2 Laboratory of Water and Air Quality, Department of Environmental Studies, University of the Aegean, Theofrastou and Alkaiou, GR-81. 100 Mytilene, Greece.
Mikrochim. Acta 136, 137±141 (2001)
A Simple Method for the Speciation of Organotin Compounds in Water Samples Using Ethylation and GC-QFAAS Nikolaos S. Thomaidis1;2 , Freddy C. Adams1; , and Themistokles D. Lekkas2 1
MiTAC, Department of Chemistry, Universitaire Instelling Antwerpen, Universiteitsplein 1, B-2610 Antwerp, Belgium Laboratory of Water and Air Quality, Department of Environmental Studies, University of the Aegean, Theofrastou and Alkaiou, GR-81 100 Mytilene, Greece 2
Abstract. A simple method for the extraction of organotin compounds from water samples was developed in which both the instrumental parameters and the extraction/derivatization step were optimized. Organotin compounds (butyl-, phenyl- and octyl-) in tap water samples were ethylated with the addition of 2.5 ml of 0.4% w/v NaBEt4 at pH 5.00 and subsequently extracted two times, for 10 min, with 3 and 2 ml of hexane. The combined extracts were analyzed with on-column capillary GC-QFAAS. The recoveries were quantitative for di- and tri- alkyltin compounds, whereas between 67 and 86% of the monoalkyltin compounds were recovered. The detection limits obtained ranged from 110 pg for monobutyltin to 500 pg for triphenyltin, as sensitivities were found to be compound dependent. The preparation of ethylated standards was also optimized. It was found that two subsequent extractions, with 1.0 and 0.5 ml of hexane were necessary for the quantitative recovery of the ethylated organotin compounds. Key words: Organotin compounds; ethylation; water samples; gas chromatography ± quartz furnace atomic absorption spectrometry.
The organotin compounds, especially butyl and phenyl species, are used in many human activities. The plastics industry is responsible for the largest single usage of organotins, amounting to approximately two-thirds of the consumption. Most of the remainder is accounted for by biocides for antifouling paints, crop protection and wood preservation [1]. The increased consumption of these chemicals, particularly as biocides, has resulted in their direct leakage into the aquatic To whom correspondence should be addressed
environment. Tributyltin is a toxic compound in the environment; concentrations of few ppt (4±50 ng lÿ1 ) could exert sublethal and lethal effects on a wide variety of organisms [2]. The monitoring of tin compounds is required by the European Community legislation. Therefore, the development of accurate, robust and compatible methodology for the determination of organotin compounds in the aquatic environment is necessary. Various analytical methods have been reported in the literature for organotin speciation and most of them are based on the use of coupled techniques using liquid or gas chromatography [1]. Gas chromatography has found more applications until now [1, 3], due to its high resolving power and its ability to be coupled with very sensitive tin-selective detectors, such as mass spectrometry (MS) [4±6], ¯ame photometric detection (FPD) [7±11], quartz-furnace atomic absorption (QFAAS) [12±17] and microwave-induced plasma ± atomic emission spectrometry (MIP-AES) [18, 19]. Due to the low volatility of the organotins, a derivatization step is required when a GC technique is going to be used. Derivatization is achieved by three different techniques: (a) hydride generation with NaBH4 [10, 12, 16], (b) alkylation with Grignard reagents [4±7, 9, 13, 15, 18] and (c) alkylation with NaBEt4 [8, 11, 14, 17, 19]. Hydride generation has the advantage that it could be performed directly in the aqueous phase and the hydrogen (produced as a byproduct) facilitates the purging of organotin hydrides [10]. However, the yield and reproducibility of the hydridization of phenyltins is low and losses of the analytes could occur [20]. Grignard derivatization
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N. S. Thomaidis et al.
yields are generally higher for all organotins [13, 21], although this was also questioned in the literature [5]. However, this derivatization procedure is complex and time consuming and analyte losses could occur during these steps [20]. The more recent procedure utilizes the one-step extraction/derivatization with NaBEt4 . This derivatization procedure has many advantages. It can be performed directly in the aqueous phase (as opposed to Grignard reagents), reducing drastically the number of analytical steps, thus saving time, and improving reliability. It appears to be more robust and accurate for extraction from environmental samples and the ethylated products can be analyzed directly by GC equipment (as opposed to hydridization using NaBH4 ). However, lower derivatization ef®ciencies were generally obtained in real samples [21], thus a careful optimization step is often required. The aim of this study was the critical examination of this extraction/derivatization step for the production of ethylated standards and the recovery of organotins from water samples. Sources of error are discussed. The instrumental parameters of a GC-QFAAS setup were also critically optimized.
Reagents All reagents were of analytical grade and the solvents were of HPLC grade. Milli-Q (Millipore, USA) water was used throughout. Bu3 SnCl (96%), Bu2 SnCl2 (95%), BuSnCl3 (95%), Ph3 SnCl (95%), Ph2 SnCl2 (96%) and PhSnCl3 (98%) were purchased from Aldrich (Milwaukee, WI, USA). Sodium tetraethylborate (NaBEt4 ) was purchased from Strem Chemicals (Bischheim, France). The aqueous solutions of this reagent were prepared daily. Acetate buffer (pH 5.00; 0.1 mol lÿ1 ) was prepared by dissolving 13.6 g of sodium acetate trihydrate (Merck, Darmstadt, Germany) in 1 l of Milli-Q water followed by pH adjustment with concentrated acetic acid (Merck). Preparation of Ethylated Standards
Experimental
Stock standard solutions of the chloride salts of the organotin compounds were prepared in 50 ml CH3 OH. Appropriate volumes of these solutions were transferred in a specially designed extraction vessel [19] with 30 ml of acetate buffer solution (pH 5.00). One ml of NaBEt4 0.4% w/v (prepared daily) and 1 ml of hexane were added and the mixture was shaken manually for 5 min. After phase separation, the hexane phase was collected with a micropipette into an Eppendorf conical vial. The extraction procedure was repeated with 0.5 ml of hexane. The combined extracts were analyzed by GC-QFAAS and GC-MIP-AES. In both cases, calibration was performed with ethylated standards, produced from the chloride salts of the organotin compounds by Grignard reaction, as described earlier [13, 19]. A blank solution was prepared by the same procedure, without the addition of organotin compounds. This was analyzed ten times and used for the determination of the limit of detection (3BL ) and limit of quantitation (10BL ) of the procedure, where BL is the standard deviation of the 10 successive injections.
Instrumentation
Determination of Organotins in Water Samples
The instrumental parameters of GC-QFAAS are summarized in Table 1. The detailed instrumentation and GC-MIP-AES parameters have been described elsewhere [18, 19].
The pH of 1 l of tap water was adjusted at 5.00 with 13.6 g of sodium acetate and few ml of acetic acid and it was transferred in a 1 l separation funnel. 2.5 ml of NaBEt4 0.4% w/v and 3 ml of hexane were added and the mixture was shaken manually for 10 min. After phase separation (5 min), the hexane phase was collected in a conical vial and the extraction procedure was repeated with 2 ml hexane for 10 min shaking. The combined hexane extracts were analyzed by GC-QFAAS.
Table 1. Instrumental parameters for the determination of organotins in water samples by GC-QFAAS GC Injection mode Injection volume Retention gap Capillary column Carrier gas Column head press. Temperature programme Transfer line temp. Heating block temp. QFAAS Light source Wavelength/slit Temperature of quartz furnace H2 ¯ow rate Air ¯ow rate
Chrompack CP 9001 cool on-column injection 2 ml deactivated fused silica capillary column HP-1 (25 m 0:32 mm) He 150 kPa initial temp. 80 C for 1 min ! 20 C minÿ1 ! 270 C for 1 min 270 C 300 C Perkin-Elmer 2380 ± MHS 20 EDL Sn (8 W) 286.4 nm/0.7 nm 900 C 600 ml minÿ1 30 ml minÿ1
Results and Discussion Optimization of Instrumental Parameters of GC-QFAAS The following instrumental parameters were carefully optimized: the ¯ow rates of H2 and air, the column head pressure and the position of the transfer line into the side arm of the quartz cell. Hydrogen was essential for the determination of organotins and its ¯ow rate was very critical for the sensitivity (Fig. 1). The optimized ¯ow rates were 600 ml minÿ1 of H2 (Fig. 1) and 30 ml minÿ1 of air. The position of the transfer deactivated capillary column into the side arm of the quartz cell is also very critical. It was optimized
139
A Simple Method for the Speciation of Organotin Compounds
Fig. 1. Effect of H2 ¯ow rate on the signal of 3.5 ng MBT, 7.7 ng of DBT, 3.5 ng TBT and 32 ng TPhT. Air ¯ow rate: 40 ml minÿ1 Table 2. Retention times (tR ) of the ethylated organotins with the GC-QFAAS system Compound
tR (min)
Compound
tR (min)
BuSnEt3 Bu2 SnEt2 PhSnEt3 Bu3 SnEt
2.59 3.65 4.19 4.61
Ph2 SnEt2 Oc2 SnEt2 Ph3 SnEt Oc3 SnEt
6.65 7.90 8.76 10.05
manually and it was found that 1±2 mm was the optimum position (higher sensitivity, minimum noise). The optimum He column pressure was found in the range of 100±150 KPa. The retention times of the ethylated organotins under the optimized GC-QFAAS conditions are given in Table 2.
tion higher than 1% should be avoided, because the lifetime of the column decreased. The extraction/derivatization step was carefully optimized. Various extraction/reaction times were tested (3-5-10-15 min) and it was found that adequate recoveries were obtained for tri-alkyl tins with 5 min shaking. However, the recoveries of mono- and di-alkyl tin compounds were not satisfactory. Higher volumes of hexane were tested (1-2.5-5 ml), but no signi®cant improvements could be obtained. On the other hand, quantitative recoveries were obtained when two successive extractions were performed, with 1 ml and 0.5 ml for 5 min each, respectively. The results from the recovery experiments are given in Table 3. The recoveries for MPhT were not reproducible and generally very low and they are not given. This could be attributed to the fact that MPhT is very sensitive to moisture in the air [21, 22]. A typical chromatogram of a mixture of organotins is shown in Fig. 2. The ®gures of merit of the analytical procedure are given in Table 4. Linear ranges up to 36 ng were obtained for most of compounds. The day-to-day precision (n 4) of the calibration sensitivity is also given in Table 4. The precision of ®ve injection of approximately 4 ng of each compound ranged from 3.5±12%. The absolute detection limits ranged from 110 pg for MBT to 500 pg
Preparation of Ethylated Standards by One-Step Extraction/Derivatization The preparation of standards was optimized in terms of the NaBEt4 concentration (0.05±2.0% w/v) and the extraction step(s). It was found that the recoveries were quantitative with the addition of 1 ml of 0.3±2% w/v of NaBEt4 solution. However, solutions with concentra-
Fig. 2. Typical GC-QFAAS chromatogram of the following mixture of ethylated organotins: 8.1 ng of MBT, 14.4 ng of DBT, 9.6 ng of TBT, 11.8 ng of DPhT and 13.6 ng of TPhT
Table 3. Recoveries of the preparation of ethylated standards (n 7) Compounds
Added (ng)
1st Extraction found (ng)
Recovery (%)
2nd Extraction found (ng)
Total recovery (%)
MBT DBT TBT DPhT TPhT
20.03 31.88 19.96 28.42 27.05
15.91 27.02 18.21 21.73 24.8
79.4 3.8 84.8 3.4 91.2 6.4 76.5 8.0 91.7 8.5
1.42 2.47 1.53 1.98 2.26
86.5 4.3 92.5 3.2 98.9 6.0 83.4 7.5 100 11
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A Simple Method for the Speciation of Organotin Compounds
Table 4. Figures of merit for the determination of organotins with ethylation and GC-QFAAS (LOD, 3BL ± LOQ, 10BL ) Compound
Sensitivity (n 4) (PH units ngÿ1 )
Correlation coef®cient
Limit of detection (LOD, ng)
Quantitation limit (LOQ, ng)
RSD (%) (n 5)
MBT DBT TBT DPhT TPhT
83.2 6.6 70.0 3.1 60.0 3.0 38.4 1.0 16.4 2.8
0.999 0.9995 0.9993 0.998 0.997
0.11 0.14 0.17 0.25 0.50
0.36 0.46 0.57 0.84 1.65
5.5 3.5 6.0 9.1 12
For app. 4 ng injected for each compound.
Table 5. Recoveries of organotins from spiked tap water samples (n 4) Compounds
Added amount (ng)
1st Extraction Found (ng)
Recovery (%)
2nd Extraction Found (ng)
Total recovery (%)
MBT DBT TBT DPhT TPhT
20.0 38.8 20.0 32.5 33.0
13.8 36.8 19.1 30.3 32.1
69.0 94.8 95.5 93.2 97.3
0.68 1.47 1.50 2.54 2.41
72.4 98.6 103 101 105
for TPhT, as sensitivities were found to be compound dependent. The recoveries were also checked by analyzing the extracts with GC-MIP-AES. Results comparable with those of GC-QFAAS were obtained.
GC-QFAAS technique. This simpli®ed method, which is a modi®cation of the method of Ceulemans et al. [19], is rapid and simple and it could be applied for the speciation of organotins in any type of water sample.
Determination of Organotins in Spiked Tap Water Samples by GC-QFAAS
Acknowledgements. N. Thomaidis thanks Ms. S. Slaets, Dr. M. B. de la Calle-Guntinas and Mr. W. Van Mol for their valuable collaboration and help.
One liter of tap water was spiked with appropriate amounts of MBT, DBT, TBT, DPhT and TPhT and the extraction/derivatization steps were performed as described under the experimental procedures. It was also found that two successive extractions were necessary for the complete recovery of the ethylated spiked compounds. The results are given in Table 5. The recovery of MBT is adequate, since the extraction of the monoalkyl-derivatives is very dif®cult. It was also observed that the recoveries were better in these experiments than in the preparation of standards. In some water samples, an unknown peak was observed at tR 10:67 min of which the identi®cation was not possible. In conclusion, these results show that extraction/ derivatization procedure for the preparation of ethylated standards and the recovery of organotins from water samples has to be carefully optimized. Two successive extractions are necessary for complete recovery. Instrumental parameters, such as H2 and air ¯ow rate and the position of transfer line into the side arm of quartz cell are critical for optimum sensitivity of the
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