focus on Mass Spectrometry & FAIMS Theory
The first report describing FAIMS (then called field ion spectrometry) in English was published by Buryakov and co-workers in 1993 [1]. The technique was then developed further in the late 1990s by the Guevremont group, who published many papers on the subject [2]. FAIMS has recently been the subject of a textbook published by CRC press in 2008 [3].
FAIMS separation occurs on the basis of the differences between the mobility of ions in alternating high and low electric fields. Ions travelling between two electrodes in a carrier gas stream at atmospheric pressure (Figure 1) are subjected to an asymmetric waveform which alternates between high field and low field conditions.
This asymmetric RF waveform is known as the dispersion voltage (DV). Figure 1 shows the ion path through the device when the lower of the two plates is earthed whilst the DV is applied to the upper plate. In the high field component of the DV, positive ions will be repelled away from the upper plate, and when the waveform switches into the low field component they will be attracted towards it.
The voltage in the high field component is higher than that of the low field component, but the time spent in the low field component is longer (2:1 ratio low field: high field) to compensate. If the high field and low field mobility are the same, the waveform should focus the ions into the centre of the electrodes.
However, in practice this does not usually occur as mobility changes under high field conditions, and ions will accumulate a net drift towards one of the electrodes, which will result in the ions colliding with the electrode and being neutralised. Ions of differing mass, charge and conformation will behave differently in the same waveform.
Spectroscopy
High-Field Asymmetric Waveform Ion Mobility Separation and its Application to Small Molecule Analysis
James C. Reynolds, Lauren J. Brown, Robert Smith, Emma L. Harry and Colin S. Creaser, Centre for Analytical Science, Loughborough University, Loughborough, LE11 3TU UK
High field asymmetric-waveform ion mobility spectrometry (FAIMS), also commonly referred to as differential mobility spectrometry (DMS), is an atmospheric pressure gas phase separation technique which exploits the difference between the mobility of an ion under high and low electric field conditions, as they pass between two electrodes. FAIMS can either be employed as a standalone mobility device or used as an orthogonal pre-separation technique hyphenated with chromatography and/or mass spectrometry. This report will discuss the development of modern FAIMS instrumentation and its application to the analysis of small molecules and peptides.
The mobility of an ion in a high-field electric waveform (Kh) can be described using this equation:
Kh(E) = K0(1+ α (E))
where K0 is the reduced mobility at low electric field, E is the high-field component of the electric waveform and alpha (α) is a coefficient for the dependence of mobility of an ion on the electric field strength. This difference in mobility is
determined by the α value, which describes which direction the overall net drift will be at any given dispersion voltage.
In order for the ions to pass through the device a voltage is applied to one of the electrodes to reverse the drift of the ions towards the electrodes [1]. This is termed the compensation voltage (CV) [2]. The CV for ion transmission is compound
specific and dependent on the α value of an ion resulting from the previously described mobility differences in high and low fields. By scanning the FAIMS device across a range of CV voltages, a differential mobility spectrum can be obtained, with species separated on the basis of the CV required to enable transmission. Figure 2 shows an example CV scan obtained using a mixture of vancomycin, reserpine and the test peptide MRFA, each of the three compounds can be partially resolved on the basis of differential mobility. The device can also be used as a filter by using a constant CV value, to transmit only ions which traverse the device at a specific CV value, whilst excluding all other species.
Figure
1.Graphical depiction of the asymmetric waveform applied to a FAIMS device, and the path of ion motion through a planar FAIMS device.
Figure 2. UltraFAIMS CV Scan (-1 to +6V) of test mixture: A = TIC, B = MRFA [M+H]+, C = Reserpine [M+H]+ D = Vancomycin [M+H]+.
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