A new type of GC–MS with advanced capabilities

Abstract

We have combined the benefits of supersonic molecular beam interface and its related fly-through electron ionization (EI) ion source with the advanced features of the Varian 1200L gas chromatography–mass spectrometry (GC–MS) and mass spectrometry–mass spectrometry (MS–MS), resulting in a new and powerful GC–MS platform with record setting performance. Electron ionization of vibrationally cold molecules in the supersonic molecular beams (SMB) (cold EI) provided mass spectra with enhanced molecular ion, yet with good library search results and superior identification probabilities. We found that high GC column flow rates lower the elution temperature for any given compounds. This allows much larger molecules to elute at the maximum temperature of standard columns. We analyzed a mixture of heavy linear chain hydrocarbons all the way to C 84H 170 with a molecular weight of 1179.3 amu, using a 4 m 0.25 mm i.d. column and 32 ml/min He flow rate. Furthermore, we obtained a dominant molecular ion to all these compounds. The lower elution temperatures also greatly enhance the ability to analyze very thermally labile compounds such as carbamate pesticides. The experimental 1200 system is capable of triple quadrupole based MS–MS. We found that MS–MS on the molecular ion is much more effective than on fragment ions, and thus, the enhancement of the molecular ion directly improves the MS–MS sensitivity. Fast GC–MS analysis was also explored, based on very high column flow rate for fast splitless injections without affecting the sensitivity, and on the high system selectivity due to the combination of enhanced molecular ion and MS–MS. We demonstrate a few seconds long GC–MS–MS analysis of diazinon, spiked at 10 ng/g in a mixed fruit and vegetable extract. The feature of enhanced molecular ion provides significant enhancement in the detection sensitivity via SIM and RSIM on the molecular ion. While octafluoronaphthalene (OFN) detection limit of below 1 fg in SIM mode is shown, the harder is the compound analysis by standard GC–MS the greater is the gain in sensitivity by 1200-SMB. For example, the use of the 1200-SMB resulted in an improvement of over 50 times in the RSIM detection limit of cholesterol on its molecular ion at m/ z = 386, and decabromobiphenylether could be analyzed in SIM mode at 1 ppb with S/N > 50 (peak-to-peak noise).