Accepted: MS Research T-wave calibrations

Introduction

Travelling Wave Ion Mobility (TWIM), since its introduction in 2004, has developed into a ubiquitous separation tool, impacting global efforts in areas ranging from food analyses to drug discovery. TWIM separates ions using a series of DC waves operating in a gas-filled ion guide. The relationship between the average ion velocity achieved and the TWIM operating parameters remains incompletely understood, so TWIM instruments are typically calibrated to obtain accurate collision cross section (CCS) values. Measurement of TWIM CCSs for large, multiply-charged species can be particularly exacting and requires careful choice of calibrant and expert evaluation of the resulting calibration curves. Here, we introduce a dramatically improved calibration methodology, yielding robust calibrations over a significantly improved range of ions and conditions.

Methods

We extend earlier work which predicted the existence of velocity relaxation effects in TWIM experiments and demonstrated that these could be accommodated in a generalised calibration function. This calibration related average ion velocity to mobility and mass-to-charge ratio, and utilised six free parameters. Using a new analytical approximation we derive a calibration requiring only two free parameters. Using an enhanced SIMION simulation of the device, we identify an additional effect related to off-axis ion distribution. We accommodate this by adding one further parameter to the calibration. Experimental data for a variety of small molecules, peptides and proteins were acquired using a Waters Synapt G2 HDMS instrument and a research Q-IMS-ToF with a cyclic TWIM cell.

Preliminary Data

Previous approaches to TWIM calibration revolved around a simple power-law fit to experimental data. Subsequent work has revealed that optimal performance for large multiply-charged molecules was achieved over a narrow range of TWIM wave amplitudes and velocities. Our efforts to develop a deeper understanding of ion transport during TWIM have directly resulted in a new six-parameter calibration expression that takes velocity relaxation effects into account to produce calibrated TWIM CCS values of enhanced accuracy and precision. Furthermore, we have developed a simplified two-parameter expression which retains the ability to account for ion velocity relaxation. Our preliminary data evaluates the effectiveness of these new expressions in comparison to power-law calibrations preformed under the same experimental conditions using a wide range of wave amplitudes and velocities. We have evaluated calibration performance in different TWIM pressure regimes, and using a large number of analytes, ranging from metabolites and small peptides (e.g. 151 Å2) to multiprotein complexes (e.g. 13,400Å2).

Our results demonstrate that these new calibration functions produce CCSs of significantly improved precision and accuracy across a much wider range of TWIM conditions than has previously been possible. For example, power-law calibrations obtained using the full range of analyte classes typically produce CCS values with relative precisions of ~5% when an optimum range of TWIM parameters is used. In contrast, our new expressions provide uniformly improved CCS precisions over any ion mixture tested, and are much more robust to changes in TWIM parameters. In particular, for our preferred two-parameter expression, we routinely achieve relative precisions that are a factor of 5 improved over typical power-law calibrated TWIM data. In light of these new results, this presentation will focus on outlining next-generation calibration workflows that are likely to impact all TWIM measurements moving forward.

Novel Aspect

Novel calibration methods for TWIM instruments, and improved experimental designs for high-accuracy CCS measurements for a wide range of ions.
Options:

A post-doc is presenting author on this abstract? No
A graduate student is presenting author on this abstract? No
An undergraduate student is presenting author on this abstract? No

Oral Choice:

Fundamentals: Ion Mobility and MS (In Memory of Al Yergey)
Second Oral Choice:

Ion Mobility: New Developments & Applications
Poster:

Ion Mobility: Fundamentals

Submitting Author:

Keith Richardson
Waters Corporation
Wilmslow,
keith_richardson@waters.com