Current Imsms Applications

APCI-MS enables ion chemistry at pressures used in typical IMS systems to be studied, but some issues remain regarding cluster formation in the interface region and this could influence the interfacing of IMS

Figure 3 An ion mobility spectrometer-mass spectrometer.

(which operates close to atmospheric pressure) to MS. However, Spangler has recently published details regarding a better understanding of the behaviour at the IMS-MS interface.

An ion mobility spectrometer may be coupled to a mass spectrometer (see Figure 3) with sample transfer via a pinhole, typically 50 | to 100 | in diameter. The mass spectrometer used in conjunction with an ion mobility spectrometer enables m/z identification of the reactant and product ions. The mass spectrometer is initially programmed to scan through the chosen mass range with the IMS shutter grids continuously open. (If the ion mobility spectrometer is used in the normal pulsed mode it may take a very long time to obtain a mass spectrum, which may then not be representative.) Thus, it is possible to record ions created in an ion mobility spectrometer, and a mass spectrum of IMS sample ions is shown in Figure 4.

The mass spectrometer is then programmed to detect ions of one chosen mass. In this 'tuned'

ion mode (with the IMS shutter grids operating normally), a drift spectrum of the selected ion species is generated (see Figure 5). Thus, it is possible to associate a particular ion mass with a particular ion mobility peak. Hence, IMS-tuned MS enables the reduced mobility for ions to be determined, but the signals are weak and a significant amount of data averaging is required. Because the signals are very low, mass spectrometers used in conjunction with ion mobility spectrometers are set to pulse counting mode. Sometimes the average of thousands of spectra is necessary to produce a mobility peak. Using IMS-quadrupole MS to determine the reduced mobilities of all the ion-molecule clusters in a mobility spectrum could take from several hours to days. IMS-TOF is much faster, because it is able to scan at 50 to 60 Hz about 1000 scans per mobility spectrum. Ew-ing and Stone have an IMS-tuned MS for investigating the kinetic thermodynamic relationship for the ion reactions.

Figure 4 A mass spectrum of IMS sample ions.

Figure 5 A drift spectrum of a selected ion species.

Figure 6 A mass spectrum of CID product ions.

IMS-MS-MS studies can be performed when IMS is coupled to a triple quadrupole mass spectrometer. An ion selected by the first quadrupole can be injected into a collision gas, for example argon, in a second quadrupole at 2x10~5Torr (subjecting the cluster ions to CID), and then the product ions can be analysed in the third quadrupole. Figure 6 shows a mass spectrum of CID product ions. Consequently, MS-MS analysis is used extensively in assigning ion identities. IMS coupled to triple quadrupole MS enables the composite identification of the ion-molecules found in a drift tube. However, the number of ions reaching the detector in IMS-MS-MS is extremely low and a large amount of averaging is required to determine structures.

See Colour Plate 46.

See also: I/Mass Spectrometry. N/Chromatography: Gas: Gas Chromatography-Mass Spectrometry.

III/Biomedical Applications: Gas Chromatography-Mass Spectrometry; Drugs and Metabolites: Liquid Chromatography-Mass Spectrometry; Explosives: Gas Chromatography; Liquid Chromatography; Thin-Layer (Planar) Chromatography; Forensic Toxicology: Thin-Layer (Planar) Chromatography; Forensic Sciences: Capillary Electrophoresis; Liquid Chromatography; Heroin: Liquid Chromatography and Capillary Electrophoresis.

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