Ion mobility spectrometry (IMS) is a technique that differentiates ions based on their size, shape, charge, and mass. While often used in small, portable devices at airports and remote locations for rapidly detecting small molecules whose mobility is known (for example, certain narcotics and explosives) when coupled with liquid chromatography-mass spectrometry (LC/MS), IMS provides an orthogonal dimension of separation and some unique, enabling capabilities.
There are three main ways to perform ion mobility separation, differential IMS, drift-tube IMS, and travelling wave IMS. There are benefits to each:
- Differential IMS allows only ions of selected mobility to be transmitted which can give increased selectivity for a targeted analysis.
- Drift-tube IMS enables Collision Cross Section (CCS) to be calculated from direct measurements. Although a time-consuming process, this is often a good way to determine the CCS of larger molecules such as proteins.
- Travelling Wave IMS allows mobility separation to occur in a shorter flight tube resulting in an instrument with a smaller footprint and is the only method of orthogonal separation by which CCS values can be measured without compromise in sensitivity. This type of IMS separation can be calibrated to allow the easy, quick measurement of CCS for all the ions seen.
Travelling wave IMS is fully compatible with LC/MS and that’s why Waters employs this technique in our IMS instruments.
The collision cross section (CCS) of an ion is its average area as it tumbles through a gas, and is typically measured in Angstroms squared. CCS is related to the chemical structure (mass and size) and three-dimensional conformation (or shape) of an ion. You can see CCS represented here by the molecule trapped within a sphere – its CCS can be thought of as the “shadow” cast by that sphere.
Under typical operating conditions, CCS is a robust and precise measurement related to an ion’s structure. Knowing an ion’s CCS helps in its identification and characterisation. The additional information provided by CCS is independent of sample matrix and chromatography method, which means analytes can be confidently identified even when sample types and chromatographic conditions change.
In many applications (such as food safety monitoring, metabolite identification, and impurity analysis) this can enable the reliable detection of analytes and reduce the occurrence of false positive and false negative results, reducing the need for additional confirmatory analyses and increasing productivity.
While previously the benefits of ion mobility and CCS have been limited to those performing complex, research-type application, Waters LC/IMS/MS instruments (SYNAPT G2-Si and Vion IMS QTof) now bring the benefits of ion mobility and CCS data to all, from research to routine.
For more information take a look at our new video explaining the fundamentals of IMS and the benefits of CCS data.