Determination of dimethyl sulphide in seawater

Ocean DMS is primarily formed by the enzymatic cleavage of dimethylsulphonium propionate (DMSP), which is several orders of magnitude faster than the abiotic decomposition of DMSP Dacey and Blough, 1987). 

Since the introduction of the flame photometric detector (FPD) (Brody and Chaney, 1966) and its first apphcation to marine DMS (Lovelock et al., 1972), gas chromatography with flame photometric detection (GC-FPD) has become the standard technique for the determination of dissolved DMS. Among other sulphur-selective detectors, thus far only the sulphur chemiluminescence detector (SCD) (Benner and Stedman, 1989 Shearer, 1992) has also been used for the determination of dissolved DMS (e.g., Ledyard and Dacey, 1994). However, detailed descriptions of this emerging analytical technique are still lacking. In contrast, the popularity of the FPD resulted in the pubUcation of a variety of methods for the determination of oceanic DMS (e.g., Andreae and Barnard, 1983 Leek and Bdgander, 1988 Turner and Liss, 1985), two of which have been compared during an inter-laboratory calibration (Turner et al., 1990). In the following, some principles of DMS determination by GC-FPD will be discussed, before the analytical procedure is described. 

The flame photometric detector is the principal component in the determination of sulphur compounds for which it offers a selectivity of about five orders of magnitude with respect to hydrocarbons. The selective sulphur detection is based on the formation of electronically excited S2 molecules in a hydrogen-rich flame. These short-lived species revert to their ground state and emit characteristic molecular band spectra with peak wavelengths at 384 and 394 nm. This chemiluminescent radiation passes an optical filter and is monitored by a UV-sensitive photomultiplier. 

An outstanding feature of the FPD in the sulphur mode is its non-linear response function. In general, the relationship between peak area. A, and analyte mass, m, is described by an exponential form  

The above mentioned practical aspects of FPD detection as well as the phenomenological FPD response models have been reviewed excellently by Farwell and Barinaga (1986). 

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