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Ion Mobility Mass Spectrometry: Another Separations Layer for Complex Applications

Today’s mass spectrometry applications have evolved to demand higher accuracy and sensitivity for complex (multi-) analyte processing. Examples include: high-throughput shotgun proteomics studies, metabolomics and lipidomics studies in native tissue or biofluids, and a host of other rigorous applications.

The need for high sensitivity, accuracy, resolution, and throughput

Technologies have evolved to address the needs for complex workflows. A range of flow rates can be employed to suit the needed sensitivity (standard-, micro-, nanoflow). A range of semi-, fully-automated components have arisen to suit the workflow scale and throughput (large sample numbers, large patient cohorts). Separations and mass analysis technologies have evolved to perform high-accuracy, high-resolution analysis using advanced LC, GC, and (hybrid) MS instrumentation – with associated data processing software.

Two-dimensional separations technologies

This evolution has extended to the separations stage of complex samples as well. Multi-dimensional separations using 2D column technologies (GCxGC, LCxLC), apart or inline with MS analysis, have seen a resurgence of recent use from early popularity. Although two-dimensional separations techniques were once thought too complex for routine analytical workflows, recent technologies have demonstrated new found accuracy, precision, and robustness for complex sample applications.

The rise in popularity of ion mobility mass spectrometry

The evolution of multi-separations (2D) technologies has increased the orthogonality and deconvolution of complex samples. With this has come the potential to uncover previously masked and low-abundance analytes and their associated biological implications and importance.

Another technique, with somewhat modest origins, is quickly gaining popularity due to its utility and implementation with modern hybrid instrument technologies.

Ion Mobility Mass Spectrometry (IM-MS) offers a further layer of separations power and adds yet another dimension for complex, high-throughput applications.

What is ion mobility mass spectrometry?

IM-MS combines the concept of ion-mobility spectroscopy with mass spectrometry analysis. Ion-mobility spectroscopy (IMS) is an analytical technique used to separate ionized (gas phase) molecules based on their mobility in a carrier buffer gas. Due to their somewhat basic architecture and operation, IMS instruments have been used for a wide range of applications, including drug and explosive detection, food monitoring, and medical diagnostics, in both laboratory and remote locations. Combination with mass analyzers such as mass spectrometers serves to greatly enhance multi-dimensional high-accuracy analysis of molecules.

How does ion mobility mass spectrometry work?

IM-MS separations are based on various principles, optimized for different applications. All are centered around the concept of movement of gas phase ions through a carrier gas using defined electric field(s).

Drift-time ion mobility spectrometry (DTIMS) involves movement of ions through a tube of defined length using an electric field gradient. Smaller ions travel faster than larger molecules, and separation is based on drift time within the tube. A feature termed the rotationally averaged collision cross section (CCS) is a physical property that reflects the shape and size of the molecules to be measured. High-resolution determination of CSS enables structural determination of large molecules, typically when coupled with a time-of-flight mass spectrometer (TOF-MS).

Differential mobility spectrometry (DMS) differs from the above in that a high-voltage asymmetric waveform at radio frequency (RF) is combined with a static (DC) waveform applied between two electrodes. The technique functions as a mass selector based upon the propensity of ions to migrate toward one electrode over the other. Instruments using DMS are typically paired with triple quadrupole mass spectrometers, which also function as mass selectors inline with mass (m/z) detection.

A third type of IM-MS, termed Travelling wave ion mobility spectrometry (TWINS), uses RF and DC voltages now applied to a series of ring electrodes or a stacked ring ion guide (SRIG). Based upon collisional frequency with gas molecules within the SRIG, smaller ions exit faster than larger molecules, thus enabling separation. CSS values can be calculated using internal standards. This technique is built into several commercial MS instruments.

How can ion mobility mass spectrometry increase sample coverage and throughput?

The various ion mobility techniques can be interfaced with different mass analyzers and ionization sources to expand the range of investigation.

Modern MS instrument designers have optimized the combination of IMS technique, ionization source, and high-performance MS detection, to arrive at state-of-the art solutions for complex applications.

Examples include those platforms employing parallel accumulation-serial fragmentation (PASEF). The PASEF technique allows precursor ions to accumulate in parallel and to be released sequentially as a function of their ion mobility. The approach provides a further level of separation upstream of the quadrupole, thus enabling substantial gains in sequencing speed for complex high-throughput proteomics.

The combination of PASEF with a novel Trapped Ion Mobility Spectrometry device, demonstrates even further increases in processing speed with high-sensitivity from very small sample sizes.

Other technologies have helped bring the approach away from complex, research applications and closer to routine analysis – for state-of-the art applications in food quality and authenticity, and ma

ny other routine applications.

Outlook for new ion mobility mass spectrometry technologies

As workflows, study sizes, sample complexity continue to escalate in scale, IM-MS will undoubtedly see further use as a complementary “new” dimension of separations power.

As mass spec analysis continues to move into more routine (and remote) applications, IM-MS will continue to contribute a “new” layer of separations efficiency.

Updated March 2022