3 Types of IMS: An At-a-Glance Guide

Published on: 28 Jun 2017, under FAIMS

What is Ion Mobility Spectrometry?

When an ion moves through a gas under the influence of an electric field, the combination of acceleration due to the field and deceleration due to collisions with gas molecules leads to it very quickly acquiring a terminal velocity v, where

v = KE

E is the electric field strength and K is known as the ion’s mobility.

The magnitude of v (and hence K) is related to the ion’s collision cross-section; the larger the ion, the more likely it is to collide with gas molecules, so the lower its terminal velocity. Ion mobility in all its forms is based upon exploiting this fact; each of these forms is briefly outlined below.

1. Drift-Tube Ion Mobility Spectrometry (DT-IMS)

Drift tube IMS uses a long, gas-filled tube with a constant (and relatively low) electric field. Ions are injected at one end, and the time taken to travel the length of the tube is recorded. From this, the velocity of travel can be directly calculated, and if the field strength is known, the ions’ mobilities can also be determined. If the travel time is to be known, ions must be injected into the system in short pulses, and then all the ions must be allowed to travel through to the detector before another pulse is injected.

Drift tube IMS at a glance

2. Travelling-Wave Ion Mobility Spectrometry (TW-IMS)

TW-IMS works along similar lines to DT-IMS. However, instead of having a constant electric field, TW-IMS uses alternating sections of positive and zero electric field, travelling parallel to the ions’ direction of travel.

While in the positive section, ions move at v=KE, while in the zero-field section, they remain stationary. Travel time is still dependent on K, but now in two ways: firstly, the higher the mobility, the higher the velocity of travel while in the positive field; and secondly, the higher the velocity of travel, the longer the ions remain within the positive section of the wave. In the case where v = velocity of wave travel, the ion is carried along at the front of the wave, effectively “surfing” it. As it is still travel time through the system that is being measured, ions must still be fed into the system in short pulses.

Travelling Wave IMS

3. Field Asymmetric Ion Mobility Spectrometry (FAIMS)/Differential Mobility Spectrometry (DMS)

Instead of applying a field parallel to the ions’ travel, FAIMS uses an asymmetric alternating electric field, perpendicular to the direction of travel. This causes ions to drift towards one or the other electrode, depending on their values of K in high (KH) and low (KL) fields – only ions with a certain differential mobility (KH-KL) will pass through the system. By applying an additional DC compensation field, the value of this differential mobility can be changed, and by scanning through compensation field strengths, a spectrum of ions separated by their differential mobility can be obtained. For more details, see our FAIMS Technology page.

ultraFAIMS is orthogonal to MS and IMS

Drift Tube IMS

Travelling Wave IMS


Collision cross-section (CCS) measurement

Direct (more accurate, based on low-field mobility)

Requires system calibration against known CCS


Can it be added to existing mass spectrometers?

Requires new mass spectrometer

Requires new mass spectrometer

Can retrofit with minor modifications to existing mass spectrometer (learn more)

Isomer separation and charge state separation




Orthogonality of separation

Poor – larger ions also have larger m/z

Poor – larger ions also have larger m/z

Highly orthogonal to MS

Linear dynamic range

Limit to size of ion packet in pulse which limits linear dynamic range (staggered pulsing can improve this)

Limit to size of pulse of ions which limits linear dynamic range (staggered pulsing can improve this)

Superior linear dynamic range (stream of ions instead of pulse, charge capacity constraint improved by multiple channels devices)

Duty cycle

Poor duty cycle (relative to other techniques, still 10s of ms timescale)

Better duty cycle than IMS because waves propel ions (1-10s of ms)

Ions analyzed as a continuous beam (fastest – μs) but in scanning mode becomes slowest (s timescale) – hopping between different static settings can make it faster for target ions

Physical effects on ion structure (aiding structural studies)

Ambient conditions for ions

Field heating effects (potential for distortion or fragmentation of molecule structures)

Field heating effects (potential for distortion or fragmentation of molecule structures)

Resolving power

Highest resolving power

High resolving power (generally worse than DTIMS)

Less resolving power but ‘high separation selectivity’ i.e. experimental conditions can be changed to increase separation


Lower sensitivity

Higher sensitivity

Highest sensitivity


Add ultraFAIMS to your mass spectrometerultraFAIMS

Add ion mobility to existing mass spectrometers to provide in-source separation of ions

Retrofit to Thermo Scientific instruments