Direct analysis of complex mixtures by mass spectrometry

Abstract

Mixture analysis can provide information on individual components if the sample is first subjected to chromatographic separation. Two critical capabilities, soft ionization and the ability to mass-select and then dissociate ions of a particular m/z, coalesced in 1975, allowing direct analysis of complex mixtures by mass spectrometry. Chemical ionization was used as the soft ionization method and mass-analysis used the ion kinetic energy spectrometer (MIKES). Soft molecular ionization produces a set of ions that are structural surrogates of the neutral molecules; they can be mass-selected and allowed to spontaneously dissociate (i.e. as metastable ions) or fragmented upon energy transfer e.g. in the course of collision-induced dissociation (CID). The second stage of mass analysis provides information about the atomic connectivity in the precursor ion and by implication in the original molecule. This review focuses on the development of complex mixture analysis by mass spectrometry and allied topics. Discussion of the activation techniques associated with (collision-induced dissociation, surface-induced dissociation and metastable ion dissociation) emphasizes the importance of energy transfer phenomena and the internal energies distributions of ions to explain the observed mass spectra. The translational to internal energy transfer in collisions is readily accessed in the MIKES where the second stage mass analyzer is a kinetic energy/charge analyzer. Collisions in the keV range can also be used to change ion the charge state via the processes of charge exchange, electron stripping, or charge inversion. New ionization sources for analysis of non-volatile compounds that were introduced during the time period (1975–1990) of this review included secondary ion mass spectrometry, plasma desorption and field ionization and they are briefly discussed. Several types of scans were developed to rapidly access information of the individual components, including chemically specific scans (e.g. neutral loss scans of mass 30 for nitro compounds).