ion mobility spectrometer

Ion Mobility Spectrometry

- How Does It Work

The ion mobility spectrometry (IMS) is a powerful analytical technique based on ion molecular interactions in homogenous electric field. The main advantages of this technique are: compact design, high sensitivity (ppb-ppt level), fast response (ms range), operation in atmospheric pressure and ability to separate isomeric compounds. Traditionally the IMS instruments consist of three major parts: ionization region, reaction region and drift tube. The schematic view of ion mobility spectrometer is shown in figure 1.

ion mobility spectrometry
Figure 1. Schematic view of ion mobility spectrometry

Ionization Region – Ionization Sources

In ionization region of IMS the formation of reactant ions (RI) occurs. The formation of RI in ion mobility spectrometry is closely related to the type of ionization source used in the instrument. The traditional and most common ionization sources for IMS are radioactive sources, especially Ni63. This ionization source is stable, has a long lifetime and does not require an additional power supply. The main drawback of this source is its radioactivity and the restrictions related to it. The RI ions generated from such type of ionization sources are O2-.(H2O)n in negative polarity and H+.(H2O)n in positive polarity.

In addition to radioactive ionization sources, there exist many others that have been successfully implemented for IMS such as corona discharge, glow discharges, low-temperature plasma ionization, atmospheric-pressure photoionization, pulsed electron sources, electro-spray ionization and secondary electro-spray ionization.

The ionization source used in ion mobility spectrometry instruments developed by MaSaTECH is the corona discharge (CD). Compared to radioactive ionization sources offering CD up to 1 order higher signal yield. The higher signal generation is closely related to higher sensitivity and better signal to noise ratio. Another great advance of corona discharge in comparison with all the other ionization sources is its ability to selectively generate RI. In positive polarity, corona discharge is able to generate H+.(H2O)n or NO+.(H2O)n reactant ions while in negative polarity generating CD O2-.(H2O)n or NO3- reactant ions. Due to different chemical ionization mechanisms of different reactant ions we are able to change the selectivity and the detection efficiency of IMS instrument.

ion mobility spectrometer MS spectrum
Figure 2. IMS and MS spectrum of selective RI generation from CD

Modification of RI by dopant gasses

The RI in IMS instruments can be also easily modified by so-called dopants. The dopants are very useful in applications where focused selectivity is required. As a result of correct selection of dopant gasses it is possible to control mechanism of product ions formation. Dopants are used in IMS in order to increase sensitivity and selectivity of the instrument for target compounds.

Reaction region

In the reaction region of IMS, the interaction between reactant ions and sample molecules occurs and new ion products are formed. There exist many effective pathways for the formation of these ions, where the most common are:

Proton transfer reaction:
H+.(H2O)n + M
M.H+ + (H2O)n
Association reaction:
NO+ + M
Charge exchange reaction:
O2- + M
M- + O2

The importance of RI and their role in the reaction region of IMS is demonstrated in figure 3. The figure 3a demonstrates the response of BTX (benzene, toluene and xylene) with H+.(H2O)n reactants ion. The ion mobility spectrometry response to this RI results in pure sensitivity (ppm level) and selectivity where the H+.(H2O)n and Toluene+ ions are hard to be recognized. The figure 3b. shows the response of BTX to NO+ reactant ions. The NO+ ions results in three order higher sensitivity of IMS instrument and significantly better selectivity.

ion mobility spectrometry response
Figure 3. response of BTX for a) H+.(H2O)n and b) NO+ reactant ions
Shutter grid

At the end of the reaction region of ion mobility spectrometer, the shutter grid is placed. The most common type of shutter grid used in IMS is Bradbury-Nielsen type. This type of shutter grid consists of parallel wires, of which those mutually neighbouring have different electric potential. This creates the potential barrier and the ions are stopped in front of the drift tube of IMS. In such case the shutter grid is closed. The parallel wires are short-circuited for a short time and the shutter grid is open. During this time the ions are injected to the drift tube for separation. The typical injection time in ion mobility spectrometry technique is from 10 µs to 100 µs and shutter grid duty-cycle is in the range of 20-100 ms.

Drift tube

The drift tube of ion mobility spectrometer is the place where the ionic species are separated. After injecting ions to the drift tube these ions are guided by homogenous electric field to the end of the drift tube. The homogenous electric field is formed by metal rings connected by resistors of the same nominal value. Figure 4a shows skeleton of IMS instrument constructed of stainless steel rings isolated by PTFE. After reaching the faraday plate collector at the end of the drift tube, the signal is amplified by current/voltage amplifier. The amplified signal is subsequently plotted as a function of arrival time measured from shutter grid injection. As ion mobility spectrometers do not work in vacuum, the ion movement is not straightforward. There occur huge numbers of ions-molecules interactions between charged ions and neutral particles of a drift gas. In this case the ion movement is not represented by time of flight but by the drift time.

skeleton of ion mobility mass spectrometer
Figure 4. a) skeleton of IMS and b) schematic view of ions movement in IMS drift tube

The drift of the ions in homogenous electric field is represented by the mobility k. The mobility is expressed as the ratio of ion’s drift velocity v and electric field intensity E:

k = v/E = (LD/tD)/(V/LD) = LD2/(tD.V)

where LD is length of the drift tube, tD is measured drift time and V is electric potential at the start of the drift tube. As the IMS instruments usually work in different conditions such as temperature and pressure, there is used correction known as a reduced mobility k0:

k0 = k. (T0/T).(p/p0)

where T is temperature in IMS drift tube, T0 = 273 K, p is pressure in the IMS drift tube and p0 = 1013 mbar The reduced mobility is a characteristic value for each kind of ions in the given drift gas.

Advanced Ion Mobility Spectrometry

The Advanced Ion Mobility Spectrometer (AIMS) developed by MaSaTECH company offers resolution up to 90 FWHM, what is sufficient for separation of isomeric compounds. The big advantage of AIMS is its ability to work with atmospheric air as a buffer gas. This advantage reduces the operating costs to zero. The high sensitivity of IMS technique is in the MaSaTECH AIMS instrument improved by the corona discharge (CD) ionization source. Among the other advantages like non-radioactivity and higher ion yield generation, offers the CD selective reactant ions generations. The CD implemented in AIMS is able to generate O2- or NO3- reactant ions in negative polarity1,2 and H3O+ or NO+ in positive polarity3. The selective reactant ions generation increases the sensitivity and selectivity of instrument3. AIMS is the only commercially available instrument, which allows the selective reactant ions generation.

The biggest advantage of AIMS consists in its capillary sample inlet. The capillary sample inlet designed by MaSaTECH company allows the use of the AIMS not just for VOC detection. This inlet technique also enables to use the AIMS for analysis of surfaces and liquids. The surface analysis can be provided by laser diode module. This sampling technique is based on desorption of semi-volatiles or non-volatiles compounds from the surface. The desorbed compounds are subsequently analyzed by AIMS instrument4. The Surface Sampling Method is a suitable combination of high resolution, high sensitivity Thin Layer Chromatography with AIMS5. The interface of Liquid sampling module (LSM) allows simple analysis of liquids with AIMS6. This technique is based on the thermal spray of the liquid and the mixture of vapours, when droplets are directly injected to the reaction region of the IMS, where the chemical ionization of the sample occure. The technique was successfully tested for detection of explosives in different solvents, as well as for the analysis of amino acids and dipeptides.

The AIMS is able to operate in atmospheric and subatmospheric pressure. It is possible to control all parameters like the shutter grid pulse width and period, drift field intensity, aperture grid electric potential, temperature and many others. All parameters of AIMS can be setup by the user. The instrument can be used for industry applications as well as for laboratory research.

Ionic mobility spectrometer laser desorption
Figure 5. The desorption profiles of isometric β− blocker analyzed by Laser Desorption AIMS