Mass spectrometric complications

For organometailic compounds, the situation becomes even more complicated because the presence of elements such as platinum, iron, and copper introduces more complex isotopic patterns. In a very general sense, for inorganic chemistry, as atomic number increases, the number of isotopes occurring naturally for any one element can increase considerably. An element of small atomic number, lithium, has only two natural isotopes, but tin has ten, xenon has nine, and mercury has seven isotopes. This general phenomenon should be approached with caution because, for example, yttrium of atomic mass 89 is monoisotopic, and iridium has just two natural isotopes at masses 191 and 193. Nevertheless, the occurrence and variation in patterns of multi-isotopic elements often make their mass spectrometric identification easy, as depicted for the cases of dimethylmercury and dimethylplatinum in Figure 47.4. 

The first complication to be considered is the presence of an electrostatic field during the mass spectrometric study of the reaction. Only few quantitative studies have allowed for the possible contribution of hard collisions to cross-section (25), and the possibility that competitive reactions of the same ion may depend on ion energy is generally neglected in assigning ion-molecule reaction sequences. These effects, however, do not preclude qualitative application of mass spectrometric results to radiation chemistry. 

The situation is somewhat better for the gas-phase chemistry of isolated transition-metal ions or complexes, and this area of research has received a lot of attention in the past. On the experimental side, comprehensive mass-spectrometric techniques allow for an explicit measurement of thermochemical and kinetic parameters of reactants, intermediates, and products occurring along the reaction pathways. These data can be obtained without the influence of ligands, counter ions, solvents etc. which would be a highly complicated enter-... 

Why then, is such a complicated and expensive set up necessary AMS combines mass spectrometric features with efficient discrimination of isobaric and molecular interferences. Therefore, it can detect and quantify atomic species of very low abundance. In the case of 14C dating, before AMS was utilized, about 1 g of carbon was needed to date an archaeological item. One gram of fresh carbon contains about 6 x 1010 14C atoms, of which 14 decay per minute. To get 0.5% statistical precision using decay counting, a 48 h acquisition time is necessary. The same result can be obtained with AMS in about 10 min and with only 1 mg of carbon. 

A further common problem the analyst faces when integrating SPC into analytical procedures for the determination of LAS is the scarcity of available reference compounds, thereby complicating their determination. Therefore, the identification of the analytes has to be performed by comparison of retention time and absolute peak area ratio between the deprotonated molecular ion and the fragment ion, relative to the ratio obtained from the authentic standard ( 20%). Retention times of SPC, for which no standards were available, can be determined once by mass spectrometric identification in full-scan mode. 

The major analytical complication in Mo isotope analysis is precise correction for isotope fractionation during Mo purification and mass spectrometric analysis. This subject is reviewed in general by Albarede and Beard (2004), and is discussed here in particular reference to Mo. It is important to recognize that this challenge is fundamentally dififerent in mass dependent stable isotope studies as compared to investigations of mass-independent Mo isotope variations produced by nucleosynthesis. The latter have received attention in recent years for high-precision determination of Mo isotope composition (e.g., Dauphas et al. 2002a,b Yin et al. 2002), but are not relevant here. 

Determination of the D/H ratio of water is performed on H2-gas. There are two different preparation techniques (1) equilibration of milliliter-sized samples with gaseous hydrogen gas, followed by mass-spectrometric measurement and back calculation of the D/H of the equilibrated H2 (Horita 1988). Due to the very large fractionation factor (0.2625 at 25°C) the measured H2 is very much depleted in D, which complicates the mass-spectrometric measurement. (2) water is converted to hydrogen by passage over hot metals (uranium Bigeleisen et al. 1952 Friedman 1953 ... 

FAB-MS (Table 2.8) has been widely used for the characterization of flavonoids solubilized in a variety of matrices, and normally involve the use of xenon or argon atoms for bombardment (Table 2.9). The matrix signals may complicate interpretation of the spectra. Nevertheless, when combined with CID of positive ions and tandem mass spectrometric techniques, FAB-MS can provide information on the aglycone moiety, the carbohydrate sequence, and... 

A gas-chromatographic mass-spectrometric method has also been described for determination of oxolinic, nalidixic, and piromidic acids in fish muscle (191). However, this method is rather complicated due to the need for derivatization (reduction) with sodium tetrahydroborate. 

It may be concluded that, in spite of the complicated mass-spectrometric behavior of sulfated oligosaccharides, the method has been successfully used many times for characterization of fragmentation products of red algal galactans, especially of enzymolysis fragments (additional examples will be given below). 

Gygi et al. claimed an offline SCX fractionation coupled with RP and mass spectrometric analysis to be optimal, as opposed to the in-line SCX-RP biphasic methodology used in MD protein identification technology (MudPlT) [35]. The authors argued that the offline SCX approach provides increased loading capacity, improved resolution, greater flexibility and repeated sample analysis. However, MudPIT s comprehensive analysis of complex peptide mixtures avoids the need for complicated switching valves and minimizes sample loss. 

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