5.5.7 Tandem Mass Spectroscopy

Mass spectroscopy is commonly referred to as a confirmatory technique since there is little doubt (error) in the identity of an analyte. To be even more certain of an analyte’s identify, two or even three, mass spectrometers can be used in series (the output of one MS is the input of another MS). Most often a soft ionization source, such as chemical ionization, is used in the first MS and allows for selection of the molecular ion in the first MS, while a harder ionization is used in the second MS to create fragments. A subsequent MS will select for a specific ion fragment from the second MS and further fragment it for identification. This technique allows a molecular ion (or ion fragment) to be isolated in the first MS, subsequently fragmented in the second and third MS, and identified based on its final fragment pattern. You should be able to see the confirmatory nature of this technique.

Mass filters of choice for use in tandem include magnetic sector, electrostatic, quadrupole, and ion trap systems. In the absence of HPLC or GC introduction, tandem MS offers many of the same advantages of a single GC-MS or HPLC-MS system but it is much faster since the analyst does not have to wait on the chromatography portion of the analysis. For example, chromatograph separations take from minutes to hours prior to entry into a MS, while tandem MS systems (without GC) require only milliseconds. But of course, this saving in time is considerably more expensive than simple chromatographic-based MS systems.

©Dunnivant & Ginsbach, 2008