PART 3. Selected issues related to the application of FAIMS

I. Electric field strength

Figure 1 reminds us that the electric field strength is important in separation by FAIMS. The traces indicate the observed compensation voltage(CV) at various applied dispersion voltages(DV) for two charge states of a selected peptide. Firstly the CV of transmission of these ions increases with applied DV. Secondly, the CV's of the two ions become more different from each other as the DV increases.

The lower pair of traces in Figure 1 were collected with electrodes spaced apart by 2.5 mm, whereas the top traces were collected using electrode spacing reduced to 1.25 mm. Increasing field strength as before, increases the observed CV and the ion separation.

Figure 1. The effect of field strength on observed CV. Upper traces were collected with 1.25 mm electrode spacing and the lower pair with 2.5 mm spacing.

II. Waveform shape and frequency

The waveform shape in FAIMS must be asymmetric, having the peak voltage different for the positive and negative halves of the waveform. The applied waveforms are usually a square wave, or a square wave approximated by addition of two or more sinusoidal waves. The fundamental FAIMS effect is based upon the difference in mobility of the ion at a low and a high strength electric field. Clearly the effect is maximized if each field strength low or high, are held constant for the duration of their part of the waveform. Any waveform that is less perfect than a square wave, including sinusoidal approximations, will be less effective.

The waveform shape will control the relative difference in the electric field during the high and low field portions of the wave. If the waveform is composed of high and low field portions having similar duration times, the difference between the two field strengths will be small. As the wave becomes more asymmetric, and the difference in electric field strengths becomes larger, the FAIMS effect, and ion separation, will increase. However this does not continue indefinitely because as the period of high voltage becomes shorter the drift distance diminishes and the FAIMS effect again diminishes.

In addition, altering the waveform towards higher voltage is limited because an electrical discharge will eventually occur. Assume that E_dis is the strongest field possible using the hardware configuration and bath gas composition selected, and t_hi and t_low are the periods of time at high and low field. The ratio of the time durations, t_hi /t_low must be equal to E_low / E_dis but beyond this the user must select the value of this ratio and must establish the waveform frequency.

The effect of waveform frequency is also less clearly defined. At the low extreme of frequency, the ions travel sufficiently far during one period of the waveform that collision with the wall is inevitable. At frequencies sufficiently high that the ion travel is a small percentage of the electrode spacing it is expected that ion transmission may be adequate. At very high frequency new effects may come into play. For example time is required for the ion to reach a velocity appropriate for the applied field strength. In some cases, for example if ion solvation is a factor determining the final ion velocity, sufficient time is required for the number of collisions that permit the ion to be suitably stably solvated. At frequencies that are too high, the FAIMS effect and separation will diminish. Normally however, the production of high frequency - high voltage waveforms is limited by the electronics available to produce the waveform.

III. Electrode configuration

There are two different electrode configurations used in FAIMS devices used for mass spectrometry. The ions may be separated in regions having either (a) curved electrodes or (b) flat electrodes. Curved electrodes are beneficial because they focus the ions together to minimize losses to walls caused by diffusion and ion-ion repulsion. Because ion loss is minimized, the devices can be larger and ion separation can be at modest speed. Flat devices are subject to the full detrimental effects of diffusion, therefore the separation must be fast to avoid ion loss to walls. In order that the separation will proceed quickly, the space holding the ions must be small so that the ions which are being rejected will be rejected quickly, retaining as many of the ions of interest as possible. Fortunately the onset of electric discharge occurs at stonger fields in smaller spaces, thus permitting the required higher electric field strengths.

Fortunately, both of these approaches work well, and both have been used successfully.

IV. Bath gas composition

Figure 2 illustrates an example of the effect of changing the composition of the bath gas. As in Figure 1 we are considering ions of different charge states of a peptide. As before, we are interested in the behavior of these ions as the applied voltage, DV, is increased. The lower of the traces were recorded while pure nitrogen was used as the bath gas. The upper traces were recorded while a 1/1 mixture nitrogen and helium was used as the bath gas. In this example the addition of a new gas, helium, significantly altered the CV's of transmission of these ions as well as slightly improving separation. It is also interesting to note that the doubly charged peptide is transmitted at a higher CV than the triply charged in pure nitrogen, whereas it is transmitted at lower CV than the other in a mixture containing helium. Why?? Many questions remain unanswered.

Figure 2. The effect of gas composition on observed CV. Upper traces were collected with a 1/1 mixture of N₂ and He, and the lower pair using N₂ [RCMS].

V. Temperature

The temperature of each of the two FAIMS electrodes plays a role in ion separation. Normally, with closely spaced flat plate FAIMS, the two plates are held at the same temperature. The selected temperature, usually slightly elevated, will ensure that the electrodes remain clean and free of surface layers. Clearly, the temperature is held low to prevent chemical modifications to the analyte in the case of biological samples.

In the case of curved electrodes the electrodes are often held at different temperatures [JASMS]. The temperature difference affects the ion separation. Normally the electrodes are held about 20^oC difference, with the electrode with smaller radius held at the lower temperature. The magnitude of this temperature difference will modify ion separation, device resolution, and ion transmission efficiency, and will be determined experimentally for the compounds under investigation. Figure 3 illustrates the separation of two compounds at different combinations of electrode temperatures. Note that the separation is better when the inner electrode is held at the lower temperature.

Figure 3. The effect of temperatures of the inner and outer electrodes in concentric cylinder FAIMS on the CV spectrum of the ions of taurocholic acid (m/z514) and methotrexate (m/z 453). Inner and outer electrode temperatures were: (top) 36^oC/76^oC, (middle) 76^oC/76^oC, and(bottom) 116^oC/76^oC.

As discussed above, the temperature is generally kept sufficiently low to prevent chemical modifications to any labile species in the sample. On the other hand, low temperatures often promotes formation of solvated ions during the low field portion of the applied waveform. In some cases formation of these complexes further increases the observed CV, and results in improved separation.

VI. Time

Time is an element that affects the ion separation in FAIMS in complex ways. At the simplest level, ions set in motion with differing electric fields would steadily become farther apart in time. Unfortunately several other factors influence the ions in addition to the electric field. Examples include diffusion and ion-ion repulsion, both of which disperse the ion cloud in time. Clearly two groups of ions set in motion to be separated, will never escape each other if the rate of diffusion keeps the two clouds of ion interspersed among each other.

Time also plays a role in determining the fraction of the original ions that will ultimately be detected. A good separation process that takes so long that all ions are lost by collision to the walls is useless. Ions are lost from small spaces rapidly. This means that if the ions are confined in small spaces the separation must be completed quickly. This also requires that the ions must be subjected to strong forces so that they rapidly separate from each other.

VII. Other effects

Ion separation in FAIMS is subject to other effects including ion-ion repulsion (space charge effects), ion diffusion, and bath gas turbulence, to mention only a few. Unfortunately these effects (in conjunction with time) never assist in the separation, rather they always try to thwart and undermine the separating process.

As discussed above in 'time' each of the two approaches, curved and flat electrodes, of FAIMS tries to minimize some of these negative components, but a final solution will gradually appear as each is further developed.

Resolution of FAIMS devices

Many of the same parameters discussed above in relation to separation, are also relevant to resolution.

In general, in a FAIMS with curved electrode geometry, the peaks get wider as the CV and DV are increased.

By comparison, in a FAIMS of planar geometry the peak width remains constant or decreases while the applied voltages CV and DV are increased. In principle therefore the resolution advantage of planar geometry over curved electrodes increases as the CV and DV of ion transmission increase.

Why do peaks in a FAIMS with curved electrode geometry get wider as the CV and DV increases? This a result of the stronger ion focusing effect at higher CV and DV, which in turn means that the point of optimum focus remains between the electrodes for a wider range of CV as the CV and DV are increased. During this range of CV the ions continue to pass through FAIMS.

The decrease in resolution with increase of DV in curved electrodes is therefore only 'apparent'. What we observe are wider peaks since the electric field between curved electrodes is not constant. This variation means that while one voltage is applied, in fact we actually have a 'range' of CV's between the surfaces of the electrodes. This means that two ions having different CV's can simultaneously be transmitted.

For this discussion the compensation voltage (CV) will continue to be defined as the actual voltage applied between the plates. The fields at which the ion is stable will be called 'compensation field' and 'dispersion field' or F_C and F_D for the purposes of this discussion. In FAIMS of planar geometry the F_D = DV/d and F_C = CV/d fields exist everywhere between the plates (d is the gap between the electrodes). In curved geometry the ion is stable and migrates to the location having F_C and F_D, but elsewhere the fields between the electrodes are higher and lower than F_C and F_D.

During a scan of the Compensation Voltage (CV) at a selected Dispersion Voltage (DV), the ions transmitted through FAIMS appear as a peak with a width of W_CV centered at CV of V_CV. In order to see many separated peaks we hope to maximize the ratio V_CV/W_CV.

How is the 'real' resolution going to be revealed? This will be resolved by improvements in the mechanisms by which ions are extracted from FAIMS.

For example Figure 4 shows two types of ions being transmitted simultaneously by a FAIMS with curved electrodes. Both ions are focused at their individual F_C fields, but the field between the electrodes covers a range that extends across the stable F_C of both ions. In the conventional approach all ions are carried by a gas flow from between the plates to the mass spectrometer, therefore we see only one peak in the scan of CV, low 'apparent' resolution.

Figure 4. Focusing of two types of ions simultaneously in curved geometry FAIMS. Each ion collects at region of F_C suitable for that ion. Both are carried by gas flow, lengthwise between the plates and exit together to appear as one peak.

Figure 5 represents a possible manner by which the resolution of FAIMS is revealed. At constant CV, the ions are extracted through an opening in one of the electrodes by application of a stepped extraction voltage. Now a signal (in time) for each ion is observed independently and the actual 'real' resolution of the curved electrode FAIMS is shown. A combination of these drift time spectra, collected at each CV, will be used to reconstitute a CV spectrum. The peak widths in this spectrum will no longer increase as the DV is increased.

Figure 5. Focusing of two types of ions simultaneously in curved geometry FAIMS. An orthogonal field (pulse) extracts ions through a hole in one of the electrodes to give two independent ion pulses arriving at different times.