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Abstract No.: TuP-149
Session: New Instrumentation
Presentation date: Tue, Aug 29, 2006
Presentation time: 09:50 – 11:20

Isotope Abundance Analysis for Improved Confidence Level in Sample Identification with the Supersonic GC-MS

Tal Alon1, Alexander Gordin1, Aviv Amirav1

1 Tel Aviv University, Tel Aviv, Israel

Correspondence address: Tal Alon, Tel Aviv University, Chemical Physics, Ramat Aviv, Tel Aviv, 69978 Israel.

Web site: http://www.tau.ac.il/~polaris

Keywords: Elemental Analysis; Isotope Distributions; Library Searching; Molecular Beam.

Novel aspect: A new isotope abundance analysis method and software for improved sample identification by GC-MS and LC-EI-MS.


It is well known that the relative abundances of the various isotopomers (molecular ion peaks with different isotopes) can provide accurate elemental formulas information. While a few isotope abundance analysis (IAA) methods are documented in the literature for the last twenty years, IAA is currently ignored and unused in combination with electron ionization mass spectrometry. The reasons for this are the facts that over 30% of the molecules do not show a molecular ion, while with the rest of the compounds self chemical ionization and vacuum background can severely hamper the isotope abundance measurement accuracy.

We have developed a novel IAA method and software that, among other things, links with the NIST MS library and automatically confirms or rejects its identification. In case of a confirmation the sample identification is unambiguous since the confirmation is done by an independent set of data and method. In the case of a rejection, the IAA software independently provides a list of elemental formulas with declining order of matching to the experimental data, in similarity to costly accurate mass measurements.

IAA seems as ideally applicable to GC-MS (and LC-EI-MS) with supersonic molecular beams (SMB) since it requires having a trustworthy and high abundance true molecular ion that is unique to the Supersonic GC-MS, plus absence of self CI and vacuum background, again unique features of our Supersonic GC-MS.

GC-MS with SMB is based on the coupling of a SMB inlet and ion sources with a bench top Agilent 6890 GC + 5972 MSD and it is named Supersonic GC-MS. A new Supersonic GC-MS was recently built and evaluated, based on the combination of SMB inlet and ion sources with the Varian 1200 triple quadrupole GC-MS-MS. The GC column output, at any column flow rate, is mixed with helium make up gas at a flow rate of 90 ml/min and this gas mixture expands from the supersonic nozzle into a differentially pumped vacuum chamber. The expanded sample compounds are skimmed and enter as a SMB into the MS vacuum chamber where the vibrationally cold sample molecules are ionized by 70 eV electrons in a homemade dual cage fly-through EI ion source, and mass analyzed by a quadrupole mass analyzer.

The confidence level in sample identification with the Supersonic GC-MS is significantly improved through: A) Enhanced molecular ion; B) Complementing cold EI (if needed) by cluster chemical ionization; C) Library identification of cold EI mass spectra with a probability that is usually higher than that obtained with standard GC-MS; D) Providing enhanced isomer MS effects; E) Extended range of thermally labile and low volatile compounds that are amenable for analysis, and F) The applicability of the IAA method and software as above.
As a result, the Supersonic GC-MS provides improved confidence level in sample identification for an extended range of compounds.