by Martine Bissonnette, M.Sc. Chemist
Rapid detection of chemicals and their concentration in the atmosphere is of prime importance during a spill as it gives an indication of the health hazards, pollution levels and sometimes, other dangers such as the possibility of an explosion. Knowing the concentration of a chemical can help in determining what level of personal protection is required, in assessing whether or not the surrounding population is in danger and can help to locate the source of a spill by monitoring the movement of the vapours since the concentration increases as one gets closer to the source. An example of a situation where rapid detection is important would be in assessing the emission limits from an anhydrous ammonia tank car are acceptable.
Indicator tubes systems can perform rapid detection of chemicals. The tubes are based on a chemical reaction generating a colour change and are commonly called indicator detectors or colorimetric tubes. The tube is made of glass and contains a solid granular material which has been impregnated with the appropriate reagent. The reagent changes colour in the presence of the product for which it has been manufactured. There are over 200 different tubes commercially available for different gases in various concentrations and new tubes are being developed as the need arises. There are also qualitative tubes which can detect a number of various compouds from the same chemical class.
Although often called "Dräger tubes" because of the fact that the Dräger company was the first one to commercialize the unit as we know it today, one has to be aware that there are other companies who also manufacture indicator tubes detection systems. Some of the other manufacturers include Gastec Corp., MSA, Matheson and Sensidyne.
The system is very simple to use. The sealed ends of the glass tube are broken and the tube is inserted into a holder which is attached to a pumping device. A specified volume of air sample (100 mL for the Drager) is pumped into the tube and the reaction takes place inside the glass tube. The most common type of tube has a concentration scale printed on it. The reagent becomes stained over a portion of the length and the length of the stain is proportional to the concentration of the contaminant, for which a value can be obtained on the concentration scale.
The tubes provide a rapid mean of determining the concentration of a gas or vapour and require minimal operator training and expertise. Too good to be true? Absolutely! The interpretation of the results and the lack of proper maintenance of the instrument are critical. The error on the final result can be up to 50%. In the best case scenario, i.e. correct interpretation and proper maintenance, the error on the result is specific to each tube and is provided by the manufacturer.
Sources of errors
The flow rate characteristics will vary from one brand of pump to another. Therefore, it is not recommended to use one brand of indicator tubes with another brand of pump. If one uses different brands, the results should be regarded as qualitative, i.e. presence or absence of a chemical.
Tubes have a typical shelf-life of two years and should not be used after the expiry date. The shelf-life can be increased if they are refrigerated (not frozen since condensation can occur). The tubes should also be allowed to reach ambient temperature before being used.
Another factor which can affect the shelf-life of the tubes is exposure to light. Such exposure can trigger or inhibit the reactivity of the reagents inside the tube. Therefore, it is important to keep the tubes in their original packaging.
Most of the chemicals used in indicator tubes are not specific to a single substance. That "non-specificity" is governed by the chemical principle on which a detector tube is manufactured. Obviously, if the substance which the tube is designed to detect is present, that substance will produce a positive result. However, a positive result does not necessarily mean that the substance for which the tube is designed is present; it may indicate the presence of another substance which undergoes similar reactions with the reagents in the tube. Therefore, the interpretation of the results demands a knowledge of all the components which can give a positive result with the indicator tube.
The various pumps cost anywhere between $250 and $400. Accessories such as adaptors, spare part kits and carrying cases are extra. The tubes typically come in boxes of 10 (sometimes 5) and a box costs between $35 and $90. Once you get over the initial investment of buying the pump, detection tube systems are a very inexpensive method for rapid, on-site detection of chemicals.
In a nutshell
Dectector tubes systems are very efficient to perform monitoring when properly used. However, at a chemical spill, there are so many factors which can interfere with the result such as fumes from surrounding vehicles, extreme temperatures and humidity, that it is wiser to use the tubes as general indicators. Is there a lot of contamination, a little or none?
Remember, the error on the concentration value obtained using a detector tube can be as low as 5% or as high as 50%.
Publication: TDG Dangerous Goods Newsletter, Vol. 16, No. 1, Spring 1996.
* Martine Bissonnette is a former Emergency Response Advisor with CANUTEC.