Key principles

In classical drift tube ion mobility spectrometry (IMS) experiments, packets of analyte ions travel through a gas-filled “drift tube” under the influence of a uniform electric field and their arrival time is recorded at a detector.  The mobility of an ion in the drift gas is governed by the relationship between the drift velocity of ionic species and the strength of the applied electric field. Mobility is influenced by the instrument operational parameters of gas temperature and pressure but more importantly for separation, the physicochemical properties of the ion and the gas, including ion charge state, the ion and gas molecule masses, and the rotationally averaged collision cross section (CCS) of the ion and gas molecules.  

Drift Tube Ion Mobility

IMS is often coupled with mass spectrometry as it extends the information that can be obtained about an ion as it delivers information about the three-dimensional structure of an ion, and increases peak capacity and confidence in compound characterization.

Travelling Wave

Waters has pioneered the use of travelling wave ion mobility spectrometry (TWIMS) and introduced SYNAPT, the first ever commercial IMS-MS instrument in 2006.  Since then several impactful SYNAPT enhancements have been introduced into commercial variants and recently both the Vion IMS Qtof and SELECT SERIES Cyclic IMS have been introduced. In TWIMS, a series of voltage pulses are applied to a stacked ring ion guide (SRIG), creating a travelling wave that ions can ‘surf’ along. This drives ions forward, carrying higher mobility ions more easily than those of lower mobility. An applied RF voltage confine ions radially, ensuring high transmission.

TWIMS systems can be easily calibrated against compounds with known CCS values, allowing ions to be distinguished with additional selectivity on the basis of their chemical structure, size and shape without compromising on sensitivity. Learn more about what the inclusion of CCS will add to your analysis.

Overview of Ion Mobility techniques

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