slider IMS technology

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.

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 IMS 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 CD 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.

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 → 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 IMS 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.

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 IMS 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.

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.
 

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