Non-linearity in the quadrupole detector system: implications for the determination of the 13C mass distribution of an ion fragment.
Some biomedical (13)C tracer applications to study glucose/lactate metabolism require the determination of up to 7 mass isotopomers of glucose, generated by the (13)C-labeling. Isotopomer ratios determined by GC-Quadrupole-MS can depend on the analyte amount being measured and may differ from expected theoretical values. A calibration is necessary to correct these effects. However, the large number of different isotopomers prevents an empirical calibration, that is based on known isotopomer mixtures. Neither insufficient mass separation in the quadrupole nor proton adducts or losses of the ionized fragment can account for the observed deviation from expected values and the dependency on the sample size. We explain the deviations with a nonlinear mapping of the isotopomer amount being analyzed to the area under the elution curve for the corresponding mass channel. The nonlinear relation allows to estimate potential errors in the determination of isotopomer ratios. It also allows to define a dynamic range as the range of signal areas, for which the systematic error in the isotopomer ratio determination is below 5% of the nominal value. It is 1 : 200 and 1 : 500 for two different fragments of glucose, measured under EI and CI and is smaller than the range imposed by the measurement precision. After correcting the nonlinearities the dynamic range improved by the factor 10 in both cases.
Vogt
Josef A
JA
Universitätsklinik für Anästhesiologie, Klinikum der Universität Ulm, Sektion Anästhesiologische Pathophysiologie und Verfahrensentwicklung, Parkstrasse 11, 89073 Ulm, Germany. josef.Vogt@medizin.uni-ulm.de
Wachter
Ulrich
U
Georgieff
Michael
M
D002247:Carbon Isotopes / Q000032:analysis*; D008401:Gas Chromatography-Mass Spectrometry; D005943:Gluconeogenesis / Q000502:physiology; D005947:Glucose / Q000032:analysis / Q000096:biosynthesis*; D007477:Ions; D019344:Lactic Acid / Q000378:metabolism; D008970:Molecular Weight; D015203:Reproducibility of Results; D021241:Spectrometry, Mass, Electrospray Ionization
D002247:Carbon Isotopes; D007477:Ions; D019344:Lactic Acid; D005947:Glucose