Isotope enrichments biomarkers

The use of species-specific spiking greatly enhances the traceability of the analytical procedure and improves the reliability of the results. On the other hand, the use of isotopically enriched metalloproteins provides a means to evaluate the analytical conditions, under which the metal center is most stable. Despite all the advantages of species-specific spiking for ICP-MS, it has to be mentioned that huge effort is necessary to produce and characterize an isotopically enriched protein spike. Therefore, this approach might be sensible in practice for the assay of only a few proteins or biomarkers. ... 

Isotope dilution MS is based on the principle of the addition of stable isotope internal standard to a liquid specimen. The standard is chemically most like the unlabeled substrate such that it is goes through the same processes of sample preparation or modification. The only real differences between the standard and unlabeled substrate is mass. MS uniquely exploits the mass differences and is one of the reasons why it is considered the gold standard in biomarker quantification. Figure 13.6 illustrates this point for some of the most common internal standards used in NBS. It is noteworthy that inappropriate use of enrichments can alter the chemical and physical properties of the labeled internal standard. For example, excessive deuterium enrichment or the use of a deuterium at an exchangeable reactive position can result in poor quantification or loss of enrichment. 

Gas chromatography, using bonded-phase, fused-silica capillary columns, is the most commonly employed procedure for the separation of minor and trace hydrocarbon components. A variety of detectors are used the most common is flame ionization for generic quantitation, magnetic sector, and quadrupole MS and MS/MS for biomarker compounds (43), element-specific detectors for het-eroatomic and metallocompounds (44), and high-resolution MS for measurement of carbon (45) and hydrogen (46) isotopic ratios. Ultra-high-resolution Fourier transform MS (FTMS) coupled with soft-ionization techniques offers the opportunity to detect trace petroleum components without prior enrichment (47). 

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