Sampling Guide for Air Contaminants in the Workplace - Irsst

IRSST – Sampling Guide for Air Contaminants in the Workplace 21
2.2.2 Colorimetric direct-reading devices
Colorimetric methods are among the simplest, fastest, and least costly methods. Their operating principle is
based on the fact that the intensity of the developing colour is proportional to the concentration of a
contaminant or a family of contaminants. Three types of devices are used, namely:
• Tubes connected to a manual or automatic pump;
• Long-duration tubes operating by passive diffusion;
• Chips containing capillary tubes requiring the use of an optical reader.
In the case of tubes connected to a pump, the concentration is a function of the sampled air volume. It is
therefore important, after having broken the ends of the tube and connecting it to the pump, to comply with
the time period necessary for the desired volume of air to pass through and the reaction to develop. Low
concentrations can be evaluated by several strokes of the pump, according to the manufacturer's instructions.
Long-duration colorimetric tubes are designed in the same way as conventional colorimetric tubes. However,
the quantity of the reactive substance in the support may differ, to allow long-term sampling without
exceeding the reaction capacity of the impregnated substances. Long-duration tubes are generally graduated in
ppm-hours. To obtain a weighted concentration, the change in colour reading is divided by the sampling time
in hours.
The most recent system consists of a chip containing capillary tubes filled
with a reactive substance. As with the above-mentioned devices, a
colorimetric reaction occurs on contact with the contaminant, whose
intensity is not read by the user but by an optical reader. The reaction time
and the mass flow rate are also taken into consideration in calculating the
concentration. Information about the contaminant, the mass flow rate and
the measurement range are included in the bar code located on the chip.
The interaction between the optical reader and the different chips is done
using the instructions included in this bar code. Chips are available for
some fifty compounds. An optical reader is available for loan from the
IRSST.
The main limitations of these devices are their non-specificity and low
accuracy. They are useful as source-detection devices, or to see variations in concentrations in space or time,
or for following a single known contaminant. They cannot be used for evaluating a worker's exposure (19).
2.2.3 Sampling media
2.2.3.1 Adsorbent tubes
Adsorbent tubes are used to collect samples in the gaseous and
vapour states such as solvent vapours, some gases, and acids.
They are glass tubes containing two sections of adsorbent.
These tubes may contain activated charcoal, silica gel, or
certain polymers. By analyzing each of the sections
individually, the efficiency of adsorption of the collecting
medium can be verified. Sampling is considered as acceptable
if less than 10% of the chemical is found in the second
section. If more than 25% of the chemical is found in it, a loss has probably occurred and the results express a
minimum concentration. However, this rule may vary when more than one substance enters the tube, thus
promoting competition for the adsorption sites.
The ends of the tube are broken on the sampling site and connected to the pump by means of special devices.
The tube must be placed with the arrow in the direction of air flow. The tube must be vertical to prevent any
channeling, which would reduce the adsorption efficiency. The flow rate and sampling volume must be