Different Behavior of Organotin Compounds by Anodic StrippingVoltammetry and their Quanti®cation after Partial Ion Exchange Separation

Klaus-Michael OchsenkuÈhn1, Maria OchsenkuÈhn-Petropoulou2;�, Fotis Tsopelas2,

and Leonidas Mendrinos2

1 Laboratory of Trace Element Studies, Institute of Physical Chemistry, NCSR `̀ Demokritos'', Aghia Paraskevi, 153 10 Athens, Greece2 Laboratory of Inorganic and Analytical Chemistry, Department of Chemical Engineering, National Technical University of Athens, Iroon

Polytechniou 9, 157 73 Athens, Greece

Abstract. The electrochemical behavior and the ana-

lytical performance of the organotin compounds mono-,

di- and tributyltin, as well as, mono-, di- and triphenyltin

were investigated by various voltammetric techniques,

such as alternating current polarography of the ®rst

harmonic (ACP1), square wave polarography at a hang-

ing mercury drop electrode (SQW) and anodic stripping

voltammetry (ASV). Differences could be observed in

the sensitivity, the detection limit and the reproducibility

of the organotins by the techniques used and in some

cases their simultaneously determination was dif®cult,

as the half-wave potentials were very close together.

However, by ASV double peaks were observed only for

the monophenyltin and dibutyltin species, distinguish-

ing them from the overlapping peaks of diphenyltin and

triphenyltin, respectively. This different electrochemical

behavior, is due to the formation of intermediate pro-

ducts during the electrolysis of these compounds with

other half wave potentials and was successfully utilized

for the separation and determination of monophenyltin

in the presence of diphenyltin. In the case of dibutyltin,

where no linear calibration curves for the two peaks

were obtained, a prior suitable ion exchange procedure

was developed for its separation and determination from

the interfering triphenyltin. The successful application

of this technique could be proved on a CRM freshwater

sediment of BCR.

Key words: Organotins; square wave polarography; anodic strip-ping voltammetry; double peaks by dibutyltin and monophenyltin;separation by ion exchange

The worldwide usage of the highly toxic organotin

compounds and the damage caused to estuarine

organisms has forced the scienti®c community and

the health organizations at local and international level,

to a systematic research for the precise and accurate

determination of these compounds, even at very low

concentrations, in the environment [1, 2]. The organo-

tin compounds mostly used are tributyltin (TBT) for

antifouling paints on vessels and as stabilizer for PVC

and triphenyltin (TPT) in fungicides. They undergo a

degradation in the environment by the progressive

removal of the butyl- or phenyl-groups from the tin

atom leading to dibutyltin (DBT), monobutyltin (MBT)

and diphenyltin (DPT), monophenyltin (MPT) species,

respectively. Because of the large differences in the

toxicity and bioavailability of these organotin species,

several analytical approaches have been used for their

accurate and precise determination in ultratrace con-

centrations in environmental samples, such as sedi-

ments, marine organisms or surface waters. The

organotin analysis is a multi-step procedure and

mostly hyphenated techniques, such as GC or HPLC,

cryogenic trapping, supercritical ¯uid extraction or

solid phase micro extraction (SPME) with an element

speci®c detection system, such as AAS, UV spectro-

metry, ICP-AES, ICP-MS, GC-MID-AED (microwave

induced-atomic emission detection) or non-speci®c

detection e.g. ¯ame photometric detection (FPD) have

Mikrochim. Acta 136, 129±135 (2001)

� To whom correspondence should be addressed

been reported [3±9]. For most of the above mentioned

techniques usually an additional step, the derivatisation

of the organotin species to more volatile compounds is

required, such as hydride generation or Grignard

alkylation [10]. Voltammetric techniques used for the

determination of organotins are, however, independent

of any derivatisation procedure, are simple to handle,

accurate and reproducible [11±19]. But in some cases

no differentiation of organotins is possible and they

have to be converted into inorganic tin prior to their

polarographic determination. In other cases very close

half wave potentials for some of the organotin species

were observed and therefore a separation and enrich-

ment is necessary prior to analysis, to reach low

enough detection limits, required for some environ-

mental matrices.

In the present work the behavior of the butyl- and

phenyltin compounds with three polarographic techni-

ques, such as the alternating current polarography of

the ®rst harmonic (ACP1), the square wave polaro-

graphy (SQW) with a hanging mercury drop electrode

(HMDE), as well as the anodic stripping voltammetry

(ASV) was investigated, with the aim of developing

a simple method for distinguishing and determining

the different organotin species. Furthermore a suitable

ion exchange procedure was developed to separate

and enrich some of the investigated species prior to

polarographic analysis. This procedure was applied to

the determination of some organotins in a candidate

CRM freshwater sediment of BCR.

Experimental

Instrumentation

The polarograph VA 746 Trace Analyzer (Metrohm) in connectionwith the VA 747 Stand equipped with a multimode electrode, asworking electrode, was used for the polarographic determination ofthe organotin compounds. The reference electrode was a Ag/AgClelectrode ®lled with 0.2 M LiClO4 in abs. ethanol and the auxiliaryelectrode a glassy carbon electrode. All polarograms were scannedin the voltage range from ÿ0.2 to ÿ1.2 V. The alternating currentpolarography (ACP) was performed with the dropping mercuryelectrode (DME) with a dropping time of 20 ms, sweep rate: 24 mV/s,amplitude of alternating current (AC) voltage: 30 mV, modulatedfrequency: 50 Hz. For the square wave polarography (SQW) withthe hanging mercury drop electrode (HMDE) a drop size: 4, t.meas:2.0 ms, sweep rate: 20 mV/s, voltage amplitude: 20 mV, modulatedfrequency: 50 Hz were used. For the anodic stripping voltammetry(ASV) in the SQW mode using the HMDE, the parameters were:drop size: 4, voltage amplitude: 20 mV, modulated frequency: 50 Hz,sweep rate: 20 mV/s, pre-electrolysis time: 180 s, pre-electrolysisvoltage: ÿ1200 mV. Especially for dibutyltin (DBT) and mono-phenyltin (MPT), where double peaks were observed, the followingpre-electrolysis times: 10, 20, 30, 60, 120, 180 and 240 s and pre-

electrolysis voltages: ÿ1000, ÿ1100, ÿ1200, ÿ1400 andÿ1700 mV were tested.

Reagents

The investigated organotin compounds, such as BuSnCl3 (MBTCl3)(95%, liq.), Bu2SnCl2 (DBTCl2) (96%, solid), PhSnCl3 (MPTCl3)(98%, liq.), Ph2SnCl2 (DPTCl2) (96%, solid) were supplied byAldrich, while Bu3SnCl (TBTCl) (97%, liq.) and Ph3SnCl (TPTCl)(� 97%, solid) by Fluka Chemica. Stock solutions in the rangeof 1 � 10ÿ2±2 � 10ÿ2 mol=L of each compound were preparedevery week in abs. ethanol and kept frozen (ÿ15 �C), and workingsolutions were prepared daily in the range of 1 � 10ÿ4±1 � 10ÿ3 mol=L. For the extraction of the organotin compounds,dichloromethane, p.a. (Merck), was used and as electrolyte 0.2 MLiClO4, prepared from LiClO4 � 3H2O, Merck p.a. dissolved inabs. ethanol (Merck p.a.). For the separation of DBT and TPT fromtheir mixtures the cation exchanger DOWEX 50 W-X8 (Serva) inthe H�-form, 100±200 mesh, packed into a glass column (i.d.0.5 cm, 15 cm long) was used. The used resin bed was 7 cm long.The elution of the organotin compounds from the resin wasperformed with 3 M HCl in methanol. The ¯ow rate of 1 ml/min wasregulated with a peristaltic pump (Minipuls 2, Gilson). For theapplication of this procedure on a real sample, the candidatereference material CRM 646, a freshwater sediment from a canalnear Amsterdam/Netherlands, was analysed for DBT and TPT.

Analytical Procedure

For the polarographic determination appropriate micro amounts ofthe investigated organotin compounds and their mixtures, werespiked in a 10 ml Metrohm polarographic cell containing as blank3 ml of dichloromethane and 3 ml of the electrolyte 0.2 M LiClO4

in abs. ethanol. The mixture was cooled at 10 �C and deoxygenatedby passing nitrogen saturated with solvent (abs. ethanol/dichlor-omethane) for 5 min. Then, the polarograms were scanned sub-sequently by changing the measurement mode. The evaluation ofthe polarograms was achieved using either calibration curves,taken under the same conditions as described above or the standardaddition method.

Ion exchange separation: 20 ml of 0.5 N HCl were spiked withmicro amounts of DBT or TPT or their mixtures and passed throughthe ion exchange column. The elution of the organotin compoundswas performed with 3 M HCl in methanol [17] and fractions of 1 mlwere taken in order to obtain the elution curves. For thepolarographic determination each fraction of the eluate, as well asthe ef¯uent, were transferred into a separatory funnel, then 5 ml ofdichloromethane and 5 ml of distilled water, for phase separation,were added and shaken for 5 min. After phase separation 3 ml of theorganic phase and 3 ml of 0.2 M LiClO4 in abs. ethanol weretransferred into the polarographic cell and the determination wasperformed as mentioned above. As blank 0.5 M HCl was used,passed through the ion exchange column and subjected to the sameprocedure as the sample.

Sediment sample: For the determination of DBT and TPT in theCRM 646 freshwater sediment, micro-amounts of DBT and TPT(4±8mg) were spiked in 5±25 g of the well mixed sediment andthe mixture was leached with 70 ml of 0.5 M HCl for 20±24 h atlow temperature (5±10 �C). After centrifugation of the slurry at2000 rpm for 20 min, the supernatant liquid was ®ltered through a0.45mm Sartorius cellulose acetate ®lter. The ®ltrate was then passedthrough the ion exchanger column with a ¯ow rate of 1 ml/min. The

130 K.-M. OchsenkuÈhn et al.

elution was performed using 3 M HCl in methanol with a ¯ow rateof 1 ml/min. Two fractions were taken, the ®rst one containing DBT(0±10 ml) and a second one containing TPT (11±20 ml), as itresulted from the corresponding elution curves. These fractions wereanalyzed as mentioned above. The evaluation of the polarogramswas achieved using the standard addition method.

Results and Discussion

In Table 1 the half wave potentials obtained by the

three polarographic techniques for each organotin

compound are presented. As it can be seen, the half

wave potentials for DBT and TPT by ACP1 and SQW

at the HMDE are very close together, as well as for

MPT and DPT. Therefore a simultaneously determina-

tion of these compounds is not possible using these

voltammetric techniques. However, by the ASV, double

peaks could be observed only for DBT and MPT, when

the electrolysis took place under certain conditions,

regarding the pre-electrolysis time and voltage. Based

on the different electrochemical behavior of these two

compounds, an attempt was made to distinguish them

and, if possible, to determine them in the presence of

the above mentioned interfering compounds. As far as

the determination of MBT with voltammetric techni-

ques concerns, it was found not sensitive enough and

the calibration curves, obtained were not linear. There-

fore this compound was not taken into consideration

for the further measurements. It is to mention that

inorganic tin did not interfere with the investigated

organotin compounds, as it was not extracted by di-

chloromethane [17].

The occurrence of a double peak can be explained

by an electrode process during the electrolysis, where

in the ®rst step an organotin radical is produced from

the organotin compound, which afterwards is recom-

bined to a dimer organotin compound [11, 14]. The

dimer compounds are obviously in the case of DBT and

MPT stable and have different half wave potentials as

their monomer forms. This effect is of interest for the

separate determination of organotin compounds, which

otherwise have about the same half wave potential.

In Fig. 1 the polarograms of MPT are demonstrated,

obtained by SQW at the HMDE (lower part (I), one

peak) and by reverse scanning (ASV, upper part (II),

two peaks). The ratio of the sensitivities of the peaks

between the direct (at ÿ0.58 V) and reverse scan (at

ÿ0.43 V) is for this case about 1:10.

Table 1. Half wave potentials of the organotin compounds DBT, TBT, MPT, DPT and TPT, obtained by ACP 1, SQW voltammetry at theHMDE and ASV

E1=2[V]

Organotin ACP SQW ASVsalts (HMDE)

Bu2SnCl2 (DBT) ÿ0:71� 0:01 0:72� 0:01 ÿ0:73� 0:01 (1st peak)�ÿ0:69� ÿ �ÿ0:62� (2nd peak)�

Bu3SnCl (TBT) ÿ0:99� 0:01 ÿ1:00� 0:01 ÿ1:02� 0:01PhSnCl3 (MPT) ÿ0:58� 0:01 ÿ0:58� 0:01 ÿ0:53� 0:01 (1st peak)

ÿ0:43� 0:01 (2nd peak)Ph2SnCl2 (DPT) ÿ0:58� 0:01 ÿ0:58� 0:01 ÿ0:55� 0:01Ph3SnCl (TPT) ÿ0:68� 0:01 ÿ0:69� 0:01 ÿ0:70� 0:01

� E1=2 is depending on the concentration.

Fig. 1. Polarogram of MPT by SQW at the HMDE (I) and by ASV(II), pre-electrolysis at ÿ1200 mV for 180 s

Different Behavior of Organotin Compounds by Anodic Stripping Voltammetry 131

In Fig. 2 the polarograms obtained by ASV for the

individual MPT and DPT compounds and a mixture of

them are presented. The direct scan (SQW, lower part)

shows one broad peak at the same half wave potential

for all compounds, due to the overlapping of the peaks

of MPT and DPT. In the reverse scan (ASV, upper part)

a double peak resulted for MPT, as well as for the

mixture. It can be seen, that the peak height of MPT at

ÿ0.43 V is undisturbed and therefore MPT can be

determined independently by using its related calibra-

tion curve at the above mentioned half wave potential.

Furthermore it is possible to calculate the ratio of this

peak to the one at ÿ0.53 V and in this way to ®nd out

the contribution of MPT on the DPT peak height in the

mixture. From the difference of the height of MPT and

DPT at ÿ0.53 V, the concentration of DPT can be

obtained, when using the calibration curve for DPT.

The calibration curves for the two MPT peaks are

linear in the working range up to 15mg MPT/ml cell

volume. The evaluation was carried out by using the

peak height and not the peak area.

The combination of DBT and TPT is another

example of organotin compounds with about the same

half wave potentials by ACP1 and SQW, while DBT

can form a double peak by ASV. In Figs. 3 and 4 the

in¯uence of the pre-electrolysis time and voltage on the

shape of the polarograms obtained for DBT by ASV is

demonstrated. As it can be seen, by increasing the pre-

electrolysis time or the pre-electrolysis voltage, the

Fig. 2. Polarograms of MPT, DPT and a mixture of them (MPT7.5mg/ml cell volume, DPT 3.5 mg/ml cell volume)

Fig. 3. Polarograms of DBT taken by ASV at different pre-electrolysis times (DBT 5 mg/ml cell volume)

Fig. 4. Polarograms of DBT taken by ASV at different pre-electro-lysis voltages (DBT 5mg/ml cell volume)

132 K.-M. OchsenkuÈhn et al.

formation of the double peak is favoured to the debit of

the peak height. A pre-electrolysis time of 180 s and a

pre-electrolysis voltage of ÿ1200 mV were selected for

the measurements with ASV of DBT, as well as of

MPT.

However the development of the double peak in

dependence of the concentration of DBT is much more

different than by MPT and the calibration curves,

obtained for the two DBT peaks, are not linear (Fig. 5).

It is assumed that by DBT during the electrolysis at

HMDE not only dimeric species but also polymer

compounds are produced, changing the half wave

potentials [14]. At concentrations less than 1.5mg/ml

cell volume a well developed peak appears at ÿ0.69 V

(curve I), which shifts at higher concentrations to

ÿ0.62 V. At about 1.3mg DBT/ml cell volume a second

peak atÿ0.72 V (curve II) becomes visible. Both peaks

pass a maximum, the 1st one at about 1.8 and the 2nd

one at about 3mg DBT/ml cell volume. After about

4mg DBT/ml cell volume the intensity of the 2nd peak

becomes higher than this of the 1st one. Under these

circumstances an analytical determination of DBT by

ASV can only be performed in the range up to 1.5mg

DBT/ml cell volume. Due to the fact that the two

peaks, obtained for DBT by ASV, have a half wave

potential close to those of TPT, it was not possible to

separate these compounds using their different electro-

chemical behavior as in the above mentioned case of

MPT and DPT.

Therefore, a chromatographic separation of DBT and

TPT was performed prior to the polarographic

determination. Using the cation exchanger DOWEX

50 WX8 in the H�-form in a 7 cm high resin bed and

3 M HCl in methanol as eluent, a separation of DBT

and TPT was possible (Fig. 6). By a stepwise elution of

both compounds it can be seen, that in the ®rst 10 min

DBT was removed from the column and after 10 min

TPT was eluted. The recovery of DBT after the ion-

exchange separation was about 70±75% and those of

TPT 25±35%. Through the optimization of the ion

exchange procedure, a higher recovery rate for TPT

(70%) could be obtained [21].

For the determination of these two compounds, the

whole volume (10 ml) in the ®rst 10 min fraction was

used for the determination of DBT and the whole

volume (10 ml) in the second 10 min fraction for the

determination of TPT.

In Table 2 the sensitivities, obtained from the slope

of the corresponding calibration graphs and the

detection limits, resulted for the three polarographic

techniques for each organotin compound are presented.

The comparison of the three polarographic techniques

shows that the highest sensitivities and the lowest

detection limits were achieved by ASV, as it is ex-

pected, due to the enrichment of the compounds

occurred by the electrolysis. However, the reproduc-

ibilities of the two other techniques, such as ACP1

Fig. 5. Dependence of the peak heights of DBT at ÿ0.69 V toÿ0.62 V (I) and ÿ0.73 V (II) from the concentration, when usingASV

Fig. 6. Separation of a mixture of DBT and TPT (DBT 18.9 mg,TPT 40.2mg) with the ion exchanger DOWEX 50 WX8, 7 cm resinbed and 3 M HCl in methanol as eluent

Different Behavior of Organotin Compounds by Anodic Stripping Voltammetry 133

(7±12%) and SQW at HMDE (4±7%) are better in

comparison to those obtained by ASV (20±25%), due

to the complex electrode processes, which take place at

the surface of the stationary electrode during the

electrolysis [11, 14].

Application of the technique was demonstrated

through the determination of DBT and TPT in a

candidate freshwater sediment, CRM 646 (BCR). Due

to the very low concentration of TPT in the investigated

sediment, its determination could be achieved only in-

direct using the spiked sediment samples and sub-

stracting the spiked amount of TPT. By the evaluation,

the recovery rates, as well as the moisture content of

the samples were taken into consideration [17, 20].

The statistical treatment of the results for the real

sample, obtained by the described method, and the

results of the certi®cation exercise show that there are

no statistical differences between the two mean values.

The reproducibility of about 20% is very satisfactory

taking into consideration the various steps required

prior to the analysis. The corresponding RSD value

obtained from all laboratory data of the certi®cation

exercise on this sediment was about 15%. The most

important result which arises from this investigation is

that now DBT and TPT, two organotins with very close

half wave potentials, could be separated and in that

way determined in the same matrix.

Conclusion

As the investigation demonstrated, the polarographic

determination of the interfering species DPT and MPT,

as well as DBT and TPT is possible through a double

peak evaluation and a prior chromatographic separation

and the detection limits for the determination of TBT,

DBT, MPT, DPT and TPT could be improved to lower

concentrations, when using ASV at a HMDE.

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Table 2. Sensitivities and detection limits of the organotin compounds DBT, TBT, MPT, DPT and TPT, obtained by ACP 1, SQWvoltammetry at the HMDE and ASV.

Sensitivity [nA/mg organotin Detection limit [mg organotincompound/ml cell volume] compound/ml cell volume]

Organotin ACP1 SQW ASV ACP1 SQW ASVsalts (HMDE) (HMDE)

273:7ÿ 13:5� CBu2SnCl2 36.6 22.6 (ÿ0.69 V) 0.28 0.11 0.009(DPT) 131:1ÿ 37� C (ÿ0.69 V)

(ÿ0.72 V)Bu3SnCl 16.1 7.4 7.3 1.00 3.12 0.5(TBT)PhSnCl3 5.9 2.9 35.2 (ÿ0.43 V) 1.83 2.05 0.15(MPT) 21.9 (ÿ0.53 V) (ÿ0.43 V)Ph2SnCl2 22.7 11.9 75.4 0.30 0.23 0.02(DPT)Ph3SnCl 10.3 5.8 15.7 0.75 0.35 0.13(TPT)

Table 3. Results of DBT and TPT determination, obtained by ASVafter ion exchange separation and enrichment (this work) andlaboratory means of the certi®cation exercise from BCR onCRM 646 fresh water sediment [20]

Organotin This work Certi®cation exercise(ng/g dry sediment) CRM 646

(ng/g dry sediment)mean of mean values [20]

DBT 654.6� 130.8 770.3� 117.2TPT 29.2� 9.0 34.9� 5.1

134 K.-M. OchsenkuÈhn et al.

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[20] Minutes of the Certi®cation Meeting on Organotin in Fresh-water Sediment. Amsterdam, 21 September 1998 (EU projectMAT 1-CT 94-071).

[21] K. M. OchsenkuÈhn, M. OchsenkuÈhn-Petropoulou, F. Tsopelas,L. Mendrinos, European Conference on AnalyticalChemistry, Euroanalysis XI. Lisbon-Portugal, 3±9 September2000.

Different Behavior of Organotin Compounds by Anodic Stripping Voltammetry 135