Solvent vapor effects in planar high-field asymmetric waveform ion mobility spectrometry: Solvent trends and temperature effects

• Solvent vapor added to planar FAIMS cell dramatically increases resolving power.
• Magnitude of the solvent effect correlates with the solvent molecular volume.
• Consistent with clustering/declustering of ions with neutral molecules.
• Heat lessens the solvent vapor effect, consistent with cluster/decluster model.

Solvent vapor added to the drift gas in a planar high-field asymmetric waveform ion mobility spectrometer (FAIMS) leads to dramatic shifts to larger CV values while retaining narrow peaks, leading to dramatically increased resolving power (up to ∼140) for a variety of ions including isomeric ions. Here we present studies designed to improve the fundamental understanding of the role of solvent vapor on performance of planar FAIMS, focusing on solvent trends with molecular size and effects of temperature. The studies employ as test compounds the [M − H]− ions of the three positional isomers of phthalic acid and the M− and [M − H]− ions of trinitrotoluene (TNT). The addition of solvent vapor to FAIMS carrier gas has a significant impact on the behavior of the ions inside the FAIMS cell and can impact the overall performance of the cell. We have shown that by comparing a range of solvent vapors (water and C1–C4 alcohols), the magnitude of the effect correlates to the molecular volume of the solvent molecule. This fits well with the model of clustering/declustering of ions with neutral molecules on the time scale of waveform used in FAIMS. We have also shown that with the addition of heat, the effect of the solvent vapor is lessened, as one would expect with the cluster/decluster model.

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