Labeling, stable isotope clusters

Although additional work clearly is required to sort out the details, it seems likely that a supernova was involved in the formation of the solar system and that the Sun originated in a region of cluster star formation. One avenue of future work will be to investigate the potential contributions to stable isotopes from a late supernova. It is difficult to establish the level of such a contribution from observations because there are no labels on the atoms of stable isotopes produced in the last supernova as there are for the short-lived radioactive isotopes. It might be possible to infer the contribution by comparing solar system... 

Unfortunately, for the majority of small molecule LC-MS/MS analyses, stable isotope labelled internal standards are not available so far. In such cases, compounds with a very similar molecular structure typically serve as internal standard ( homologues or analogues ). Since the ionization properties are substantially determined by functional groups of a molecule, ionization behaviour may differ significantly—even between compounds with very similar over-all molecular structure. Differential clustering, e.g. with sodium, ammonium or formate ions often present in mobile phases may as well impact the parity of ionization yield between analyte and internal standard. Hence the availability of an appropriate homologue is crucial and critical for the development of reliable LC-MS/MS methods in TDM [51]. 

A more complex situation arises when a label is used with a high natural abundance of stable isotopes, e.g., 13C that occurs in nature as 1.07% (compared to 0.016% for 2H). In that case the cluster will have to be adapted for each variant. Again, a computer program can be used to calculate the theoretical clusters for each variant and thus build up a corrected matrix [A] (Jonckheere, 1982). 

Following HPLC separation of different folate forms, a number of detection systems can be used including UV, fluorometric, or electrochemical activity (65). Due to its sensitivity and selectivity, fluorescence detection is most commonly used, particularly for reduced folate forms (86). Quantitation is usually based on external standard methods, although an internal method using a stable isotope label has been recently reported (see below) (87). More cumbersome identification and quantifications are needed in methods in which folates are eluted in clusters (69). It is important that the actual concentrations of folate calibration solutions are determined using known extinction factors (30) because most commercially available folate calibrants are of variable purity (7). 

Isotopic labelling often shifts the mass of one isobar relative to another for the purposes of identification. In ionized mixtures of methyl fluoride with a trace of ammonia, the weakly hydrogen-bonded cluster ion CH3F- HNHj (m/z 52) has the same mass as a much more stable ammonia cluster ion, (NH3)3H+. With NH3 the mass-to-charge ratios become m/z 53 and m/z 55, respectively. Moreover, the isotopic substitution identifies the product of the m/z 52 m/z 53 fragmentation as FCH. ... 

Classical feeding experiments with both stable and radioactive isotopic labels (7) enabled the biosynthetic origin of the polyethers to be elucidated and for a general stereochemical model to be proposed (1). More recent work on this class of compounds has focused on a genetic approach, and unusual and interesting genes specific to polyether biosynthesis have been isolated from these clusters. 

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