Slurry Sampling—An Analytical Strategy for the Determination of Metals and Metalloids by...

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Slurry Sampling—An Analytical Strategy for the Determination of Metalsand Metalloids by Spectroanalytical TechniquesSergio Luis Costa Ferreira a; Manuel Miró b; Erik Galvão Paranhos da Silva c; Geraldo DominguesMatos a; Pedro Sanches dos Reis a; Geovani Cardoso Brandao a; Walter Nei Lopes dos Santos c; AlvaroTavares Duarte d; Maria Goreti Rodrigues Vale d; Rennan Geovanny Oliveira Araujo e

a Instituto de Química, Universidade Federal da Bahia, Salvador, Bahia, Brazil b Faculty of Sciences,Department of Chemistry, University of the Balearic Islands, Palma de Mallorca, Spain c Departamentode Ciências Exatas e Tecnológicas, Universidade do Estado da Bahia, Salvador, Bahia, Brazil d Institutode Química, Universidade Federal do Rio Grande de Sul, Porto Alegre, RS, Brazil e Departamento deQuímica, Universidade Federal de Sergipe, Aracajú, SE, Brazil

Online publication date: 14 January 2010

To cite this Article Ferreira, Sergio Luis Costa, Miró, Manuel, da Silva, Erik Galvão Paranhos, Matos, Geraldo Domingues,dos Reis, Pedro Sanches, Brandao, Geovani Cardoso, dos Santos, Walter Nei Lopes, Duarte, Alvaro Tavares, Vale, MariaGoreti Rodrigues and Araujo, Rennan Geovanny Oliveira(2010) 'Slurry Sampling—An Analytical Strategy for theDetermination of Metals and Metalloids by Spectroanalytical Techniques', Applied Spectroscopy Reviews, 45: 1, 44 — 62To link to this Article: DOI: 10.1080/05704920903435474URL: http://dx.doi.org/10.1080/05704920903435474

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Applied Spectroscopy Reviews, 45:44–62, 2010Copyright © Taylor & Francis Group, LLCISSN: 0570-4928 print / 1520-569X onlineDOI: 10.1080/05704920903435474

Slurry Sampling—An Analytical Strategyfor the Determination of Metals and Metalloids

by Spectroanalytical Techniques

SERGIO LUIS COSTA FERREIRA,1 MANUEL MIRO,2 ERIKGALVAO PARANHOS DA SILVA,3 GERALDO DOMINGUESMATOS,1 PEDRO SANCHES DOS REIS,1 GEOVANICARDOSO BRANDAO,1 WALTER NEI LOPES DOS SANTOS,3

ALVARO TAVARES DUARTE,4 MARIA GORETI RODRIGUESVALE,4 AND RENNAN GEOVANNY OLIVEIRA ARAUJO5

1Universidade Federal da Bahia, Instituto de Quımica, Salvador, Bahia, Brazil2University of the Balearic Islands, Faculty of Sciences, Department ofChemistry, Palma de Mallorca, Spain3Universidade do Estado da Bahia, Departamento de Ciencias Exatas eTecnologicas, Salvador, Bahia, Brazil4Universidade Federal do Rio Grande de Sul, Instituto de Quımica, Porto Alegre,RS, Brazil5Universidade Federal de Sergipe, Departamento de Quımica, Aracaju, SE, Brazil

Abstract: This article critically overviews the state-of-the-art of slurry sampling as anapproach for the minimization of sample preparation prior to the determination of metalsand metalloids in complex matrices by spectroanalytical techniques. Relevant factorsinvolved in the optimization of slurry-based analytical procedures and the dependenceof the quality of the results on the calibration method selected are discussed in detail.The advantages and limitations compared to solid sampling for the analysis of solidmatrices are highlighted and discussed.

Analytical applications of slurry sampling reported in the literature emphasizingpublications between 2004 and 2009 are comprehensively compiled covering detectionby flame atomic absorption spectrometry (FAAS), electrothermal atomic absorptionspectrometry (ET-AAS), cold vapor atomic absorption spectrometry (CV-AAS), hydridegeneration atomic absorption spectrometry (HG-AAS), hydride generation atomic flu-orescence spectrometry (HG-AFS), inductively coupled plasma optical emission spec-trometry (ICP-OES), and inductively coupled plasma mass spectrometry (ICP-MS).

Keywords: Slurry sampling, sample preparation, complex samples, solid sampling,spectroanalytical techniques

Address correspondence to Sergio L.C. Ferreira, Universidade Federal da Bahia, Instituto deQuımica, 40170-290, Salvador, Bahia, Brazil. E-mail: [email protected]; or Manuel Miro, Department ofChemistry, University of the Balearic Islands, Carretera de Valldemossa Km 7, 5 E-07122 Palma deMallorca, Spain. E-mail: [email protected]

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Slurry Sampling 45

Introduction

The determination of mineral species in environmental and biological matrices currentlyposes few problems to the analytical chemist because a vast number of selective, pre-cise, sensitive, and accurate analytical methods are available (1–3). Sample preparation ishowever still the Achilles’ heel of any analytical process for assurance of traceability ofanalytical results (4–8) and involves a given number of steps to present the sample in anappropriate manner to the detector.

If total sample decomposition processes are employed, this gives improved homo-geneity for analyte distribution in an aqueous sample. However, researchers have givenparticular attention to the simplification of sample preparation procedures. The focus insimplifying the analytical process is the development of alternative methods in order toavoid/minimize processes related to total sample decomposition. In the determination oftrace elements in solid substrates, efforts have been directed to circumvent mineralizationprotocols based on dry ashing and ultrasound or microwave-assisted wet chemical diges-tion methods. Two analytical approaches, the so-called slurry sampling and direct solidsampling combined with either AAS (9–12) or inductively coupled plasma (ICP)-basedtechniques (9, 13, 14) have significantly decreased the number of preliminary operationsin the analytical process for the handling of solid samples. In this context, these processesappear as good alternatives for sample pretreatment.

Slurries are solid dispersions in a liquid phase that can be transported as solutions,enabling the direct determination of analyte, reducing the time required for analysis andminimizing the risks of contamination by circumventing sample decomposition with wetor dry oxidation procedures. The smaller the volume of suspension, the greater the enrich-ment factor attained. Therefore, ultratrace concentrations can be detected. Thus, to obtainhomogeneous and stable slurries, which influence directly accuracy and precision, manyexperimental parameters such as particle size, solid mass to total slurry volume, additionof stabilizing regeants, etc., should be optimized. In fact, several problems including cal-ibration difficulties, weighing errors, sample inhomogeneity, etc., may become important,unless certain rules are strictly fulfilled. The direct analysis of solids as slurries offers ad-vantages over more conventional sample preparation procedures. Among these advantagesare the shorter sample preparation time, reduced sample contamination risk, increased sen-sitivity (less dilution), lower analyte loss through volatilization prior to analysis, and thepossibility of selective analysis of micro-amounts of solids (15).

For the determination of trace elements in harsh aqueous matrices, the developmentof on-line sample pretreatment procedures exploiting flow injection analysis, and relatedapproaches have opened new avenues regarding automation and miniaturization of samplehandling. The flow systems are entirely enclosed, thereby preventing sample contamina-tion and analyte losses, with the added advantage compared to manual procedures of adecreased sample and reagent consumption and subsequent minimization of waste genera-tion. Readers are referred to the monograph by Miro and Hansen (16) and review articles(17–19) for a critical evaluation of a vast number of on-line sample processing meth-ods, viz. liquid–liquid extraction, solid–liquid extraction, (co)precipitation, chemical vaporgeneration (CVG), membrane-based extraction, and sample digestion/mineralization forsimplification of sample processing, prior to determination by atomic absorption spectrom-etry (AAS), inductively coupled plasma optical emission spectrometry (ICP-OES), or ICPmass spectrometry (ICP-MS).

The present article overviews significant contributions in the field of slurry sampling forminimization of sample decomposition prior to determination of trace metals and metalloids


46 S. L. Costa Ferreira et al.

using detection by flame AAS (FAAS), electrothermal (ET) AAS, CVG-AAS, CVG atomicfluorescence spectrometry (CVG-AFS), ICP-OES, or ICP-MS. Critical variables to bethoroughly investigated for proper performance of slurry-based procedures are commentedon in detail along with the most appropriate calibration methods for accurate quantification.Also described are relevant applications of slurry sampling for trace element determinationreported in recent years (emphasizing between 2004 and 2009), with particular referenceto environmental, geological, food, and biological matrices. Whereas direct solid samplinghas been the subject of several recent reviews (20, 21), no comprehensive review articlecovering the coupling of slurry sampling to the overall spectroanalytical techniques hasbeen reported to date to the best of our knowledge.

Slurry Sampling

Several procedures for minimization or avoidance of the processing of solid samples havebeen established. Slurry sampling involves the handling of the sample as a finely dividedsolid suspension (15) and features several advantages over the classical procedures of prepa-ration of solid samples it (1) minimizes the risk of sample contamination; (2) eliminates orreduces to a large extent loss of target analytes that can be eventually volatilized during thepretreatment step; (3) reduces the use of hazardous or corrosive reagents; (4) can be equallyapplied to the determination of organic and inorganic samples; (5) is readily applicable tothe determination of volatile elements following chemical vapor generation approaches; (6)fosters speciation analysis (as opposed to direct solid sampling) with appropriate derivatiza-tion reactions or separation procedures; and finally, (7) is readily automated or mechanizedvia flow-based approaches (9, 14).

Considering the aforementioned advantages, slurry sampling has been the subject ofintense research for trace element determinations using FAAS, ET-AAS, cold vapor AAS(CV-AAS), hydride generation AAS (HG-AAS), HG-AFS, ICP-OES, and ICP-MS, asreviewed in the following sections. It should, however, be borne in mind that the accuracyof analytical methods involving slurry sampling is closely related to the homogeneity ofthe sample analyzed. This should be regarded as the major bottleneck of this techniquebecause the sample amount slurried may not be representative of the actual composition ofthe bulk sample.

Critical Factors in Slurry Preparation

A number of chemical and physical factors should be thoroughly investigated when opti-mizing slurry-based methods as pinpointed in the following.

Grinding Methods

There are several grinding methods available in the literature. The appropriate choice de-pends directly on the sample matrix, the analyte itself, grinding time, and the analyticaldetection technique. In this context, the bottle and bead method, micronizing mill, mixingmill, puck-type grinder, grinding mill, and vibration pot mill are worth mentioning. Ap-plications, advantages, and limitations of the overall grinding procedures are detailed inearlier review papers (9, 12). The grinding is necessary to obtain low particle size andto ensure that whenever the analyte is not homogeneously distributed into the solid. This


Slurry Sampling 47

implies an increase in slurry homogeneity; consequently, the aliquot will be more rep-resentative. The main disadvantages are the likelihood of contamination and an increasein the turnaround time. Beside, when very small particles are weighed problems of staticelectricity can be observed (22, 23). An alternative method, not discussed in earlier reviews(9, 12), is the cryogenic grinding (brittle fracture) technique, which is performed at lowtemperature with frozen samples. This technique, which has not been widely accepted,despite the fact that good sample homogenization may be accomplished, was establishedby Iyengar (24), and its first application for slurry sampling was proposed by Mierzwa etal. (25). A comparative determination of some metals in tea leaf samples by ET-AAS andICP-OES employing slurry sampling was performed. The accuracy was checked by use ofcertified reference material and good results were obtained at the 95% confidence level.Methods using cryogenic grinding for slurry sampling have been proposed for analysis ofseeds and plant reference materials (26), hair (27), food (28), and human teeth (29), allusing ET-AAS as the analytical technique. A method for analysis of seafood samples hasbeen recently proposed using cryogenic grinding and FAAS, where particle sizes below 84µm were injected into the instrument nebulizer (30).

Diluents

Chemical diluents are crucial components of the slurry because of their dependence uponthe stability of the solid sample. Diluents might work as extractants as well and thus improvethe accuracy and precision of the analytical process by transfer of target elements into theliquid phase. Selection of the diluent is commonly done on the basis of the sample matrixand analyte characteristics. The most frequently exploited diluent is nitric acid, whichconcomitantly assists in analyte extraction (9, 12). Miller-Ihli (23) reported that 75–90%of lead was extracted into the liquid phase of sediment slurries, and that the precisionapproached that obtainable with liquid digests when a high percentage of the analyte wasin the liquid phase.

Some authors reported the combination of nitric and hydrochloric acid or hydrogenperoxide and hydrochloric acid as suitable alternatives. Vinas et al. (31) proposed a pro-cedure for cadmium, lead, and selenium determination in baby food samples by ET-AAS.In this procedure, suspensions were prepared in a medium containing 0.1% (w/v) TritonX-100, 30% (v/v) concentrated hydrogen peroxide, 1% (v/v) concentrated nitric acid, and amatrix modifier (0.5% (w/v) nickel for selenium, 0.2% (w/v) nickel plus 1% (w/v) ammo-nium dihydrogenphosphate for cadmium, and 1% (w/v) ammonium dihydrogenphosphatefor lead).

A procedure for the determination of phosphorus in honey, milk, and infant formulasusing slurried samples by ET-AAS was proposed by Lopez-Garcıa et al. (32). The suspen-sions were prepared in a medium containing 50% (v/v) concentrated hydrogen peroxide,1% (v/v) concentrated nitric acid, 10% (m/v) glucose, 5% (m/v) sucrose, and 100 mg L−1

of potassium.Alkaline solutions are not usually employed as diluents but some papers mentioned

the use of alkyl ammonium hydroxides as solubilizers for biological matrices. Tan andMarshall (33) used tetramethylammonium hydroxide to investigate selenium residues fromzoological and botanical matrices prior to slurry introduction to ET-AAS. The same diluentwas used by Sola-Larranaga and Navarro-Blasco (34) for minerals and trace elementdeterminations in infant formula by ICP-OES and FAAS.



Stabilizing Reagents

The aim of stabilizing chemicals is to disperse agglomerates and/or prevent sedimentationof particles. A slurry can be stabilized using a highly viscous liquid medium or surfactantentities. Rahman et al. (35) employed ultrasonic slurry sampling ET-AAS with a metal tubeatomizer for the determination of lead in fish samples. In this work various parameters wereevaluated, among them a slurry stabilizing agent. Some authors proposed the addition of afew drops of antifoaming agent to the sample suspension to minimize Triton X-100 foamas recommended by Bermejo-Barrera et al. (36) for lead determination in mussel samplesby ET-AAS. Ethanol, KO300G, the nonionic surfactant Triton X-100, and glycerol havebeen the most frequent choices as stabilizing agents for slurries, as can be observed inTable 1.

Sample Mass–to-Diluent Volume Ratio

The solid sample amount and diluent volume should be selected taking into account bothsample homogeneity and concentration level of analyte in the sample. For samples con-taining high concentrations of elements (e.g., contaminated soils or sediments) the samplemass–to-diluent volume ratio can be set to low values that most likely lead to optimumanalytical properties (e.g., accuracy, precision, and ruggedness). Yet increased ratios areneeded for trace element determinations. It is important to state that slurries with identicalsample mass–to-slurry volume ratio but different diluent volumes might provide completelydifferent analytical results. This is a consequence of the dependence of the measured vol-ume of diluent upon the optimal slurry homogenization time. To affix a suitable ratio for agiven analytical method, a linear relationship of the amount of sample slurried against theanalytical signal should be expected.

The mass of the weighed portion of the sample depends on the total content of met-als. Baralkiewicz (37) optimized the sample weights for slurry preparation testing dif-ferent masses and in four different liquid media for lead determination in lake sedimentsamples.

Table 1Stabilizing agents used in slurry sampling

Analyte Sample Stabilizing agent Technique References

Pb Water Triton X-100 ET-AAS (73)Cd Wheat flour Triton X-100 ET-AAS (72)Mn and Cu Various

samplesTriton X-100 ET-AAS (74)

Si, Ca, Mg, Fe, Al,Mn, and S

Cement,gypsum andbasic slag

Glycerol ICP-OES (75)

Al, Ca, Fe, Mg, S, Si,Ti Ba, Cr, Mn, Ni,Sr, V, Zn, and Zr

Coal Glicerol andTriton X-100

ICP-OES (76)

V Soil KO300G ET-AAS (37)


Slurry Sampling 49

Particle Size

Particle size greatly influences the transport and atomization efficiencies of the analytecontaining slurry in ICP and FAAS. Knowledge of particle size distribution is fundamentalto assure slurry homogeneity and stability over the sampling time. Variables to be accountedfor in the exploration of this parameter are the slurry homogenization system, sample nature,sample homogeneity (with respect to the analyte), sample density, preparation time of theslurry, and analytical technique employed. For example, particle size is not an importantparameter for ET-AAS, but it is a critical factor in FAAS because of the risk of clogging ofthe nebulizer capillary. Methods employing slurry sampling and FAAS have been frequentlyapplied to relatively homogeneous samples with particle dimensions < 30 µm (38, 39).In ICP-based techniques the upper limit in particle size depends primarily on the typeof nebulizer. V-groove Babington-type nebulizers are the most popular for use in slurrysampling and have been applied to detection by ICP-OES and ICP-MS (40, 41). For systemscoupling electrothermal vaporization (ETV) with ICP-based detection the effect of particlesize is identical to that described for ET-AAS.

Analyte Partitioning

The efficiency of the extraction process occurring within the time frame of slurry prepa-ration influences the precision and accuracy of the method. The extraction yield for agiven analyte might be influenced by the sample matrix, the analyte nature, the bindingstrength between element and matrix, the solid particle size, the type and concentrationof diluent, the homogenization efficiency, and the exposure time to the diluent. These fac-tors are crucial in the assessment of calibration techniques for use in FAAS or ICP-basedslurry sampling techniques. For instance, the standard calibration technique using aqueousstandard solutions has been proven suitable for determination of trace elements in seafoodsamples when extraction efficiencies were >70% (28).

Slurry Homogenization System

There are a vast number of alternatives reported for suspension homogenization, all hav-ing advantages and limitations. The most simple and cost-effective approach is manualshaking. This is only considered to be efficient for low-density materials in the presenceof a stabilizing agent. Mechanical agitation employing magnetic stirring or vortex mixingis another option. It is suitable for large slurry volumes and features easy instrumentaloperation, wide availability, and low cost when compared with instrumental counterparts.Magnetic stirring is inappropriate for agitation of ferrous matrices. In this case vortex mix-ing can be used satisfactorily. The so-called gas bubbling approach is a simple alternativeprior to ET-AAS analyses. An argon stream is directly applied to the autosampler vials forcontinuous homogenization of the suspensions to be analyzed.

Application of ultrasonication as an external energy source via ultrasonic bath or ultra-sonic probe has gained momentum over the past few years for breaking up of agglomerates(42, 43). Ultrasonic baths are less expensive but lack high sonication power compared toprobes. Ultrasonic probes are incomparably superior to baths because slurry preparationand agitation can be effected directly in individual ET-AAS autosampler cups, as a conse-quence of their minute dimensions. In addition, probes can be used for handling both low-and high-density solid materials. Further, the acoustic cavitation effect of probes reducesthe particle size to a large extent and improves analyte extractability compared to bath



sonication. Use of hydrofluoric acid as a diluent is incompatible with titanium probesbecause of accelerated probe corrosion (42).

Calibration Techniques

An important issue for appropriate validation of slurry sampling–based analytical methodsis the evaluation of calibration strategies. Quantification of target elements using spectro-analytical techniques may differ from one aggregation state of the matrix to another. Thus,calibration procedures for aqueous solutions cannot be directly extrapolated to slurries. Thecalibration model should be, in the latter case, investigated for each individual analysis.

External Calibration with Aqueous Standards

The determination of target analytes in slurries against aqueous standard solutions wouldgreatly simplify the analytical procedure. However, this calibration technique is solelyapplicable when no statistically significant differences are found between the slope ofthe calibration curve using aqueous standard solutions and that of the analyte additiontechnique established using slurry samples spiked with analyte concentrations within thelinear dynamic range. This calibration model is particularly useful when coupling slurrysampling with ET-AAS because the preliminary pyrolysis step in the temperature programaimed at removing matrix constituents renders virtually identical atomization processes forelements regardless of the aggregate state of the sample (44). Likewise, calibration modelsfor ETV-ICP techniques generally involve aqueous standards as described by de Loos-Vollebregt and coworkers in a recent comprehensive review article (11). The application ofthis calibration method for FAAS and ICP techniques can be easily employed but dependson several factors as described in the literature (45).

Lopez-Garcıa et al. (32) reported a procedure for determination of phosphorus infoodstuffs by ET-AAS. The samples were directly introduced into the atomizer as slurriesin the presence of hydrogen peroxide. Quantification was made using aqueous standardsprepared in the same suspension medium.

Analyte Addition and Addition Calibration Techniques

Biased results are to be expected when using aqueous standards in those cases wheresignificant differences between the slopes of the external calibration and the analyte additiontechnique are encountered. It has been demonstrated that if the matrix effect is consistentfor a given number of samples, an average slope can be estimated from the results of theanalyte addition technique and employed for the analysis of samples of similar nature.This method is called addition calibration (9). On the other hand, if matrix effects divergebetween individual samples, the slopes will be significantly different, and thus this methodis inapplicable. This is the case of slurried samples with significantly different concentrationlevels of suspended particles (46). Hence, the analyte addition technique has to be used forall samples despite being tedious and time consuming (47).

Internal Reference Technique

Two eventual shortcomings of slurry sampling compared to the analysis of digested samplesare the decrease of transport efficiencies and the incomplete atomization of the target ana-lyte. To tackle these potential drawbacks, internal references have been added to the sample


Slurry Sampling 51

prior to multielemental analysis via ICP techniques, fast-sequential FAAS, or continuumsource AAS. It should be taken into account that the success of this approach relies uponthe similarity in behavior between the analyte and the internal reference. For example,a slurry sampling method has been proposed for the determination of arsenic, mercury,antimony, selenium, and tin in sediments by CVG-ICP-OES using germanium as a vaporforming internal reference (48). Isotope dilution mass spectrometric methods are valuablealternatives for accurate quantification of target metal species.

Certified Reference Materials for Calibration/Validation

Several researchers have analyzed slurries against sample-matched matrixes employingcertified reference materials (CRM) (25, 49–53). This calibration mode can be applied intwo different ways: (1) use of several CRMs of identical/comparable nature with variableanalyte concentrations and (2) use of different masses of a single CRM. For example, thequantification of zinc in chocolate by slurry sampling FAAS was successfully accomplishedwith a CRM of rice flour (54). Some examples of CRMs used for investigation of methodaccuracy on the basis of the nature of samples analyzed are given below.

Tseng et al. (49) applied slurry sampling electrothermal vaporization dynamic reactioncell inductively coupled plasma–mass spectrometry (ETV-DRC-ICP-MS) to determineiron, cobalt, nickel, copper, and zinc in biological samples. This method was validatedusing NIST SRM 1573a tomato leaves reference material and NRCC DORM-2 dogfishmuscle reference material and applied to the analysis of tea and swordfish samples. Theauthors reported precision between sample replicates better than 6% for all determinations.

Afridi et al. (50) developed a method based on ultrasonic-assisted acid slurry for thedetermination of cadmium and lead by ET-AAS in biological samples (blood and scalphair) and the validation was made using certified materials BCR 397 human hair and BCR185R bovine liver. Burylin et al. (51) used certified reference samples of the composition ofmarine algae (laminarias) VMl-01 8243-2003 and ground wheat grain ZPM-01 244-2003for validation of a procedure for determination of cadmium and lead in slurry sampling ofcarbonized samples.

An electrothermal atomic absorption spectrometric procedure for zinc determinationin animal tissues was proposed by Munoz-Delgado et al. (52). To check the reliability, threestandard reference materials (SRMs) were used: bovine muscle (SRM 8414), bovine liver(SRM 1577b), and dogfish liver (DOLT-2).

A paper published by Potgieter and Maljanovic (53) described the analysis of thechloride content in various South African cements and cementitious materials by ICP-OES. In this procedure, the samples were introduced into the plasma as slurries and thecalibration was performed by using aqueous solutions of reference materials.

A procedure for the metals determination in the edible parts of freshwater fish wasproposed by Arain et al. (55), whose accuracy was evaluated by analysis of BCR 185Rbovine liver and compared with conventional wet acid digestion methodology.

Surrogate Matrix for Calibration

An alternative model utilized for reliable construction of the calibration curve when noCRMs are available involves the preparation of standard slurries in a synthetic matrix ofphysicochemical composition as close as possible to that of the real sample to be analyzed.Prior to standard preparation, the synthetic matrix should be made free from analyte. Forexample, in the development of a slurry-based method for determination of manganese



in wheat flour by FAAS (38), the authors observed that the calibration curve could beestablished with spiked slurries made of commercial rice flour as a surrogate samplematrix.

Slurry Sampling and Spectroanalytical Techniques

Slurry sampling is a sample processing/injection technique that can virtually be hyphenatedto the overall spectroanalytical techniques. Slurry sampling procedures coupled to ET-AASare well consolidated (9), yet attempts to introduce suspensions in FAAS (38, 56) (seeTable 2 for further details), ICP-OES (57–59), ICP-MS (60), and CVG-AAS/AFS (61–63)have been reported over the past few years. The success of the slurry-ET-AAS marriage isa consequence of the integration of an electrothermal vaporization (pyrolysis) step withinthe analytical measurement. This facilitates the removal of matrix components prior to theatomization stage. In addition, the particle size does not pose problems for ET-AAS asmentioned earlier. Table 3 compiles recent analytical methods involving slurry samplingcombined with ET-AAS for determination of trace elements in complex matrices.

The hyphenation of electrothermal vaporization (ETV) with ICP-OES and ICP-MSopened new avenues for handling slurries while maintaining the sensitivity inherent toICP-based techniques. Table 4 lists relevant research dealing with slurry sampling-ICP-based detection. Readers are referred to the recent review by Resano et al. (11) for acritical comparison of ET-AAS, ETV-ICP-MS, and ETV-ICP-OES, in terms of sensitivity,instrumental cost, multielement detection capability, matrix interferences, and calibrationprocedures when applied to solid sample analyses.

The use of slurry sampling in the determination of volatile elements is of particularrelevance to overcome analyte losses frequently encountered with conventional batch-wisewet-chemical mineralization procedures (12). The design of entirely enclosed flow-basedsetups where unit operations (e.g., gas–liquid separations) have been optimized and readilyimplemented into the flow network has been the driving force in exploitation of slurrysampling with AAS-, AFS-, or ICP-based techniques aimed at determination of volatilespecies without introduction of suspensions into the detection device (46, 47, 61, 64).The reproducible and precise timing for the manipulation of slurried samples and reagentzones in flow setups facilitated the development of automated and cost-effective elementalspeciation analysis with no need for chromatographic separation (62, 65–67). It should bestressed that in these particular applications, the analytes should be completely extractedinto the aqueous phase during the slurry preparation and handling for accurate quantificationof metal and metalloid species.

In Table 5, the analytical features of recent methods exploiting slurry sampling fordetermination of volatile species of metal and metalloid species are shown (46, 68–71).

Slurry Sampling Versus Direct Solid Sampling

Direct solid sampling methods involving the introduction of the solid sample into thegraphite tube of the ET-AAS atomizer have been proposed as an alternative to slurrysampling for the acceleration and simplification of analytical procedures of solid samples(10, 11, 20, 21). However, both techniques are not free from drawbacks resulting fromhigh background matrix absorption, carbon deposition into the tube, inaccurate calibration,irreproducibility, and sample inhomogeneity.

In a recent publication, Araujo et al. (72) critically compared the analytical performanceof slurry sampling and direct solid sampling for the determination of cadmium in wheat


Tabl

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appl

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ofsl

urry

sam

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elem

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ratio

n(%

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Cu,

Mn,

FeSe

afoo

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3/H

Cl(

1m

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Ext

erna

lcal

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tion

<3.

8(3

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nW

heat

flour

HN

O3

(2m

olL

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Ext

erna

lcal

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tion

<3.

5(3

8)M

n,Z

nC

hoco

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HN

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(2m

olL

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CR

Mri

ceflo

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(39)

Cu,

Fe,M

n,Z

nK

rill

HN

O3

(2–4

mol

L−1

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xter

nalc

alib

ratio

n<

5(7

7)C

u,Z

n,Pb

Riv

erse

dim

ent

HN

O3

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NH

4C

l(2%

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nalc

alib

ratio

n<

5(4

5)C

uC

hoco

late

HC

l(2

mol

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xter

nalc

alib

ratio

n<

2.5

(78)

Zn,

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Hum

anha

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3(2

mol

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ratio

n<

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53


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Cd,

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frui

tsan

dfr

uitt

eas

0.2%

CTA

Can

d3%

HN

O3

NR

Stan

dard

addi

tion

met

hod

(79)

Mn,

Ni

Lak

ean

dm

arin

ese

dim

ent

3%(v

/v)

HN

O3

and

10%

(v/v

)H

2O

2

<7.

0fo

rM

nan

d<

8.8

for

Ni

Ext

erna

lcal

ibra

tion

(80)

PdY

east

Sacc

haro

myc

es0.

3m

olL

−1th

iour

ea-1

mol

L−1

HC

l-0.

5%T

rito

nX

-100

6.5

Stan

dard

addi

tion

met

hod

(81)

InSo

il1%

(v/v

)H

NO

3an

d10

%(v

/v)

HF

2.8

Ext

erna

lcal

ibra

tion

(82)

Cd

Whe

atflo

ur0.

014

mol

L−1

HN

O3+

0.1%

(v/v

)H

2O

2

9–23

Ext

erna

lcal

ibra

tion

(72)

PbSe

awat

eran

dw

aste

wat

er1.

0%T

rito

nX

-100

<10

Ext

erna

lcal

ibra

tion

(73)

Sn,P

bSe

dim

ent

7%(v

/v)

HN

O3-0

.02%

(v/v

)T

rito

nX

-100

(for

Pb);

10%

(v/v

)H

F-1%

(v/v

)H

NO

3

(for

Sn)

1–8

for

Pb;

3–16

for

SnE

xter

nalc

alib

ratio

nfo

rPb

and

stan

dard

addi

tion

met

hod

for

Sn

(83)

NR

=no

trep

orte

d,C

TAC

=ce

tyltr

imet

hyla

mm

oniu

mch

lori

de.

54


Tabl

e4

Rec

enta

pplic

atio

nsof

slur

rysa

mpl

ing

for

trac

eel

emen

tdet

erm

inat

ion

bydi

rect

inje

ctio

nin

toIC

P-ba

sed

dete

ctio

nte

chni

ques

Det

ectio

nPr

ecis

ion

Ana

lyte

Sam

ple

tech

niqu

eD

iluen

t(%

)C

alib

ratio

nR

efer

ence

Cu,

Fe,M

g,M

n,an

dZ

nB

ovin

eliv

erIC

P-O

ES

2.0

mol

L−1

HN

O3

—E

xter

nalc

alib

ratio

n(5

7)

Ca,

Mg,

Mn,

Fe,

Cr,

Al,

Ag,

Ba,

Bi,

Cd,

Co,

Cu,

Ga,

In,N

i,Z

n,A

s,an

dSe

Mul

tivita

min

form

ulat

ions

ICP-

OE

S0.

8M

HN

O3

0.4–

2.9

Stan

dard

addi

tion

calib

ratio

n(5

8)

Na,

K,C

a,M

g,S,

Fe,M

n,C

u,an

dZ

n

Whe

atflo

uran

dflo

ur-b

ased

food

sIC

P-O

ES

0.1%

(w/v

)T

rito

nX

-100

and

6%(v

/v)

HN

O3

5–10

Ext

erna

lcal

ibra

tion

(59)

As,

Cd,

Cr,

Cu,

Ni,

Pb,a

ndZ

nSu

rfac

ew

ater

sw

ithsu

spen

ded

solid

sIC

P-M

S1%

(v/v

)H

NO

310

–15

Inte

rnal

stan

dard

(115 In

)(6

0)

55


Tabl

e5

Rec

enta

pplic

atio

nsof

slur

rysa

mpl

ing

for

trac

em

etal

and

met

allo

idde

term

inat

ion

follo

win

gfo

rmat

ion

ofvo

latil

esp

ecie

s

Det

ectio

nPr

ecis

ion

Ana

lyte

Sam

ple

tech

niqu

eD

iluen

t(%

)C

alib

ratio

nR

efer

ence

As,

Sb,S

e,Te

,an

dB

iM

ilkH

G-A

FSA

qua

regi

a<

6.4

Ext

erna

lcal

ibra

tion

(47)

As

Sedi

men

tH

G-A

AS

Aqu

are

gia

and

HF

<13

Ext

erna

lcal

ibra

tion

(84)

As

Soil

HG

-AFS

HC

l2.

1E

xter

nalc

alib

ratio

n(8

5)A

s,Sb

,and

SeC

oalfl

yas

hH

G-A

FSH

Cl

<8.

0—

(86)

Cd

Lea

ves

HG

-AA

SH

Cla

ndH

2O

25.

7E

xter

nalc

alib

ratio

n(6

3)A

s,H

g,Sb

,Se,

and

SnB

iolo

gica

land

envi

ronm

enta

lC

VG

-MIP

-OE

SH

Cla

ndde

cano

l<

12St

anda

rdad

ditio

nm

etho

d(4

6)

Hg

and

SeB

iolo

gica

lIC

P-O

ES

Var

ious

<19

Ext

erna

lcal

ibra

tion

(48)

PbSe

dim

enta

ndse

wag

esl

udge

HG

-IC

P-O

ES

Aqu

are

gia

<15

Ext

erna

lcal

ibra

tion

(69)

As,

Ge,

Hg,

Pb,

Sb,S

e,an

dSn

Coa

lC

VG

-ET

V-I

CP-

MS

Aqu

are

gia

and

HC

l<

14E

xter

nala

ndis

otop

icdi

lutio

nca

libra

tion

(70)

Hg

Geo

logi

cals

ampl

esC

VA

AS

Form

icac

id2.

6E

xter

nalc

alib

ratio

n(7

1)

56


Slurry Sampling 57

flour. It was proposed that both methods were simple, faster, and more sensitive thanwet digestion procedures and were both suitable for the routine screening of cadmium infoodstuffs. Direct solid sampling methods showed improved sensitivity and repeatabilityover slurry sampling because of the avoidance of analyte dilution. However, dedicatedautomatic systems are needed for reliable injection of minute amounts of solids into theatomizer. Solid sampling essentially uses no reagents, as opposed to slurry sampling, whereacid solutions are frequently employed as diluents. Another distinct advantage of directsolid sampling is that samples of different texture can be readily analyzed without furtherhindrance. The analytical properties of slurry sampling methods are dependent to a largeextent on the size and density of the particles to be analyzed.

Conclusions

The fundamentals and recent analytical applications of slurry sampling methods for deter-mination of trace elements in solid samples by atomic absorption or emission spectrometryhave been reviewed. Several schemes for slurry preparation and presentation of the sampleto the detector and calibration methods for analyte quantification have been discussed indetail. The use of slurries either in flow systems or directly injected into the atomic spectrom-eter expedites the analytical process compared with wet-chemical digestion counterparts,which are prone to sample contamination and analyte losses because of the requirement ofserial manual operations. Moreover, introduction of slurried samples into atomic spectrom-eters, particularly ET-AAS, is fairly straightforward with no need for dedicated interfaces,which are mandatory in approaches involving direct solid sampling. The major hindrancein the development of slurry-based methods is the preparation of stable suspensions withinthe time frame of the assays whereby experimental variables including particle size dis-tribution, sample-to-diluent ratio, chemical composition and volume of diluent, type ofdispersant, and slurry homogenization mode need to be thoroughly investigated for propermethod performance.

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Slurry Sampling 61

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