Measurement of mass isotopomer distributions

MS can measure the ratio between molar fractions of mass isotopomers. The ratio between two mass isotopomer pools of masses nti and m2 is defined in the present work as intensity ratio Jmi/m2- K identical with a mass spectral intensity ratio. If more than two mass isotopomer pools are assessed, their relative ratios, normalized to the sum, are named mass isotopomer distribution. The mass distribution of a compound can be thus obtained from the analysis of ions, which contain the intact carbon skeleton of the analyte. In the area of me-tabohc flux analysis, mass distributions of various metaboHtes have been assessed by MS. The major method used is GC/MS, whereby the analytes are deriva-tized into forms with desired physico-chemical properties such as increased volatihty, thermal stabiHty and suitable MS properties [62]. The mass of the formed derivate must be sufficiently high (usually above 175 apparent mass units) to avoid background interference [48]. To obtain the mass distribution of a compound, ions with the entire carbon skeleton of the analyte have to be present. For accurate quantification of the mass distribution of such ions, they should occur in high abundance and preferably be unique species, thus being formed by only one fragmentation pathway. 

The compounds analyzed by GC/MS comprise e.g. amino acids [34, 39, 64 - 69], organic acids [33,63,65,66,69], sugars [39,70,71 ], lipids and fatty acids [72, 73]. Moreover, mass distributions of polymers and their building blocks, obtained via hydrolysis of the polymer, were assessed. Examples are glycogen [39, 70], cell protein [8,10,17], or DNA [74]. Most of the analytical methods have been developed for tissue samples. Since most of the compounds studied are polar or even charged molecules, derivatization is necessary in most of the cases of GC/MS analysis. The derivatization method of choice clearly depends 

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