Ion mobility Mass Spectrometry

As a prelude to my next article on SMAD (Single-Injection Multi-Omics Analysis by Direct Infusion Mass Spectrometry) I thought it wise to first understand the concept of Ion-Mobility Spectrometry-Mass Spectrometry (IMS-MS). IMS-MS has rapidly become a core tool I metabolomics, lipidomics, proteomics and structural biology, offering fast separation, improved signal filtering and enhanced confidence in identifications.

What is Ion Mobility Spectrometry (IMS)?

The core principle of Ion Mobility Spectrometry (IMS) is to separate ions in an inert gas, known as a buffer gas under the influence of an electric field. The applied electric field forces ions to migrate through the buffer gas with a velocity correlated to the specific analyte’s mobility, governed by its size, shape, mass, charge and interactions with the buffer gas.

Ions are separated by their differences in mobility through either space or time, depending on the IMS method. Smaller, more mobile ions travel faster in a specific electric field strength than larger, less mobile ions. Mobility for each ion is measured as a function of the experimental parameters (temperature and pressure) which are often normalised to standard conditions in order to calculate the reduced mobility.

Once separated by mobility, ions are transmitted to the mass analyser where their mass to charge ratios (m/z) ca be determined on a microsecond timescale. The effective separation of analytes achieves with this method makes it widely applicable in the analysis of complex samples such as in proteomics and metabolomics.

IMS-MS is now widely employed in proteomics, metabolomics, environmental analysis, lipodomics and structural biology.

Commercial Adoption and Industry Applications

MaSaTECH from Slovakia have used IMS in the field for real time explosives detection.

IMSinnovation (a spin off from MaSaTECH) and are using the technology to provide innovative analytical solutions that enhance the quality and safety of food and beverages.

Owlstone Medical use FAIMS sensor technology to look at biomarkers via breathalysers

Fasmatech specialises in the design and development of custom mass spectrometry and ion mobility instrumentation.

Sample Introduction and Ionization

The first step is to convert the sample into gas ions. There are a few different ways of doing this (Fig 1.), gas phase samples are typically ionizes with radioactive ionization, corona discharge ionisation and photoionization techniques. Electrospray ionisation is a common method for ionising samples in solution. For solid phase analytes, they are typically ionised by either matric-assisted laser desorption ionisation (MALDI) for large mass molecules or laser desorption ionisation (LDI) for molecules with smaller masses.

Sample Type	Ionisation methods
Gas-Phase Samples	-          Radioactive ionisation -          Corona discharge ionisation -          Photoionization
Liquid Samples	-          Electrospray Ionisation (ESI)
Solid Samples	-          Matrix-Assisted Laser Desorption / Ionisation (MALDI) -          Laser Desorption Ionisation (LDI)

Figure 1. Common ionisation method for different sample types

Differences between IMS Instrumentation

Figure 2. Variations of IMS technology with representative descriptions of applied field and gas dynamics. Bullet points describe the relative parameters of each IMS platform with key attributes reflecting the ability to measure CCS information, ion packet distribution, instrument footprint, and modularity among other relevant descriptors. Also included are the main instrument manufacturers which currently market each IMS method. Instrument diagrams have been adapted/reprinted from Ref. 7 “Lipid analysis and lipidomics by structurally selective ion mobility-mass spectrometry” 1811, Kliman, M. et. al. 935–945. 2011, with permission from Elsevier.

DTIMS

Drift tube ion mobility spectrometry (DTIMS). In this technique ions are drifted through a tube varying from 5 – 300 cm in a uniform electric field. Smaller ions travel fast through the drift tube than ions with larger collision cross section. The uniform field enables DTIMS to measure K (measure of mobility for each ion) and hence calculate the corresponding CCS (Collision cross section) values for analytes from the Mason-Schamp equation. DTIMS is widely used for structure analysis, and is usually coupled with TOF (time of flight) mass spectrometers.

Strengths – Highest measurement accuracy for CSS, simple physics and excellent reproducibility

Applications – Structural biology, metabolomics, standards development

TWIMS

Travelling Wave Ion Mobility Spectrometry (TWIMS) uses a series of moving electric potential waves that push ions through a gas-filler cell. Separation arises from how ions ‘surf’ these waves, smaller ions keep up with the waves more effectively. CCS Measurement is indirect and requires calibration with standard compounds.

Strengths – Compact design, high speed and compatible with high throughput MS systems. Applications – lipidomics, proteomics, small-molecule profiling.

TIMS

Trapped Ion Mobility Spectrometry (TIMS) uses an electric field gradient to trap ions against a counterflow of gas. By reducing the electric field, ions elute based on their mobility. It enables PASEF (Parallel Accumulation Serial Fragmentation) dramatically increasing MS/MS acquisition speed.

Strengths – Very high resolution, ability to accumulate ions and low gas consumption Applications – High throughput proteomics, clinical metabolomics.

FAIMS / DMS / DIMS

Field Asymmetric-waveform ion mobility spectrometry (FAIMS) a.k.a. Differential Ion Mobility Spectrometry (DIMS). Is a technique where ions are separated by the application of a high-voltage asymmetric waveform at radio frequency (RF) combined with a static (DC, direct current) waveform applied between two electrodes. These devices are extremely small, usually just a few square centimetres in surface area. They are easily implemented on existing MS platforms. Though it’s unable to provide CCS values, due to the differences in separation characteristics, FAIMS/DMS/DIMS devices are able to provide a high degree of selectivity.

Strengths – Extremely compact, high selectivity, filters chemical noise Applications – Targeted analysis, isomer filtering and signal cleanup prior to MS

DMA

Differential mobility analysers (DMA) operated similarly to DTIMS (Drift tube ion mobility spectrometry) as both systems utilize a constant electric field and are able to measure K (measure of ion mobility). However, it operates at ambient pressure and is able to detect very large analytes like antibodies and other macromolecules.

Strengths – Large mass range, precise charge-state manipulation Applications – Biotherapeutics, nanoparticles, polymer analysis

Applications of IMS-MS

IMS methods are typically conducted in three principle application settings

Isomer Separation -> Signal filtering -> Untargeted Annotations by IMS-MS

Isomer Separation

In order to separated structurally similar isomers like lipids, carbohydrates and amino acids, various IMS methods are employed. For isomeric lipids (same molecular formular, different arrangement of atoms in space) FAIMS may be used to separate them. Bowman and coworkers were able to separate isomeric lipids that differed in their double bond orientation (cis/trans) and chain length.

Signal Filtering by IMS-MS

Signal to noise ratio is always a concern with spectroscopic methods. This can be a particular problem if background ions are in high relative abundance compared to the ion of interest. Though all IMS methods which can significantly increase the signal-to-noise ratio for specific ions and decrease background noise, such as DMS, FAIMS, DIMS or DMA, are well suited for this purpose because they can operate as intrinsic mobility filters.

Untargeted Annotations by IMS-MS

One of the great advantages of IMS-MS is that it separated ions on a millisecond timescale, making it a fast method for complex solution elucidation (Fig 3). IMS-MS enhances untargeted workflows by providing, orthogonal separation dimensions (m/z + drift time), Collision cross section (CCS) values as an additional molecular descriptor and improved annotation confidence.

Figure 3. Illustrative workflow of untargeted identifications by IMS-MS with incorporation of additional confidence gained with subsequent chemical information acquired from each analytical technique.

Conclusion

Ion Mobility Mass Spectrometry adds a crucial structural-separation dimension to traditional MS. By differentiating ions based on their movement through gas under electric fields, IMS-MS enables rapid isomer separation, superior signal filtering, and more confident untargeted analysis. With the rise of DTIMS, TWIMS, TIMS and FAIMS technologies across major MS platforms, IMS is now a central tool for modern multi-omics research and a foundational pillar for emerging approaches like SMAD, which I will cover in the next article.

References

1.  Fernandez-Lima, F., et al. Ion Mobility Spectrometry: Fundamental Concepts, Instrumentation, Applications and the Road Ahead. J. Am. Soc. Mass Spectrom. (2019). doi:10.1007/s13361-019-02288-2

2. Bowman, A. P., Abzalimov, R. R., & Shvartsburg, A. A. Broad Separation of Isomeric Lipids by High-Resolution Differential Ion Mobility Spectrometry with Tandem Mass Spectrometry. JASMS 2017, 28(8), 1552–1561.

3.  Lipid analysis and lipidomics by structurally selective ion mobility-mass spectrometry. Kliman et al., Anal. Bioanal. Chem. (2011).