The Thermo DSQ II GC-MS instruments are our most up-to-date instruments for analysis of compounds that are sufficiently volatile to enter the gas phase intact. The Mass Spectrometer of the Trace DSQ II GC-MS instrument at our Upper Campus facility can be operated in either electron impact (EI+) positive ionization mode, positive ion chemical ionization (CI+) mode, or electron capture negative ionization mode (ECNI or CI-). The Quadrupole MS gives nominal m/z assignments, accurate to +/- 0.2 Da. The associated gas chromatograph is equipped with both a split / splitless heated inlet and a programmable temperature vaporization (PTV) inlet. Samples are normally injected dissolved in organic solvents, using a Thermo Triplus Autoinjector. Gaseous headspace sampling and solid phase microextraction (SPME) capabilities will soon be added to the instrument, facilitating automated sampling of the gaseous headspace volume above liquids. This will be particularly useful for analyzing traces of volatile organic compounds in an aqueous matrix.
Electron impact ionization (EI+) is the ‘standard’ ionization mode used with GC-MS and provides characteristic ion fragmentation mass spectra, that can be compared with standard library reference spectra to identify unknown sample components. In some cases, involatile or thermolabile analytes can be chemically modified or derivatised to increase their volatility and/or thermal stability, thereby making them suitable for GC-MS analysis. Trimethylsilylation of –OH and –COOH is a frequently used approach. Chemical derivatization of samples prior to analysis can also be used to make analytes more electrophilic and amenable to negative ionization by electron capture (ECNI). This usually involves addition of perflourinated groups like pentafluorobenzyl- or hexafluoroisopropyl- to the analyte molecule. This approach can yield very high sensitivities with femtogram detection limits for some analytes and forms an excellent basis for trace quantitative analysis of target compounds.