Ion Mobility Spectrometry
The Tool of Civil Forfeiture
The detection of trace amounts of chemicals on various surfaces has become increasingly important in several areas of law enforcement. A key laboratory technique applied to help with the detection and identification of these compounds is Ion Mobility Spectrometry (IMS). IMS instruments have been used in a variety of applications including explosive detection, airport security, chemical weapons monitoring and drug detection (2). One specific use of IMS in Pennsylvania is as a tool in the seizure of US currency accused of being connected to illegal drug transactions.
The analysis begins with sample collection. In the case of drug residues, this will be achieved using some sort of unreactive, uncontaminated swab. The IMS instrument requires that samples are in the gas phase, which means solid collected on the swab will have to be converted to a gas in order to be analyzed. This is accomplished using heat. Once the sample is in the gas phase it can then be swept into the main chamber of the IMS using a flow of gas, most commonly clean, dry air.
Figure 1: IMS schematic. Source: Smiths Detection (www.smithsdetection.com)
The first step to analysis using these instruments is ionization. This is the process through which drug molecules with no charge becomes charged. Charged molecules, known as ions, can be either positively or negatively charged. In IMS, ions are most commonly given a positive charge. This is achieved by passing the sample in the gas phase over a weakly radioactive source, Ni63 in Figure 1 above, that emits particles which collide with the drug molecules to cause ions to form.
The second step in IMS is separation. Once the ions are formed, they flow into a drift tube under an electric field and through a series of focusing rings. Ions will travel at different speeds depending on their shape and size, thereby separating compounds from one another as some move quicker through the tube than others.
The last step is detection. Once the ions are separated in the drift tube, they are collected and strike an electrode. That signal is amplified and recorded by the operating software. The result, a spectrum, is a plot of ion intensity current vs. drift time in milliseconds (Figure 2).
Figure 2: IMS Detector Output as a Function of Drift Time (2)
Drift time refers to the time it takes the ions to move down the tube and reach the detector. Standards of known drug or explosive material are run on an instrument to calculate the reference drift times. This drift time is stored in a library, and when an unknown sample is run, drift times for the sample can be compared to those in the library to determine the presence of target substances.
IMS is only a presumptive testing technique because drift time is not unique to an individual compound. Ions of the same shape and/or size may travel at the same speed resulting in their instantaneous detection. This means the method cannot differentiate between certain sets of compounds. As a result, IMS must be accompanied by another testing method of higher selectivity to ensure results are accurate. Other more selective methods, such as mass spectrometry, can be used to further support the presumptive identification of compounds identified using IMS.
References:
- Smiths Detection Canada. IONSCAN 500DT Operation Manual. May, 2013
- Cumeras, R. et al. Review of Ion Mobility Spectrometry. Part 1: Current Instrumentation (2015). Analyst, 140(5), 1376-1390.
- Hill, H.H. & Simpson, G. Capabilities and Limitations of Ion Mobility Spectrometry for Field Screening Applications (1997). Field Analytical Chemistry and Technology, 1(3), 119-134.