Two-dimensional mass spectrometry

Abstract

When we survey two-dimensional mass spectrometry or MS/MS applied to the analysis of biomolecules, direct mixture analysis of individual components without prior chromatographic separation is found to play a dominant role. This type of application is frequently also referred to as targeted compound analysis. Obviously, such an analysis represents a potential substitute for other hyphenated mass spectrometric techniques that rely on ancillary chromatography, i.e. gas chromatography/mass spectrometry (GC/MS) or, more recently, liquid chromatography/ mass spectrometry (LC/MS). As far as methodology for direct mixture analysis by MS/MS is concerned, the use of an additional dimension of mass analysis serves to compensate for the sacrifice of chromatographic resolving power. Hence, two mass analyzers coupled to a tandem are required as the chief parts of an MS/MS instrument consisting of a total of five building blocks. These are i) a "primary" ion source, ii) a first mass analyzer, iii) a collison cell serving as a "secondary" ion source, iv) a second mass analyzer and v) a detector. In order to eliminate time-resolving (and, hence, time consuming) chromatography the mass dispersion (spatial resolution) of the first analyzer is employed to separate M-like ions (ca. 10-s s by MS) instead of neutrals (rain to h by GC or LC). For selecting the mixture component of interest (one by one) the first mass analyzer only has to be set (successively) to the proper fixed mass (e.g. o fMH + as the M-like species in question). When high yields of M-like ions are essential and unnecessary complexity of primary ion current is to be avoided (e.g. trace analysis in complex mixtures) "early" fragmentation in the ion source can be curtailed by "soft" ionization techniques such as chemical ionization,(CI) with suitable reagent gases. Fragmentation, thus suppressed, is, however, a prerequisite to the eventual MS characterization of the mass-selected species. Consequently, fragmentation into daughter ions is to be enacted as a "late" event in a separate step, e.g. by collisional activation (CA) in the collision cell filled with (unreactive) target gas causing collision induced dissociation (CID). The resulting secondary ion current is mass analyzed in the customary way by Scanning the second analyzer over the required range. The detector will then allow the recording of the secondary (CID) spectrum of an ion selected from a primary spectrum. When the mass-selected parent beam was free of isobaric and/or isomeric foreign contributions a CID spectrum of a pure compound is obtained despite the presence of many other components in the sample in large excess. It can be directly compared with reference CID spectra for identification or other evaluation. Several combinations of different mass analyzers (magnetic and electric sector analyzers, quadrupole mass filters) are presently in use allowing CID processes within a wide range of collision energies. Combinations of quadrupoles have certain important advantages over others as to sensitivity and selectivity, but also ease and flexibility of (computer controlled) operation. Applications using a triple stage quadrupole (QQQ) instrument and aimed at structure elucidation of complex biomolecules (peptides, antibiotics) will be discussed to illustrate current trends.