Species-specific ICP-IDMS

General Remarks For species-specific ICP-IDMS, the elemental species needs to be well defined in terms of composition and structure. This is feasible for most of the low molecular weight elemental species of environmental or biochemical importance, such as the inorganic compounds chromate and iodate, but also for organo-elemental species, such as monomethylmercury (MMM) (MeHg ), 

As an important example of a biochemically relevant species, Se-labeled selenomethionine can be prepared, for example, by isolation from selenized yeast, grown in a Se-labeled selenite solution. After digestion of the yeast with pro-tease/lipase solution, Se-labeled selenomethionine is isolated by size-exclusion chromatography (SEC) [72, 73]. Another important example is the synthesis of Fe-labeled transferrin [74]. The synthesis of other biologically relevant molecules, especially those of higher molecular weight, is usually too complicated, and the species-unspecific spiking mode is more convenient in these cases. 

Speciation Analysis Simultaneous Determination of Multiple Species of the Same Element At the beginning of the development of species-specific isotope dilution, only a single spike compound was used [77]. Later, species-spedfic IGP-IDMS has also been used for the determination of more than one spedes of the same element, using different spike spedes labeled with the same isotope. Examples include the determination of mono- and dimethylmercury (MMM and DMM) and of the three butyltin compounds TBT, DBT, and MBT [80-83]. Various spike isotopes, usually of medium natural abundance, such as Hg or ° Hg and Sn, Sn, or Sn, have been applied. The reason for selecting this type of spike 

An application of technical interest is the determination of different thiophene derivatives, the major sulfur impurities in petroleum products. S-labded thiophene, dibenzothiophene, and 4-methyldibenzothiophene spikes in hexane were used to determine these sulfur compounds in different gasoline samples, naphtha, diesel fuel, gas oil, and heating oil by species-spedfic GC-ICP-IDMS [55]. The accuracy of the results thus obtained was demonstrated by successftd analysis of the reference material NIST SRM 2296. The LOD was limited to 7 ng of sulfur per gram of sample by the background noise in the isotope ratio chromatograms. 

Speciation Analysis Simultaneous Determination of Species of Different Elements 

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Hyphenated techniques are the analytical methods most frequently used today for elemental speciation. For that purpose, a separation technique, such as high-performance liquid chromatography (HPLC), gas chromatography (GC), capillary electrophoresis (CE), or gel electrophoresis (GE), is coupled on-line with an elemental detection method, such as ICP-MS [67]. Hyphenated ICP-MS techniques can also be combined with the isotope dilution method for quantification of elemental species. Two different spiking modes are possible, the species-specific and spedes-unspedfic mode. Rottmann and Heumann were the first to present a setup for H PLC-ICP-IDMS enabling the use of these two spiking modes [68] (Figure 8.14). 

Figure 8.14 Elemental speciation via HPLC-ICP-IDMS using the species-specific or species-unspecific spiking mode. Reproduced with permission from [68].

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