Saturated isotopic labeling

Exchange reactions between bulk and adsorbed substances can be studied by on-line mass spectroscopy and isotope labeling. In this section the results on the interaction of methanol and carbon monoxide in solution with adsorbed methanol and carbon monoxide on platinum are reported [72], A flow cell for on-line MS measurements (Fig. 1.2) was used. 13C-labeled methanol was absorbed until the Pt surface became saturated. After solution exchange with base electrolyte a potential scan was applied. Parallel to the current-potential curve the mass intensity-potential for 13C02 was monitored. Both curves are given in Fig. 3.1a,b. A second scan was always taken to check the absence of bulk substances. 

Considerable interest in the subject of C-H bond activation at transition-metal centers has developed in the past several years (2), stimulated by the observation that even saturated hydrocarbons can react with little or no activation energy under appropriate conditions. Interestingly, gas phase studies of the reactions of saturated hydrocarbons at transition-metal centers were reported as early as 1973 (3). More recently, ion cyclotron resonance and ion beam experiments have provided many examples of the activation of both C-H and C-C bonds of alkanes by transition-metal ions in the gas phase (4). These gas phase studies have provided a plethora of highly speculative reaction mechanisms. Conventional mechanistic probes, such as isotopic labeling, have served mainly to indicate the complexity of "simple" processes such as the dehydrogenation of alkanes (5). More sophisticated techniques, such as multiphoton infrared laser activation (6) and the determination of kinetic energy release distributions (7), have revealed important features of the potential energy surfaces associated with the reactions of small molecules at transition metal centers. 

Ionization changes can be efficiently corrected with the use of an isotopically labeled IS, which possesses identical ionization response and fragmentation pattem. Therefore, deuterated IS can be used to correct both the overall method variability (e.g., sample preparation, injection, electrophoretic process, etc.) as well as matrix effects since the amount of suppression from interferents is expected to be similar. However, the total concentration of analyte and IS should be below the saturation of the ionization process. Guidelines to obtain a reproducible CE—MS method were published by Ohnesorge et al. and took into account the use of an isotopically labeled IS. 

Apart from the 1,5 isomer, there is experimental evidence only for the intermediate formation of 1,2-didehydroindane (93), the saturated analogue of 79. The FVP of nona-l,3,8-triyne (94) yields indane (95) and indene (96), which is formed from 95 by loss of Results from isotopic labeling suggest an unusual (1,3-... 

An additional type of derivative that is often used for glycan profiling and analysis is the modification of the reducing end with a chromophore, usually achieved by the formation of a Schiff base with an aromatic amine, and subsequent reduction with sodium borohydride to stabilize the initially formed product by its conversion to a saturated amine. This procedure was initially developed for HPLC analysis with ultraviolet detection, but it is also appropriate for MS analysis because the substituted amino group represents a site for charge localization that simplifies the fragmentation pattern and also provides the opportunity to introduce a stable isotope label for quantitative purposes (54). 

From feeding experiments with isotopically labelled 5-aminolevu-linic acid (18) and methionine (142) it became clear that the carbon skeleton of the naturally occurring highly saturated porphyrins are formed from uroporphyrinogen III (14) and that the additional methyl groups are transferred from 5-adenosyl methione (77, 72). 

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