Why is ultraFAIMS orthogonal to other forms of IMS and MS?

Identifying more components of complex mixtures is of fundamental interest for analytical chemists in many fields. Mass spectrometry (MS) sorts ionized chemical compounds on the basis of their mass-to-charge ratio ( m/z ). This process is often enhanced by hyphenating MS with other techniques such as gas or liquid chromatography that provide ‘orthogonal’ separation (i.e. they separate based on unrelated properties of the compounds).

Ion mobility techniques been used widely in tandem with MS to help separate compounds in complex mixtures. ultraFAIMS separates ions based on their differential mobility in alternating high and low electric fields. Changes in mobility at varying field strength is due to alterations in ion collision cross section. This is caused by a number of factors ( see ultraFAIMS whitepaper for more information ), including the de-clustering of drift gas molecules under high field conditions.

ultraFAIMS allows the selection of ions by the application of a compensation field, which ‘steers’ ions to the mass spectrometer inlet. As differential mobility is only weakly related to  m/z  the applied compensation field does not correlate with m/z , making the separation orthogonal to MS.

The orthogonality of ultraFAIMS contrasts with traditional ion mobility techniques like linear drift tube IMS and travelling wave IMS. These techniques distinguish ionized compounds according to the speed that they migrate through a buffer gas under low field conditions. This separates ions based on their absolute mobility, which is closely related to m/z . In a linear drift tube, this means that the ‘drift time’ separation of ions correlates strongly with m/z . Thus ions which are not well separated based on m/z by MS are also likely to be poorly resolved by traditional IMS.

To see how the ultraFAIMS’ orthogonality to MS can help find more peaks in real-world complex biological samples take a look at this recent publication -   Increasing peak capacity in non-targeted omics applications by combining full scan field asymmetric waveform ion mobility spectrometry with liquid chromatography-mass spectrometry ,   Kayleigh L. Arthur, Matthew A. Turner, James C. Reynolds and Colin S. Creaser, Anal. Chem., 2017 Mar 21;89(6):3452-3459, doi: 10.1021/acs.analchem.6b04315