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pollution problems. Speci¢c research efforts involve thedevelopment of mathematical models that relate pollutantsource emissions to ambient pollutant concentrations forairborne particles and photochemical oxidants. His

research group also executes ¢eld experiments for thepurpose of gathering the emissions and air quality dataneeded to evaluate air quality model performance.

end of special issue

Ultrasonic extraction of heavy metals fromenvironmental and industrial hygienesamples for their subsequentdeterminationKevin AshleyUS Department of Health and Human Services, Centers for Disease Control and Prevention,National Institute for Occupational Safety and Health, Cincinnati, OH 45226, USA

Ultrasonic extraction (UE) is an effectivemethod for the extraction of a number ofheavy metals in environmental samples. Inmany cases, UE offers a means for quantita-tive recoveries of heavy metals. However, UEhas been underutilized for sample prepara-tion purposes in environmental analysis.Nevertheless, the use of UE for extraction ofheavy metals in the industrial hygiene arena isincreasing, with applications in the ¢eld aswell as in the laboratory. In this article someexamples of applications of UE to heavy met-als extraction ( for their subsequent determi-nation) in environmental and industrialhygiene samples are presented. It is expectedthat the use of UE for sample preparationpurposes in environmental analytical chemis-try will become more widespread, owing toits simplicity, ease of use, speed, andenhanced safety when compared with other,more traditional sample preparation proce-

dures. z1998 Published by Elsevier ScienceB.V.

Keywords: Ultrasonic extraction; Heavy metals;Environmental analysis; Industrial hygiene

1. Introduction

The objective of this article is to review early andrecent studies of ultrasonic extraction (UE) for heavymetal determinations, and to demonstrate its potentialfor environmental and industrial hygiene applications.The use of ultrasonic energy to extract contaminantspecies of analytical interest from environmental sam-ples has been applied successfully in organic analysis[ 1^3 ]. UE forms the basis of a US EnvironmentalProtection Agency (EPA) method for the extractionof organics from soils [ 4 ]. Ultrasound has also seenextensive use in effecting chemical reactions for syn-thetic purposes, principally in organic and organome-tallic chemistry [ 5 ]. However, despite its potentialutility, the use of ultrasonic energy for extractingheavy metals from solid environmental matrices hasbeen underutilized [ 6 ].

Ultrasonic energy, when applied to solutions,causes acoustic cavitation: that is, bubble formationand implosion. The collapse of bubbles formed byultrasonic energy results in the generation of

0165-9936/98/$19.00 ß 1998 Published by Elsevier Science B.V. All rights reserved.PII S 0 1 6 5 - 9 9 3 6 ( 9 8 ) 0 0 0 1 8 - 1

This article was prepared by US government employees aspart of their official duties, and therefore legally may not becopyrighted in the United States of America.Disclaimer: Mention of company names or products doesnot constitute endorsement by the Centers for DiseaseControl and Prevention.

366 trends in analytical chemistry, vol. 17, no. 6, 1998

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extremely high temperatures and pressures at the inter-face of the collapsing bubble and another phase, lead-ing to enhanced chemical reactivity [ 5,7 ]. The in£u-ence of extremely high effective temperatures andpressures at the interface between an aqueous solutionsubjected to ultrasonic energy and a solid matrix, com-bined with the oxidative energy of hydroxyl radicalsand hydrogen peroxide created during sonolysis ofwater, results in high extractive power. This power,elicited by sonication of environmental samplesimmersed in aqueous solutions, has been used to effectin the quantitative extraction of a number of heavymetal species. These species can be subsequentlydetermined by using appropriate analytical tech-niques.

In this article, some examples of the use of ultra-sound as a sample preparation method for analyticalchemistry purposes are given. It is hoped that thisreview will stimulate further work in this ¢eld. Thisarticle may also serve to advertise the utility of ultra-sonic energy for the purpose of heavy metals extrac-tion, for the subsequent analytical determination ofthese species.

2. Ultrasonic extraction for singleelement analysis

2.1. Determination of lead

In an early investigation of Long et al., UE at ambi-ent temperature was applied to the dissolution of leadfrom glass ¢ber ¢lter ambient air samples [ 8 ]. In thiswork, an interlaboratory evaluation of UE with atomicspectrometric determination of lead was conducted.The results from UE were compared against paireddata (using split samples ) from a reference methodinvolving a strong acid ashing sample preparation pro-cedure. It was shown in this work that the results fromusing UE were statistically equivalent to the dataobtained from using the reference method. As a resultof this and related studies, UE became an acceptedEPA method for the extraction of lead from glass¢ber ¢lters used in sample collection for ambient airanalysis [ 9 ].

Recently a number of studies have demonstrated theef¢cacy of UE for lead dissolution from a number ofenvironmental matrices, including paint, soil, dust,and workplace air ¢lter samples [ 10^16 ]. In oneinvestigation by Ashley, recoveries of lead were deter-mined from representative National Institute of Stand-ards and Technology (NIST) Standard Reference

Materials (SRMs) subjected to UE (at ambient tem-perature for 30 min) in diluted (10% v /v) nitric acid;portable anodic stripping voltammetry (ASV) wasused for lead determination in extracted samples[ 11]. Lead recoveries obtained from UE /ASV werecompared to the recoveries obtained by reference sam-ple preparation and analytical methods utilizing con-centrated acids with high heat and atomic spectromet-ric determination of lead [ 17 ]. One reference methodused for this study was National Institute for Occupa-tional Safety and Health (NIOSH) Method 7300 [ 18 ],which is a hot plate digestion employing concentratednitric acid and perchloric acid, followed by induc-tively coupled plasma atomic emission spectrometric( ICP-AES) lead determination. The results of thecomparison between the UE /ASV results and datafrom the reference method (NIOSH 7300) are shownin Table 1. The data of Table 1 show that the leadrecoveries from SRMs when using the UE /ASVmethod are statistically equivalent to those of the refer-ence method. In related work, lead in ultrasonicallyextracted NIST SRMs and other performance evalua-tion materials (again at ambient temperature, 30 minextraction time, using diluted nitric acid ) was deter-mined by using ICP-AES, and the recoveries werefound to be quantitative [ 16 ].

An evaluation of UE /ASV for the determination oflead in workplace air samples was performed by Ash-ley et al. [ 14 ]. Air ¢lter samples collected in the ¢eldusing cellulose ester membrane ¢lters were subjectedto UE in diluted nitric acid (10% v /v ) and ambienttemperature for 30 min, and the lead was subsequentlydetermined by portable ASV. The ultrasonic bath usedwas a 115 V, 60 Hz laboratory unit. For reference leadmeasurements, the same sample extracts (plus undis-solved ¢lter and particulate material ) were thendigested in strong nitric acid /30% hydrogen peroxideusing a hot plate, and the lead determined by ICP-AES. The results of this comparison for a portableASV instrument are shown in Fig. 1. The data arevery well correlated (R2 = 0.99), with the y-interceptnear zero and the slope close to unity. The estimatedlimit of detection (LOD) is in the order of a few micro-grams of lead per sample. This investigation shows theutility of UE /ASV for monitoring lead levels in work-place air. A new NIOSH method for workplace mon-itoring of lead in air, which is based on this procedure,has been submitted to the NIOSH Manual of Analyt-ical Methods [ 19 ].

Based on research demonstrating the utility of ultra-sonic extraction of lead from environmental referencematerials, an American Society for Testing and Mate-

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rials (ASTM) provisional standard has been approvedthat describes UE procedures for lead from paint, dust,soil and workplace air [ 20 ]. This provisional standardcan be used for preparation of samples in the ¢eld aswell as in the laboratory. Lead extracted from samplescan be determined by using atomic spectrometry [ 21],ASV [ 22 ], or other equivalent techniques that have

been published as national and international stand-ards.

2.2. Chromium speciation

UE has been shown to be useful for the extraction ofhexavalent chromium from environmental and indus-

Table 1Lead recoveries determined from representative NIST SRMs (data from [ 11,17 ])

SRMa Percent recovery ( þ standard deviation)

UE /ASVb Reference methodc

1579, lead-based paint 94.5 þ 5.7 (n = 11) 98.3 þ 5.2 (n = 6)1648, urban particulate 91.4 þ 3.7 (n = 12) 93.5 þ 4.9 (n = 7)2704, river sediment 82.4 þ 7.6 (n = 12) 82.1 þ 4.6 (n = 8)

aSRMs 1579, 1604, and 2704 are 11.83%, 0.655%, and 0.0161% by weight, respectively.bUltrasonic extraction followed by anodic stripping voltammetric (ASV) determination.cThe reference method employed was NIOSH Method 7300 [ 18 ], modi¢ed for solid ( bulk ) samples.

Fig. 1. Plot of results from 10% (v /v) nitric acid ultrasonic extraction /portable ASV determination of lead vs. data obtained byreference lead analysis method (concentrated nitric acid and 30% hydrogen peroxide hot plate digestion / ICP-AES), forworkplace air samples. The line through the graph indicates unity slope and zero intercept. From [ 14 ].

368 trends in analytical chemistry, vol. 17, no. 6, 1998

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trial hygiene samples [ 23,24 ]. In one investigation byJames et al., sonication (at 25³C for 30^45 min) of soilsamples spiked with Cr(VI ) and immersed in a basicextraction solution was found to give quantitativerecoveries of hexavalent chromium [ 23 ]. The per-formance of UE for dissolution of Cr(VI ) comparedfavorably with other, more severe sample preparationprocedures ( i.e., more traditional methods employingconcentrated acids and hot plate or microwave diges-tion).

Recently, a UE /£ow injection analysis (FIA) pro-cedure was evaluated by Wang et al. for the determi-nation of hexavalent and total chromium in samples of

industrial hygiene interest [ 24 ]. In this work, Cr(VI )was ultrasonically extracted ( in a 40^50³C bath for1 h) from spiked solid samples and performance eval-uation samples in a basic buffer solution of ammoniumsulfate and ammonium hydroxide. Both insoluble(PbCrO4 ) and soluble (K2CrO4 ) forms of chromatewere used for spiking. Hexavalent chromium was thenretained on a strong ion-exchange resin, and subse-quently separated as an anion from (cationic ) trivalentchromium and other metal cations by elution with pH8 ammonium sulfate / ammonium hydroxide buffer.The eluate was then acidi¢ed with HCl, and the chro-mium complexed with 1,5-diphenylcarbazide (DPC)

Fig. 2. Responses for FIA of various amounts of Cr(VI ) (ng), detected as the Cr^DPC complex by UV /VIS at 540 nm withMeOH^water (1:1) as the mobile phase. From [ 24 ].

Table 2Determination of hexavalent chromium in spiked cellulose ester ¢lters, spiked acid-washed sand, and performance evaluationmaterials (data from [ 24 ])

Samplea Cr(VI ) content, Wg /g ( þ % RSDb ) Recovery (%)

Spiked ¢lters, 40 Wg Cr(VI ) 37.9 ( þ 3.2) 95.6Spiked sand, 25 Wg Cr(VI ) 23.8 ( þ 4.5) 95.2Spiked sand, 25 Wg Cr(VI )+250 Wg Cr( III ) 23.3 ( þ 7.6) 93.8EPA CRMc 013-050 paint 54.4 ( þ 3.4) N /Ad

NIST SRMe 1633a £y ash 0.19 ( þ 3.9) N /A

aSamples were treated by ultrasonication in ammonium sulfate /ammonium hydroxide buffer, and were then subjected to anionexchange separation. Eluate was acidi¢ed and reacted with DPC, and the Cr^DPC complex determined by FIA using UV /VISdetection.bRelative standard deviation.cCerti¢ed reference material.dNo reference data for Cr(VI ) content available.eStandard reference material.

trends in analytical chemistry, vol. 17, no. 6, 1998 369

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prior to £ow injection analysis using UV /visibledetection.

Fig. 2 shows FIA responses for different concentra-tions of the Cr^DPC complex [ 24 ]. A linear regres-sion is obtained, and the method is highly sensitive,with a detection limit in the sub-nanogram range. Thismethod was used to determine chromium in samplesspiked with various concentrations of Cr(VI ) and sub-jected to the aforementioned sample preparation pro-cedure. Analytical results from spiked samples andperformance evaluation samples such as an EPA Cert-i¢ed Reference Material (CRM) paint and a NISTSRM coal £y ash are summarized in Table 2. Resultsfrom spiked sample matrices were quantitative, butCr(VI ) recoveries in performance evaluation materi-als cannot be determined since the reference values arenot known.

Total chromium was determined in the EPA CRMand the NIST SRM, along with real-world £y ash andpaint samples, by ultrasonication of the materials(again in a 40^50³C bath for 1 h) in a solution con-taining cerium(IV) [ 24 ]. Ce( IV) causes oxidation ofCr( III ) to Cr(VI ), enabling determination of totalchromium species in a given sample subjected toUE. After subsequent sample preparation steps ( ionexchange separation and complexation with DPC),chromium concentration was determined by FIAusing UV /VIS detection. The results from this treat-ment were compared to results obtained using a refer-ence method employing nitric acid hot plate digestion(Table 3). It can be seen from the data presented inTable 3 that the performance of ultrasonic extraction iscomparable to that of the reference sample preparationprocedure, which employs a much more severeapproach ( that is, concentrated nitric acid hot platedigestion ). A stepwise determination of ¢rst Cr(VI )and then total Cr allows for the determination of

Cr( III ) concentration ( if desired) by difference. Fur-ther experiments were conducted to verify that Cr(VI )was not being reduced to Cr( III ) during the extractionprocess [ 24 ].

3. Ultrasonic extraction formultielement analysis

One of the ¢rst applications of UE to heavy metalsdissolution for multielement analysis was performed

Table 4Recoveries of target elements from NIST SRM 1648 sub-jected to ultrasonic extraction for 50 min at 100³C in a mixtureof hydrochloric and nitric acids (data from [ 25 ])

Elementb Percent recovery ( þ % RSDa )

SRM No. 1648only

SRM No. 1648 spiked ontoglass ¢ber ¢lters

As 130 þ 2.2 (nc = 3) 96 þ 2.9 (n = 48)Ba 80 þ 0.8 (n = 3) 100 þ 3.7 (n = 15)Cd 114 þ 8.5 (n = 3) 78 þ 5.0 (n = 32)Co 96 þ 5.4 (n = 3) 96 þ 4.1 (n = 49)Cr 23 þ 1.4 (n = 3) NAd

Cu 100 þ 1.4 (n = 3) 69 þ 5.0 (n = 32)Fe 68 þ 1.4 (n = 3) 97 þ 4.3 (n = 49)Mn 88 þ 1.6 (n = 3) 94 þ 5.6 (n = 49)Ni 90 þ 9.0 (n = 3) 97 þ 4.0 (n = 49)Pb 95 þ 1.1 (n = 3) 99 þ 3.0 (n = 49)Sr NA 98 þ 5.9 (n = 32)Ti 12 þ 5.0 (n = 3) NAV 79 þ 1.9 (n = 3) 92 þ 6.3 (n = 49)Zn 97 þ 3.8 (n = 3) 88 þ 8.4 (n = 49)

aRelative standard deviation.bElements determined by ICP-AES.cNumber of replicate samples.dNot available.

Table 3Determination of total chromium in EPA CRM paint and NIST SRM coal £y ash with pre-oxidation and /or nitric acid digestionprocedures (data from [ 24 ])

Sample Chromium concentration, Wg /g ( þ % RSDa )

Oxidation with Ce( IV) Nitric acid digestion

EPA CRM 013-050 (paint )b 587.9 ( þ 4.5) 596 ( þ 3.8)NIST SRM 1633a (£y ash)c 185.0 ( þ 5.7) 192.0 ( þ 4.9)Coal £y ash samples 23.2 ( þ 6.3) 24.8 ( þ 6.7)Paint chips 358.2 ( þ 7.1) 361.1 ( þ 5.3)

aRelative standard deviation.bReference value = 617.6 ( þ 6.1) Wg /g.cReference value = 196 ( þ 3.1) Wg /g.

370 trends in analytical chemistry, vol. 17, no. 6, 1998

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by Harper et al. [ 25 ]. In this investigation, a NISTSRM was subjected to ultrasonic irradiation in acidmixtures at different bath temperatures and durations,following a simplex optimization procedure [ 26 ].SRM #1648, Urban Particulate, was subjected to UEin varying acid mixtures for various ultrasonic irradi-ation periods in order to optimize the extractability of asuite of target heavy metals. After optimization of theextraction procedure, the elements were determinedusing inductively coupled plasma atomic emissionspectrometry ( ICP-AES). Recoveries from ultrasonicextraction at elevated temperature for 1 h were foundto be quantitative for a number of heavy metals fromboth SRM No. 1648 by itself ( analyzed in a singlelaboratory ) and glass ¢ber ¢lters spiked withSRM 1648 (analyzed by multiple laboratories ); seeTable 4.

In a more recent study, Sanchez et al. investigatedthe use of UE for the analysis of the heavy metals Cd,Cr, Cu, Mn, Ni, Pb, and Zn in several European refer-ence materials [ 27 ]. UE in aqua regia, followed byatomic spectrometry, was used to determine theabove heavy metals in BCR CRM 146 (sewagesludge), and the values determined were all veryclose to the certi¢ed reference values. A modi¢ed sim-plex optimization procedure [ 28 ] was used to deter-mine optimum bath temperatures and sonication peri-ods. UE in a mixture of aqua regia and hydro£uoricacid, followed by atomic spectrometry, was used todetermine the same suite of heavy metals in BCRCRMs 320 and 141 ( river sediment and soil, respec-tively ). HF was used for these last two CRMs sincethese are known to contain high levels of silicates,which would be insoluble if HF were not used forextraction purposes. Again, the values obtained fromUE /atomic spectrometry were comparable to thereference values, thereby demonstrating the utility ofthis extraction technique.

Elsewhere, UE with hydro£uoric acid and nitricacid has been used by Demange et al. for the determi-nation of heavy metals in workplace air samples[ 29,30 ]. The acid mixture is added directly to the¢lter cassette ( that is, the ¢lter is not removed), andthe contents subjected to ultrasonic energy. Metals aresubsequently determined by ICP-AES. The procedureis used by the French Institut National de Re-cherche et de Securiteè ( INRS) for the determinationof metals in workplace air samples [ 31]. It is desiredby the INRS to determine metals in silicate materials,hence the use of hydro£uoric acid in the extractionmedium.

4. Conclusion

In this article, an overview has been given of theapplication of ultrasonic extraction (UE) for the pur-pose of dissolution of heavy metals for their subse-quent determination by appropriate analytical tech-niques. Numerous studies have demonstrated theusefulness of UE for extraction purposes for metalsdetermination, and several examples have been citedhere. It has been shown that UE of environmental andindustrial hygiene samples in diluted acids can be usedeffectively for single or multielement determinations.The performance of UE procedures has been demon-strated to be comparable to that of reference methodsemploying severe conditions ( i.e., high acid concen-trations and high heat or pressure ). The utility of UEfor ¢eld-based extractions has been put into use forindustrial hygiene single element analysis (of, forexample, lead and hexavalent chromium). Further-more, UE has been demonstrated to perform well forlaboratory-based sample preparation purposes formultielement determinations of heavy metals.

Some drawbacks of UE include: (a ) ¢nite lifetimesof ultrasonic probes [ 5 ]; ( b ) inapplicability to theextraction of certain metals [ 25 ], and (c ) and theinability to quantitatively extract heavy metals fromvery large bulk environmental samples [ 9,11]. But inview of the successes demonstrated, laboratory-basedUE of heavy metals from environmental and industrialhygiene samples may ( in many cases ) replace moresevere sample preparation procedures that require theuse of concentrated acids with the application of highheat and /or pressures ( that is, hot plate and /or micro-wave extraction). Additionally, in some instances, UEis an excellent tool for ¢eld-based sample preparationfor on-site heavy metal analysis.

Acknowledgements

Drs. Jeff Seeley (Procter and Gamble Co. ) and PeterEller (CDC /NIOSH) critically reviewed the draftmanuscript. Helpful discussions with MartineDemange ( INRS, France ) and Alan Howe (Healthand Safety Laboratory, UK) are gratefully acknowl-edged.

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trends in analytical chemistry, vol. 17, no. 6, 1998 371

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[ 10 ] US EPA, Standard operating procedure for the ¢eld anal-ysis of lead in paint, soil and dust by ultrasonic, aciddigestion and colorimetric measurement (EPA 600R-93/200), EPA Of¢ce of Research and Development(ORD), Research Triangle Park, NC, 1993.

[ 11] K. Ashley, Electroanalysis 7 (1995) 1189.[ 12 ] US EPA, Evaluation of the performance of re£ectance and

electrochemical technologies for the measurement of leadin characterized paints, bulk dusts, and soils (EPA 600/R-95-083). EPA /ORD, Research Triangle Park, NC, 1996.

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[ 21] ASTM E1613, in Annual Book of ASTM Standards, Vol.04.11. ASTM, West Conshohocken, PA, 1998.

[ 22 ] ASTM PS 88, in Annual Book of ASTM Standards, Vol.04.11. ASTM, West Conshohocken, PA, 1998.

[ 23 ] B.R. James, J.C. Petura, R.J. Vitale, G.R. Mussoline,Environ. Sci. Technol. 29 (1995) 2377.

[ 24 ] J. Wang, K. Ashley, E.R. Kennedy, C. Neumeister, Ana-lyst 122 (1997) 1307.

[ 25 ] S.L. Harper, J.F. Walling, D.M. Holland, L.J. Pranger,Anal. Chem. 55 (1983) 1553.

[ 26 ] S.N. Deming, S.L. Morgan, Anal. Chem. 45 (1973)278A.

[ 27 ] J. Sanchez, R. Garcia, E. Millan, Analusis 22 (1994) 222.[ 28 ] J.A. Nelder, R. Mead, Computer J. 7 (1965) 308.[ 29 ] M. Demange, J.C. Gendre, B. Herve-Bazin, B. Carton, A.

Peltier, Ann. Occup. Hyg. 34 (1990) 399.[ 30 ] M. Demange, B. Herve-Bazin, B. Carton, A. Boulet, J.M.

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[ 31] M. Demange, Chemical dissolution methods for analysisof sampled air particles (Doc. No. 297047 /MD), InstitutNational de Recherche et de Securiteè ( INRS), Vandoeu-vre, 1988.

Puri¢cation and characterization of plantecdysteroids of Silene speciesMaè ria Baè thoriDepartment of Pharmacognosy, Albert Szent-Gyoë rgyi Medical University,P.O. Box 121, H-6701 Szeged, Hungary

Methods and results of the isolation, analysisand structural elucidation of ecdysteroidsoccurring in plants of Silene species are sum-marized. Emphasis is placed on the chromato-graphic procedures used for preparativepuri¢cations and analyses of Silene ecdy-steroids, and a brief survey is given of the

other preparative and analytical methods.Experimental data from TLC and HPLC analy-ses are also given. z1998 Elsevier ScienceB.V.

Keywords: Ecdysteroids; Silene plant ecdysteroids; Dropletcounter-current chromatography

0165-9936/98/$19.00 ß 1998 Elsevier Science B.V. All rights reserved.PII S 0 1 6 5 - 9 9 3 6 ( 9 8 ) 0 0 0 2 1 - 1

372 trends in analytical chemistry, vol. 17, no. 6, 1998