The potential of mass spectral analysis is amplified by using a series of MS analyzers in tandem, or MS-MS and MSn. This affords a higher degree of sensitivity, lowers the possibility of interference from contaminants, and aids in structural identification of the molecule for the chemical analysis; but it also adds substantially to the cost of the procedure. The general process most commonly used in MS-MS is similar to linking multiple quadrupoles together and assigning each a separate function. Described as MS-MS in space, three quadrupoles are typically set to analyze the sample in series. The first quadru-pole filters the analyte ions in the traditional sense of a mass analyzer by allowing only the ion with the desired m/z to pass. Fragmentation of the chosen ion, referred to as the precursor ion, occurs in the second quadrupole by impact with collision gas molecules to form product ions. These product ions can then be scanned by the third quadrupole or selectively allowed into the detector. Alternatively, MS-MS may be performed by MS-MS in time, which uses the ion trap as its mass analyzer. After the ejection of all but the desired precursor ion from the trap, the fragmentation of ions with m/z values that resonate with the particular applied voltage occurs to generate product ions. This multistage entrapment and fragmentation of ions can hypothetically continue as many times as desired, but is usually unnecessary and cost prohibitive. Nevertheless, the multiple steps of MS-MS virtually eliminate contamination by undesired compounds that may co-elute from the GC column and enable the fragmentation analysis of isolated ions. These are both positive results that will help untangle complex molecular structures and raise the confidence of identification.
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