Sulfur isotope dilution analysis

The principle of the isotope dilution analysis (IDA) is described in Section 6.4. Due to its advantages as a definitive and accurate analytical method for the determination of element concentration via isotope ratio measurements, IDA is being increasingly applied in mass spectrometry, especially in ICP-MS and LA-ICP-MS as one of the most frequently used techniques. For example, the isotope dilution technique is employed in species analysis in biological systems, " e.g., for the determination of mercury species in tuna material,or in aquatic systems. Further applications of the isotope dilution technique are the determination of selenomethionine in human blood serum by capillary HPLC-ICP (ORC) MS ° or sulfur speciation in gas oil, diesel or heating fuel by LA-ICP-MS. Evans and co-workers have reported on the high accuracy analysis of sulfur in diesel fuel by IDA. ICP-SFMS has been employed for Si species analysis in biological or clinical samples and... 

The isotope dilution technique in LA-ICP-MS has been applied for the direct analysis of sulfur species in gas oil, diesel or heating fuel as described by Boulyga et al. In addition, these authors have developed an analytical method for the direct determination of halogens (Cl, Br and I) in powdered geological and environmental samples using isotope dilution laser ablation ICP-MS. The detection limits of LA-ICP-IDMS are in the low and sub-p,gg range for halogens. ... 

Figure 8.9 Signal intensities and corresponding sulfur and lead isotope ratios of an isotope-diluted diesel fuel sample as a function of time during LA-ICP-IDMS analysis. Reproduced with permission from [47,48].

Chapters on sample introduction and hyphenated sample treatment and ICP systems have also been further updated since the last edition. No doubt that chromatographic, electrophoresis, flow injection and field flow fraction separations have extended ICP-MS (and AES) measurements as the mainstay of elanental specia-tion measurements in biological and environmental fields. Without the combination of these separation techniques and ICP measurements, elemental speciation applications would be severely hampered... if not impossible (Chapter 18). The ability to measure P and S with high sensitivity has opened up new opportunities in proteomics, for example. Species-specific and unspecific isotopic dilution (ID-MS) has been critical in quantifying speciation analysis and revealing recovery errors (Chapter 13). Species-specific techniques have been applied to identify species transformations, resulting in the development of multi-species methods whereas, hyphenated species-unspecific ICP-ID-MS determinations of heteroatoms such as sulfur have become a common quantification technique in proteomics. 

An important criticism of the use of combustion trains is that combustion is not site specific, that is all atoms in the analyte end up in the gas transferred to the IRMS. For studies of carbon isotope effects this is invariably C02. The question is especially important for carbon isotope analysis because analyte molecules of interest usually contain several different kinds of carbon atoms and therefore combustion methods average or dilute the IE s of interest. Should site specific isotope ratios be required another method of sample preparation (usually much more tedious) is necessary. Combustion methods, however, are frequently used to study nitrogen and sulfur IE s because many organic molecules are singly substituted with these atoms. Obviously, oxygen isotope effects cannot be determined using combustion trains because external oxygen is employed. Rather some type of pyrolytic sample preparation is required. 

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