Detection of explosives by analysis of their vapors and particles.

Detection of explosives by analyzing their vapors and particles.

Vandyshev Boris Alekseevich, Candidate of Technical Sciences

DETECTION OF EXPLOSIVES BY ANALYZING THEIR VAPORS AND PARTICLES

Terrorism using explosives (HE) has become widespread throughout the world in recent years, the fight against this illegal phenomenon has been elevated to the rank of an international problem.

The use of explosive devices (ED) by terrorists, skillfully camouflaged in household items, hidden in cars and even under a person’s clothing (kamikaze terrorists), usually results in a large number of victims and causes significant material damage.

For example, according to data published in the magazine “Security” (No. 9, 1995), in the USA in 1993, 1,880 terrorist acts were committed using ED, as a result of which 70 people died, 1,375 were injured, and property damage amounted to 526 million dollars.

According to information from the Ministry of Internal Affairs of the Russian Federation (the newspaper “Moskovsky Komsomolets” from December 3, 1997), in 1997 in Russia “740 criminal explosions were carried out, in which 460 people were injured, including 150 killed.”

Specialists from many countries are working on creating devices that allow for the timely detection of explosive devices and their neutralization.

It is difficult to name a scientific and technical area whose achievements would not be used to solve this problem.

Among the devices that allow detecting hidden explosive devices, a prominent place is occupied by equipment for direct detection of explosives by detecting their vapors and particles present in certain quantities near or on the surface of a terrorist “bomb”.

In order to have an idea of ​​the quantities of explosives that must be detected in the air using a gas analysis detector, Table 1 provides approximate data on the pressure of their saturated vapors at normal atmospheric pressure and room temperature.

As can be seen from the table, the sensitivity of explosive vapor detectors must be high enough, especially since industrial and military products are manufactured using various binders (such as American C-4), which significantly complicates the process of evaporation of explosives from them.

Gas chromatography, ion drift spectrometry and mass spectrometry are used to detect explosives.

The first two directions have advanced most successfully in terms of manufacturing commercial detectors of explosive vapors and particles.

The developers have created a fairly wide range of corresponding devices, some of which are presented in Table 2.

The sample being analyzed is introduced into the detector either by sucking air from the surface or from cracks in the object being examined, or by presenting particles captured by the sampler or sorbed vapors of explosives.

The design of the air sampler in the «Shelf» and «MO-2» devices is quite original: it creates a tornado-like vortex, inside which an air vacuum tube is formed, which provides conditions for «sucking» air samples from cracks and hard-to-reach places of the controlled object.

In stationary and mobile explosive detectors, air samples for analysis are taken using a remote hand sampler with a preliminary concentration of the substance to be registered.

Products with a developed sorbent surface are used as concentrators: paper filters, bulk materials, metal spirals, nets, etc.

A built-in microcomputer is used to control the analysis process. In order to increase the efficiency of the analysis, several columns are used, or (as in the EKNO device) a monoblock consisting of thousands of short parallel capillary columns. Other methods of recording the vapor phase of explosives are also used.

The chemiluminescent method used in the EGIS device is very effective. Here, the molecules of explosives undergo pyrolysis with the formation of nitrous oxide NO, which, reacting with the ozone O3 obtained in the device, forms excited molecules NO2.

When these molecules enter the ground state, they emit infrared radiation, which is recorded by a photomultiplier. The entire analysis process from sample injection to the final result takes no more than 30 seconds.

The device has proven itself in conditions of mass explosion hazard control.

For example, tests of two devices conducted in Germany by security services showed that the false alarm rate was about 0.03% out of 400,000 analyses. All major European airports are equipped with these devices.

The molecular condensation nuclei method (MCNM) used in the Edelweiss-4 device has high sensitivity.

In this case, ionized molecules of explosives contribute to the formation of aerosol particles in the reaction chamber, the presence of which is recorded by a change in light transmission.

The device is equipped with a remote manual vortex sampler with a concentrator and radiant heating of the surface being examined. The analysis cycle time after introducing the sample into the device is 120 seconds.

It should be noted that gas chromatographic detectors of vapors and particles of explosives require carrier gases for their operation, of which high-purity nitrogen and argon are most often used.

This is often the reason for the skepticism of users towards devices of this class, who fear that their successful operation depends on the availability of the required gas, especially in areas remote from its production sites.

In this regard, “EGIS” looks more advantageous, in which the carrier gas (hydrogen) is produced in the device itself by electrochemical decomposition of water.

Devices based on the method of ion mobility spectrometry in an electric field (drift spectrometers) are made in both portable and mobile versions.

Ionized molecules of explosives (usually by irradiation with a beta-particle flow of weakly radioactive tritium or nickel-63 sources) enter the drift chamber, where they move to the collector under the influence of an electric field of a certain configuration.

When they hit it, they create a current pulse in the electric circuit, which is amplified and processed by the electronic unit.

The drift time to the collector depends on the mobility of the ions and the parameters of the electric field, which is the basis for identifying the substance being analyzed.

Sampling for analysis is carried out both by directly sucking air into the device (“Shelf”, “MO-2”), and using a remote sampler (“IONSCAN”, “ITEMISER”).

In the latter case, a paper filter is used as a concentrator, which absorbs explosive vapors or retains their particles when air is pumped through it using a turbine, or a smear sample is taken from the surface of the controlled object.

For example, the mass spectrometric detector (MSD) of explosive devices “CONDOR”, created by SCIEX in collaboration with British Aerospace, is a rather large stationary device costing over 1 million US dollars.

The MSD “TOP 2000”, developed by Sensar (USA), has smaller dimensions and weight characteristics and cost (180x90x60 cm; 360 kg; 300,000 US dollars).

Its sensitivity reaches 1 ppt of explosives in a sample with an analysis time of about 1 sec. The company is working on improving the device in order to simplify its maintenance, optimize the sampling operation and reduce the cost.

The simplest and most accessible way to detect trace amounts of explosives is the method of colored chemical reactions.

Its essence lies in the formation of colored products during the interaction of certain reagents with a sample taken by smearing from the surface of an object suspected of being explosive.

The domestic chemical kit consists of a set of three reagents, paper filters and packaging that easily fits in a pocket.

The surface of the controlled object is wiped with a paper filter (gauze, cotton wool, etc.).

Then, solutions from the vials are dripped onto the filter at the contaminated site in a certain sequence, and the presence of explosives in the sample is determined by the appearance of a red-violet, orange or pink color.

The sensitivity of the method is: for TNT — 10-8 g in the sample; for tetryl, hexogen, octogen — 10-6 g; for TEHN — 10-5 g.

The vials with reagents are made both in the form of droppers and sprayers. The set can also be used in investigative activities at the site of the explosions.

In conclusion of the article, it is appropriate to draw attention to another aspect related to the detection of hidden caches of explosives.

As can be seen from Table 1, the concentration of hexogen and PETN vapors in the air, which are part of most plastic explosives (PE), is quite low and requires high sensitivity from explosive detectors, which leads to a complication of their design, an increase in their weight and size characteristics and cost, and a decrease in monitoring performance.

In order to improve the efficiency of inspection operations, simplify, lighten and reduce the cost of equipment for detecting hidden explosive caches, experts proposed introducing volatile additives (markers) into the PE, the evaporation of which would be several orders of magnitude higher than the evaporation of hexogen and PETN and would not affect the main performance characteristics of plastic explosives.

One such marker may be, for example, ethylene glycol dinitrate (EGDN), which meets these requirements.

To facilitate the detection of explosive substances, the international community adopted the Convention on the Labeling of Highly Volatile Substances in 1991.

This project is aimed at the future, when unmarked explosive substances whose shelf life has expired will be replaced by marked ones.

It is known that some manufacturers have already switched to producing only marked explosive substances.

This example shows how the combined efforts of nations can fruitfully solve the problem of combating terrorism.

Table 1. Some information on the volatility of explosives.

* — according to various sources

Table 2. Equipment for detecting vapors and particles of explosives (according to advertising data)