HEXAVALENT CHROMIUM BACKUP DATA REPORT (ID-103)

This report was revised June, 1991

Introduction

The general procedure for collection and analysis of air samples of Cr(VI) is described in OSHA method no. ID-103 (11.1.). Briefly, the Cr(VI) sample is collected on a 37-mm polyvinyl chloride (PVC) filter and submitted to the laboratory for analysis. Any Cr(VI) on the filter is extracted with a hot sodium carbonate (Na₂CO₃) / sodium bicarbonate (NaHCO₃) buffer solution. The extract is then analyzed for Cr(VI) by differential pulse polarography (DPP) using a dropping mercury electrode. The analytical method has been validated using soluble and insoluble chromate compounds and a selected time weighted average (TWA) concentration range of about 0.005 to 0.01 mg/m³ as Cr(VI) or 0.009 to 0.02 mg/m³ as CrO₃ (840-L air sample). The TWA concentration selected for evaluation was considered a very low level at the time of the evaluation. When this method was first developed, the OSHA TWA permissible exposure limit (PEL) was 0.1 mg/m³ as CrO₃ (11.2.). The PEL at the time of this revision is 0.1 mg/m³ as a ceiling determination.

1. Evaluation Protocol

Unless mentioned otherwise, PVC filters obtained from Mine Safety Appliances (37-mm diameter, 5-µm pore size, model FWS-B, part no. 625413, Pittsburgh, PA) were used for all sample filter preparations. All filter samples were prepared and analyzed according to procedures described in the method (11.1.).

The experiments performed were:

1. Buffer/Extraction/Electrolyte (BEE) solution
2. Analysis - Desorption Efficiency
3. Analytical precision and accuracy
4. Interferences
5. Detection limits
6. Method comparison
7. Extraction efficiency
8. Mixed-cellulose ester filters

All samples were analyzed using a model 384 Polarographic Analyzer [Princeton Applied Research (PAR), Princeton, NJ) with the exception of the interference experiment. A model 374 Analyzer with a model 316 cell sequencer was used for this experiment.

All results were statistically examined using OSHA Inorganic Methods Evaluation Statistical Protocol (11.3.).

2. Buffer/Extraction/Electrolyte (BEE) Solution

It has been reported (11.4.) that the recovery of insoluble lead chromate in the presence of reducing agents such as magnetite (Fe₃O₄) is largely dependent on solution pH. At a low pH, less than 1% Cr(VI) was recovered; even at neutral pH, at least 10% of the Cr(VI) appeared to be reduced to Cr(III). Acceptable recoveries of the Cr(VI) species occurred when a 7% Na₂CO₃ extraction solution was used. The most useful analytical pH range for Cr(VI) appeared to be between 10 and 11.

For the evaluation of OSHA method no. ID-103, a pH in this range was achieved by using a 10% Na₂CO₃ / 2% NaHCO₃ buffer solution. Although a 7% Na₂CO₃ solution was used in the original study (11.4.), the BEE solution was thought to offer greater stability and solubility for the more insoluble chromate compounds. The BEE solution should satisfactorily prevent reduction to Cr(III) [or potential oxidation of any Cr(III) to Cr(IV)]. This 10% / 2% buffer was used as an extracting as well as a supporting electrolyte solution for all of the experiments in this backup report.

3. Analysis - Desorption Efficiency Study

Procedure:   An analysis of a total of 18 spiked samples (6 samples at each of three test levels) was performed for each of four different chromate compounds. The compound used for spiking were lead, zinc, potassium, and calcium chromates. The spiked concentrations corresponded to about 0.0048, 0.009, and 0.019 mg/m³ of CrO₃ when using an 840-L air sample volume. The step-by-step procedure used is listed:

3.1. Preparation of Stock Solutions

Each chromate compound was weighed on PVC filters, transferred to a 125-mL Phillips beaker, and the appropriate volume of BEE solution was added. The beakers were slowly heated with occasional swirling for 30 min. The solutions were cooled and then quantitatively transferred with deionized water (DI H₂O) rinses to individual volumetric flasks. The final amount of chromate in each solution was:

Stock Solution		Concn (µg/mL) as Cr(VI)
Lead chromate (PbCrO4)		13.68
inc Chromate (4ZnO· CrO3.3H2O)		24.68
Potassium Chromate (K2CrO4)		252.5
Calcium Chromate (CaCrO4)		179.05

3.2. Preparation of Known Spiked Samples

Three sets of six spiked samples were prepared for each chromate compound studied by spiking appropriate volumes of stock solutions onto PVC filters. The filters were then placed into 125-mL Phillips beakers, 10 mL of BEE solution added, and the samples were heated and prepared as mentioned in the method (11.1.).

3.3. Analytical Procedure

Standards were prepared from 0.1 to 1 µg/mL Cr(IV). Samples and standards were analyzed according to Ref. 11.1.

Results:   The analytical recoveries for the chromate compounds are presented in Tables 1 to 4. Recoveries and precision were excellent for all four compounds tested.

4. Analytical Precision and Accuracy

The analytical precision and accuracy data for results in Tables 1 through 4 are presented below. The pooled coefficients of variation (CV₁) and the average analytical method recovery (AMR) over all test levels for individual chromate compounds are:

5. Interferences

Procedure:   An experiment to test the potential interference from various amounts of Cr(III) and magnetite (Fe₃O₄) in the BEE solution was conducted. These reducing substances may coexist with Cr(VI) compounds in some workplace atmospheres and may also interfere with the analysis of Cr(VI) (11.4.). Differing amounts of Cr(VI), Cr(III), and Fe₃O₄ were spiked onto PVC filters. The concentrations of the spikes varied from 0 to 50 times the Cr(VI) concentration. Potassium chromate and chromium chloride (CrCl₃) solutions were used for the Cr(VI) and Cr(III) spikes, respectively. As shown in Table 5, eleven different mixture combinations and six samples of each combination were prepared and analyzed.

Results:   The recoveries for Cr(VI) in solution with varied amounts of Cr(III) or Fe₃O₄ are shown in Table 5. The recovery range is between 97 and 103%. For the DPP method, there appears to be no significant effect on recovery even when Cr(III) and Fe₃O₄ are present in excess together as much as 10 and 50 times, respectively, over Cr(VI).

6. Detection Limits

Procedure:   The qualitative and quantitative analytical detection limits of the method were determined by preparing BEE solutions containing varied amounts of K₂CrO₄ and then analyzing by DPP. The Rank Sum Test (11.5.) was used to determine the qualitative detection limit for Cr(VI). Blank samples and standards were analyzed and the results were ranked from lowest to highest signals. The standard concentrations ranged from 0.01 to 0.04 µg/mL as Cr(VI). The quantitative limit was determined by examining the recoveries and coefficients of variation of five sets of six standards. The concentration of these standards ranged from 0.02 to 0.1 µg/mL as Cr(VI).

Results:   The results of the Rank Sum Test are shown in Table 6. As shown, the qualitative detection limit is 0.19 µg as CrO₃ (10-mL sample volume) and was determined at the 99% confidence level. Table 7 shows recoveries and CVs for the five sets of low concentration standards. Using conservative limits of:

        CV   <   10%
recovery   <   ±10% from theoretical

7. Method Comparison

Procedure:   A comparison of this polarographic analytical method with another method was conducted. The NIOSH colorimetric method (S317) for chromate (11.6.) was modified to allow use of the base extraction during sample preparation instead of the 0.5 N sulfuric acid (H₂SO₄) extraction. Samples were prepared by spiking PVC filters with solutions of PbCrO₄. The samples were extracted and an aliquot was analyzed according to the method (11.1.). Another aliquot was acidified with 6 N H₂SO₄ and analyzed by the following procedure:

7.1. A total BEE sample solution volume of 10 mL was slowly and carefully acidified with 5 mL of 6 N H₂SO₄. After liberation of CO₂, each sample was diluted to a 25-mL volume with DI H₂O.

7.2. A 15 mL aliquot of the sample was taken, 0.5 mL of s-diphenylcarbazide (DPC) was added, and then analyzed colorimetrically at 540 nm as described in reference 11.6.

Results:   The sample comparison data of the two methods at about 0.5, 1, and 2 times the selected TWA concentration are shown in Table 8. This data indicates the modified NIOSH colorimetric/DPC method and the DPP method will give similar results over the concentration range tested.

8. Extraction Efficiency

Procedure:   An extraction efficiency study of Cr(VI) on PVC filters was conducted by spiking solutions of K₂CrO₄ onto FWS-B filters. These spikes were allowed to dry overnight, extracted, and then analyzed by polarography. Spikes were made with approximately 3.3, 4.9, and 9.8 µg as Cr(VI).

Results:   The results of the extraction efficiency study are presented in Table 9. The average recovery over the three concentrations tested was 100.9%.

9. Mixed-Cellulose Ester (MCE) Filters

Procedure:   A study of the stability of Cr(VI) on a MCE filter (type HA, 24-mm diameter, 0.45-µm pore size, cat. no. HAWP-024-00, Millipore Corp., Bedford, MA) was conducted. The reduction of Cr(VI) to Cr(III) on this type of media has been mentioned in the literature (11.4., 11.6.). To assess if the amount of Cr(III) would have any effect on Cr(VI) stability, differing amounts of Cr(III) were also added to the MCE filters.

Five different mixture combinations of Cr(VI) and Cr(III) were prepared from K₂CrO₄ and CrCl₃ solutions. Six samples of each mixture combination were prepared by spiking these solutions on MCE filters. These samples were extracted and analyzed according to Ref. 11.1.

Results:   The results are reported in Table 10. A decrease of 20 to 40% in recovery was noted with the larger decrease occurring when no Cr(III) was present.

10. Additional Information and Conclusions

10.1. The collection efficiency of PVC filters for chromic acid was reported to be 0.945 ±0.035. The experiment was performed at a generated chromic acid concentration of 0.192 mg/m³ (11.7.).

10.2. The molecular formula for the zinc chromate compound used in Section 3.1. was preliminarily determined by atomic absorption and then confirmed by X-ray diffraction analysis.

10.3. Analysis of other metals extracted into the BEE solution: Many metals, if extracted, can be analyzed by DPP. The peak potentials of lead and zinc salts in the BEE solution were experimentally determined to be -0.628 and -1.354 V, respectively. Since the peak is dependent on analytical conditions, these potentials may be slightly different with different instruments.

10.4. An additional evaluation of storage stability was recently conducted to determine if Cr(VI) is stable on PVC filters manufactured by Omega Specialty Instrument Co., Chelmsford, MA (cat. no. P-503700, 5-µm pore size, 37-mm diameter). Six filters were spiked with solutions of potassium dichromate. Three filters were analyzed by polarography after 1 week and three after 1 month of storage. The filters were stored in petri dishes and placed in a laboratory drawer. Recoveries were approximately 100%, indicating no significant storage problems for this PVC product.

10.5. Conclusions

This analytical method has been shown to be precise and accurate when analyzing four different chromate compounds commonly found in the workplace. Detection limit experiments indicated reasonable recoveries at concentration levels near 0.06 µg/mL as CrO₃. This is adequate for ceiling or TWA occupational exposure determinations; however, at least 15-min air samples should be taken for ceiling determinations since the detection limit may not be achievable for all polarographic instruments (0.06 µg/mL CrO₃ would equal 0.02 mg/m³ for a 30-L air volume). Results compared well to those obtained using a modified colorimetric/DPC method, which indicates the modified method could possibly be used to analyze samples if a polarograph is unavailable. Spiked quality control samples should be prepared with the specific chromate compound(s) and taken through this alternate procedure first to assure no loss of Cr(VI). The spiked samples should be prepared in a matrix closely matching the industrial process being sampled.

A gain or decrease in Cr(VI) recoveries [possibly due to oxidation of, or reduction to Cr(III)] was not noted in any of the experiments performed with the exception of the MCE filter study. Filters composed of MCE appear unacceptable for collecting Cr(VI).

11. References

11.1. Occupational Safety and Health Administration Technical Center: Hexavalent Chromium by J. Ku (USDOL/OSHA-SLTC Method No. ID-103). Salt Lake City, UT. Revised 1989.

11.2. Occupational Safety and Health Administration: Industrial Hygiene Field Operations Manual (IHFOM) (OSHA Instruction CPL 2-2.20, 4/2/79, II-47). Office of Field Coordination, U.S. Dept. of Labor, OSHA.

11.3. Occupational Safety and Health Administration Analytical Laboratory: Precision and Accuracy Data Protocol for Laboratory Validations. In OSHA Analytical Methods Manual. Cincinnati, OH: American Conference of Governmental Industrial Hygienists (Pub. No. ISBN: 0-936712-66-X), 1985.

11.4. Thomsen, E. and R.M. Stern: A Simple Analytical Technique for the Determination of Hexavalent Chromium in Welding Fumes and Other Complex Matrices. Scand. J. of Work, Environ. and Health 5: 386-403 (1979).

11.5. Dixon, W.J. and F.J. Massey, Jr.: Introduction to Statistical Analysis. 2nd ed. New York: McGraw-Hill Book Co., Inc., 1957. pp. 289-292, 445-449.

11.6. National Institute for Occupational Safety and Health: NIOSH Manual of Analytical Methods. 2nd ed., Vol. 3 (DHEW/NIOSH Pub. No. 77-157-C). Cincinnati, OH: National Institute for Occupational Safety and Health, 1977. pp. S317-1-S317-6.

11.7. National Institute for Occupational Safety and Health: Documentation of the NIOSH Validation Tests by D. Taylor, R. Kupel and J. Bryant (DHEW/NIOSH Pub. No. 77-185). Cincinnati, OH: National Institute For Occupational Safety and Health, 1977.