Derivatized mercury species

Because of differences in ecotoxicity between the different mercury species and as many mercury species are volatile or can easily be transformed into volatile compounds, they can readily be separated by gas chromatography and detected by MIP optical emission spectrometry for speciation. Freeh et al. [520] compared the Bee-nakker microwave-induced plasma (MIP) and a furnace atomization plasma excitation spectrometry (FAPES) source for the determination of derivatized mercury species in natural gas condensate with coupling to high-resolution GC for sample introduction and monitoring the emission of the 253.6 nm mercury hne. The precision of replicate measurements for dimethyl-, methylbutyl-, and dibutyhnercury... 

Several workers have chosen to derivatize the mercury species to their corresponding nonpolar, alkylated analogs such as butyl derivatives, which can then be separated on nonpolar packed or capillary columns. 

Figure 8.17 Comparison of CC-ICP-MS chromatograms for mercury species in a river water sample after species-specific isotope dilution with a ° Hg-labeled MMM spike and derivatization with (a) NaBEt4 and (b) NaBPr4. Reproduced with permission from [88]. Only chromatogram (a) indicates conversion of MMM into Hg°.

Dual-electrode LCEC is very useful for the selective detection of chemically reversible redox couples. In this case, two electrodes are placed in series (Fig. 27.1 OB). The first electrode acts as a generator to produce an electroactive species that is detected more selectively downstream at the second electrode, which is set at a more analytically useful potential. One excellent example of the use of a dual-electrode detector for electrochemical derivatization is the detection of disulfides [34]. In this case, the first electrode is used to reduce the disulfide to the corresponding thiol. The thiol is then detected by the catalytic oxidation of mercury, described earlier. Because of the favorable potential employed at the second electrode, the selectivity and sensitivity of this method are extremely high. In addition, thiols can be distinguished from disulfides by simply turning off the generator electrode. 

Methyl mercury is of much greater concern when health effects are considered, as it is much more toxic than ionic mercury or free mercury. Methyl mercury is also much more likely to be bioaccumulated, leading to serious contaminations, especially of fish. The speciation for mercury can be accomplished by derivatizing the methyl mercury and Hg2+ with sodium tetraethylborate, NaBEt4. The volatile MeHgEt, from methyl mercury, and HgEt2, from Hg2+, species formed are purged from the sample solution and separated in a GC column. An atomic emission spectrometer is used as a detector. 

The Grignard reaction is one of the most widely used derivatization techniques for the speciation of a number of elements [7,8]. The main advantage of this reaction is that different alkyl groups can be chosen to make fully alkylated species. Reactions for mercury analysis can be described as follows ... 

DME (an analyte which is not responsive within these limits may be derivatized to a chemical species which is, discussed later). The limits of the voltage window are set by (1) the oxidation of mercury (>0.1 V - SCE) and (2) the reduction of the hydro-nium ion ( — 0.8 to — 2.0 V depending on the solution pH, increasing the pH will shift the reduction potential cathodically). 

GC has been used for the separation of Hg spedes and MC-ICP-MS to measure Hg isotope ratios. Krupp and Donard [11] coupled capillary GC with different MC-ICP-MS instruments via a home-made transfer line for the measurement of the Hg isotope ratios in two Hg species, Meng" " and IHg (Table 17.1). A T1 standard was added continuously to the plasma by using a T-piece for the cormection of the GC unit and the spray chamber with the plasma torch. The five most abundant Hg isotopes were measured simultaneously in the SPEX (IHg + spedes) and STREM (MeHg+ species) mercury standards. These spedes were first derivatized to Et2Hg and EtMeHg, respectively. Then the species were separated from one another using GC and introduced diredly into the MC-ICP-MS instrument, with which isotope ratios were measured based on the short transient signals obtained. 

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