Gas-phase acid-base chemistry and its effects on mass isotopomer abundance measurements of biomolecular ions
✍ Scribed by Fagerquist, Clifton K.; Schwarz, Jean-Marc
- Publisher
- John Wiley and Sons
- Year
- 1998
- Tongue
- English
- Weight
- 466 KB
- Volume
- 33
- Category
- Article
- ISSN
- 1076-5174
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✦ Synopsis
Various parameters which a †ect mass isotopomer abundance measurements of derivatives of palmitic acid ionized by electron ionization (EI) and electron-capture negative chemical ionization (ECNCI) were tested on a sector-Ðeld double-focusing mass spectrometer. Results on methyl palmitate ionized by EI are as follows : (i) sample size had a signiÐcant e †ect on mass isotopomer abundance ratios (MIARs) ; (ii) the electron multiplier gain of the detector also had an e †ect on MIARs ; and (iii) ion scattering by ion-neutral collisions in the mass analyzer did not appear to have any signiÐcant e †ect on MIARs (under standard analysis conditions). However, "reagentÏ gas pressure (methane) had a signiÐcant e †ect on MIARs of pentaÑuorylbenzyl palmitate ionized by ECNCI. It was concluded that there are two compensatory e †ects which alter MIARs of methyl palmitate ionized by EI : (i) gas-phase acid-base chemistry in the source (speciÐcally, proton transfer between fragment cations and neutral molecules) ; and (ii) detector non-linearities, speciÐcally, underestimation of less abundant isotopomers due to their signal disproportionately falling within the signal-to-noise ratio level of the electron multiplier. Gas-phase chemistry is the dominant cause of inaccuracy in MIAR measurements for large sample sizes, while detector non-linearity is the dominant cause of inaccuracy in MIAR measurements at small sample sizes. However, in a narrow intermediate range of sample size, these two e †ects balance each other and result in MIARs which are "acceptableÏ when compared with the known MIAR values. It is emphasized that these two e †ects are present regardless of the type of mass analyzer used, e.g. quadrupole, sector-Ðeld. Improvements in the accuracy of MIAR measurements will require developments in mass spectrometry aimed at eliminating each of the contributory e †ects.