Ion mobility spectrometry

History

IMS was first developed primarily by Earl W. McDaniel of Georgia Institute of Technology in the 1950s and 1960s when he used drift cells with low applied electric fields to study gas phase ion mobilities and reactions. In the following decades, he coupled his new technique with a magnetic-sector mass spectrometer, with others also utilizing his techniques in new ways. IMS cells have since been attached to many other mass spectrometers, gas chromatographs and high-performance liquid chromatography setups. IMS is a widely used technique, and improvements and other uses are continually being developed.

Applications

Perhaps ion mobility spectrometry's greatest strength is the speed at which separations occur—typically on the order of tens of milliseconds. This feature combined with its ease of use, relatively high sensitivity, and highly compact design have allowed IMS as a commercial product to be used as a routine tool for the field detection of explosives, drugs, and chemical weapons. Major manufacturers of IMS screening devices used in airports are Morpho and Smiths Detection.

In the pharmaceutical industry IMS is used in cleaning validations, demonstrating that reaction vessels are sufficiently clean to proceed with the next batch of pharmaceutical product. IMS is much faster and more accurate than HPLC and total organic carbon methods previously used. IMS is also used for analyzing the composition of drugs produced, thereby finding a place in quality assurance and control. As a research tool ion mobility is becoming more widely used in the analysis of biological materials, specifically, proteomics and metabolomics. For example, IMS-MS using MALDI as the ionization method has helped make advances in proteomics, providing faster high-resolution separations of protein pieces in analysis.

Outside of laboratory purposes, IMS has found great usage as a detection tool for hazardous substances. More than 10,000 IMS devices are in use worldwide in airports, and the US Army has more than 50,000 IMS devices. In industrial settings, uses of IMS include checking equipment cleanliness and detecting emission contents, such as determining the amount of hydrochloric and hydrofluoric acid in a stack gas from a process. It is also applied in industrial purposes to detect harmful substances in air.

In metabolomics the IMS is used to detect lung cancer, Chronic obstructive pulmonary disease, sarcoidosis, potential rejections after lung transplantation and relations to bacteria within the lung (see breath gas analysis).

Ion mobility

The physical quantity ion mobility K is defined as the proportionality factor between an ion's drift velocity v_d in a gas and an electric field of strength E.

Ion mobilities are commonly reported as reduced mobilities, correcting to standard gas density n₀, which can be expressed in standard temperature T₀ = 273 K and standard pressure p₀ = 1013 mbar. It should be noted that this does not correct for other effects than the change in gas density and the reduced ion mobility is therefore still temperature dependent.

The ion mobility K can, under a variety of assumptions, be calculated by the Mason-Schamp equation.

where Q is the ion charge, n is the drift gas number density, μ is the reduced mass of the ion and the drift gas molecules, k is Boltzmann constant, T is the drift gas temperature, and σ is the collision cross section between the ion and the drift gas molecules. Often, N is used instead of n for the drift gas number density and Ω instead σ for the ion-neutral collision cross section. This relation holds approximately at a low electric field limit, where the ratio of E/N is small and thus the thermal energy of the ions is much greater than the energy gained from the electric field between collisions. With these ions having similar energies as the buffer gas molecules, diffusion forces dominate ion motion in this case. The ratio E/N is typically given in Townsends (Td) and the transition between low- and high-field conditions is typically estimated to occur between 2 Td and 10 Td. When low-field conditions no longer prevail, the ion mobility itself becomes a function of the electric field strength which is usually described empirically through the so-called alpha function.

Ionization

The molecules of the sample need to be ionized, usually by corona discharge, atmospheric pressure photoionization (APPI), electrospray ionization (ESI), or a radioactive source, e.g. a small piece of ⁶³Ni or ²⁴¹Am, similar to the one used in ionization smoke detectors. ESI and MALDI techniques are commonly used when IMS is paired with mass spectrometry.

Doping materials are sometimes added to the drift gas for ionization selectivity. For example, acetone can be added for chemical warfare agent detection, chlorinated solvents added for explosives, and nicotinamide added for drugs detection.

Analyzers

Ion mobility spectrometers exist based on various principles, optimized for different applications. A review from 2014 lists eight different ion mobility spectrometry concepts.

TOF-IMS / Drift tube IMS

In its simplest form, an IMS system measures how long a given ion takes to traverse a given length in a uniform electric field through a given atmosphere. In specified intervals, a sample of the ions is let into the drift chamber; the gating mechanism is based on a charged electrode working in a similar way as the control grid in triodes works for electrons. For precise control of the ion pulse width admitted to the drift tube, more complex gating systems such as a Bradbury-Nielsen or a Field Switching Shutter are employed. Once in the drift tube, ions are subjected to a homogeneous electric field ranging from a few volts per centimeter up to many hundreds of volts per centimeter. This electric field then drives the ions through the drift tube where they interact with the neutral drift molecules contained within the system and separate based on the ion mobility, arriving at the detector for measurement. Ions are recorded at the detector in order from the fastest to the slowest, generating a response signal characteristic for the chemical composition of the measured sample.

The ion mobility K can then be experimentally determined from the drift time t_D of an ion traversing within a homogeneous electric field the potential difference U in the drift length L.

A drift tube’s resolving power R_P can, when diffusion is assumed as the sole contributer to peak broadening, be calculated as

where t_D is the ion drift time, Δt_D is the Full width at half maximum, L is the tube length, E is the electric field strength, Q is the ion charge, k is Boltzmann’s constant, and T is the drift gas temperature. Ambient pressure methods allow for higher resolving power and greater separation selectivity due to a higher rate of ion-molecule interactions and is typically used for stand-alone devices, as well as for detectors for gas, liquid, and supercriticial fluid chromatography. As shown above, the resolving power depends on the total voltage drop the ion traverses. Using a drift voltage of 25 kV in a 15 cm long atmospheric pressure drift tube, a resolving power above 250 is achievable even for small, single charged ions. This is sufficient to achieve separation of some isotopologues based on their difference in reduced mass μ.

Low pressure TOF-IMS / Drift tube IMS

Reduced pressure drift tubes operate using the same principles as their atmospheric pressure counterparts, but at drift gas pressure of only a few torr. Due to the vastly reduced number of ion-neutral interactions, much longer drift tubes or much faster ion shutters are necessary to achieve the same resolving power. However, the reduced pressure operation offers several advantages. First, it eases interfacing the IMS with mass spectrometry. Second, at lower pressures, ions can be stored for injection from an ion trap and re-focussed radially during and after the separation. Third, high values of E/N can be achieved, allowing for direct measurement of K(E/N) over a wide range.

Travelling Wave IMS

Though drift electric fields are normally uniform, non-uniform drift fields can also be used. One example is the travelling wave IMS, which is a low pressure drift tube IMS where the electric field is only applied in a small region of the drift tube. This region then moves along the drift tube, creating a wave pushing the ions towards the detector, removing the need for a high total drift voltage.

Trapped ion mobility spectrometry

In trapped ion mobility spectrometry (TIMS), ions are held stationary (or trapped) in a flowing buffer gas by an axial electric field gradient (EFG) profile while the application of radio frequency (rf) potentials results in trapping in the radial dimension. TIMS operates in the pressure range of 2 to 5 mbar and replaces the ion funnel found in the source region of modern mass spectrometers. It can be coupled with nearly any mass analyzer through either the standard mode of operation for beam-type instruments or selective accumulation mode (SA-TIMS) when used with trapping mass spectrometry (MS) instruments.

Effectively, the drift cell is prolonged by the ion motion created through the gas flow. Thus, TIMS devices do neither require large size nor high voltage in order to achieve high resolution, for instance achieving over 250 resolving power from a 4.7 cm device through the use of extended separation times. However, the resolving power strongly depends on the ion mobility and decreases for more mobile ions. In addition, TIMS can be capable of higher sensitivity than other ion mobility systems because no grids or shutters exist in the ion path, improving ion transmission both during ion mobility experiments and while operating in a transparent MS only mode.

DMS / FAIMS

DMS (differential mobility spectrometer) or FAIMS (field asymmetric ion mobility spectrometer) make use of the dependence of the ion mobility K on the electric field strength E at high electric fields. Ions are transported through the device by the drift gas flow and subjected to different field strengths in orthogonal direction for different amounts of time. Ions are deflected towards the walls of the analyzer based on the change of their mobility. Thereby only ions with a certain mobility dependence can pass the thus created filter

DMA / Aspiration Condenser / Aspirating IMS

DMA differential mobility analyzer make use of a fast gas stream perpendicular to the electric field. Thereby ions of different mobilities undergo different trajectories. This type of IMS corresponds to the sector instruments in mass spectrometry. They also work as a scannable filter. Examples include the DMD (Differential Mobility Detector), first commercialized in Varian CP-4900 MicroGC. Aspiration IMS operates with open-loop circulation of sampled air. Sample flow is passed via ionization chamber and then enters to measurement area where the ions are deflected into one or more measuring electrodes by perpendicular electric field which can be either static or varying. The output of the sensor is characteristic of the ion mobility distribution and can be used for detection and identification purposes.

Drift gas

The drift gas composition is an important parameter for the IMS instrument design and resolution. Often, different drift gas compositions can allow for the separation of otherwise overlapping peaks. Elevated gas temperature assists in removing ion clusters that may distort experimental measurements.

Detector

Often the detector is a simple Faraday plate coupled to a transimpedance amplifier, however, more advanced ion mobility instruments are coupled with mass spectrometers in order to obtain both size and mass information simultaneously. It is noteworthy that the detector influences the optimum operating conditions for the ion mobility experiment.

Combined IMS methods

IMS can be combined with other separation techniques.

GC-IMS : gas chromatography - ion mobility spectrometry

When IMS is coupled with gas chromatography, common sample introduction is with the GC capillary column directly connected to the IMS setup, with molecules ionized as they elute from GC. A similar technique is commonly used for HPLC. A novel design for corona discharge ionization ion mobility spectrometry (CD–IMS) as a detector after capillary gas chromatography has been produced in 2012. In this design, a hollow needle was used for corona discharge creation and the effluent was entered into the ionization region on the upstream side of the corona source. In addition to the practical conveniences in coupling the capillary to IMS cell, this direct axial interfacing helps us to achieve a more efficient ionization, resulting in higher sensitivity.

IMS-MS : ion mobility spectrometry - mass spectrometry

When IMS is used with mass spectrometry, ion mobility spectrometry-mass spectrometry offers many advantages, including better signal to noise, isomer separation, and charge state identification. IMS has commonly been attached to several mass spec analyzers, including quadropole, time-of-flight, and Fourier transform cyclotron resonance.

LC-IMS-MS : liquid chromatography - ion mobility spectrometry - mass spectrometry

Coupled with LC and MS, IMS has become widely used to analyze biomolecules, a practice heavily developed by David E. Clemmer, now at Indiana University (Bloomington).