Field Method for the Determination of Insoluble or Total 

Hexavalent Chromium in Workplace Air 

Kyle J. Hazelwood,1 Pamela L. Drake,1 

1U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National 

Institute for Occupational Safety and Health, Spokane, Washington 

Kevin Ashley,2 

2U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National 

Institute for Occupational Safety and Health, Cincinnati, Ohio 

and Dale Marcy3 

3North Idaho College, Coeur D’Alene, Idaho 

Address correspondence to: Pamela L. Drake, U.S. Department of 

Health and Human Services, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, 315 East 

Montgomery Avenue, Spokane, WA 99207; e-mail: PDrake@cdc.gov. 

Mention of company names or products does not constitute endorsement by the Centers for Disease Control and Prevention. 

National Institute for Occupational Safety and Health 

method 7703 is a portable field procedure for the analysis of 

workplace air filter samples for hexavalent chromium (CrVI) 

content immediately after the samples are collected. The field 

method prescribes CrVI extraction from air filter samples with 

an ammonium sulfate/ammonium hydroxide extraction buffer 

using ultrasonic extraction (UE). Strong anion-exchange solidphase extraction (SAE-SPE) is then used to separate CrVI from 

trivalent chromium and other interferences. Portable spectrophotometric measurement of CrVI is then conducted using 

the 1,5-diphenylcarbazide (DPC) method. However, it has been 

found that the ammonium extraction buffer does not adequately 

bring insoluble CrVI compounds into solution during the UE 

process. Thus, it was deemed necessary to modify the field 

method so that it would provide acceptable recoveries for insoluble CrVI compounds. To this end, a more alkaline extraction 

solution—sodium carbonate/sodium bicarbonate buffer—was 

investigated. The modified procedure using the highly alkaline 

extraction solution was demonstrated to be compatible with 

SAE-SPE cartridges when determining insoluble CrVI in air 

filter samples. It was found that the carbonate/bicarbonate 

buffer was equally effective for complete dissolution of both 

insoluble and soluble forms of CrVI. Furthermore, the modified 

procedure met desired performance criteria established for air 

sampling and analytical methods.



The carcinogenic risks from chromium exposure are 

linked closely to the specific valence states of chromium compounds.(1) In the environment the most 

common chromium valences found are trivalent 

(CrIII) and hexavalent (CrVI). While CrIII is an essential trace 

element for both animals and humans,(2,3) CrVI compounds 

are considered harmful, owing to observations of carcinogenic 

and/or allergenic responses.(1) Primary routes of exposure to 

CrVI include inhalation, absorption through the skin, and 

ingestion.(2) CrVI is readily absorbed by the body via many 

routes, but CrIII is not as easily absorbed.(2,4) Metabolic breakdown of CrVI involves its reduction to CrIII by various enzymes, with concomitant generation of reactive species such 

as peroxides.(1) Toxic effects can be caused by the reaction 

of these reactive species with various biomolecules, notably 

DNA.(1,4) 

In the body the highest concentrations of chromium are 

found in the liver, kidney, and lungs.(2) The capacity of CrVI 

compounds to cause cancer as a result of occupational exposure 

has been well documented.(3,4) Although lung cancer is most 

frequently caused by occupational exposure to CrVI, other 

cancers such as oral, nasal, and gastrointestinal tumors have 

also been reported.(1) Water-insoluble CrVI compounds are 

thought to have the greatest capability for causing cancer, but 

water-soluble CrVI compounds also appear to be carcinogenic 

and allergenic.(3,5) 

While occupational exposure to CrVI occurs predominantly 

as a result of inhalation,(2) dermal contact is another potential 

route of worker exposures.(5) The primary manufacturing processes that involve exposure to CrVI are stainless steel welding, 

chromate production, chrome plating, ferrochrome production, 

spray painting, and tanning and leather working.(3) Airborne 

concentrations of CrVI in these industrial processes range anywhere from 0.005 to 1 mg/m3.

(2) 

The American Conference of Governmental Industrial Hygienists (ACGIH®R

) has set threshold limit values (TLVs®R) for 

water-soluble and water-insoluble CrVI compounds at 

0.05 mg/m3 and 0.01 mg/m3, respectively.(6) Given the severity 

and extensive health effects that CrVI compounds can cause, 

analytical methods are needed that can be used to assess exposure levels quickly and accurately. Furthermore, since the 
TLVs for CrVI are different for water-soluble and insoluble CrVI 

compounds, analytical methods that can differentiate between 

the two are needed. 

BACKGROUND 

Field sampling and determination of airborne CrVI employ a 

collection method that uses sampling filter materials such 

as polyvinylchloride (PVC) or mixed cellulose ester (MCE) 

filters.(7) However, since MCE filters and some PVC filters 

have been found to promote the reduction of CrVI to CrIII,

(8) 

on-site field methods that can be used to assess occupational 

exposure immediately would help to bypass the problem of 

sample instability. 

In previous work a quick and effective field method for 

determination of soluble CrVI was developed.(9) This method 

uses ultrasonic extraction (UE) to extract CrVI from the sampling media, and strong anion-exchange solid-phase extraction 

(SAE-SPE) to separate CrVI from CrIII and other interferences. 

SAE-SPE has proved to be a reliable separation and isolation 

technique for the subsequent determination of CrVI in workplace samples.(7–10) 

For some matrices and elements, UE has been shown to be 

an effective means of aiding in the extraction of the metal analytes from filter samples.(11,12) Earlier studies have shown that 

UE is an effective technique for dissolution of CrVI compounds 

collected from workplace atmospheres.(9,13) For chromates, – basic buffer solutions have been used for extraction,(12 14) 

since CrVI compounds are strongly oxidizing in acidic conditions and may become reduced to CrIII.

(2) In basic solutions, 

CrVI species are relatively stable and exist as anions, in contrast 

to other heavy metal ions, which are ordinarily cationic under 

such conditions.(3,5) Therefore, CrVI is easily separated from 

other metal cations by SAE-SPE.(9) 

Using the National Institute for Occupational Safety and 

Health (NIOSH) field method,(10) after extracted CrVI is retained on the SAE cartridges (while potential interferences 

are washed through), the isolated CrVI is eluted with a 0.50 M 

(NH4)2SO4/0.10 M NH4OH elution buffer. The eluate solution 

containing CrVI is subsequently acidified, and 1,5-diphenylcarbazide (DPC) is added to form a colored complex ion, that 

can be measured with high sensitivity using visible spectrophotometry.(9,10,14) 

Although the field method using the ammonium sulfate/ 

ammonium hydroxide extraction buffer is effective for the extraction of soluble and sparingly soluble CrVI compounds,(9,12,13,15) this buffer been found to be inadequate for 

dissolving insoluble CrVI compounds.(16) The NIOSH procedure is therefore not capable of providing accurate quantitative determination of total CrVI if insoluble chromate compounds are present in the sampled matrix. Because of this 

limitation a modified version of the NIOSH field method was 

developed using a sodium carbonate/sodium bicarbonate extraction buffer, which is known to dissolve insoluble CrVI 

compounds.(14,16) The modified method enables measurement 

of total CrVI in occupational air samples. Alternatively, separate on-site measurement of soluble and insoluble CrVI species 

in workplace atmospheres is possible by using different extraction buffer systems in sequence. To our knowledge no other 

field method is capable of measuring insoluble hexavalent 

chromium compounds. 

EXPERIMENTAL SECTION 

Instrumentation and Equipment 

Sonication was carried out with a Crest Ultrasonics 

(Trenton, N.J.) 117-V, 50-Hz ultrasonic bath. The 12-port solidphase (SPE) extraction manifold used in this work was purchased from Supelco (Bellefonte, Pa.). Disposable polytetrafluoroethylene (PTFE) valve liners (Supelco) were used for the 

ports of the SPE manifold. The manifold was connected to a 

Gast (Benton Harbor, Mich.) vacuum pump through a pressure control valve. Strong anion-exchange solid-phase extraction (SAE-SPE) cartridges were purchased from Varian (Palo 

Alto, Calif.). The cartridges contained 0.5 g (lower capacity) 

or 1 g (higher capacity) of quaternary amine-bonded silica 

sorbent with chloride ion functioning as the counterion. A 

portable, battery-powered spectrophotometer (Hach DR/2000, 

Loveland, Colo.) was used for spectrophotometric measurements at 540 nm (sample path length 1 cm). A Fisher Scientific 

(Chicago, Ill.) Accumet 50 pH meter was used to measure 

solution pH. Where applicable, materials were weighed to the 

nearest 0.0001 g using a Mettler (Greifensee, Switzerland) 

Model AE 163 analytical balance. 

Reagents and Materials 

Reagent grade chromic oxide and potassium chromate 

solids, as well as Cr(III), Fe(II), and Cr(VI) standard solutions 

(∼1000 μg/mL), were obtained from Aldrich (Milwaukee, 

Wis.). Ammonium sulfate, ammonium hydroxide, lead chromate, sodium carbonate, sodium bicarbonate, 1,5-diphenylcarbazide (DPC), acetonitrile (used to dissolve DPC), and hydrochloric acid were reagent grade and were obtained from 

Fisher Scientific. Additionally, 15 mL polypropylene centrifuge tubes, scintillation vials, and volumetric flasks were all 

purchased from Fisher Scientific. Certified reference material (CRM) filters, consisting of glass fiber filters containing certified amounts of hexavalent and total chromium in 

welding dust (CRM 545), were obtained from the Institute 

for Reference Materials and Measurements (IRMM) of the 

European Commission (Brussels).(17) Preloaded plastic filter 

cassette samplers housing mixed cellulose ester (MCE) or 

polyvinyl chloride (PVC) filters with cellulose backup pads 

were obtained from SKC (Eighty Four, Pa.). 

Air Sample Collection 

Air samples containing CrVI were collected onto air filters from fumes generated during welding stainless steel. The 

samples were collected using a homemade multi-port sampler connected to a Gilian (Clearwater, Fla.) AirCon2, highvolume, air-sampling pump calibrated up to an airflow rate 

of 15 L/min. The multiport sampler contained eight separate 

ports with each port calibrated at a flow rate of approximately 
1.7 L/min. Gravimetric analysis using PVC filters had previously demonstrated the precision of the mass collected onto 

separate filters using the multiport sampler to be less than 5% 

relative standard deviation (RSD) for aerosol loadings ranging 

from ∼0.5 to ∼5 mg/filter. After the samples were collected, 

they were immediately prepared for analysis. 

Preparation of Air Filter Samples 

Ultrasonic Extraction 

Air filter samples were removed from the sampling cassettes 

and placed into 15-mL centrifuge tubes using PTFE-coated 

forceps. Clean plastic or glass rods were used to push the 

filters to the bottom of each centrifuge tube. Next, 10 mL 

of 0.10 M Na2CO3/0.02 M NaHCO3 extraction buffer was 

added to each centrifuge tube; the extraction buffer had a 

measured pH of 10.7 (±0.1). The centrifuge tubes containing 

the filter samples were then immersed in the ultrasonic bath 

and subjected to sonication for 30 min at ambient temperature. 

The water level in the bath was adjusted so that it was above 

the level of sample solution inside the centrifuge tubes. In 

some experiments, an alternative buffer was used (i.e., 0.05 M 

ammonium sulfate/0.05 M ammonium hydroxide, pH 8.0).(10) 

Strong Anion-Exchange Solid-Phase Extraction 

SAE-SPE cartridges were first conditioned with 3 mL of distilled water,(18) thereby activating the functional groups of the 

sorbent. Aliquots of each sample solution were subsequently 

loaded onto separate SAE-SPE cartridges. Each SAE-SPE cartridge had a sorbent mass of 0.5 g or 1 g and accommodated a 

sample volume of 3 mL or 6 mL. Distilled water was used in 

three 1-mL fractions to wash away CrIII and other interferences, 

and scintillation vials were used to collect extraction solution 

and water. Afterward, these vials were removed and replaced 

with clean scintillation vials, which were used to collect eluted 

CrVI. For elution, 9 mL of 0.50 M (NH4)2SO4/0.10 M NH4OH 

elution buffer was used in three 3-mL fractions to elute CrVI 

from the SAE-SPE cartridges. 

Analysis 

The eluate was acidified with 100 μL of 37.5% (v/v) HCl solution, and DPC indicator solution added. The colored 

chromous-diphenylcarbazone complex ion develops within 

minutes after the addition of 2 mL of 20-mM DPC complexing 

solution. The absorbance of each sample was then measured at 

540 nm using the Hach DR/2000 spectrophotometer. Distilled 

water blanks were used to zero the instrument. 

RESULTS AND DISCUSSION 

Extraction Buffers for CrVI Compounds 

The composition of the carbonate/bicarbonate extraction 

buffer mixture was varied in order to determine the concentration ratio that was most compatible with the SAE-SPE cartridges, while also giving good recoveries of insoluble CrVI. 

Through trial and error (by trying buffer mixtures of different 

ratios and strengths), a buffer consisting of 0.10 M Na2CO3/ 

0.02 M NaHCO3 was found to meet both criteria. Using this 

carbonate/bicarbonate buffer system, sonication of known 

masses of lead chromate and/or potassium chromate was 

investigated. 

Calibration 

A calibration line was prepared from concentrations of CrVI 

ranging from ∼0.05 to ∼3.0 μg/mL CrVI (prepared from lead 

chromate in carbonate/bicarbonate buffer), and from a reagent 

blank. Aliquots from a solution of 20 g/mL CrVI μ in 0.10 M 

Na2CO3/0.02 M NaHCO3 were taken and pipetted into 

15-mL centrifuge tubes, and then diluted up to 10 mL with 

the Na2CO3/NaHCO3 extraction buffer to yield the desired 

concentrations. These samples were then prepared using SAESPE, acidified and complexed with DPC, and measured spectrophotometrically. The resulting calibration line is described 

by the expression: Absorbance A 

2 

= 0 VI .7789[Cr ] + 0.0006; 

R = 0.9999. 

Buffer Compatibility with SAE-SPE 

In NIOSH Method 7703,(10) SAE-SPE cartridges with a 

sorbent mass of 500 mg and a retention capacity of 25 mg 

are used. However, the 0.10 M Na2CO3/0.02 M NaHCO3 

extraction buffer is more alkaline than the 0.05 M (NH4)2SO4/ 

0.05 M NH4OH (pH 8) extraction buffer used for soluble CrVI 

compounds. It was found that larger cartridges with a sorbent 

mass of 1 g and a retention capacity of 50 mg performed well 

in the method modified for insoluble chromates. Spiked CrVI 

samples were prepared from potassium chromate standards, 

and a test was run to compare the 1-g and 500-mg cartridges 

under all conditions specified by NIOSH method 7703. A 

t-test(19) revealed no significant differences in the recoveries 

of soluble CrVI using both the 1-g and 500-mg cartridges 

(p = 0.9936), indicating the equivalent performance of the 

cartridges. 

As no breakthrough was observed, the 0.10 M Na2CO3/ 

0.02 M NaHCO3 extraction buffer did not exceed the capacity 

of the 1-g SAE cartridges. The performance of this extraction buffer for the determination of insoluble CrVI was investigated using the following modification of NIOSH method 

7703. A 20 μg/mL CrVI (from PbCrO4) solution was prepared 

by sonication in 0.10 M Na2CO3/0.02 M NaHCO3, while a 

20-μg/mL CrVI (from K2CrO4) solution was prepared by sonication in 0.05 M (NH4)2SO4/0.05 M NH4OH. To test the 

effectiveness of the carbonate/bicarbonate buffer for the analysis of soluble CrVI, a 20-μg/mL CrVI solution (from K2CrO4) in 

0.10 M Na2CO3/0.02 M NaHCO3 was also prepared. Aliquots 

were taken from each of these stock solutions to make up 

CrVI solutions with concentrations of 0.1 μg/mL, 0.5 μg/mL, 

1.0 μg/mL, and 2.0 μg/mL in a total of 10 mL of the corresponding extraction buffer. Aliquots (3 mL) from each sample 

were then prepared using SAE-SPE and analyzed by portable 

visible spectrophotometry, as described previously. 

Table I presents the results for CrVI measurements from the 

above experiments. Plots of absorbance versus concentration 

for each of the three different solutions yielded coefficients 
of determination (R2) approaching 0.999 (Table I). As can 

be seen from the near-unity correlation for each solution and 

high precision (as demonstrated by low relative standard deviations), there are no significant differences in the performances 

of the carbonate/bicarbonate buffer for extracting insoluble 

CrVI and that of the ammonium sulfate/ammonium hydroxide buffer for the extraction of soluble CrVI. Thus, 

the 0.10 M Na2CO3/0.02 M NaHCO3 buffer was found to be effective in using SAE-SPE for isolation of insoluble 

CrVI. 

Soluble and Insoluble CrVI 

The results of Table I also indicate that the carbonate/ 

bicarbonate extraction buffer can be used in concert with SAESPE to measure both soluble and insoluble CrVI. 

TABLE I. Comparison of Extraction Buffers on Chromate Compounds of Different Solubility 

Absorbance 

0.1 μg/mL CrVI 

Absorbance 

0.5 μg/mL CrVI 

Absorbance 

1.0 μg/mL CrVI 

Absorbance 

2.0 μg/mL CrVI 

20 μg/mL CrVI (PbCrO4) 

in carbonate/bicarbonate buffer 

0.023 0.117 0.206 0.406 

R2 = 0.9986 

20 μg/mL Cr 0.397 VI (K2CrO4) 

in carbonate/bicarbonate buffer 

0.025 0.113 0.211 

R2 = 0.9986 

20 μg/mL Cr 0.399 VI (K2CrO4) 

in sulfate/bisulfate buffer 

0.023 0.114 0.199 

R2 = 0.9988 

0.401 ± 0.005 

1.2% 

Mean absorbance ± SDA 

RSDB 

0.024 ± 0.001 

4.9% 

0.115 ± 0.002 

1.8% 

0.205 ± 0.006 

2.9% 

Note: Results from calibration solutions prepared after treatment with extraction buffers for insoluble (PbCrO4) and soluble (K2CrO4) CrVI compounds. 

(Carbonate/bicarbonate buffer for insoluble chromates: 0.10 M Na2CO3/0.02 M NaHCO3; sulfate/bisulfate buffer for soluble chromates: 0.05 M (NH4)2SO4/0.05 

M NH4OH.) AStandard deviation. 

B Relative standard deviation. 

Furthermore, 

the agreement of the results indicates acceptable compatibility 

of the carbonate/bicarbonate extraction buffer with the ammonium sulfate/ammonium hydroxide elution buffer. In the field, 

these buffer systems could be used separately to measure (1) 

soluble + sparingly soluble CrVI, or (2) soluble + sparingly 

soluble + insoluble = total CrVI.

(20)

CrVI – Spiked Filters at 0.1–2.0 × TLV 

To validate the method according to criteria previously established for air sampling and analytical method development,(21) replicates of each loading level of interest (0.1, 0.5, 

1.0, and 2.0 times the TLV for insoluble CrVI) were prepared. 

For the method to meet NIOSH accuracy criteria, observed 

percent recoveries must be within ±25% of the true concentration at least 95% of the time. Amounts of PbCrO4 yielding 

the desired concentrations of CrVI when diluted in 10 mL of 

carbonate/bicarbonate extraction buffer, i.e., 0.0010, 0.0050, 

and 0.0100 mg (giving 1, 5, 10 and 20 μg/filter) were spiked 

onto two types of filters (PVC and MCE). (These amounts correspond to desired levels with respect to the TLV, assuming 8­

hour time-weighted-averaged sampling at a flow rate of about 

2 L/min). These filters were prepared and analyzed as already 

described. After correcting for the media blank, percent CrVI 

recoveries were calculated from the absorbance values. 

Results from these experiments are presented in Table II. 

TABLE II. Percent CrVI Recoveries 

Filter Medium 

Percent Recovery 

@ 0.1 × TLV 

Percent Recovery 

@ 0.5 × TLV 

Percent Recovery 

@ 1.0 × TLV 

Percent Recovery 

@ 2.0 × TLV 

PVCA 

MCEB 

92.2 ± 1.6 

97.9 ± 3.6 

102 ± 4.3 

99.8 ± 3.4 

98.7 ± 2.1 

97.6 ± 2.8 

101 ± 2.9 

95.1 ± 5.0 

Note: Percent CrVI mean values ± standard deviations, n ≥ 3; obtained from air filters spiked with PbCrO4 (extracted in carbonate/bicarbonate buffer) at ∼0.1 to 

∼2.0 times the ACGIH®R TLV®R for insoluble CrVI. Samples were subjected to ultrasonic extraction in 0.10 M Na2CO3/0.02 M NaHCO3 (carbonate/bicarbonate 

buffer) prior to SAE-SPE treatment and DPC spectrophotometric measurement. APolyvinylchloride. 

B Mixed cellulose ester. 

Calculated percent recoveries (within 100% ± 10%) satisfied 

performance criteria established for the development of air 

sampling and analytical methods.(21) 
= = = = 

Analysis of Welding Fume Samples 

The method described above was used to determine the 

mass of total CrVI in air samples produced from fumes generated while welding stainless steel (UNS-304). Filter cassettes 

housing MCE or PVC filters were attached to the multiport 

sampler, which was connected to the air sampling pump. The 

multiport sampler was positioned above the welding location in 

order to collect the fumes produced. Four trials were performed 

in which either two or three welding rods (AWS A5.4 Class 

E 312-16) were used, and the air samples were collected for 

different time periods, in order obtain a range of CrVI filter 

loadings. 

After sample collection the cassettes were removed from the 

multiport sampler, and the filters were then subjected to the 

sample preparation and analysis procedure described above 

(for total, including insoluble, chromates). Calculated mean 

CrVI masses from all four trials are shown in Table III. 

TABLE III. Analysis of Air Samples for CrVI Collected from Fumes Generated When Welding Stainless Steel 

Mean Mass of CrVI± SD, 

A

Expt. 1 (PVC;B n = 8) 

Mean Mass of CrVI± SD,A 

Expt. 2 (MCE;C n = 8) 

Mean Mass of CrVI± SD,A 

Expt. 3 (MCE;C n = 8) 

Mean Mass of CrVI± SD,A 

Expt. 4 (MCE;Cn = 8) 

7.71 ± 0.24 μg 6.48 ± 0.21 μg 2.89 ± 0.11 μg 1.56 ± 0.02 μg 

RSDD 3.1% RSDD 3.2% RSDD 3.8% RSDD 1.3% 

Note: Samples were subjected to ultrasonic extraction in 0.10 M Na2CO3/0.02 M NaHCO3 (carbonate/bicarbonate buffer) prior to SAE-SPE treatment and DPC 

spectrophotometric measurement. AStandard deviation. 

B Polyvinylchloride. 

C Mixed cellulose ester. 

D Relative standard deviation. 

Determined masses of CrVI in the welding fumes collected ranged 

from ∼1.5 μg for Trial 4 to ∼7.7 μg for Trial 1. Although 

these data cannot be used to assess overall method accuracy, 

the high precision of the method over a range of CrVI measurements is demonstrated by observed RSDs of less than 

5%. 

Analysis of Reference Materials 

A certified reference material (CRM) was used to evaluate the effectiveness of the field procedure for determining either total CrVI. This reference material, IRMM CRM 

545,(17) contains approximately 3 mg of total welding dust 

loaded onto glass fiber filters, with ∼100 μg of CrVI per sample. Table IV shows CrVI recoveries obtained from CRM 545 

samples which were prepared using the carbonate/bicarbonate 

extraction buffer and analyzed by SAE-SPE and DPC spectrophotometry. 

TABLE IV. Percent Recoveries of CrVI Obtained from 

IRMM CRM 545 Filter Samples 

Sample 

No. 

Certified 

Value 

(μg CrVI) 

Measured 

Value 

(μg CrVI) 

Percent CrVI 

Recovery 

1 116.86 ± 1.74 107.84 92.3 

2 121.33 ± 1.81 114.36 94.3 

3 115.50 ± 1.72 94.46 81.8 

4 116.54 ± 1.74 109.23 93.7 

5 119.47 ± 1.78 115.60 96.8 

6 122.85 ± 1.83 117.97 96.0 

Mean ± SDA = 

92.4% ± 5.5% 

Note: Samples were subjected to ultrasonic extraction in 0.10 M 

Na2CO3/0.02 M NaHCO3 (carbonate/bicarbonate buffer) prior to SAE-SPE 

treatment and DPC spectrophotometric measurement. AStandard deviation. 

Recoveries of over 90% were obtained from all 

but one of the CRMs. The lowest value of ∼82% was identified 

as a statistical outlier (Q-test, p =0.05),(19) but was nonetheless 

not discarded from the sample set. 

Investigation of Interferences 

Test for Oxidation of CrIII 

A positive interference could potentially result from air 

oxidation of CrIII during sample extraction at higher temperatures.(22) Hence, experiments were conducted in order to test 

for conversion of CrIII during sonication in the carbonate/ 

bicarbonate buffer. Solutions of different concentrations of 

CrIII in the 0.10 M Na2CO3/0.02 M NaHCO3 extraction buffer 

were prepared after sonication of ∼0.005 g Cr2O3 plus MCE 

or PVC filter material. Aliquots of the test solutions were then 

subjected to SAE-SPE, eluted and measured as above using 

the DPC portable spectrophotometric technique. 

Table V shows results from these trials. It can be seen that 

the measured absorbance is not significantly different from 

that of blank solutions, thus showing no discernable oxidation 

of CrIII to CrVI as a result of sonication in the more alkaline 

extraction buffer. 

Cr III Interference Check 

Tests were conducted to investigate potential interference 

from CrIII if this species were present in solution along with 

CrVI. About 0.002 g PbCrO4 and ∼0.003 g Cr2O3, plus PVC 

or MCE filters, were sonicated in 0.10 M Na2CO3/0.02 M 

NaHCO3 extraction buffer. Solutions were then prepared over a 

range of concentrations, and aliquots were analyzed by 
SAE-SPE and DPC spectrophotometry in the same manner 

as described above. 

TABLE V. Absorbance Measurements from CrIII

Solutions 

[CrIII], μg/mL 

Absorbance 

(PVC)A 

Absorbance 

(MCE)B 

0 (media blanks) 0.003 0.003 

0.1 0.005 — 

1.5 0.006 0.003 

3.0 0.004 0.005 

6.0 — 0.004 

Mean ± SDC 0.005 ± 0.001 0.004 ± 0.001 

Note: Solutions (n = 3), with PVC or MCE filters, sonicated in carbonate/bicarbonate buffer, treated using SAE-SPE, and measured by DPC 

spectrophotometry. APolyvinylchloride. 

B Mixed cellulose ester. 

C Standard deviation. 

Results from these experiments are given in Table VI. 

TABLE VI. Results from [CrIII] = [CrVI] Solutions 

[CrIII] + (CrVI), 

μg/mL 

Percent CrVI 

Recovery (PVC)A 

Percent CrVI 

Recovery (MCE)B 

0 (media blanks) NDC NDC 

0.1 92.5 101.1 

0.5 103.1 94.7 

1.0 100.3 98.1 

2.0 99.4 99.8 

4.0 101.6 101.3 

Mean SDD ± 99.4 4 99.0 2.7 ± ± 

Note: Solutions (with PVC or MCE filters) sonicated in carbonate/bicarbonate buffer, treated using SAE-SPE, and measured by DPC 

spectrophotometry. APolyvinylchloride.
 

B Mixed cellulose ester.
 

C None detected.
 

D Standard deviation.
 

The 

calculated CrVI recoveries for MCE and PVC filters are 92– 

103%, indicating no apparent interference from CrIII on these 

filter media. 

FeII Interference Check 

Similar experiments as described above were carried out 

in order to investigate interferences from FeII if this species 

were present in the carbonate/bicarbonate extraction buffer 

solution along with CrVI. About 0.003 g of ferrous ammonium 

sulfate and ∼0.002 g of lead chromate were sonicated in the 

carbonate/bicarbonate buffer along with MCE or PVC filters. 

Results of these trials are shown in Table VII.

TABLE VII. Results from [FeII] [CrVI = ] Solutions 

[FeII] + (CrVI), 

μg/mL 

Percent CrVI 

Recovery (PVC)A 

Percent CrVI 

Recovery (MCE)B 

0 (media blanks) NDC NDC 

0.1 65.7 65.4 

0.5 71.6 66.6 

1.0 72.4 64.6 

2.0 74.1 69.1 

Mean ± SDD 70.9 ± 3.7 66.4 ± 1.9 

Note: Solutions (with PVC or MCE filters) sonicated in carbonate/bicarbonate buffer, treated using SAE-SPE, and measured by DPC 

spectrophotometry. APolyvinylchloride. 

B Mixed cellulose ester. 

C None detected. 

D Standard deviation. 

 It can be seen that 

CrVI recoveries are appreciably below 90%, thereby showing 

a significant interference from FeII to CrVI measurement if 

these species are present in the extraction solution at similar 

concentrations. These observations are similar to results from 

analogous experiments where hot plate extraction was used.(14) 

Thus, it is shown that UE in carbonate/bicarbonate extraction 

buffer does not prevent reduction of CrVI by FeII. 

CONCLUSION 

The field method for insoluble CrVI, using an extraction 

buffer consisting of 0.10 M Na2CO3/0.02 M NaHCO3, 

was found to meet NIOSH accuracy criteria established for 

workplace air sampling methods. The method was shown to 

be effective for of CrVI in air samples generated from welding 

fumes. The method was also demonstrated to be effective 

for the determination of CrVI in CRM welding dust samples. 

This procedure offers an alternative to NIOSH method 7703 

for the on-site analysis of CrVI compounds when the CrVI 

species of interest are insoluble. It is pointed out that the 

ammonium sulfate/ammonium hydroxide buffer can be used to 

extract soluble and sparingly soluble CrVI compounds from air 

filter samples. Thus, by taking side-by-side samples, the field 

method using either extraction buffer would enable separate 

determination of CrVI compounds of different solubility in 

workplace air. If desired, total chromium concentration could 

be determined in the field by oxidizing CrIII with CeIV, as has 

been described in previous work.(13,23) 

ACKNOWLEDGMENTS 

The authors thank Paul Pierce for generating welding samples, and Mandi Hopkins for conducting interference check 

experiments; both are with the CDC/NIOSH Spokane Research Laboratory. This research was conducted under the 

auspices of a CDC/NIOSH project on field measurement of 

metals in workplace air. 
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L.W. Chang (ed.). Boca Raton, Fla.: CRC Press, 1996, pp. 48–229. 

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