Detection of Chemical Weapons
An overview of methods for the detection of chemical warfare agents
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Source: A FOA Briefing Book on Chemical Weapons

Choice of correct protective equipment, and correct behaviour, may be decisive for a unit's possibilities to operate in CW contaminated environment. Detection is vital in obtaining information on the risks. Detection implies that evidence is obtained on the types and quantities of CW agents in the area. The questions asked may be whether, for example, protective masks are required, whether body protection is necessary, if normal behaviour should be modified in any special way, and whether equipment will require decontamination. Detection may be needed for several different purposes, e.g.,

verification and identification,
mapping of ground contamination,
mapping of decontamination requirement.
Different types of detection require different types of equipment and methods. In some cases we must determine whether the gas concentration in the air is at a dangerous level. In other situations, investigations are made of whether soil or equipment is contaminated with liquid agent, i.e., is dangerous to handle.

The simplest form of alarm is based on direct observations. A visible cloud drifts towards the observer, someone in the unit shows symptoms of poisoning, observations are made of dead animals, etc. Earlier and more reliable alarms require continous and automatic monitoring of the gas concentration. For this purpose, instrumentation is today being developed and introduced.

An alarm detector must be capable of operating continously for long periods. It should require no particular attention except for changing batteries, etc., and might be operated by personnel with short training periods. At a predetermined concentration of the relevant substance, the instrument emits signals either visually or acoustically. On the other hand, no direct reading of the concentration is required. An advantage is if different agents cause different alarms, e.g., different types of sound or different signal colours.

It must always be considered that other substances than CW agents may cause the alarm. It is important not only that these are as few as possible but also that they are known so that situations with false alarms can be avoided.

Detection paper placed onsite in advance is a simple manual method. It can be used as a warning for CW agents in the form of liquid fallout. In such cases, it is necessary to have continous monitoring with special personnel.

Wearing a protective mask and protective clothing implies a major physical and mental strain for personnel. Consequently, it is of major importance to sound the all-clear as soon as possible after a CW attack. Protective measures can be terminated when the concentration of CW agents in the air has fallen below the threshold limit.

A continuously operating all-clear system is again preferable. The need to sound the alarm may soon arise again if the conditions change. As regards the all-clear, an instrument which can be directly read-off is of great value. Such instruments, e.g., the British CAM (Chemical Agent Monitor, see illustration) show the air concentration for different types of substances. Changes can then be observed and a temporary fall in concentration can be utilized to decrease the level of protection, e.g., in order to eat. Correct utilization of an instrument with direct-reading facilities requires an operator and also someone with sufficient knowledge to reach correct decisions.

Provided there is access to other information, e.g., mapping of the ground contamination has been completed, manual methods using detection tubes or detection tickets may be sufficiently accurate when reaching decisions on sounding the all-clear.

Verification and Identification
In order to reach decisions on the level of protection required, it is necessary to know the type of agent present in the area and whether the concentration exceeds the threshold level. If alarms are sounded on the basis of vague detection reports (direct observations), the suspicions should be confirmed by verification.

Detection (enzyme) tickets and detection tubes are sufficient to demonstrate nerve agents and mustard agent under field conditions. A manual suction pump is used to draw air through the detection tube or against the ticket. Subsequently, the material is developed in order to see whether CW agents have affected the tube or ticket.

In order to reach tactical decisions, more detailed information on CW agents is necessary. This identification can be done to some extent by means of a combination of manual vapour detection (tickets and tubes) and detection paper. Information on an even higher reliability level will require analysis in a laboratory. Samples can then be taken in the field for subsequent analysis.

Mapping of Ground Contamination
In some cases, it is necessary to map which parts of an area are and which are not contaminated with CW agents in liquid form. This is essential if, for example, a convoy of vehicles has to pass through an area where CW agents have been used. Several methods are available. Traditionally, detection paper has been used. Detection papers prepared before an attack could give the information required. In such cases, this must have been made over the entire area in question and at several different places in the area. However, such information is probably not available or deficient.

Detection paper can be used in post factum detection. However, this method is not entirely reliable, particularly if a long period has passed between the contamination and detection occasions. Considerable quantities of CW agents may have been absorbed into the soil layer and still imply danger without giving any response on detection paper. Consequently, soil detection with paper should be complemented with some kind of vapour detection.

Another problem is that traditional methods of soil detection take a long time. The speed of a convoy of vehicles will decrease to a few km/h if detection must be conducted at the same time as the convoy makes progress. In this way, an aggressor achieves a considerable tactical advantage already with a low or moderate CW attack. By vapour detection with detectors or monitors, the speed of vehicles could be increased to about 30 km/h, provided that the instruments react sufficiently rapidly and are sufficiently sensitive.

Vapour detection is not a fully reliable method, and particularly not for CW agents with low volatility or at low temperatures. There is a risk that vehicles, ordnance, etc., may be contaminated after transport without this being detected during the actual transport.

Mapping of Decontamination Requirement
Mapping of the decontamination requirement involves approximately the same problems as mapping ground contamination although generally on a smaller scale. Detection with paper generally works well but is not sufficient in all situations. CW monitors are important in controlling the need for decontamination.

Persons suspected or known to be contaminated must be decontaminated immediately without previous time-consuming controls. This should also apply to personal equipment if it is reasonably easy to decontaminate. Checks of the decontamination requirement should be concentrated to heavier and more difficultly decontaminated equipment.

Developmental Trends
Development of detection methods today is mainly concentrated on instruments. New manual methods, e.g., for toxins, may be developed but the development mainly concerns instruments for detection and monitoring. In some cases, instruments capable of both tasks are being constructed.

Several lines of development are presently being followed as regards detection principles. The most common line of development is some form of ion mobility detector IMS (Ion Mobility Spectroscopy). The Chemical Agent Monitor (CAM) also belongs to this group, as well as detectors for warning such as the Finnish M86 and the more recent M90. Another principle used is flame photometry FPD (Flame Photometric Detector). A flame of hydrogen is allowed to burn the sample of air after which the colour of the flame is investigated by a photometer. In this way, the presence of phosphorus and sulphur can be demonstrated. Examples of instruments using this principle are the French monitor AP2C and Israeli combined detector and monitor CHASE.

A third principle is to use enzymes, as in the manual methods for nerve agent detection. Detectors operating on this principle have been developed in the United Kingdom, in the Netherlands and the former Soviet Union, among others.

Methods for long-range monitoring using optical methods (IR) are being developed in France and the U.S.A.

A research sector attracting great interest is the use of biologically active molecules as sensors. These biosensors are believed to have extremely great potential and research is ongoing in several countries. The advantage of biosensors is that, at least in theory, they can be given the sensitivity and specificity desired. This is possible since the biosensor uses the same mechanisms that influence the human body when exposed to poisoning. A simple type of biosensor is the enzyme ticket.

A more general type of biosensor may also be useful in the early detection of potential threats. Instead of studying toxic substances, investigations can be made of which receptors in the body may be sensitive to, e.g., a toxin. These receptors could then be used in a biosensor.


Manual Detection Methods
Detection Paper

Detection paper is based on certain dyes being soluble in CW agents. Normally, two dyes and one pH indicator are used, which are mixed with cellulose fibres in a paper without special colouring (unbleached). When a drop of CW agent is absorbed by the paper, it dissolves one of the pigments. Mustard agent dissolves a red dye and nerve agent a yellow. In addition, VX causes the indicator to turn to blue which, together with the yellow, will become green/green-black.

Detection paper can thus be used to distinguish between three different types of CW agents. A disadvantage with the papers is that many other substances can also dissolve the pigments. Consequently, they should not be located in places where drops of, e.g., solvent, fat, oil or fuel can fall on them. Drops of water give no reaction.

On the basis of spot diameter and density on the detection paper, it is possible to obtain an opinion on the original size of the droplets and the degree of contamination. A droplet of 0.5 mm diameter gives a spot sized about 3 mm on the paper. A droplet/cm2 of this kind corresponds to a ground contamination of about 0.5 g/m2. The lower detection limit in favourable cases is 0.005 g/m2.


Detection Tubes

The detection tube for mustard agent is a glass tube containing silica gel impregnated with a substrate (DB-3). Detection air is sucked through the tube using a special pump. The reaction between the mustard agent and substrate (see below) is speeded up by heating the tube with, e.g., a cigarette lighter. A developer is then added, and the result can be read-off. If the silica gel in the tube turns blue, then the vapour in the sample contains mustard agent.

Cl(CH2)2S(CH2)2Cl (mustard agent) + pyridine-CH2-p-phenylidene-NO2 (4-(4-nitrobenzyl)pyridine, colourless)
reacts at 110 oC in the presence of NaOH to give

Cl(CH2)2S(CH2)2N =CH-pyridine -NO2 (1-[1-[2-(2-chloroethylthio)etyl]-1,4-dihydro-4-pyridylidenmethyl]-4-nitrobenzene, blue)


Detection Tickets

Detection tickets for nerve agents are used in a similar way. The ticket consists of two parts, one with enzyme-impregnated paper and the other with substrate-impregnated paper. When the package is broken and the enzyme paper wetted, the substrate part of the ticket is exposed to the test vapour by means of a pump.

Subsequently, the two parts are put together for two minutes. If the enzyme part of the ticket has turned a weak blue colour, nerve agent is not present in the air. The detection limit is 0.02-0-05 mg/m3 depending on the number of strokes of the pump. The ticket can also be used witout a pump (by waving it in the air) but this gives a slightly poorer sensitivity.

An example of the enzyme substrate reaction used in detection tickets for nerve agents can be seen below. Note that the blue change of colour requires an active enzyme - some form of cholinesterase. In the presence of nerve agents, the enzyme is inhibited and no change of colour occurs. Detection tickets of this kind cannot distinguish between the different nerve agents.

2,6-dichloroindophenylacetate (red) + cholinesterase produces 2,6-dichloroindophenol (blue)

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Last modified 29 April 1997 by ICA Division, OPCW

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