Chemical ionisation mass spectrometry: A survey of instrument technology

Compatibility of electron ionization and soft ionization methods in gas chromatography/orthogonal time-of-flight mass spectrometry

Orthogonal acceleration time-of-flight (oa-TOF) mass spectrometry (MS) was coupled to gas chromatography (GC) to measure ion yields (ratio of ion counts to number of neutrals entering the ion source) and signal-to-noise (S/N) in the electron ionization (EI) mode (hard ionization) as well as in the soft ionization modes of chemical ionization (CI), electron capture negative ion chemical ionization (NICI) and field ionization (FI). Mass accuracies of the EI and FI modes were also investigated. Sixteen structurally diverse volatile organic compounds were chosen for this study. The oa-TOF mass analyzer is highly suited for FI MS and provided an opportunity to compare the sensitivity of this ionization method to the more conventional ionization methods. Compared to the widely used quadrupole mass filter, the oa-TOF platform offers significantly greater mass accuracy and therefore the possibility of determining the empirical formula of analytes. The findings of this study showed that, for the instrument used, EI generated the most ions with the exception of compounds able to form negative ions readily. Lower ion yields in the FI mode were generally observed but the chromatograms displayed greater S/N and in many cases gave spectra dominated by a molecular ion. Ion counts in CI are limited by the very small apertures required to maintain sufficiently high pressures in the ionization chamber. Mass accuracy for molecular and fragment ions was attainable at close to manufacturer's specifications, thus providing useful information on molecular ions and neutral losses. The data presented also suggests a potentially useful instrumental combination would result if EI and FI spectra could be collected simultaneously or in alternate scans during GC/MS.

Ammonia laser desorption/chemical ionization with ammonium bromide: Fourier transform ion cyclotron resonance mass spectrometry of aromatic hydrocarbons

Laser desorption/ionization combined with Fourier transform ion cylcotron resonance mass spectrometry (LD/FT/ICR/MS) is a proven technique for the analysis of nonvolatile materials. Unfortunately, LD tends to produce a large excess of neutral species compared to ions. Laser desorption followed by chemical ionization (LD/CI) by use of a reagent gas is a seIective and sensitive means of control in the analysis of nonvolatile compounds. In this article we demonstrate the technique of ammonia LD/CI by addition of a small amount of ammonium bromide (NH4Br) to an involatile sample, i.e., the ammonium salt is used in place of ammonia reagent gas. For various aromatic hydrocarbons, abundant (M + H)(+) ions are produced as a result of CI A primary advantage of this method in FT/ICR/MS is that selective LD/CI experiments may be conducted at low pressure as in pulsed valve CI (but without the need for pulsed valve operation), thereby providing the potential for obtaining high-resolution FT/ICR mass spectra.