Ion Mobility Spectrometry (IMS)
Ion Mobility Spectrometry, or IMS, is an instrumental analytical method that is similar to Time-of-Flight MS. Instead of responding to molecular fragments, however, IMS uses soft ionization. Ionized molecules travel through a drift tube at distinct speeds that are related to their mass and geometry. Selectable positive and negative ionization enhances identification or sensitivity.
The ions in question are generated by atmospheric-pressure chemical ionization. Sample material is heated to yield a vapor that is swept into a small drift chamber where a beta radiation source ionizes the molecules. The resulting ions – separated according to size, mass and geometry – accelerate towards a detector.
On impact, each ion generates a specific signal, which is a function of ion mobility. Mobility (K) is determined from the drift velocity (vd) attained by ions in a weak electric field (E) in the drift tube, according to the equation, vd = K x E. The distribution of these signals forms an ion spectrum, with an ion mobility band corresponding to each of the unique ionic species. The spectrum is a fingerprint of the parent compound.
In the IONSCAN software, libraries of these ion spectra remain resident and are in the screening process. During analysis, the IONSCAN will look for ions that take the same amount of time and peak within an expected range of the target substances. If it finds ions that fit those parameters, it will detect the substance and “alarm’. If the IONSCAN does not find ions that fit those parameters, it will not alarm.
For more information on this technology, consult Ion Mobility Spectrometry by Gary Alan Eiceman and Zeev Karpas, published in 1994. This book covers theory, ionization processes, instrumentation, and qualitative and quantitative analyses.
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