTEN BY: Kevin As hley, Ph .D., N IOS H/D AR T; Jin W ang , Ph.D ., NIO SH /HE LD ; David M arlow , NIO SH /DA RT ; James Boiano, CIH, NIOSH/DSHEFS.
NIOSH Manual of Analytical Methods (NMAM), Fourth Edition