Plate-height model of ion mobility-mass spectrometry.

Márkó Grabarics,Kevin Pagel,Tim J Causon,Ansgar T Kirk,Gert Von Helden,Maike Lettow

doi:10.1039/d0an00433b

Abstract

In the past decade, ion mobility spectrometry (IMS) in combination with mass spectrometry (IM-MS) became a widely employed technique for the separation and structural characterization of ionic species in the gas phase. Similarly to chromatography, where studies on the mechanism of band broadening and adequate plate-height equations have been aiding method development and promoting advancements in column technology, a suitable resolving power theory of drift tube ion mobility-mass spectrometry (DTIM-MS) is essential to stimulate further progress in this emerging field of separation science. In the present study, therefore, we explore dispersion processes in detail and present a plate-height model of ion mobility-mass spectrometry. We quantify the effects of five major dispersion processes that contribute to zone broadening and determine the resolving power in DTIM-MS: diffusion, Coulomb repulsion, electric field inhomogeneities, the finite initial spread of the ion cloud and dispersion outside the mobility cell. A solution is provided to account for the nonuniform separation field in IM-MS in the presence of multiple compartments. The equations - derived from first principles - serve as the basis for formulating an expression that is similar in nature to van Deemter's plate-height equation for chromatography. A comprehensive set of experiments was performed on a custom-built DTIM-MS instrument to evaluate the accuracy of the plate-height model, resulting in satisfactory agreement between experiment and theory. Building on these findings, the plate-height equation was employed to explore the influence of drift gas pressure, injection pulse-width and the mobility of ions on resolving power from a theoretical point of view. Our findings may aid instrument design and development in the future, as well as the optimization of measurement conditions to improve ion mobility separations. By employing the plate-height concept and the general formalism of differential migration processes to describe zone spreading in IM-MS, we aim to find a common ground between this emerging method and such well-established techniques as HPLC or CZE. We also hope that the work presented here will facilitate a broader acceptance of IMS as a separation method of great potential by the communities of chromatography and electrophoresis, as well as that of mass spectrometry.

Highlights

With the advent of commercially available instruments of various designs, ion mobility-mass spectrometry (IM-MS) has PaperIn analogy to the description of distillation and extraction in fractionating columns,[1] Martin and Synge developed the plate theory of chromatography[2,3,4] to describe the elution process and the evolution of chromatographic bands
The purpose of the present study is to assess the efficiency of ion mobility spectrometry (IMS)/IM-MS as a separation method that is based on the differential migration of ions through a mobility cell
As the aforementioned criteria are satisfied in drift tube ion mobility spectrometry (DTIMS) and drift tube ion mobility-mass spectrometry (DTIM-MS) as well, the plate-height concept can be utilized to address peak broadening

Summary

Introduction

With the advent of commercially available instruments of various designs, ion mobility-mass spectrometry (IM-MS) has PaperIn analogy to the description of distillation and extraction in fractionating columns,[1] Martin and Synge developed the plate theory of chromatography[2,3,4] to describe the elution process and the evolution of chromatographic bands. More advanced macroscopic and microscopic theories have been developed since, such as the differential mass balance equations[5] (forming the basis of the Van Deemter equation6) or the stochastic theory of chromatography,[7,8,9,10,11,12,13] the concept of plate numbers and plate heights remained central in the field. When nonlinear effects are absent, chromatographic peaks converge toward a Gaussian profile, and the total variance of this distribution is the sum of the variances corresponding to the individual dispersion processes These conditions form the basis for the application of plate-height models. The dispersion processes that determine broadening in chromatography and IM-MS, are fundamentally different, but the framework is the same and the fruitful analogy between the two aforementioned techniques can be harnessed as long as the linearity of the system is maintained

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