Overview of Ion Mobility techniques

Author: Emma Marsden-Edward

Ion mobility spectrometry (IMS) is a rapid, gas-phase separation technique which has been around for many years and has found utility as low cost, stand-alone, portable detection devices which are widely used to detect explosives and narcotics at airports, contaminants in foodstuffs, and pollutants in the environment.  While standalone IMS has a number of uses, the hybridization with mass spectrometry (IMS-MS) creates a significantly more powerful analytical tool, enhancing complex mixture analysis and facilitating ion structural determination through elucidation of CCS values.  

The Key Factors of Each IMS

There are three main categories of ion mobility; time-dispersive, spatially dispersive and confinement with selective release [1,2]. Time-dispersive includes linear drift tube IMS (DTIMS) and travelling wave IMS (TWIMS), whilst in the spatially dispersive category there are differential mobility analyzers (DMA) and differential mobility spectrometers (DMS).  Trapped IMS (TIMS) and cyclic IMS are examples of a confinement and selective release approach. 

DTIMS is the oldest and the simplest technique to consider. The separation in DTIMS is well characterized, making it possible to derive collision cross section (CCS) values from first principles using this technique.  In practice multiple measurements are often taken under different fields to accurately determine CCS values.   

IMS has come a long way since the first instruments were created. TWIMS generally allows mobility separation of a similar resolution to occur in a shorter device than DTIMS, resulting in an instrument with a smaller footprint.  Whilst ion motion in a TWIMS is relatively complex, the resulting IMS separation can be calibrated to allow the easy, quick measurement of CCS for all of the ions present.

DMA and DMS are typically used as filtration devices as they only allow ions of selected mobility or mobility characteristics to be transmitted at a given time, which can give increased selectivity for targeted analyses but are ultimately slower and lower sensitivity for broad range mobility analysis.  Whilst CCS values can be determined from DMA, this is not the case for DMS. 

TIMS is a more recent development and can provide high ion mobility resolution. Although the resolving power of trapped ion mobility spectrometry decreases for more mobile ions, CCS values can be determined using this approach through calibration.  Another recent technique is cyclic IMS, which is TWIMS-based, and this can provide selectable mobility resolution and enables IMSn experiments.  

The Science Behind the IMS Techniques

The differing IMS techniques each make use of slightly different operational principles, but they all separate ions using a combination of electric fields and a background (buffer or drift) gas. 

In DTIMS, ions travel through a drift tube made of ring electrodes with a voltage gradient applied and containing a buffer gas. The time taken for the sample ions to drift through the device corresponds to the mobility (size) of the ions being analyzed (larger ions take longer than smaller ions).

In TWIMS, ions are propelled through a gas-filled confining stacked ring ion guide (SRIG) using superimposed travelling voltage waves. As the waves pass along the device, ions can ‘surf’ on the wave front for a period of time before being overtaken by the wave. Ions are separated as they travel through the device as higher mobility ions undergo less ‘roll over’ events on the waves than the lower mobility ions.

DMA uses two planar electrodes to generate an electric field and has a cross-flowing buffer gas. Ions enter the device through one electrode and are drawn across the flow of gas to an exit aperture in the other electrode. Ions of different mobility can be transmitted by varying the electric field strength.  DMS also uses two electrodes and a gas flow perpendicular to the fields generated, but in this instance an oscillating, asymmetric, high/low field is used for separation. Ions enter the device with the buffer gas and their high and low field mobility characteristics determine whether they drift towards, and are lost to, the electrodes or transmitted. The use of a supplemental voltage across the electrodes allows tuning of the device to allow selection of specific ions for transmission. 

TIMS employs an electric field with spatially increasing strength to trap ions against a flow of gas. The ions are trapped at different axial positions, dependent on their mobility. Ions are released by slowly decreasing the electric field, with low mobility ions exiting before high mobility ions.

The Importance of Collision Cross Section (CCS)

IMS provides a characteristic and reproducible measure of an analyte’s structure through derived CCS values. CCS values can be used to increase confidence in identification of known compounds. CCS can also serve as a valuable parameter in the identification of unknowns when compared with theoretically derived values. 

You can find out more about the benefits of travelling wave and CCS values by exploring our current range of advanced IMS technologies here.


References:

  1. Lanucara F. et al, The power of ion mobility-mass spectrometry for structural characterization and the study of conformational dynamics, Nature Chemistry6, (2014), 281-294.
  2. May, J. C.; McLean, J. A. Ion Mobility-Mass Spectrometry: Time-Dispersive Instrumentation. Analytical Chemistry87 2015, 1422–1436.

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