LC-MS/MS is a highly selective technique for bioanalysis, but for challenging assays, more selectivity may be beneficial. Background interferences, co-eluting isobaric ions, long analytical run times, and poor ion fragmentation characteristics lead researchers back to the drawing board. Redeveloping and validating assays is time consuming and costly, with no guarantee of success.

High field asymmetric waveform ion mobility spectrometry (FAIMS) is a unique problem solving tool for LC-MS method development enabling many of the aforementioned challenges to be solved quickly and cost effectively.  For increased selectivity, the FAIMS device is installed in the atmospheric pressure region between the ion source and the mass spectrometer. In just a few minutes the system is ready to exploit three complementary dimensions of separation: LC, FAIMS, and MS.

The patented FAIMS technology separates gas-phase ions at atmospheric pressure. Desolvated ions enter the annular region between two concentric cylindrical FAIMS electrodes and are transported toward the mass spectrometer entrance via a flow of clean, dry gas.  Within the gas filled region of the electrodes, the ions oscillate as a result of a waveform causing alternating high and low electric fields. The waveform is asymmetric; the high field is applied for one time unit followed by an opposite-polarity low field component applied for twice as long.  Separation of ions during the two opposite polarity phases of the waveform is caused by behavior differences of the ions under the high- and low-field conditions.

Over time, ions travel toward one or the other electrode.  A low dc voltage compensates for this migration resulting in a select ion subset being transmitted into the MS. The magnitude of the so-called ?compensation? voltage (CV) is compound dependent. Ions requiring a different CV for transmission are removed from the ion beam, thus selectivity is achieved. The CV may be set to a constant value or stepped during an LC-MS run to coincide with the elution of specific analytes. High analyte transmission efficiency is achieved with this increase in selectivity by means of a unique ion focusing mechanism arising from the cylindrical geometry of the FAIMS electrodes.

An LC-MS method with increased selectivity makes FAIMS a valuable tool for solving common method development challenges.  For example, insensitive SRM methods may be due to ions with poor fragmentation characteristics. Instead of SRM, analyses using SIM may be sensitive but not selective enough when using generic sample preparation methods and fast chromatography.  FAIMS combined with SIM may have sufficient selectivity to offer a higher sensitivity method than SRM alone; this selectivity may include separation of endogenous isobaric species or reduction in high chemical.  In another such example, the analysis time for many LC methods is determined by chromatographic resolution of isobaric or isomeric compounds. The increased selectivity of FAIMS methods may allow a decrease in run time because FAIMS separation occurs on a millisecond timescale instead of the chromatographic timescale.

The simplicity of the electrode design, the ease of installation and use, and the speed in which challenges can be solved make FAIMS a technique that should be used early in the LC-MS method development process, conserving resources that may otherwise be put into reinvestigating sample preparation and LC separation conditions. The increased selectivity will lead to accelerated method development and improved robustness in LC-MS analyses.

The Finnigan^TM TSQ^TM Quantum Ultra mass spectrometer will be the first mass spectrometer available with the FAIMS interface.