High-Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS)
Function: Compounds (Ions) are Separated by Mobility
- Function of the size, shape, charge
- Direct, real-time separation
- Continuous sample introduction
An Ion filter – only molecules of interest are allowed to be detected
FAIMS Receptor Gallery
- The FAIMS device is like a coin sorter, but ions are filtered via electric fields.
- Once filtered they can be identified.
FAIMS Technical Overview
Breathtec Biomedical aims to assist the development of novel instrumentation and methodologies for high-field asymmetric waveform ion mobility spectrometry (FAIMS). FAIMS can be performed on a standalone device, or, more prominently, coupled to a mass spectrometer (MS). FAIMS is a separation technique which allows continuous sample introduction, enabling direct, real-time analysis.
FAIMS (or differential mobility spectrometry, DMS) involves separation based on the inherent differences in an ion’s mobility in high and low electric fields, and can be performed at atmospheric pressure and room temperature (unlike its chromatographic counterparts). In conventional ion mobility spectrometry (IMS), ions are pulsed through a drift tube by an applied electric field in the presence of a buffer gas which opposes the ion’s motion. As the field is held constant, the velocity, and therefore the mobility, of an ion is dependent upon its mass, charge, size, and shape as it traverses the drift tube and collides with the buffer gas molecules. This is demonstrated by the following equation:
where ν is the velocity of the ion, Κ is the mobility of the ion, and Ε is the electric field strength. Therefore, the migration time through the tube is characteristic of each ion.
However in FAIMS, the applied RF voltage is alternated between a high and low field as the ions are swept through the cell by a carrier gas. As the electric field increases in intensity, the velocity does not increase proportionally with the field strength; thus mobility becomes field-dependent at higher electric fields. At such higher fields (above 10,000 V/cm, as opposed to the 200 V/cm considered low-field), the mobility may now be expressed by the following equation:
where Κh is the high-field mobility, Κ is the low-field mobility, α and ß are compound specific values which account for high-field mobility effect, N is the gas number density of carrier gas, and Ε is the electric field strength. This distinct difference in mobility between high and low fields, Κh/Κ, is the basic operational principle of FAIMS.
FAIMS is typically performed by utilizing one of two cell geometries: curved or planar. To best illustrate the separation principles of FAIMS, a planar geometry cell is discussed, however, both geometries are fundamentally similar. Separation is achieved by maintaining one electrode at (or near) ground, while applying a waveform to the other electrode, as described by the application of a dispersion voltage to create a high-field environment for a short amount of time, before applying the opposite polarity voltage to create a low-field environment for a greater amount of time (A). While subjected to a high field, an ion has a greater mobility, causing it to drift faster toward the lower plate. When the field is switched, the ion’s mobility decreases, and it drifts toward the upper plate. Due to the asymmetry of the waveform, there will be a net displacement of the ion after one period of the waveform as the ion will not cover the same vertical distance when in low-field as it did in high-field. With each continuous period, the ion will move closer and closer to one of the electrodes until it makes contact and is annihilated.
To compensate for this drift, specific ions of interest can be made to reach the detector by applying a secondary DC voltage to the electrodes, called the compensation voltage, as seen in (B).