In drift-tube ion mobility spectrometry (DT-IMS) as used in the TOFWERK IMS-TOF, a static uniform electric field is used to pull ions through the ion mobility drift tube which is filled with buffer (drift) gas.
The time an ion takes from the start of the drift tube to the detector is referred to as the drift time. As the ion travels through the drift tube, it undergoes collisions with the buffer gas; this observational property which averages all geometric orientations and ion-gas interaction types during the experiment is referred to as the collision cross section (CCS). The larger an ion, the larger also its CCS. This means that ions with a large CCS require more time to traverse the drift tube and therefore exhibit a longer drift time.
The CCS can be directly calculated from the measured drift time and other known experimental parameters using the so-called Mason–Schamp equation. Consequently, DTIM systems operating at low electric fields are still preferred for the direct determination of accurate and precise determination of CCS values. The CCS is specific for an analyte in a given drift gas and can therefore be used as a molecular identifier in addition to accurate mass, fragmentation ions and LC retention time in compound databases.
Isomeric molecules cannot be distinguished by mass spectrometry due to identical mass. Yet, they usually display small differences in CCS (on the order of 1-2% for many isomeric metabolites). This means an ion mobility system should have a resolving power which is sufficiently high to separate analytes with CCS differences of about 1% to be useful as an analytical separation device.
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