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What is High-Resolution Ion Mobility Spectrometry?

In drift-tube ion mobility spectrometry (DT-IMS) as used in the TOFWERK IMS-TOF, a static uniform electric field is used to pull ions through the ion mobility drift tube which is filled with buffer (drift) gas.

The time an ion takes from the start of the drift tube to the detector is referred to as the drift time. As the ion travels through the drift tube, it undergoes collisions with the buffer gas; this observational property which averages all geometric orientations and ion-gas interaction types during the experiment is referred to as the collision cross section (CCS). The larger an ion, the larger also its CCS. This means that ions with a large CCS require more time to traverse the drift tube and therefore exhibit a longer drift time.

The CCS can be directly calculated from the measured drift time and other known experimental parameters using the so-called Mason–Schamp equation. Consequently, DTIM systems operating at low electric fields are still preferred for the direct determination of accurate and precise determination of CCS values. The CCS is specific for an analyte in a given drift gas and can therefore be used as a molecular identifier in addition to accurate mass, fragmentation ions and LC retention time in compound databases.

Ω represents the integrated CCS, µ the reduced mass of the analyte ion and the drift gas, k Boltzmann’s constant, q the elementary charge, z the charge number, E the electric field, td the drift time, L the length of the drift tube and N the neutral gas number density.
Schematic representation of a DT-IMS.

Isomeric molecules cannot be distinguished by mass spectrometry due to identical mass. Yet, they usually display small differences in CCS (on the order of 1-2% for many isomeric metabolites). This means an ion mobility system should have a resolving power which is sufficiently high to separate analytes with CCS differences of about 1% to be useful as an analytical separation device.

The figure shows the separation of two analytes with a difference of 1 percent in their CCS value with three different IMS resolving powers. The ion mobilograms for the single analytes is shown with dashed grey lines, the resulting spectrum of a 1:1 mixture in red. It is obvious that a high resolution IMS with resolving power of 200 is required to baseline-separate such a mixture. The TOFWERK IMS-TOF is the only drift-tube IMS-MS instrument that can deliver such resolving power.

Further Reading

Hill, H. J., Eiceman, G. A. & Zeev Karpas. Ion Mobility Spectrometry, Third Edition. (CRC Press, 2013). doi:10.1201/b16109

Lanucara, F., Holman, S. W., Gray, C. J. & Eyers, C. E. The power of ion mobility-mass spectrometry for structural characterization and the study of conformational dynamics. Nat. Chem. 6, 281–294 (2014).

May, J. C., Morris, C. B., & McLean, J. A. (2016). Ion Mobility Collision Cross Section Compendium. Analytical chemistry, 89(2), 1032-1044.

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