Mass Spectrometry Mass Spectrometry MSMS

Mass spectrometry-mass spectrometry, also known as tandem mass spectrometry, is the term used to describe mass spectrometric methods employing instruments that contain more than one mass analyser. Such instruments may be used to increase the amount of structural information obtained or to introduce more specificity.

The simplest tandem mass spectrometer to consider is the triple quadrupole mass spectrometer. This com-

Figure 5 GC-MS analysis of dibenzothiophene (DBT) in a crude oil using low resolution (top) and high resolution (bottom) selected ion monitoring. (Reproduced from Tibbets and Large (1988) by kind permission of John Wiley and Sons.)

prises two quadrupole mass filters and a multipole collision cell and is shown schematically in Figure 7. In principle the operation of such an instrument follows the following sequence: ion selection in the first quadrupole mass filter, ion dissociation in the collision cell and separation of the products of ion dissociation in the second quadrupole mass filter. For illustration, Figure 2C, the NCI mass spectrum of 2-nitrofluorene, only contains the M!' ion at m/z 211. Hence the only information contained in this mass spectrum is the relative molecular mass of the compound. In order to generate structural information a product ion scan could be carried out. To do this the first quadrupole mass filter would be set to transmit only m/z 211. This ion would then be subjected to collisions with a gas held in the collision cell (collisonally induced decomposition (CID)) and the resulting product ions recorded using the second quadrupole mass filter. The resulting product ion spectrum then shows only ions that have arisen

Figure 6 A comparison of the GC-HRMS-SIM fingerprints of methyldibenzothiophenesand C2-dibenzothiophenesin three crude oils from two North Sea fields. (Reproduced from Tibbets and Large (1988) by kind permission of John Wiley and Sons.)

directly from the fragmentation of m/z 211 and contains structural information. It can be seen that this method is also useful for clearing up ambiguities in the interpretation of EI spectra, since it allows precur-sor¬°product ion relationships to be clearly defined.

A second application of tandem mass spectrometry often used in conjunction with GC-MS utilizes a technique called multiple reaction monitoring (MRM). This technique, like high resolution SIM, is used to increase the specificity of an analysis. After first recording product ion mass spectra of the analyte(s) of interest, one or more precursor/product ion relationships are chosen. The criteria for this are that the product ions selected are intense and characteristic of the specified analyte. Then in order to carry out the MRM experiment, the first quadrupole mass analyser is set up in SIM mode, to switch between the precursor ions of interest. The collision cell is operated in

Ion source (ion formation)

Ou a dru pole mass filter 2 (product ion analysis)

rrf-t

Data system p-i

Chromatograph (component selection)

| Collision cell ¬°(formation of product ions) Quadrupole mass filter 1 (precursor ion selection)

Detector

Figure 7 A triple quadrupole mass spectrometer. On leaving the GC column sample components are first ionized, then ions selected by the first quadrupole mass filter are fragmented in the collision cell for analysis by the second quadrupole mass filter. Such instruments may yield greater structural information than single stage instruments and allow further selectivity to be introduced into GC-MS experiments.

the normal way, and the second quadrupole mass filter is set up to switch between the characteristic product ions.

The output from such an experiment is chromatographic, producing one or more plots of signal intensity against time. Peaks are only observed in such chromatograms when an ionized compound yields an ion of the selected precursor ion m/z value, which also subsequently fragments, under CID, to give a product ion of the selected product ion m/z. The two stages of mass selection make this a highly selective technique and it has been proposed as a viable alternative to high resolution SIM for a variety of applications.

Recent advances in ion trap technology have meant that similar modes of operation also available on these compact, relatively low cost instruments. In this case ions other than the precursor ion of interest are selectively ejected from the trap. The selected precursor ion is then subjected to CID in the trap and a product ion mass spectrum may be recorded by ejecting these ions. For a fuller discussion of tandem mass spectrometry see Busch et al. (1988).

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment