Microchemical Journal 98 (2011) 129–134

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Microchemical Journal

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Tartaric acid extraction of organotin compounds from sediment samples

Marcos Flores a,⁎, Manuel Bravo b, Hugo Pinochet b, Paulette Maxwell c, Zoltán Mester c

a Departamento de Ciencias Básicas, Universidad Santo Tomas–Talca, Avenida Carlos Schorr 255, Talca, Chileb Laboratorio de Química Analítica y Ambiental, Instituto de Química, Pontificia Universidad Católica de Valparaíso, Avenida Brasil 2950 Valparaíso, Chilec Institute for National Measurement Standard, National Research Council Canada, Ottawa, Ontario, Canada K1A 0R6

⁎ Corresponding author. Tel.: +56 71 342418.E-mail address: marcosflores@santotomas.cl (M. Flo

0026-265X/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.microc.2010.12.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 10 December 2010Accepted 14 December 2010Available online 29 December 2010

Keywords:Focused microwave extractionButyltin compoundsSulfur interferencesSelective extractionSolid environmental samples

A new extractionmethod for the determination of tributyltin (TBT), dibutyltin (DBT) andmonobutyltin (MBT) insediments based on extraction with tartaric acid and methanol has been developed. Tin species were extractedfrom sediment samples using focused microwave technology, then ethylated with sodium tetraethylborate(NaBEt4) and analyzed by isotope dilution (ID) gas chromatography–mass spectrometry (GC-MS). Theadvantages of such methodology in comparison with other established extraction methods for the routinespeciation analysis of organotin compounds are discussed with respect to sulfur interferences co-extracted fromcomplex matrices.Interferences from elemental sulfur are normally found with acetic acid extraction, but with tartaric acidextraction these interferences were eliminated, demonstrating selective extraction.The accuracy of the analytical procedure was established by analyzing a certified reference material (CRM)(PACS-2, marine sediment) and comparing the results to the certified values. Good agreement betweendetermined and certified values for butyltin compounds was obtained. Finally, some complex sediment samplescollected from San Vicente's Bay, Chile, were analyzed with the proposed methodology, demonstrating itspotential value for monitoring butyltins in environmental samples with high concentrations of sulfurcompounds.

res).

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Aquatic pollution by organic compounds of Sn (IV) (OTCs) is aserious concern in many countries, because OTCs are toxic and persistin aquatic ecosystems particularly in sediments where they areconcentrated [1]. The use of OTCs, particularly tributyltin (IV) (TBT),as an additive in antifouling paints has been mainly responsible fortheir widespread introduction into the aquatic environment [2].Thetoxicity of organotin compounds depends on the number and natureof the organic groups. Trialkyltin compounds (R3SnX) are much moretoxic to mammals and aquatic organisms than monoalkyltin com-pounds (RSnX3) [3].

Many analytical procedures have been reported over the years forthe determination of TBT and its degradation products [4]. Most ofthem combine a separation technique such as gas chromatography(GC), with selective detectors such as atomic absorption spectrometry(AAS), atomic emission spectrometry (AES), mass spectrometry (MS),flame photometric detection (PFD), or pulsed flame photometricdetection (PFPD) [5,6].

For less specific detectors, such as PFPD or PFD, high concentrations ofsulfur and/or organosulfur compounds present in some environmental

samples such as sediments can produce interferences which influenceorganotin determination [7–10]. Cai et al. [11] reported alkyl sulfideinterferences in the determination of OTCs byGrignard alkylation and gaschromatography–mass spectrometry (GC-MS) in the scan mode. It isclear that an effective method to eliminate and/or decrease elementalsulfur and organosulfur compounds would benefit these analyses.

Extraction is usually performed by mechanical agitation/shaking[12], reflux [13], sonication [14], accelerated solvent extraction (ASE)[15] or microwave assisted methods [16].

Major efforts to decrease the presence of sulfur interferences havebeen focused on the development of desulfurization procedures,including the use of activated copper, oxidation by dimethyldioxirane(DMD), absorption by Al2O3 and the addition of a derivatization step[17–22]. The elimination of sulfur compounds can be accomplishedduring the extraction step using pressurized liquid extraction (PLE)[23] but this is an expensive technique for many laboratories and lowrecoveries for MBT have been observed.

Over the last 20 years, extractionmethods using non-polar solvents,non-polar solvents plus acid, polar solvents, and supercritical fluids [6]have been reported. Typically, the extraction studies have been focusedon the improvement of extraction efficiency for the various organotincompounds; however, little effort has been dedicated to improving theselectivity of the extraction process.

The objective of this work was to develop a simple selectiveextraction method based on tartaric acid and methanol for butyltin

130 M. Flores et al. / Microchemical Journal 98 (2011) 129–134

compounds in complex sediment samples to be used as a routineprocedure and compare it with traditional acetic acid extraction. ANational Research Council of Canada (Ottawa, Ontario, Canada) PACS-2Sediment certified reference material (CRM), certified for mono-, di-,and tributyltin content, was analyzed to assess the accuracy, precisionand utility of the extraction method. The new extraction protocol wasthen applied to sulfur-containing sediment samples collected in SanVicente's Bay, Chile.

2. Experimental section

2.1. Instrumentation

For the analysis of organotin compounds. A Hewlett Packard HP6890 (Agilent Technologies Canada Inc., Mississauga, ON, Canada) gaschromatograph fitted with a DB-5MS capillary column from AgilentJ&W Scientific (30 m×250 μm i.d.×0.25 μm coating) was used for theseparation of the organotin species. Detection was achieved with amass selective detector (MS) HP model 5973 (Agilent TechnologiesCanada Inc., Mississauga, ON, Canada). Typical GC/MS operatingconditions are presented in Table 1.

A Discover focusedmicrowave system, with an Explorer autosamplersystem (CEM, Matthews, NC, USA) was used to extract the organotinspecies from the solid samples.

2.2. Reagents and standard solutions

The organotin standards, monobutyltin trichloride (MBT, 95%),dibutyltin dichloride (DBT, 97%), tributyltin chloride (TBT, 96%) andtripropyltin chloride (TPrT, 98%)were purchased fromAlfaAesar (WardHill, USA). Organotin standard stock solutions (5000mgL−1 as Sn)wereprepared in methanol and kept refrigerated until used. Workingstandard solutions (5 mg L−1 as Sn)were prepared from stock standardsolutions by dilution in high-purity deionized water (DIW) obtainedfrom a Nanopure mixed bed ion exchange system fed with reverseosmosis domestic feedwater (Barnstead/Thermolyne Corp., IA, USA). Allstandards were stored in the dark at 4 °C.

Methanol, ammonia, elemental sulfur (S8, 97%), acetic acid, andtartaric acid were purchased from Sigma-Aldrich (Oakville, ON,Canada). Tartaric acid solutions (0.5 M) containing 20% methanolwere prepared weekly by dilution in DIW. Optimization of the tartaricacid/methanol extraction was previously developed for mechanicalshaking and later adapted for the microwave system [24].

Sodium tetraethylborate (NaBEt4) was purchased from StremChemicals (Newburyport, USA). NaBEt4 was dissolved in DIW daily toprovide a 2% (wt/v) ethylating solution. A 2 M sodium acetate (FisherScientific, Nepean, ON, Canada) buffer was prepared by dissolving65 g of sodium acetate in 400 mL DIW and 25 mL glacial acetic acid.The pH was adjusted to 5 with glacial acetic acid.

117Sn-enriched for TBT and DBT stock solution (97% purity) withisotopic composition and uncertainties provided at a nominal concen-tration of 100 mg kg−1 in methanol was provided by LGC Inc.

Table 1GC-MS operating conditions.

Column DB-5MS: 30 m×0.25 mm i.d, 0.25 μm df

Injector system Split/splitless injector, splitless modeInjector temperature 250 °CCarrier gas: flow rate Helium: 1.2 mL min−1

Transfer line temperature 290 °CMS HP model 5973 mass selective detectorSIM parameters Measured ions: m/z 179, 263 and 291; dwell times:

100 ms for each m/zMS quad temperature 150 °CMS source temperature 250 °C

(Teddington, UK). A working standard solution containing 0.79 mg L−1

(as Sn) of TBT and 0.91 mg L−1 (as Sn) of DBT were prepared byvolumetric dilution of the stock in methanol. The concentrations of the117Sn-enriched spike were quantified by reverse-spike isotope dilution(ID) against high-purity natural abundance of TBT and DBT standards.

2.3. Analytical procedures

2.3.1. Sampling and sample treatmentThe samples were collected on the coastline of Chile and consisted of

sediments. Surface sediments (10 cm depth) were acquired with aBierge–Ekman dredge (15×15×15cm). Approximately 3 kg of sedimentwas collected from each site and placed in a polycarbonate bottle. Thesampleswere stored frozen at−20 °C and lyophilized. The dried sampleswere sieved to 1 mm. The fractionsb1 mmwere stored at−20 °Cprior toanalysis.

2.3.2. Extraction from sediment samplesThe focused microwave system operating conditions were as

follows [16]. Briefly, 500 mg of sediment sample was placed in a glassmicrowave vial and 5 mL of extractant solution, either acetic ortartaric acid, was added. The vial was placed in the autosampler andthe focused microwave extraction was performed over 4 min.Maximum irradiation power was set to 200 W, and the holdtemperature was 100 °C. The sample was then cooled to roomtemperature and the vials were centrifuged at 2000 rpm for 10 min.

2.3.3. Sample preparation for PACS-2 using a standard addition methodAn internal standard, 100 μL of 5 mg L−1 TPrT, was added to each

500 mg subsample of PACS-2. Extractant (5 mL), either glacial aceticacid or tartaric acid, was added to the sample. OTC standards wereadded to one set of PACS-2 samples before extraction and to a secondset of PACS-2 samples after extraction, i.e., into the acetic acid ortartaric acid extract. Focused microwave extraction was performed asdescribed previously for 4 min at a maximum power of 200 W.Following centrifugation, the acidic supernatant was transferred to asample vial for derivatization.

2.3.4. Sample preparation for PACS-2 using isotope dilution (ID)The sample preparation for PACS-2 using isotope dilution was

carried out as reported elsewhere [25]. In this experiment threeblanks and six samples of PACS-2were prepared at the same time. ThePACS-2 sediments were spiked with solutions containing di- andtributyltin enriched in 117Sn. Acetic or tartaric acid was added and thesamples were placed in the microwave system at a maximum powerof 200 W for 4 min. After derivatization, isotope ratios weremeasuredby GC-MS and the ratios of intensities at m/z 235 and 232 were usedfor quantification of TBT and DBT in PACS-2. The equations used forthese calculations are reported elsewhere [26–28].

2.3.4.1. Derivatization and extraction by isooctane prior to analysis. Allsamples were derivatized prior to analysis. The derivatization stepinvolves the ethylation of organotin compounds to obtain thermallystable volatile tetra-substituted species for GC separation [29]. Briefly,2 mL of sediment extract was placed in a glass vial. TPrT was added asan internal standard (I.S.). The ethylation reaction was performed in10 ml of buffer (pH 5), to which 5 mL of ammonia, 1 mL of NaBEt4(2%), and 2 mL of isooctane were added. After manual shaking for5 min, the vial was centrifuged for 10 min, allowing separation ofphases. The isooctane layer was then transferred to a 2 mL glass vialand 2 μL of the organic phase was injected onto the head of the GCcolumn for analysis.

131M. Flores et al. / Microchemical Journal 98 (2011) 129–134

3. Results and discussion

3.1. Selection of extractant solution

To develop a method for the extraction of organotin compoundsfrom sediments, several parameters which may affect performancemust be considered. These parameters include stability of theanalytes, extraction efficiency, selectivity for the target analytes,formation of stable complexes [30], and especially co-extraction ofinterfering compounds from complex matrices such as the sedimentsexamined in this study.

No degradation products were observed when butyltin standardswere submitted to tartaric or acetic acid extraction procedures.

3.1.1. GC-MS studyThe determination of OTCs in complex environmental matrices

following derivatization with NaBEt4 can suffer from the presence ofinterferences such as sulfur and organosulfur compounds which areoften present in sediment samples. Such interferences may result inunsatisfactory chromatographic resolution when non-element or massspecific detectors are used [10]. In order to study this problem duringthederivatizationprocedure for sedimentextracts, elemental sulfurwasadded to both extracting liquids. Acetic and tartaric acids (5 mL) werespiked with 1 mg each of elemental sulfur to mimic the extractionprocedure. The influence of theadded sulfurwasevaluated todeterminethe advantages of tartaric acid as compared to acetic acid for theextraction of sediments with a high sulfur content. Each spikedextracting liquid (2 mL) was subjected to the derivatization procedure.Typical GC-MS chromatograms are shown in Fig. 1. Fig. 1A shows thetotal ion chromatogram (TIC) of the derivatized acetic acid extract andFig. 1B shows the TIC of the derivatized tartaric acid extract. Severalcompounds were identified by comparing the mass spectra with thosein the NIST library, as well as with the spectra recorded on the same

Fig. 1. Typical chromatograms obtained by GC-MS in full-scan mode of (A) an ethylatedsulfur spiked acetic acid extract and (B) a tartaric acid extract.

instrument by subjecting known standards to chromatography underthe same conditions. Four intense peaks due to sulfur compounds werefound in the chromatogram of the acetic acid extract.

Potential sulfur-containing molecular ions at m/z 154, 186, 256,and 192 were found in the acetic acid extract by extracting the ionsfrom the full-scan chromatogram. Fig. 2 shows the extracted ionchromatograms of m/z 154, shown in panel A and m/z 186, shown inpanel B. These ions are attributed to organosulfur compounds. Theintensity of these ions was dramatically decreased in the tartaric acidextract, as compared to the acetic acid extract. For the first threemolecular ions, identical mass spectra were previously found insediment extracts from San Vicente's Bay, Chile [10]. In Fig. 3A, thepeak eluting at 7.4 min can be attributed to diethyltrisulphide (Et2S3),m/z 154. The peak eluting at 10.9 min in Fig. 3B is particularlyinteresting because its retention time is very close to that of DBT. Thispeak was attributed with high probability to diethyltetrasulfide(Et2S4) m/z 186. A third peak RT 13.5 min (m/z 192) was attributedto the molecular ion of elemental sulfur (s6). The peak at RT 18.6 min(m/z 256) was also attributed to the molecular ion of elemental sulfur(s8). For elemental sulfur, an equilibrium exists among the formsconsisting of 6, 7 and 8 sulfur atoms and S6 is more stable in solventswith a low value of the dielectric constant than in solvents with a highvalue of the dielectric constant such as acetic acid [31].

High concentrations of these compounds could lead to misinter-pretation of data, because the retention times forOTCs are close to them,8.3 min for MBT, 9.2 min for TPrT, 10.5 min for DBT, and 12.3 min forTBT. This is particularly true for DBTwith a retention time of 10.5 min ascompared to 10.9 min for diethyltetrasulfide (Et2S4).

Based on these results, the tartaric acid extraction method is themost selective and it was therefore applied to the samples collected inSan Vicente's Bay, Chile (Fig. 4).

Fig. 2. Extracted ion GC-MS chromatograms of an ethylated acetic acid extract spikedwith elemental sulfur. Panel A shows the extracted ion chromatogram of m/z 154 andpanel B shows the extracted ion chromatogram of m/z 186.

Fig. 3. EI-MS spectra obtained by GC-MS from elemental sulfur spiked acetic acid extract: (A) diethyltrisulphide and (B) diethyltetrasulphide.

Table 2Comparison of analytical performance as a function of extractionmethod (LOD and LOQ inng(Sn)L−1).

Acetic acid Tartaric acid

LOD LOQ R2 LOD LOQ R2

TBT 52 120 0.9998 55 138 0.9995DBT 42 93 0.9992 36 80 0.9995MBT 43 95 0.9990 116 270 0.960

132 M. Flores et al. / Microchemical Journal 98 (2011) 129–134

3.2. Analytical figures of merit

It was observed that the extraction efficiencies for the organotinspeciesMBT, DBT and TBTwere 45, 73 and 50% for tartaric acid and 82,100 and 60% for acetic acid, respectively. The comparison of the twoextraction methods shows that the acetic acid extraction resulted in amore efficient extraction of the organotin, but the extractionefficiencies observed for tartaric acid for all three organotin speciesare sufficient to obtain accurate quantitative results.

LOD, LOQ and precision were compared for the two extractionmethods. Precision was calculated from repeated measurements ofthe relative integrated response for each OTC and TPrT (normalized tothe TPrT internal standard). The results are presented in Table 2.

The linear response for both extraction methods ranged from LOQto 50μgL-1.

Generally, the analytical performances (LOD or LOQ) of bothextraction methods are very similar, no statistically significantdifference appears except for MBT. This is expected since MBT is themost polar of the OTCs and is expected to have a lower solubility in

less polar solvents resulting in a higher LOD and LOQ for the methodbased on tartaric acid. The LOD values obtained in this work areacceptable for the determination of butyltin species in highlycontaminated sediments.

The repeatability (evaluatedbyrelative standarddeviation,RSD)of thewhole analytical process, i.e. from the extraction procedure to analysis,ranges from2 to 6% for acetic acid depending on the species and from7 to8% for tartaric acid. Both methods lead to satisfactory extractionrepeatability. However, the tartaric acid extraction was expected to

Table 4Concentration of TBT and DBT in certified reference material (PACS-2 marine sediment)determined by isotope dilution using GC/MS in SIMmode for acetic acid (A) and tartaricacid (B) extractions.

Concentration (μg(Sn)g−1 (dry mass)±σ a)

DBT TBT

A 1.129±0.015 0.871±0.023B 1.133±0.031 0.846±0.018Certified value 1.047±0.064 0.890±0.105

a Standard deviation (n=6).

Table 5Determination of MBT, DBT and TBT in samples from San Vicente Bay by liquid–liquidextraction–GC–MS.

Sample Concentration (μg(Sn)g−1 (dry mass)±σa)

MBT DBT TBT

SA 0.301±0.04 0.447±0.03 1.024±0.17S1 0.09±0.02 0.139±0.03 0.291±0.05

a Standard deviation (n=4).

133M. Flores et al. / Microchemical Journal 98 (2011) 129–134

exhibit poorer repeatability because it has a highwater content andhencethe OTCs are less soluble in this extracting liquid. Finally in bothmethodsthe highest value for repeatability was for DBT.

The quantification based on acetic acid extraction seems to havebetter precision for the TBT and MBT species as estimated by R2.

3.3. Quantification of TBT, DBT and MBT in PACS-2 sediment CRM usingstandard addition calibration and acetic and tartaric acids extractions

The accuracy of the analytical procedure has been evaluated byanalyzing a certified reference material.

The focused microwave extraction method using acetic or tartaricacid and a GC/MS analytical method were applied to the determinationof OTCs in PACS-2 using a standard addition technique for quantifica-tion. TPrT was used as an internal standard. The mass selective detectorwas used in single ion monitoring mode (SIM). Mass-to-charge ratios179 forMBT, 249 for TPrT, 263 forDBTand291 for TBT exhibited thebestsignal-to-noise ratio and were monitored for all measurements. Theresults are presented in Table 3. Excellent agreement with the certifiedvalues for DBT and TBT was obtained for both extraction methods;however, the acetic acid extraction for theMBT produced a significantlylower concentration than the certified value. This ismight be due to lowextraction efficiency.

3.4. Quantification of TBT and DBT in PACS-2 sediment CRM by isotopedilution with acetic and tartaric acid extractions

Since the accuracy and precision provided by ID methods allowscontrol of every single speciation analysis step independently, evenpossible loss of substance of the isotope-diluted sample will have noinfluence on the final result [32]. The concentrations of TBT and DBTare presented in Table 4. Good agreement with the certified valueswas obtained for both TBT and DBT using both acetic acid and tartaricacid extraction protocols.

3.5. Application

3.5.1. Quantification of OTCs in sediment samplesButyltin species weremeasured in two surface sediment samples, SA

and S1, collected from Chile's San Vicente's Bay. Results obtained in thisstudy are presented in Table 5. All three OTCswere found in the analyzedsamples. In both samples, the TBT concentration was higher than thatobserved for DBT andMBT. It may be that the higher TBT concentrationsare a result of the continuous discharge of this contaminant, especiallyfrom the port on the bay. Concentration values (ppm range) obtained inthis work are in good agreement with the values reported by otherauthors for the same bay [12]. Stuer-Lauridsen and Dahl [33] haveproposed thatwhen the concentration ratio TBT/DBT ismore than1.5 thesitemaybe consideredhighly contaminated. Applying this criterion, bothsediments from San Vicente bay are highly contaminated.

3.5.2. Evaluation of complex matrix effectsThe main advantage of the tartaric acid extraction method is the

selectivity of the extraction compared to the acetic acid extraction;

Table 3Concentrationof TBT,DBTandMBT in certified referencematerial (PACS-2marine sediment)determined by standard addition using GC/MS in SIM mode for acetic acid (A) and tartaricacid (B) extractions.

Concentration (μg(Sn)g−1 (dry mass)±σa)

MBT DBT TBT

A 0.303±0.150 1.079±0.240 0.846±0.018B 0.647±0.174 1.018±0.124 0.865±0.038Certified 0.6b 1.047±0.064 0.890±0.105

a Standard deviation (n=4).b Certified value.

therefore, these procedures were applied to real samples with highconcentrations of sulfur and organosulfur compounds, as reportedearlier [10]. These samples were collected in San Vicente's Bay, Chile.

The sample quantified previously for butyltin species (SA) wasstudied to evaluate the selective extraction for sulfur and organosulfurcompounds. The extractions were performed for tartaric and aceticacid. Typical GC-MS chromatograms are shown in Fig. 4. Fig. 4A showsthe (TIC) of the derivatized acetic acid extract and Fig. 4B shows theTIC of the derivatized tartaric acid extract.

Fig. 4. Typical chromatogram obtained by GC-MS in full-scan mode of (A) an ethylatedacetic acid extract and (B) tartaric acid extract from sample SA.

134 M. Flores et al. / Microchemical Journal 98 (2011) 129–134

Note that the intensity of several signals is dramatically decreasedin the tartaric acid extract, as compared to the acetic acid extract of areal sediment sample, SA.

Acknowledgements

M. Flores acknowledges the doctoral fellowship from the CONICYT(Comision Nacional de Ciencia y Tecnología, Gobierno de Chile).

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