Isotope ratio measurements accurate determination

The use of accurate isotope ratio measurement is exemplified here by a method used to determine the temperature of the Mediterranean Sea 10,000 years ago. It is known that the relative solubility of the two isotopic forms of carbon dioxide COj) in sea water depends on temperature... 

Secondary isotope effects are small. In fact, most of the secondary deuterium KIEs that have been reported are less than 20% and many of them are only a few per cent. In spite of the small size, the same techniques that are used for other kinetic measurements are usually satisfactory for measuring these KIEs. Both competitive methods where both isotopic compounds are present in the same reaction mixture (Westaway and Ali, 1979) and absolute rate measurements, i.e. the separate determination of the rate constant for the single isotopic species (Fang and Westaway, 1991), are employed (Parkin, 1991). Most competitive methods (Melander and Saunders, 1980e) utilize isotope ratio measurements based on mass spectrometry (Shine et al., 1984) or radioactivity measurements by liquid scintillation (Ando et al., 1984 Axelsson et al., 1991). However, some special methods, which are particularly useful for the accurate determination of secondary KIEs, have been developed. These newer methods, which are based on polarimetry, nmr spectroscopy, chromatographic isotopic separation and liquid scintillation, respectively, are described in this section. The accurate measurement of small heavy-atom KIEs is discussed in a recent review by Paneth (1992). 

A major attraction is the ability to perform isotope ratio measurements, e.g. in many geological applications to determine the age of rocks, and isotope dilution analysis. The latter in particular is gaining-popularity as a highly accurate, precise and hence traceable, method of analysis, so it is worthwhile describing these techniques in more detail. 

Isotope ratio measurements are performed whenever the exact ratio, or abundance, of two or more isotopes of an element must be known. For example, the isotopic ratios of lead are known to vary around the world, so it is possible to determine the source of lead in paint, bullets and petrol by knowing the isotopic abundances of the four lead isotopes 204, 206, 207, 208. Another example is the use of stable isotopes as metabolic tracers, where an animal is both fed and injected with an element having artificially enriched isotopes and the fractional absorption of the element can be accurately determined. 

A multiple ion collector device is required for the simultaneous determination of separated ion beams in precise and accurate isotope ratio measurements in order to study, for example, isotope fine variation in Nature or during tracer experiments using enriched stable isotope tracers. In thermal ionization mass spectrometers or in ICP-MS, mostly a system of several Faraday cups (up to 16) and/or ion counters (electron multipliers) is applied. In the photographs in Figures 4.7 and 4.8 examples of multiple ion collector systems are shown from the mass spectrometers MC-ICP-MS... 

Van den Boom et al.150 have reported on the determination of silicon isotope ratio measurements in silicate materials by MC-ICP-MS (at a mass resolution of 2500 to resolve isobaric interferences) after sodium hydroxide sample digestion and purification of silicon. 829Si and 830Si have been determined for several silicon isotope standard reference materials. A precision for 830Si of 0.18-0.41 %o was achieved. Precise and accurate measurements of isotope ratios on transient signals by HPLC-MC-ICP-MS for nuclear application was performed by Giinther-Leopold et al.151... 

An interesting approach is the application of multiple ion collector mass spectrometry (MC-TIMS and MC-ICP-MS) for the determination of Cd and T1 in high purity zinc metal after trace matrix separation by the certification of reference materials from the Bureau Communie de Reference (BCR).29 Accurate and precise element concentrations in high purity zinc metal have been obtained with both mass spectrometric techniques via precise isotope ratio measurements using the isotope dilution strategy the analytical data. 

ICP-MS has been employed, as discussed in Section 9.5, for the determination of platinum originating from cisplatin, carboplatin and oxaliplatin in human plasma ultrafiltrate. The method developed was successfully used to support pharmacokinetic studies in cancer patients treated with cisplatin, carboplatin or oxaliplatin.5 Counterfeit products on the drug market, which have important implications for pharmaceutical companies and human health, can be clarified by mass spectrometric isotope ratio measurements. For example, precise and accurate sulfur isotope measurements (a 54S) by MC-ICP-MS, were employed to study the isotope variation of pharmaceuticals and to detect to the origin of counterfeits by Clough el al.6... 

Isotope Ratio Measurements The analysis of milk samples for the accurate determination of its constituent elements has also been done by isotope dilution analysis [34, 102, 103]. For that purpose, an enriched isotope of the element or elements to be determined is spiked to the sample, altering the natural isotopic composition of the element to be analyzed. By measuring the altered... 

In addition, the occurrence of isobaric interferences of analyte ions with isobaric polyatomic ions can hamper the accuracy and precision of isotope ratio measurements (see also Section 6.1.3). The main factors affecting the accurate and precise determination, for example, of using ICP-MS, are the isobaric interference of the molecular ion on 236pj+ analyte ions, and... 

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... 

Stable isotope (see Box 1.1) abundances cannot at present be determined with sufficient accuracy to be of use in studies of their natural variations. Mass spectrometers can, however, measure the relative abundances of some isotopes very accurately, resulting in stable isotope ratio measurements, for example oxygen —180/160, carbon — 13C12C and sulphur—3%/32S. Stable isotope ratios are reported in delta notation (8) as parts per thousand (%o per mil) relative to an international standard, i.e. ... 

Quantitative analysis Inorganic MS can determine elements at ppt concentrations or below. Inorganic MS is used for simultaneous multielement analysis for metals and nonmetals. Inorganic MS provides the isotope distribution of the elements. Special mass spectrometers are used for accurate isotope ratio measurements used in geology and geochemistry. 

Isotope dilution mass spectrometry (ID-MS) is widely accepted as a quantification procedure of proven accuracy in elemental analysis and isotope ratio measurements [4]. Several areas of research in nuclear science, geochronology, medicinal chemistry, environmental science, and agricultural science have benefited from this technique. ID-MS is applicable to all elements that have at least two stable isotopes. Monoisotopic elements can be analyzed only if they have a long-lived natural or artificial radioisotope. For example, iodine and thorium have been determined with spikes of the long-lived isotopes 29i and 25 Th, respectively [44]. TI-MS and ICP-MS are the methods of choice for accurate ID-MS analysis. ICP-MS has the advantage that several elements can be analyzed simnltaneously under the same experimental conditions. Other ionization techniqnes discussed in this chapter have also been coupled with ID-MS. 

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