Evaluation of sequential extraction procedures for soluble and insoluble

hexavalent chromium compounds in workplace air samples†

† This article was prepared by U.S. Government employees as part of 

their official duties and legally may not be copyrighted in the United 

States of America. 

‡ 

‡ Disclaimers: Mention of company names or products does not 

constitute endorsement by the Centers for Disease Control and 

Prevention. The findings and conclusions in this paper are those of the 

authors and do not necessarily represent the views of the National 

Institute for Occupational Safety and Health. 

Kevin Ashley,*a Gregory T. Applegate, a

a

U.S. Department of Health and Human Services, Centers for Disease 

Control and Prevention, National Institute for Occupational Safety and 

Health, 4676 Columbia Parkway, Mail Stop R-7, Cincinnati, OH, 

45226-1998, USA. E-mail: KAshley@cdc.gov; Tel: +1(513)841-4402 

x

x Current Address: Procter & Gamble Company, 5299 Spring Grove 

Ave., St. Bernard, Ohio 45217, USA 

Pamela b L. Drake, Paul A. Pierce,b A. Dale Marcy,b

b

U.S. Department of Health and Human Services, Centers for Disease 

Control and Prevention, National Institute for Occupational Safety and 

Health, 315 E. Montgomery Avenue, Spokane, WA, 99207, USA 

c

c

North Idaho College, 1000 W. Garden Avenue, Coeur d’Alene, ID, 83814, 

USA 

Nathalie Carabind and Martine Demanged

d

Institut National de Recherche et de Se ́curite ́, De ́partement Me ́trologie des 

Polluants, Rue du Morvan, CS 60027, 54519 Vandœuvre-les-Nancy, France 

Because toxicities may differ for Cr(VI) compounds of varying solubility, some countries and 

organizations have promulgated different occupational exposure limits (OELs) for soluble and 

insoluble hexavalent chromium (Cr(VI)) compounds, and analytical methods are needed to determine 

these species in workplace air samples. To address this need, international standard methods ASTM 

D6832 and ISO 16740 have been published that describe sequential extraction techniques for soluble 

and insoluble Cr(VI) in samples collected from occupational settings. However, no published 

performance data were previously available for these Cr(VI) sequential extraction procedures. In this 

work, the sequential extraction methods outlined in the relevant international standards were 

investigated. The procedures tested involved the use of either deionized water or an ammonium sulfate/ 

ammonium hydroxide buffer solution to target soluble Cr(VI) species. This was followed by extraction 

in a sodium carbonate/sodium hydroxide buffer solution to dissolve insoluble Cr(VI) compounds. 

Three-step sequential extraction with (1) water, (2) sulfate buffer and (3) carbonate buffer was also 

investigated. Sequential extractions were carried out on spiked samples of soluble, sparingly soluble 

and insoluble Cr(VI) compounds, and analyses were then generally carried out by using the 

diphenylcarbazide method. Similar experiments were performed on paint pigment samples and on 

airborne particulate filter samples collected from stainless steel welding. Potential interferences from 

soluble and insoluble Cr(III) compounds, as well as from Fe(II), were investigated. Interferences from 

Cr(III) species were generally absent, while the presence of Fe(II) resulted in low Cr(VI) recoveries. Twostep sequential extraction of spiked samples with (first) either water or sulfate buffer, and then 

carbonate buffer, yielded quantitative recoveries of soluble Cr(VI) and insoluble Cr(VI), respectively. 

Three-step sequential extraction gave excessively high recoveries of soluble Cr(VI), low recoveries of 

sparingly soluble Cr(VI), and quantitative recoveries of insoluble Cr(VI). Experiments on paint pigment 

samples using two-step extraction with water and carbonate buffer yielded varying percentages of 

relative fractions of soluble and insoluble Cr(VI). Sequential extractions of stainless steel welding fume 

air filter samples demonstrated the predominance of soluble Cr(VI) compounds in such samples. The 

performance data obtained in this work support the Cr(VI) sequential extraction procedures described 

in the international standards. 

Introduction

Worldwide, millions of workers are potentially exposed to 

hexavalent chromium, Cr(VI), during their work activities. 

Occupational exposures to Cr(VI) can cause lung cancer and/or 

numerous other deleterious health effects, primarily through 

inhalation.1,2 In recognition of the carcinogenic properties of 

inhaled chromate compounds, the Occupational Safety and 

Health Administration (OSHA) recently effected new regulations that reduced (by more than tenfold) the Permissible 

Exposure Limit (PEL) for Cr(VI) in the United States.3

Besides carcinogenic effects, some Cr(VI) compounds also can 

act as contact allergens, for instance in leather tanning and in 

work activities involving cement.4 In occupational settings, 

Cr(VI) exposures have been associated with numerous industries 

including metal plating, chemical production, refining, chromite 

ore processing, spray painting, and welding operations, to

name a few.5 Occupational exposure to Cr(VI) remains a serious 
problem in the U.S. and internationally, and there is a need to 

reduce or eliminate these exposures. 

Exposure assessment methods are necessary to assist in the 

prevention or reduction of worker exposures to Cr(VI). Thus 

sampling and analytical methods have been developed in order to 

provide needed tools for exposure assessment purposes.6 For 

many years, sampling and analytical methods for airborne Cr(VI) 

have been developed and refined so as to provide means for 

occupational exposure assessment of this highly toxic species. 

Monitoring methods for Cr(VI) are continually being optimized 

in terms of sensitivity, speed, reliability, robustness, field portability, and so on. Improved monitoring methods are required in 

order to meet the challenges associated with measuring airborne 

Cr(VI) at trace levels, and the recent literature includes a number 

of examples thereof.6–16

Because toxicities may differ for Cr(VI) compounds of varying 

solubility,4,5 some organizations and countries have established 

different occupational exposure limits (OELs) for soluble and 

insoluble Cr(VI) compounds.17,18 Recently, international standards have been published describing methods for determining 

these compounds in workplace air samples.7,8 Sequential 

extraction procedures are described in ASTM D68327 and ISO 

167408 and are meant to enable serial measurement of soluble 

and insoluble Cr(VI) compounds collected onto workplace air 

filter samples. Separate measurement of soluble and insoluble 

Cr( ) compounds has been successfully demonstrated6,9 VI and 

sequential extraction procedures have been applied to workplace 

samples for subsequent determination of elemental chromium 

and other metals.10 Other work reported the use of sequential 

extraction followed by voltammetric measurement of chromium 

species.11 However, supporting validation data for the 

standardized sequential extraction methods for subsequent 

measurement of Cr(VI) have not been previously reported in the 

peer-reviewed literature. In an effort to fill this data void, the aim 

of the present work was to evaluate and validate the sequential 

extraction procedures for Cr(VI) that are described in the relevant 

published international standards.7,8

Experimental

Reagents and materials

Reagent grade potassium chromate, chromic nitrate, chromic 

oxide, sodium hydroxide and phosphoric acid were obtained 

from Sigma-Aldrich (Milwaukee, WI, USA). Spectroscopic 

grade Cr(VI) standard solution (1,000 mg l-1

) came from Inorganic Ventures (Lakewood, NJ, USA). Ammonium sulfate, 

ammonium hydroxide, lead chromate, strontium chromate, 

ferrous sulfate, sodium carbonate, sodium bicarbonate, 1,5­

diphenylcarbazide (DPC), methanol, and acetonitrile (used for 

dissolution of DPC), all reagent grade, were obtained from 

Fisher Scientific (Waltham, MA, USA). Nitric, hydrofluoric and 

hydrochloric acids were spectroscopic grade and were also 

purchased from Fisher Scientific. Paint pigments were obtained 

from a local hardware store (Nancy, France). 

Polypropylene centrifuge (‘‘Falcon’’) tubes with screw caps 

(15 ml and 50 ml) were purchased from Becton Dickinson 

(Franklin Lakes, NJ, USA). Two- and three-piece polystyrene air 

sampling cassettes, polyvinyl chloride (PVC) filters (37 mm 

diameter, 5 mm pore size), glass fiber filter backup pads (37 mm 

diameter), and binder-free quartz fiber filters (37 mm diameter) 

were obtained from SKC (Eighty-Four, PA, USA). Strong 

anion-exchange solid-phase extraction (SAE-SPE) cartridges, 

containing 1 g of quaternary amine-bonded silica sorbent (with 

chloride as counterion), were obtained from Varian (Palo Alto, 

CA, USA). Where applicable, Bu ̈chner funnel filtration was 

performed using plastic filtration funnels (Fisher Scientific) and 

47 mm diameter polytetrafluoroethylene (PTFE) filters (SKC) to 

fit the funnels. Plastic or borosilicate glass laboratory ware was 

used in all procedures. Deionized water (18 MU cm resistivity), 

prepared using a Millipore MilliQ® water purification system 

(Billerica, MA, USA), was used for all experiments. 

Instrumentation and equipment

Spectrophotometric measurements at 540 nm (sample path 

length 1 cm) were carried out using a Hach DR/2800 spectrophotometer (Loveland, CO, USA). Plastic cuvettes, 1 cm path 

length, were also obtained from Hach. Where required, an 

Accumet 50 pH meter (Chicago, IL, USA), calibrated using pH 

4.0, 7.0 and 10.0 buffers (Fisher Scientific), was used to measure 

solution pH. Where applicable, materials were weighed to the 

nearest 0.0001 g using a Mettler Model AE 163 analytical 

balance (Greifensee, Switzerland). Mechanical pipettes of 

various sizes (100 ml to 10 ml, with corresponding plastic pipette 

tips) were obtained from Eppendorf (Hamburg, Germany). A 

Labquake® rotator (Barnstead, Dubuque, IA, USA), placed in 

a Precision Model 45EG laboratory oven (Winchester, VA, 

USA) set at 37 ± 1 °C, was used for mechanical agitation of 

samples undergoing the soluble extraction procedure. For 

extraction of insoluble Cr(VI) compounds, sonication at 40 ±

2 °C was performed using a Branson Ultrasonics Model 

3510 ultrasonic bath (Danbury, CT, USA). 

A 12-port solid-phase extraction (SPE) manifold, used in some 

experiments, was purchased from Supelco (Bellefonte, PA, 

USA). Disposable PTFE valve liners (Supelco) were used for the 

ports of the SPE manifold. The manifold was connected to 

a Gast vacuum pump (Benton Harbor, MI, USA) through 

a pressure control valve. For some samples, inductively coupled 

plasma – atomic emission spectrometry (ICP-AES) was 

performed using an Applied Research Laboratories 34000 

instrument (Sunland, CA, USA). Where applicable, ion chromatographic (IC) experiments were carried out with a Dionex 

Series 4500i system (Sunnyvale, CA, USA). 

Collection of air samples

Airborne particulate samples containing Cr(VI) were collected 

onto PVC filters, which were housed in plastic air sampling 

cassettes, from fumes produced during stainless steel welding. 

Fumes were generated from manual metal arc welding for 

various periods between 5 and 15 minutes using AWS A5.4 E312­

16 rods (Harris Welco, Kings Mountain, NC, USA) on a section 

of AISI Type 304 stainless steel rail (NIOSH, Spokane, WA, 

USA). Stainless steel welding fume samples were collected using 

a homemade cylindrical multi-port sampler (16.5 cm inner 

diameter with a 38 cm tall mixing chamber; overall height of 

86.4 cm). The sampler was connected to a Gilian AirCon2 
high-volume sampling pump (Clearwater, FL, USA) calibrated 

to an airflow of up to 15 l min-1

. The multi-port sampler contained eight separate sampling ports, with each port calibrated to 

a flow rate of 1.7 ± 0.1 l min-1

, and enabled simultaneous 

collection of uniform welding aerosol loadings onto the filters. 

Previous work has reported that the precision of the mass of 

Cr(VI) collected onto separate filters using this multiport sampler 

was 1.3% to 3.8% relative standard deviation (RSD) for 

measured Cr(VI) masses of ~ 1.5 mg to ~ 8 mg per filter.9

Sample preparation and analysis

Airborne particulate samples containing Cr(VI) that were 

collected onto PVC filters from stainless steel welding fumes were 

prepared in accordance with procedures described in international standards for separate or sequential extraction of soluble 

and insoluble Cr(VI) compounds.7,8 PVC filters spiked with 

potassium chromate, lead chromate and combinations thereof 

were prepared similarly by using procedures for soluble extraction, insoluble extraction, or both (in series). In order to investigate the potential influence of trivalent chromium on extraction 

and analysis, some experiments also included spikes of chromic 

nitrate and/or chromic oxide (soluble and insoluble Cr(III) 

compounds, respectively), along with spikes of the aforementioned soluble and/or insoluble Cr(VI) compounds. For some 

experiments, ferrous sulfate was spiked onto the filters (along 

with Cr(VI) compounds, but in a separate area of each filter) in 

order to investigate analytical interference from Fe(II) present in 

solution. Spiking levels were generally in the range of 0.5 to 3 mg 

of each material per filter sample. 

Details regarding sample preparation procedures for soluble 

and insoluble Cr(VI) compounds are described in the published 

international standards.7,8 Briefly, soluble extraction entailed 

treatment of the filter samples in either water or in a buffer of 

0.05 M ammonium sulfate and 0.05 M ammonium hydroxide. To 

do this, the filters were placed into 15 ml centrifuge tubes and 

5 ml of extraction solution (water or ammonium sulfate buffer) 

was added; the centrifuge tubes were then capped. Alternatively, 

5 ml of extraction solution was added directly to the sampling 

cassette housing the filter (and glass fiber filter backup pad, if 

applicable), and the sampler was capped. The centrifuge tubes or 

cassettes were then affixed to the rotator, which itself was placed 

in the laboratory oven held at 37 °C. The rotator was turned on 

to effect mechanical agitation and extraction was carried out for 

a period of one hour. To extract insoluble Cr(VI) compounds, the 

filters (either within centrifuge tubes or sampling cassettes) were 

treated by sonication in a buffer of 3% (m V-1

) sodium carbonate 

and 2% (m V-1

) sodium hydroxide for 30 minutes at ~ 

° 40 C. 

Filters that underwent sequential extraction were first subjected 

to soluble extraction in either water or sulfate buffer. Removal 

of the soluble extraction solution was afforded by means of 

a Bu ̈chner suction filtration apparatus8 or via direct filtration 

through the PVC filter housed within the cassette.12 An alternative procedure entailed, after carrying out extraction within 

a 15 ml centrifuge tube, careful manual removal of the soluble 

extract by decantation/pipetting. Then the remaining undissolved residue and filter were treated as outlined above by 

ultrasonic extraction in carbonate buffer (either in the 15 ml 

centrifuge tube or within the sampling cassette). 

In order to investigate the application of a three-step sequential extraction procedure, some experiments were carried out 

which entailed: (1) extraction of filter samples with water to 

dissolve soluble Cr(VI) compounds (e.g., potassium chromate), 

(2) extraction in sulfate buffer to target sparingly soluble Cr(VI) 

species (e.g., strontium chromate), and (3) extraction in 

carbonate buffer to dissolve insoluble Cr(VI) compounds (e.g., 

lead chromate). This three-step procedure was applied to filter 

samples spiked with the above representative soluble, sparingly 

soluble and insoluble Cr(VI) compounds, and also to some 

samples of stainless steel welding fumes. 

Analysis of aliquots of extract solutions for Cr(VI) content by 

the diphenylcarbazide (DPC) method was carried out in accordance with one of three techniques: (1) a German procedure that 

includes phosphoric acid in the DPC detection solution, which 

eliminates potential measurement interferences from iron after 

extraction has taken place;13 (2) a NIOSH procedure that entails 

SAE-SPE prior to DPC detection to eliminate potential metallic 

ion interferences;14–16 or (3) a standardized IC procedure that is 

used to isolate Cr(VI) from other potential interferants prior to 

detection with DPC.7,8,19 Aliquots from both soluble (whether 

water or sulfate buffer) and insoluble (i.e., carbonate buffer) 

extract solutions were analyzed by one of these three DPC 

detection methods; sample aliquots were diluted in water prior to 

analysis, if necessary. The equivalence of the SAE-SPE and IC 

methods for Cr(VI) determination has been previously demonstrated.20 For purposes of comparison, selected samples were 

analyzed for chromium content by ICP-AES after hotplate 

digestion in aqua regia. 

21

Results and discussion

Extraction of soluble Cr(VI) from spiked filter samples

To verify the performance of water to extract soluble Cr(VI), 

initial experiments were carried out on spiked samples of 

potassium chromate (in both liquid{

{ Note: It was found that Cr(VI) recoveries from liquid spikes that were 

allowed to air dry were significantly low, presumably due to reduction 

to Cr(III). This phenomenon was observed with several different lots of 

PVC filters. 

and solid forms) on PVC 

filters; spiking levels were in the range of 0.5–1.5 mg. Extractions 

were carried out using 5 ml of water either in 15 ml centrifuge 

tubes or directly within the sampling cassettes, as described in the 

experimental section. After water extraction, Cr(VI) measurements were carried out using the detection technique described in 

the German procedure.13 High recoveries (ranging from 94% to 

122%) of soluble Cr(VI) were obtained in all instances, including 

situations where insoluble Cr(VI) (as lead chromate) was also 

present. These trials served as an initial check of the soluble 

extraction with water. Extraction of potassium chromate with 

the 0.05 M ammonium sulfate/0.05 M ammonium hydroxide 

buffer has been previously demonstrated to be effective,9,22 and 

additional experiments with this buffer that were carried out here 

for verification purposes yielded quantitative recoveries of 

soluble Cr(VI) (98% to 104%). These experiments served to 

demonstrate that, in the absence of interferants, water could 

indeed be used successfully to obtain quantitative recoveries of 

soluble Cr(VI) from spiked PVC filters. 
Further water extraction experiments in the presence of 

soluble and insoluble Cr(III) compounds (as chromic nitrate and 

chromic oxide, respectively) revealed no apparent influence of 

these Cr(III) compounds on recoveries of soluble Cr(VI). In these 

trials, water extractions were carried out within sampling 

cassettes using PVC filters with glass fiber filter backup pads. 

Cr(VI) measurements after extraction of PVC filters were performed using the SAE-SPE method described in NIOSH method 

7703.15 Observed recoveries of Cr(VI) from potassium chromate 

spikes in the presence of soluble and/or insoluble Cr(III) remained 

quantitative (i.e., 89% to 106%). Similar results have been 

reported for the use of the ammonium sulfate/ammonium 

hydroxide buffer, where recoveries of soluble Cr(VI) were quantitative and were unaffected by the presence of Cr(III) 

species.9,14,23 These experiments demonstrated that quantitative 

recoveries of soluble Cr(VI) can be obtained in the presence 

of other chromium compounds, both trivalent and hexavalent, of 

varying solubility. This work also verified that sulfate buffer can 

be used successfully to extract soluble Cr(VI) in the presence of 

insoluble Cr(VI), as previously reported.9

However, extractions with water that were carried out on 

quartz fiber filters spiked with soluble Cr(III) and soluble Cr(VI) 

produced low recoveries of soluble Cr(VI) (29–33%); extracts 

from these filters were analyzed in accordance with the French 

(INRS) method.12 It was found after further investigation that 

a water-insoluble Cr(VI) species was produced in this case.24

Transformations between Cr(III) and Cr(VI) species have been 

reported in isotope dilution mass spectrometry studies,25 but the 

generation of a water-insoluble Cr(VI) compound from soluble 

predecessors is surprising and not clearly explained. However, 

this water-insoluble Cr(VI) compound was soluble in the 

ammonium sulfate/ammonium hydroxide extraction solution, 

the use of which gave the ‘‘true’’ soluble Cr(VI) measurement in 

this case. 

As a further performance check of the chromium mass balance 

on spiked PVC or glass fiber filters, measurements of total 

chromium concentrations were carried out on filter samples 

containing various combinations of potassium chromate, lead 

chromate, chromic nitrate and chromic oxide. In these trials, 

total chromium in PVC filters was determined in accordance 

with the NIOSH aqua regia digestion/ICP-AES method,21 while 

chromium in quartz fiber filters was measured by using the 

sonication/ICP-AES procedure described in an INRS procedure.26 Observed recoveries of chromium in all cases were found 

to be quantitative (96% to 107%). These experiments provided 

verification that all sources of chromium were accounted for 

during the analysis. 

Interference from Fe(II) to the soluble extraction of Cr(VI) was 

investigated by spiking ~ 250 mg of ferrous (Fe(II)) sulfate onto 

PVC filters in the presence of ~ 10 mg of soluble chromate (as 

K2CrO4); spikes of ferrous sulfate and potassium chromate were 

placed onto different areas of the filters. Extraction with water in 

the presence of Fe(II) resulted in poor recoveries (24–32%) of 

soluble Cr(VI). Extraction with sulfate buffer in the presence 

of Fe(II) yielded better recoveries of soluble Cr(VI), albeit not 

quantitative (66–88%). Previous work has shown that the presence of Fe(II) during ultrasonic extraction of lead chromate in 

carbonate buffer leads to lower recoveries of insoluble Cr(VI) in 

that instance as well.9 The use of sulfate buffer for soluble 

extraction assists in alleviating the interference from Fe(II) due 

to its redox reaction with Cr(VI), but does not completely 

eliminate it. 

Sequential extraction of Cr(VI) from spiked PVC filters

Results from the sequential extraction of potassium chromate 

and lead chromate spiked onto PVC filters, using mechanical 

extraction in first (A) water or (B) ammonium sulfate buffer, with 

subsequent sonication in carbonate buffer, are summarized in 

Table 1. 

Table 1 Recovery data for Cr(VI) from two-step sequential extraction of potassium chromate and lead chromate spiked onto PVC air sampling filters; ±

values are standard deviations for n 1⁄4 number of samples analyzed using the DPC method 

Sequential extraction 

sequence 

Cr(VI) recovery from 

K2CrO4 (%) 

Cr(VI) recovery from 

PbCrO4 Filtration/isolation method (%) 

(1) Water Decanting/pipetting 107 ± 8 (n 1⁄4 6) 99 ± 6 (n 1⁄4 6) 

(2) Carbonate buffer 

(1) Water Bu ̈chner funnel filtration 100 ± 4 (n 1⁄4 6) 79 ± 7 (n 1⁄4 6) 

(2) Carbonate buffer 

(1) Water Within-cassette filtration 102 ± 1 (n 1⁄4 5) 99 ± 9 (n 1⁄4 5) 

(2) Carbonate buffer 

(1) Sulfate buffer Decanting/pipetting 98 ± 2 (n 1⁄4 6) 96 ± 7 (n 1⁄4 6) 

(2) Carbonate buffer 

(1) Sulfate buffer Bu ̈chner funnel filtration 95 ± 6 (n 1⁄4 6) 90 ± 8 (n 1⁄4 6) 

(2) Carbonate buffer 

(1) Sulfate buffer Within-cassette filtration 101 ± 1 (n 1⁄4 7) 98 ± 4 (n 1⁄4 7) 

(2) Carbonate buffer 

As explained previously, these procedures are options 

described for sequential extraction of Cr(VI) in relevant international standards.7,8 Analysis of both soluble and insoluble Cr(VI) 

extracts was performed in accordance with the German detection 

procedure;13 spiking levels of the chromate compounds were 0.5 

to 1.5 mg per filter. 

As described earlier, experiments carried out entailed one of 

three procedures: (1) manual decanting/pipette removal of the 

solution after soluble extraction of the spiked PVC filters within 

15 ml centrifuge tubes; (2) Bu ̈chner funnel filtration after soluble 

extraction within 15 ml centrifuge tubes;7,8 or (3) vacuum filtration through the filter itself after soluble extraction within the 

sampling cassette.12 These procedures enabled separation of the 

soluble extract and isolation of undissolved lead chromate (along 

with the PVC filter) after initial treatment with the soluble 
extraction medium (water or sulfate buffer), thereby enabling 

subsequent treatment with the insoluble extraction solution 

(carbonate buffer). 

Quantitative recoveries of soluble and insoluble Cr(VI) species 

were obtained in almost all situations tested (Table 1). However, 

Bu ̈ chner funnel filtration after soluble extraction with water 

resulted in lower insoluble Cr(VI) recoveries. Yet this phenomenon 

was not observed if sulfate buffer is used for soluble extraction. 

Experiments with a different Bu ̈chner filtration apparatus led to 

even lower recoveries of insoluble Cr(VI) (66% ± 5%) after initial 

soluble extraction with water. Unfortunately no satisfactory 

explanation can be surmised for these surprising results. 

It was considered of interest to investigate the use of a threestep extraction procedure employing the above extraction media. 

PVC filters were spiked (0.5 to 1.5 mg of solid material per 

sample) with potassium chromate, strontium chromate and lead 

chromate to represent soluble, sparingly soluble and insoluble 

Cr(VI) compounds, respectively. Sequential extraction was 

carried out within the sampling cassettes, in accordance with the 

INRS method.12 As above, aliquots of sample extracts were 

analyzed by the German detection method employing phosphoric acid.13 Results obtained from three-step sequential 

extraction of spiked PVC filters using first water, secondly sulfate 

buffer and lastly carbonate buffer, are illustrated in Table 2. It is 

noted that total Cr(VI) determined from similarly spiked samples, 

extracted using carbonate buffer and measured by the DPC 

method, yielded quantitative Cr(VI) recoveries (Table 2). These 

results demonstrate that water extracts sparingly soluble Cr(VI) 

species to an appreciable extent, thus the three-step extraction 

method is not appropriate for its intended purpose. 

Table 2 Recovery data for Cr(VI) from three-step sequential extraction 

of potassium chromate, strontium chromate and lead chromate spiked 

simultaneously onto PVC air sampling filters; and total Cr(VI) measurement by carbonate extraction from PVC filters spiked with all three 

compounds; ± values are standard deviations for n 1⁄4 number of samples 

analyzed using the DPC method 

Extractant(s) 

Target Cr(VI) 

compound(s) 

by extractant 

Cr(VI) recovery 

from target 

compound(s) (%) 

(1) Water K2CrO a 4	 121 23 (n 6)

(2) Sulfate buffer SrCrO

± 1⁄4

4	 44 11 (n 6)a 

(3) Carbonate buffer PbCrO4	 107 

±

6 (n 

1⁄4

6)a 

Carbonate buffer only	 K2CrO + 

± 1⁄4

4 SrCrO b 4 + 97 ± 6 (n 1⁄4 3)

PbCrO4 

a Sequential extraction. b Single extraction. 

Sequential extraction of Cr(VI) from paint pigment samples

Sequential extraction with first water and secondly carbonate 

buffer were carried out on samples of paint pigment (~1 mg) 

spiked onto quartz fiber filters. Cr(VI) was determined by the 

INRS DPC method,12 and total chromium in parallel spiked paint 

pigment samples was determined by the INRS ultrasonic extraction/atomic spectrometry procedure.26 The results of these trials 

are presented in Table 3.

Table 3 Cr(VI) analysis results from sequential extraction of paint pigment samplesa using (1) water and (2) carbonate buffer, followed by DPC analysis; 

and (3) total chromium analysis by acid digestion/ICP-AES analysis 

Pigment 

% Soluble Cr(VI) (water 

extraction) 

% Insoluble Cr(VI) 

(carbonate extraction) 

% Total Cr (acid 

digestion) 

Zinc chromate 0.6 22.5 21.5 

Zinc chromate 7.8 15.2 25.0 

Zinc tetraoxychromate 0.1 9.7 9.7 

Chrome yellow <0.01 8.4 7.8 

Chrome yellow 0.3 12.4 14.4 

Chrome yellow 0.1 14.2 12.8 

Molybdenum orange 2.2 10.2 13.4 

Molybdenum orange 7.3 3.3 13.5 

Molybdenum red 5.3 6.7 12.4 

Strontium chromate 2.7 21.5 22.6 

Barium chromate 1.3 14.6 19.7 

a Pigments listed are primary constituents of the paint samples analyzed. 

 In all but one of the pigment samples 

analyzed, insoluble Cr(VI) is the preponderant Cr(VI) species 

found. It can be seen that in all cases (Table 3), the percentage of 

total chromium determined is roughly equal to the sum of the 

percentages of soluble and insoluble hexavalent chromium 

measured: [(percent total Cr(VI))/(percent total Cr)] x 100% 

ranges from 79% to 112% (mean 1⁄4 97%; standard deviation 1⁄4

11%). However, even for similar pigments, significant differences 

were observed in the percentages of soluble and insoluble Cr(VI) 

obtained. This is probably due to the presence of other constituents within the paint samples, which can influence solubility and, 

as a result, the sequential extraction of target Cr(VI) compounds. 

Nevertheless, as the total percentage of chromium is similar to the 

percentage of total Cr(VI) obtained by summing the percentages of 

soluble and insoluble Cr(VI) (Table 3), the measurement of total 

Cr(VI) species appears to remain unaffected. 

Sequential extraction of Cr(VI) from welding fume samples

Sequential extraction experiments were carried out on stainless 

steel welding fume samples generated and collected using the 

multiport sampler as described in the experimental section. 

Because aerosol loadings were uniform for each set of eight filter 

samples,9 this enabled reliable comparisons to be made between 

individual samples within a given sample set which were treated 

by different extraction and/or analysis procedures. Different 

options for sample preparation that were investigated for various 

sets and subsets of welding fume samples included the following: 
(A) two-step sequential extraction with (1) water and (2) 

carbonate buffer; (B) two-step sequential extraction with (1) 

sulfate buffer and (2) carbonate buffer; (C) three-step sequential 

extraction with (1) water, (2) sulfate buffer and (3) carbonate 

buffer; and (D) single extraction using carbonate buffer only. For 

these experiments, water or sulfate buffer extractions were 

carried out with mechanical agitation at 37 °C, while carbonate 

buffer extraction was performed using an ultrasonic bath, as 

described previously for Cr(VI)-spiked samples. Also in these 

trials, extractions were carried out directly within the sampling 

cassettes. Unless otherwise indicated, Cr(VI) in aliquots of extract 

solutions was measured using the German DPC method.13

Results from the above experiments are summarized in Tables 

4–6 for ten different sample sets collected onto PVC filters during 

stainless steel welding. If possible, measurement of total Cr(VI) 

was carried out on selected filter samples from each set. This 

allowed for comparisons of data for total Cr(VI) to Cr(VI) analysis 

results from other samples within the same set that were treated 

by using sequential extraction procedures. Measured Cr(VI) 

amounts ranged between 5 and 35 mg per sample (Tables 4–6) 

and depended largely on the fume generation and sampling time, 

which varied from ~ 5 to ~ 15 minutes. 

Table 4 demonstrates results from two-step sequential 

extraction with (1) water and (2) carbonate buffer, along with 

data from the single extraction procedure using carbonate buffer. 

Table 4 Cr(VI) analysis data from stainless steel welding fume samples: 

two-step sequential extraction with (1) water (to target soluble Cr(VI)) 

and (2) carbonate buffer (to target insoluble Cr(VI)), and (3) single 

extraction with carbonate buffer (to obtain total Cr(VI)); Cr(VI) analyses 

by DPC method. n 1⁄4 number of samples run for each subset 

Sample set/extractant(s) Measured Cr(VI) content 

Set I 

(1) Water 5.9 0.5 mg (n 5)a 

(2) Carbonate buffer 1.1 

±

0.2 

1⁄4

mg (n 5)a 

Carbonate buffer only 6.6 

±

0.3 g (n 

1⁄4

± m 1⁄4 3)b 

Set II 

(1) Water 10.1 ± 0.3 mg (n 

(2) 

1⁄4 4)a 

Carbonate buffer 0.6 ± 0.3 mg (n 1⁄4 4)a 

Carbonate buffer only 9.9 ± 1.2 mg (n 1⁄4 4)b 

Set III 

(1) Water 13.9 0.7 mg (n 5)a 

(2) Carbonate buffer 1.3 

± 1⁄4

Carbonate buffer only 15.1 

± 0.2 mg (n 1⁄4 5)a 

± 0.5 mg (n 1⁄4 3)b 

Set IV 

(1) Water 19.1 

(2) Carbonate buffer 0.42 

± 1.0 mg (n 1⁄4 3)a 

0.09 mg (n 3)a 

Carbonate buffer only 19.3 

±

± 1.8 

1⁄4

mg (n 1⁄4 5)b 

Set V 

(1) Water 23.6 ± 0.3 mg (n 

(2) Carbonate buffer 0.27 0.03 g (n 

1⁄4 4)a 

± m 1⁄4 4)a 

Carbonate buffer only 25.6 ± 1.0 mg (n 1⁄4 4)b 

a Sequential extraction. b Single extraction. 

In each case, the total Cr(VI) measured in parallel samples from 

the same sample set is close to the sum of soluble and insoluble 

Cr(VI) fractions. The percentage of soluble Cr(VI) measured in 

these welding fume samples ranged from 84% to 98%. Similar 

results have been previously reported, where predominately 

soluble Cr(VI) was measured (vs. insoluble Cr(VI)) in welding 

fumes collected during work activities.27,28 These data demonstrate the robustness of the sequential extraction procedure as 

carried out within sampling cassettes. 

Table 5 shows data from two-step sequential extraction with 

(1) sulfate buffer and (2) carbonate buffer, and also includes 

single extraction results with carbonate buffer. 

Table 5 Cr(VI) analysis data from stainless steel welding fume samples: 

(A) two-step sequential extraction with (1) sulfate buffer (to target 

soluble Cr(VI)) and (2) carbonate buffer (to target insoluble Cr(VI)); and 

(B) single extraction with carbonate buffer (to obtain total Cr(VI)); Cr(VI) 

analyses by DPC method. n 1⁄4 number of samples run for each subset 

Sample set/extractant(s) Measured Cr(VI) content 

Set VI 

(1) Sulfate buffer 8.8 ± 0.6 mg (n 1⁄4 3)b 

(2) Carbonate buffer None detecteda (n 1⁄4 3)b 

Carbonate buffer only 7.7 ± 0.4 mg (n 1⁄4 2)c 

Set VII 

(1) Sulfate buffer 8.6 ± 0.5 mg (n 

(2) Carbonate buffer None detected (

1⁄4 3)b 

n 1⁄4 3)b 

Carbonate buffer only 10.7 ± 0.3 mg (n 1⁄4 2)c 

Set VIII 

(1) Sulfate buffer 14.6 

(2) 

± 0.8 mg (n 4)b 

Carbonate buffer None detected (n 

1⁄4

4)b 

Carbonate buffer only 11.9 ± 0.7 g (

1⁄4

m n 1⁄4 4)c 

a Limit of detection 1⁄4 0.1 mg Cr(VI) per sample. b Sequential extraction. c Single extraction. 

Table 6 Cr(VI) analysis data from stainless steel welding fume samples: 

(A) three-step sequential extraction with (1) water (to target soluble 

Cr(VI)), (2) sulfate buffer (to target sparingly soluble Cr(VI)), and (3) 

carbonate buffer (to target insoluble Cr(VI)); or (B) single extraction with 

carbonate buffer (to obtain total Cr(VI)); Cr(VI) analyses by DPC method. 

n 1⁄4 number of samples run for each set/subset 

Sample set/extractant(s) Measured Cr(VI) content 

Set VI 

(1) Water 8.3 ± 0.4 mg (n 1⁄4 3)b 

(2) Sulfate buffer None detecteda (n 3)b 

(3) Carbonate buffer None detected (n 

1⁄4

3)b 

Carbonate buffer only 7.7 ± 0.4 mg (n 

1⁄4

1⁄4 2)c 

Set VII 

(1) Water 8.0 ± 0.8 mg (n 1⁄4 3)b 

(2) Sulfate buffer None detected (n 1⁄4 3)b 

(3) Carbonate buffer None detected (n 1⁄4 3)b 

Carbonate buffer only 10.7 ± 0.3 mg (n 1⁄4 2)c 

Set VIII 

(1) Water 13.0 ± 1.2 mg (n 8)b 

(2) Sulfate buffer 0.61 ± 0.07 g (

1⁄4

m n 1⁄4 8)b 

(3) Carbonate buffer 1.03 ± 1.2 mg (n 1⁄4 8)b 

Carbonate buffer only – c 

Set IX 

(1) Water 31.6 ± 1.8 mg (n 

(2) 

1⁄4 8)b 

Sulfate buffer 0.14 0.08 mg (n 8)b 

(3) Carbonate buffer 0.18 

± 1⁄4

± 0.02 mg (n 1⁄4 8)b 

Carbonate buffer only – c 

a Limit of detection 1⁄4 0.1 mg Cr(VI) per sample. b Sequential extraction. c Single extraction. 

In these cases no 
insoluble Cr(VI) is measured, and results for total Cr(VI) from 

parallel samples are similar to results for soluble Cr(VI). Insoluble 

Cr(VI) cannot be measured here because the sulfate buffer 

effectively dissolves all of the Cr(VI) species present. The implication is that the sulfate buffer is completely dissolving both 

soluble and sparingly soluble Cr(VI) compounds. Thus if it is 

known that no insoluble Cr(VI) species are present in test 

samples, the sulfate buffer is a useful option for extracting total 

Cr(VI). However, it must be recognized that extraction with 

sulfate buffer will extract more than just water-soluble Cr(VI) 

compounds. Hence if only water-soluble Cr(VI) species are of 

interest and interfering reducing agents are largely absent, the use 

of water for extraction is the obvious choice. 

Table 6 presents data from three-step sequential extraction 

with (1) water, (2) sulfate buffer and (3) carbonate buffer, along 

with single extraction results for two sample sets by using the 

carbonate buffer only. As in the above cases (Tables 4 and 5), the 

majority of the Cr(VI) species present in stainless steel welding 

fume samples are water-soluble. However, data for sparingly 

soluble Cr(VI) should be viewed as unreliable, since it is known 

from experiments previously described (Table 2) that water 

partially dissolves sparingly soluble Cr(VI) species during the 

initial step of the extraction sequence. 

Conclusion

This work has demonstrated the utility of sequential extraction 

procedures for determining soluble and insoluble Cr(VI) species in 

workplace air samples. Two-step extraction involving either water 

or sulfate buffer for the dissolution of soluble Cr(VI) compounds, 

followed by sonication in carbonate buffer to obtain insoluble 

Cr(VI) species, yielded results that demonstrated acceptable 

performance. However, three-step extraction with first water, 

then sulfate buffer, and lastly carbonate buffer is not recommended, owing to partial dissolution of sparingly soluble Cr(VI) 

species by water during the first step of the sequence. Applications 

of sequential extraction procedures to paint pigment samples and 

to stainless steel welding fume samples were successfully demonstrated. It is recognized that airborne Cr(VI) compounds can react 

with other chemicals in workplace atmospheres, e.g., species 

emanating from stainless steel welding fumes;6,10,27–30nevertheless, 

our efforts here have focused on analyses of particulate Cr(VI) 

samples that have been collected using filtration and stabilized to 

the extent possible. Another advantage of two-step sequential 

extraction (with either water or sulfate buffer) is that in the first 

extraction step, soluble Cr(III) is largely extracted along with 

soluble Cr(VI), so there is minimized possibility of oxidizing 

soluble Cr(III) to Cr(VI) during the second extraction step. Despite 

some pitfalls, the results of this study have served to provide 

performance data in support of the procedures described in the 

international standard methods,7,8 for which such results were 

previously unavailable in the peer-reviewed literature. 

Acknowledgements

We thank Dr Michael Goldcamp (Wilmington College), Michael 

Brisson (Savannah River Nuclear Solutions), Dr Kenneth Brown 

(NIOSH) and Dr Robert Streicher (NIOSH) for their critical 

review of the draft manuscript. Technical assistance by Tamara 

Wise (NIOSH) is gratefully acknowledged. This work was 

partially funded under the auspices of a National Occupational 

Research Agenda (NORA) project on hexavalent chromium in 

stainless steel welding. 

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