Organotin Speciation Analyses in Marine Biota using NaBEt₄ Ethylation and GC-FPD

Introduction

Butyl- and phenyltin compounds are used worldwide mainly as antifouling agents and biocides. The International Maritime Organisation recently resolved that TBT should also be banned from use on larger vessels within the next decade. Interest in these contaminants remains high because TBT is a persistent pollutant, especially in marine sediments.

For speciation analyses of the butyltin compounds, they must be extracted from the matrix and derivatised into suitable forms for analysis by gas chromatographic. There are three common derivatisation procedures.

Hydride generation in the aqueous phase using sodium borohydride. However, the instability and volatility of butyltin hydrides can lead to losses of compounds and consequently to an underestimation of the determinand.
Extraction of organotin complexes with tropolone or diethyldithiocarbamate followed by Grignard derivatisation. The Grignard derivatisation requires scrupulously dry conditions and is rather time consuming.
Derivatisation in the aqueous phase using sodium tetraethylborate (NaBEt₄) and simultaneous extraction into an organic phase. The determination of organotin compounds in complex matrices, such as biological matrices with a high lipid content, has led to several problems including low recovery and low derivatisation efficiency.

This work describes an optimisation of the derivatisation and extraction technique of organotin compounds for marine biota.

Sample Preparation

Using approximately 0.5 g of freeze-dried tissue, samples were dissolved in 10 mL of tetramethylammonium hydroxide. To optimise the dissolution, samples were stirred in an ultrasonic bath for 1.5 h at 50°C. After complete dissolution of the tissue, buffer and acetic acid were added to stabilise the pH between 4 and 5. The samples were simultaneously derivatised and extracted using 1 mL of a freshly prepared sodium tetraethylborate (NaBEt₄) solution and 5 mL of n-hexane. The samples were mechanically shaken for 20 min, then centrifuged at 5000 rpm for 15 min at a temperature of 0°C.

Following a double extraction procedure, the combined extract was dried with activated sodium sulfate and concentrated to ~0.5 mL by evaporation. The samples were then cleaned up using SPE (Solid Phase Extraction) Florisil cartridges and eluted with 10 mL of n-hexane. The purified samples were again concentrated to about 0.5 mL prior to analysis by GC-FPD.

Analysis by GC-FPD

An HP 5890 Gas Chromatograph equipped with a Flame Photometric Detector (FPD) (610 nm filter and with a hydrogen-rich flame) was used for all organotin determinations. A 30 × 0.25 mm × 0.25 mm HP-5 (Hewlett-Packard) capillary column (5% phenyl methyl silicone) was used with splitless injection (250°C) and the FPD was maintained at 270°C. Helium, at a flow of 1 mL min^-1, was used as the carrier gas. The GC temperature program was 60°C for 2 min, then 60-270°C at 6°C min^-1 and 270°C for 20 min.

Quality Control Procedures

A strict quality control system based on the use of different internal standards has been introduced. Tripropyltin was used as internal standard to control the derivatisation reaction. Tetraoctyltin was used to check the overall extraction efficiency. Tetrabutyltin was used to verify the GC-FPD performance throughout the analyses.

Technique Validation

The technique was validated using Certified Reference Materials (NIES-11 and BCR-477) and has been successfully applied to different biological matrices (fish, mussels, oyster and barnacles). Figure 1 reports chromatograms of a spiked blank, oyster tissue and mussel tissue (BCR 477). Both baseline and peaks are very well defined which allow an accurate and precise quantification.

Figure 1 GC-FPD chromatograms of samples spiked with the three internal standards (tripropyltin, tetrabutyltin and tetraoctyltin: a) blank, b) oyster tissue, c) mussel tissue BCR 477

After optimising and validating the method by analysing Certified Reference Materials, this method was subsequently successfully applied to real marine biota samples: mussels, oysters, fish and barnacles. Table 1 reports the results of five independent analyses on the same oyster tissue, demonstrating the excellent reproducibility of the technique.

Table 1 Results (ng g^-1 as chloride) of five independent determination of organotin compounds in oyster tissue.

Reference

R. Cassi, I. Tolosa, J. Bartocci, S.J. de Mora (2002) Organotin Speciation Analyses in Marine Biota using NaBEt₄ Ethylation and GC-FPD, Applied Organometallic Chemistry, 16, 355-359.