Sampling Guide for Air Contaminants in the Workplace - Irsst

18 IRSST – Sampling Guide for Air Contaminants in the Workplace
• Calculate the exposure concentration expressed as mass per volume (at the sampling site and not adjusted
for conditions at NTP). This concentration must be directly compared to the PEV in the ROHS expressed
as mg/m³ or as fibres/cm³;
• If the exposure concentration is expressed in ppm (vol/vol) for conditions other than NTP (PT), this
concentration must then be adjusted to bring it to NTP before comparing it to the PEV in the ROHS. A
computer-based tool available on the IRSST's Web site easily performs this conversion for comparison to
a PEV in the ROHS (16).
Equation 12: Conversion of ppm PT to ppm NTP
ppm
NTP
=
ppm
PT
P: pressure at the sampling site (mm of Hg)
T: temperature at the sampling site (°K)
It is important to emphasize the fact that the concentration in mg/m³ or in fibres/cm³ must calculated with the
sampling volume under site conditions and this concentration compared directly to the value of the PEV. Care
must be taken to know the volume sampled when using a flow measuring instrument and to ensure that the
flow rate displayed by the instrument corresponds to the flow rate under site conditions. If an instrument
displays an equivalent flow rate under certain calibration conditions, as is the case for hot-wire flow meters,
this flow must be converted to the site conditions (see section 2.6.2). A reader who wants to investigate the
subject in more detail can consult the Mémento sur l'utilisation des pompes et des débitmètres (17).
2.2 Gases and vapours
The term gas is reserved for substances that are effectively in the gaseous state at 25°C and 101.3 kPa. Gases
have no shape; they occupy the space available to them. Vapours are compounds in the gaseous state, which,
under normal conditions of temperature and pressure, are in the liquid state in equilibrium with the gaseous
state. Several portable direct-reading instruments are available on the market for sampling gases and vapours.
The most commonly used collecting mediums are sorbent tubes. Impingers, filters impregnated with reagents,
and bags are also used for some compounds. (18)
2.2.1 Electronic direct-reading instruments
Technological improvements, the miniaturization of electronic devices, and developments in computer
science have resulted in high-performing and portable direct-reading instruments (DRI). Computerized data
acquisition and processing systems are integrated into the instruments, allowing the exposure doses to be
displayed for variable periods. Detection techniques that have been used only in the laboratory can now be
used in the field as a result of miniaturization. Table 4 gives the list of DRI available at the IRSST for
evaluating gases and vapours. The interferences specific to each instrument are mentioned in their user's
manual. These instruments can be affected by electromagnetic fields. However, some of them are intrinsically
shielded against radiofrequencies. Other instruments can also be shielded against radiofrequencies if an
exterior casing is used. Below is the list of the five operating principles for these DRI, with a brief description.
• amalgamation
• combustion
• electrochemistry
• infrared spectrophotometry
• photoionization
⎛ P ⎞ ⎛ 298 ⎞
∗⎜
⎟∗⎜
⎟
⎝ 760 ⎠ ⎝ T ⎠
• Amalgamation
Amalgamation is the phenomenon by which mercury forms an alloy with another metal. Even at very low
concentrations in the air, mercury amalgamates with metals such as gold and silver. In the detector, the
mercury vapours present in the air come in contact with a gold filament, and an amalgam forms, the effect
being to increase the resistance of the filament. This increase in resistance is proportional to the amount of
mercury amalgamated. By knowing the sampling volume, it is then possible to calculate the average