5 FAIMS electrode designs
There are several different electrode designs used in FAIMS and these have been discussed in more detail elsewhere [4], in brief, they can all be broadly categorised into two groups, those that have cylindrical (coaxial) electrodes or those with planar electrode geometry. Planar FAIMS have greater resolving power than cylindrical systems at the cost of reduced transmission. Residence time in a planar FAIMS (<10s to scan full CV range) is much shorter compared to that in a cylindrical system (~1 min to scan full CV range). The shorter scan makes it much easier to interface planar systems into LC-MS experiments while still maintaining adequate chromatographic resolution. Ultra-FAIMS is a recent development in planar FAIMS, this technology uses a microchip with multiple channels for ion transmission. In this miniaturised system the gap between the electrodes is an order of magnitude smaller than that in previous planar configurations enabling higher electric field intensities to be achieved [5]. The ultra-FAIMS chip, showing the scale of the device against a human finger, is shown in Figure 3.
Figure 4. Mass spectra obtained at three different CV values from LC-ESI-FAIMS-MS analysis of a rat urine extract (9).
Metabolite ID/Metabolomics
Metabolomics is an area where FAIMS-MS can be used in several ways to enhance and/or simplify the complex data sets produced from metabolomic experiments. Here at Loughborough we have used a cylindrical Thermo FAIMS system in conjunction with a LTQ linear ion trap to identify biomarkers of ageing in rat urine samples [10].
The FAIMS was set to transmit ions at specific CV values sequentially in the range - 12V to -22V, yielding mass spectra of the FAIMS-selected fractions of the complex urine sample across the LC run. Figure 4 shows the mass spectra obtained at 3 separate CV values following a LC-FAIMS-MS analysis. It can clearly be seen that the spectra obtained at CV values of -12, -18 and -22V are different, enhancing the analytical space compared to MS alone.
The FAIMS-selected fractions were submitted for artificial neural network informatics analysis, which showed that urine from rats of different age groups could be distinguished with >80% confidence based on the data obtained from FAIMS-MS analysis allowing candidate biomarker ions of ageing could be identified.
Figure 3. The Owlstone Ultra-FAIMS chip (5). Peptide analysis – Proteomics FAIMS-MS
The first reported use of FAIMS as an orthogonal separation technique for mass spectrometry was in 1998, when a cylindrical FAIMS device was interfaced to a quadrupole mass spectrometer using an electrospray source [2]. Since then there has been a significant body of work using FAIMS devices used both in series and in parallel with mass spectrometry. In this review we will concentrate on the application of FAIMS to the analysis of small molecules including peptide analysis. The use of FAIMS hyphenated to mass spectrometry has been covered in several reviews and more detail about environmental and security applications can be found there [5,6].
FAIMS has been used in proteomics applications primarily as a pre-separation method used to enhance and simplify complex mass spectra. Another interesting area in which FAIMS has also found an application is in separating different peptide conformers giving structural information not available from standalone MS analysis. The first reported use of ESI-FAIMS-MS for peptide analysis was in 1999, when a study using leucine enkephalin was conducted by Purves et al [11].
This work showed that using FAIMS as a filter and selecting the protonated leucine enkephalin peak at its specific CV value enhanced the S/N ratio considerably by removing interfering background ion species. In addition, it was also possible to use the FAIMS to select a specific charge state of the leucine enkephalin, a capability which could prove very useful when used to preselect ions for collision induced dissociation.
Pharmaceuticals
Ion mobility spectrometry (IMS) and FAIMS have been used in a number of studies related to pharmaceutical applications, and a review was published in 2008 which covers FAIMS and IMS applications [6]. FAIMS-MS as a rapid alternative to LC-MS methods shows potential as a fast screening technique for detecting drugs of abuse in complex matrices. Experiments performed on amphetamines in urine using a cylindrical FAIMS device demonstrated quantification down to 0.2ng/mL [7]. In addition, the FAIMS based approach is much faster than established LC based methods, with samples being able to be run and quantified within 20 minutes of arriving at the analytical laboratory.
Thanks to its differential mobility based mode of separation FAIMS has the capability of resolving isomeric species which cannot be separated using MS alone. This has been explored in two separate publications, McOoeye demonstrated that FAIMS-MS can be used to to separate and quantify diastereoisomers and applied the technique to isomers of ephedra in natural health products [8]. A chiral separation has also been demonstrated using FAIMS-MS, divalent metal ions were added to a racemic mixture of terbutaline producing diastereoisomers which were resolvable with a cylindrical FAIMS system [9].
Figure 5. CV scan from the ultraFAIMS-MS ion trap analysis of the bradykinin [M+2H]2+ ion showing the separation of gas-phase conformers of bradykinin (11).
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