5.3. Direct Reading Instrumental Analysis: Colorimetric Detector Tubes
A colorimetric tube is also known as a “length of stain” tube, due to the fact
that the concentration of the chemical being tested produces a color stain in
the analytical material, proportional to the concentration in the air. These tubes
also are called Dräger tubes®, named after the company that introduced them in
1937 (Drägerwerk AG). Colorimetric detector tubes are fl ame-sealed glass tubes
containing a chemical reagent that reacts with a specifi c compound or group of
compounds, causing color change. A sample is collected by attaching the detector
tube to a special bellows-type pump that draws a known volume of air with
each stroke. If the target chemical is present, the reagent in the tube changes
color and the length of the color change typically indicates the measured concentration.
Sampling times are generally short and equipment is portable and
relatively inexpensive, hence they can be used as quick indicators of poor control
to assess a short-term event or for scoping studies. Because they are simple
to use, detector tubes have been used in a broad range of applications. Directreading
colorimetric detector tubes also were designed for use in testing workplace
air and for determining compliance with occupational exposure limits, but they
also are used widely to demonstrate compliance with emission limits for specifi
c substances. Today a considerable number of tubes are available from several
manufacturers covering a wide range of substances, although it is not possible to
differentiate between different chemical species within a generic group. Despite
the simplicity of the method, correct use is essential or results can be highly
misleading. Further, more interference with other chemical species is possible,
leading to false results. It should be mentioned here that colorimetric tubes are
not suitable for identifying unknown substances, and the correct tube needs to
be selected for a particular situation. Although the limits of detection for this
technique are typically between 0.2 to 1 ppm, detection limit can vary greatly,
depending on the tube used (IPPC H4-Part 2, 2002). According to the manufacturers’
literature, the relative standard deviation of detector tubes varies between
5 and 20%. Unused tubes must be stored correctly (some require refrigeration)
and have a fi nite shelf life, which must be observed.