Field Asymmetric Ion Mobility Spectrometry Based Plant Disease Detection: Intelligent Systems Approach

IMS Instrument

IMS generally refers to the principles, techniques and equipments designed to analysing gaseous chemical substances based on the transport of ions in electric fields. The foundational studies of IMS started in late 1940s followed by the development of practical IMS instruments in the 1970s (Eiceman & Karpas, 2005). IMS has been used as an inexpensive and powerful technique for the detection of many chemical compounds. For example, commercial IMS units are used at airports worldwide to detect explosives in carryon luggage for aviation security (Eiceman, 2002; Eiceman et al., 2004); tens of thousands of IMS units have been used by military on battlefield to determine chemical warfare agents (Eiceman, 2002); IMS can also be used to characterise drugs (Lawrence, 1989). Over the last decades, the interests of scientific researchers and engineers have been shifted from the conventional IMS to FAIMS, also known as Differential Ion Mobility Spectrometry (DIMS) or Nonlinear Ion Mobility Spectrometry (NIMS) (Eiceman & Karpas, 2005; Shvartsburg, 2009).

Conventional IMS

Conventional IMS, also called linear IMS as linear voltage gradients were used in the IMS units, was based on the determination of the velocities of the ions (Creaser et al., 2004). The basic components of a conventional IMS unit include ionization unit, drifting tube and detection plate. General ionization sources used in the ionization unit include corona discharge, photoionisaiton, electrospray ionisation, and radioactive source (Creaser et al., 2004). In the drifting tube, purified drift gas flows from the detection plate to the drafting tube entrance and a homogeneous electric field, which is generated by a series of charged rings of different voltages, is applied along the drifting tube to attract ions towards the detection plate. The electric field gradient can be alternated to allow the detection of both positive and negative ions generated in the ionization unit (Borsdorf & Eiceman, 2006). Figure 1 illustrates the systematic structure of a simple drifting tube.

Figure 1.

Structure of a simplified drifting tube (Borsdorf & Eiceman, 2006)

The sample molecules are taken into the functional unit by carrier gas and the molecules are ionised to carry positive or negative charges when passing through the ionization unit. The ionised sample molecules and the non-ionised molecules are separated at the entrance of the drifting tube by the drifting gas flow and the electric field applied in the tube. The ions move towards the detection plate along the electric field gradient at random paths and hit the detection plate at different times depending on their shapes, sizes and the strength of electric field gradient. When the ions hit the detection plate, weak current is generated; the strength of the current and the time of occurrence are recorded as the signature of the test sample. The current strength is proportional to the number of ions hit the detector.