Ion mobility spectrometry explained ...
Ion mobility spectrometry (IMS) is a technique for chemical analysis. It is characterized by low detection limits and short measurement times. Unlike mass spectrometry (MS), it can be performed at ambient pressure, but IMS is less selective. The international working group "Scientific Working Group for the Analysis of Seized Drugs" (SWGDRUG) rates analytical techniques according to their selectivity, i.e. the ability to distinguish individual substances from one another. (A denotes the highest, C the lowest selectivity) The IMS is classified in category B (medium capability).
Functional principle of IMS
The standard IMS instrument has an ionization unit, a drift tube filled with a gas (usually air) and a detector area. The sample to be analyzed is first ionized, i.e. converted into charged particles. This causes the ions to "move" toward the detector in an electric field against the gas in the drift tube. The electric field ensures that the ions in the drift tube are constantly accelerated – driven, so to speak – (drift). In the process, they collide with the air molecules, are thus slowed down and then accelerated again by the electric field. The larger the ions in the sample, the more frequently they collide, are slowed down more and arrive at the detector later. The ion mobility spectrometer (IMS) measures the drift velocity of the ions through the drift tube. The longer the drift time, the higher the molecular weight of the ion.
Length of the drift tube determines the resolution
In this method, the length of the drift tube determines the resolution of the measurement, i.e. the ability to interpret ions with very similar mass or size as different signals. With a compact design and thus shorter drift tube, this often results in lower selectivity. This can lead to false-positive and false-negative results. Resolution can be improved by using a deflector. A deflector is a component that throws the ions back so that they must pass through the drift tube again. However, long drift tubes as well as deflectors are difficult to accommodate in compact instruments.
For the determination of the drift velocity, an accurate time recording is necessary with the IMS standard design. For this purpose, the measurement must proceed step by step one after the other, i.e. sample input, ionization, drift phase, detection. Since the instruments are very sensitive, precise attention must be paid to sample presentation. Overloading the instrument will cause serious malfunctions. As a result, in the worst case, the device cannot be used for several hours.
Instruments that offer orthogonal capabilities, besides just evaluating the drift phase, are much more selective and output reliable results.