Environmental Engineering Reference
In-Depth Information
media being taken into the field. Two field blanks for each of 10 environ-
mental samples are generally recommended.
Replicate samples are often collected to assess the accuracy and precision
of analytical results, split samples to compare the performance of different
analysts, and spiked samples to assess analytical performance relative to a
known sample concentration.
2.
Resource limitations
In addition to performance characteristics described above, selection of a
sampling/analytical procedure will often be determined by the availability
of resources. Despite significant performance limitations, sampling/analyt-
ical procedures are often chosen because of their relatively low cost. This is
particularly true for gas sampling tubes and, in some cases, passive samplers.
Other factors affecting the selection of sampling/analytical procedures
include equipment availability, portability, and degree of obtrusiveness.
3.
Sampling procedures
Samples of airborne contaminants can be collected and analyzed by utilizing
both dynamic and passive sampling procedures.
In dynamic sampling, vapor-phase substances
are drawn (by means of a pump) at a controlled rate through a liquid or
solid sorbent medium or into a sensing chamber. In sampling for particulate-
phase substances, air is drawn through a filter, impacted on an adhesive-
coated surface, attracted to collecting surfaces by electrostatic or thermostatic
processes, or brought into a sensing chamber. The sample volume is deter-
mined from the known flow rate and sampling duration. Concentrations can
be calculated or read directly from electronic real-time instruments.
Dynamic sampling is conducted using two approaches. In the first, sam-
pling and analysis are discrete events. Sampling is conducted with pumps that
collect vapor-phase substances in a liquid medium (absorption) or onto one or
more solid sorbents (adsorption) such as charcoal, tenax, silica gel, etc. Exposed
sorbent media must be analyzed by a laboratory. Depending on laboratory
schedules, results may be available in a matter of days or up to several weeks.
In the case of particulate matter samples, gravimetric analysis may be con-
ducted within a day or so after samples have been sent to a laboratory. The
relatively long period between sample collection and availability of results is
an obvious disadvantage of this type of sampling. For many contaminants,
alternatives may not yield acceptable results or may be too expensive.
Dynamic sampling (and analysis) can also be conducted using direct-
read, real-time (instantaneous) or quasi-real-time instruments. Such instru-
ments are available for a limited number of common indoor contaminants.
Sampling and analysis are combined so that results are immediately avail-
able to individuals conducting investigations and testing (providing an obvi-
ous advantage).
a.
Dynamic sampling.
