PNNL

Abstract

Traveling-wave ion mobility spectrometry (TW IMS) is a new method for IMS analyses implemented in the Synapt IMS/time-of-flight mass spectrometry system. Despite wide adoption of this platform, the fundamentals of TW IMS were understood only qualitatively and factors governing the ion transit time (the separation parameter) and resolution have remained murky. Here we develop the basic theory of TW IMS using first-principles derivations and ion dynamics simulations. The key parameter is the ratio (c) of ion drift velocity at the wave slope to wave speed (s). At low c, the ion transit velocity is proportional to the squares of mobility (K) and electric field intensity (E), as opposed to linear scaling in drift tube (DT) IMS and differential mobility analyzers (DMA). At higher c, the scaling deviates from quadratic in a manner dependent on the waveform profile, becoming more gradual with the idealized triangular profile but first steeper and then more gradual for realistic profiles with variable E. At highest c, the transit velocity asymptotically approaches the wave speed. Unlike with DT IMS, the resolving power of TW IMS depends on mobility, scaling as K1/2 in the low-c limit and slower at higher c. A nonlinear dependence of the transit time on mobility means that the true resolving power of TW IMS differs from the apparent value, often substantially. In the result, a near-optimum resolution is achievable over a ~300 - 400% range of ion mobilities. The major predicted trends are in agreement with TW IMS measurements for peptide ions as a function of ion mobility, wave amplitude, and gas pressure. The issues of proper TW IMS calibration and ion distortion by field heating are also discussed. Quantitative understanding of TW IMS separations gained in this work will allow rational optimization of instrument design and operation, and improved calibration of spectra.