Explosive vapor detectors (EVD) are explosives detection instruments whose principle of operation is the selective analysis of collected vapor, samples from the air, in contrast to the explosives trace detectors (ETD) which requires the physical collection of particulate samples from surfaces. EVDs are not limited to explosives, but also to narcotics and other illicit or dangerous substances such as biological agents or chemical warfare.

Depending on the requirements of the final application, EVDs can be classified into two types: portable or handheld EVDs vs. EVDs with sample preconcentration. Portable units have detection limits in the range of ppm or ppb, similar to ETDs or explosive detection dogs). Some portable EVDs are already commercially available. Pre-concentrating EVDs have sensitivities of parts per quadrillion (10-15).

Principle of operation

The detection process is based on two steps:

  1. Sampling
  2. Analysis

However, for the portable instruments, the sampling and the analysis is done on the same device, with no need for preconcentration.

Sampling

A sample of air is collected from the interior of the target volume (truck, container, pallet, box, etc.) using a vacuum aspiration system, typically named sampler.

In the case of portable instruments, the air sample is directly injected into the detector. Butin the technologies based on preconcentration, the air crosses a collection cartridge where a percentage of the sampled vapors are retained, the cartidge is typically based on a chemical absorbent like TENAX[1] among some other techniques, for example the innovative polimer-based solutions. One important characteristic of the sampling process is the need for a high-flow collection technology (> 100 liters per minute).[2] since one of the main advantages of EVDs is the screening of complete trucks or containers in just one analysis, this type of samplers are denominated High Volume Sampler (HVS). Once the sample is taken, it is ready for the analysis.

Analysis

The sampled cartridge is inserted in the analyzer where the first step is to release the trapped vapors thanks to a vapor liberation process such as thermal desorption or Laser desorption among some other alternatives. In the case of portable instruments, the vapors are directly injected and in both cases, vapors are carried to the ionization region in a clean gas media where the vapors are ionized, different techniques are available such as radioactive sources, Secondary Electrospray or Photoionization as examples of the most common ones. These ions are then analyzed, this analysis can be done with different instrumentation; however, for the case of highly sensitive instruments, the most common technique is mass spectrometry due to the high requirements in terms of sensitivity and selectivity. The analysis provides the result of Alarm or not, although most of the techniques provide a quantitative analysis of the level of concentration detected. In the case of portable devices, the detector is typically an ion mobility spectrometer. .

Sensitivity and selectivity

The number of compounds (N) in a multi-component media, such as the atmosphere increases exponentially with the partial pressure (p). For instance, analyzing the atmosphere with a detector sensitive to a partial pressure p = 0.8 atm, only one compound is detectable (N = 1); at p = 0.01 atm, 5 compounds (N=5) can be classified (N2, O2, Ar, CO2, H2O), etc. For an analytical instrument to distinguish these N components from each other at decreasing p, one needs to increase the sensitivity, as well as the resolution or the selectivity.[3] For the case of EVDs, the detection requirements in terms of sensitivity and selectivity are quite demanding since the vapor pressure of most of the explosives or drugs is in the order of ppt or even ppq, where the number of available compounds is around millions or even billions (no solid data about this is available). In addition, if the target substance (explosive or drug) is well packed and wrapped, the real vapor pressure is even lower, typically from 3 to 5 orders of magnitude lower. The portable instruments don't have the capacity to detect at this level and only the sample preconcentration techniques can reach these requirements. This extremely high selectivity capacity is the main reason why not many EVD technologies with detection capabilities in the order of ppq are available yet, and current solutions are based on the combination of different complex and innovative analytical techniques.

Advantages and disadvantages

Portable/handheld EVDs have similar performances as the case of ETDs; this section is focused only in the EVDs based on sample preconcentration.

Advantages

  • EVD technology allows the scanning of complete trucks or containers in one analysis which reduces the screening costs significantly versus other techniques like imaging techniques or explosives trace detectors where the cargo must be unloaded and individually screened.
  • EVDs generate automatic results avoiding human interpretation which is prone to errors, reducing the false alarm rate
  • The extremely high sensitivity generates high detection rate
  • EVDs preserve the privacy of the items to be scanned since it is a non-intrusive technology versus the imaging techniques.

Disadvantages

  • The technical requirements of the EVDs are very demanding and the technology is typically based on mass spectrometry, so they are expensive instruments. However, the operation costs are lower, so the pay-off periods are lower than other technologies, but the initial expenditure is high.
  • The vapor pressure is highly dependent on the temperature, so the detection capacity of EVDs at low-temperature scenarios (<10 °C) is limited.

Applications

Even though the main application of EVDs is security, some other direct applications can be considered such as atmospheric analysis, food fraud control, medical applications such as the detection of illnesses like cancer, tuberculosis, or even diabetes by breath analysis and demining processes in the military domain.

Regulations

ECAC has been the first organization in the world to approve the implementation of EVDs in the EU for air cargo screening according to the COMMISSION IMPLEMENTING REGULATION (EU) 2023/566 [4] that entered in force in April 2023 with the main objective to increase the level of security in the EU.

References

  1. ↑ Cao, X.-L.; Hewitt, C. N. Atmos. Environ., Part A 1993, 27 (12), 1865−1872.
  2. ↑ Xiaofeng Xie, H. Dennis Tolley, Milton L. Lee, Concentrically packed high-flow air sampler for parts-per-trillion volatile and semi-volatile organica compounds, Journal of Chromatography A, Volume 1502, 2017, Pages 1-7, ISSN 0021-9673, https://doi.org/10.1016/j.chroma.2017.04.020.
  3. ↑ Reaching a Vapor Sensitivity of 0.01 Parts Per Quadrillion in the Screening of Large Volume Freight D. Zamora, M. Amo-Gonzalez, M. Lanza, G. Fernández de la Mora, and J. Fernández de la Mora Analytical Chemistry 2018 90 (4), 2468-2474 DOI: 10.1021/acs.analchem.7b00795
  4. ↑ https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32023R0566
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