Environmental Engineering Reference
In-Depth Information
occur even when a sampling/analytical procedure has relatively high spec-
ificity. This is true for the DNPH-HPLC method used for HCHO and other
aldehydes. It is subject to significant negative interference from ozone (O
).
3
The use of sampling/analytical procedures to
conduct sampling for vapor-phase substances should be a relatively conser-
vative one. It is desirable to use reference, approved, or recommended meth-
ods that have been systematically evaluated by the National Institute of
Occupational Safety and Health (NIOSH), the U.S. Environmental Protection
Agency (USEPA), the Occupational Safety and Health Administration
(OSHA), or the American Society for Testing and Materials (ASTM). The use
of approved methods provides a relative (but not absolute) degree of confi-
dence in the accuracy, precision, and reliability of sampling/analytical pro-
cedures being employed.
e.
Reference methods.
Contaminant measurements conducted in prob-
lem buildings or in systematic research studies need to be quality assured
to provide confidence in their accuracy and reliability. Quality assurance
procedures include instrument calibration, and, when appropriate, use of
field blanks, media blanks, replicate samples, and split or spiked samples.
They focus on both field and analytical aspects of contaminant collection
and measurement.
f.
Quality assurance.
i. Calibration. Calibration is a process whereby measured values of
air flow and/or contaminant levels are compared to a standard. In the case
of air flow, the standard may be primary or secondary, with the latter trace-
able to the former. Primary standards can be traced directly to those at the
National Institute of Standards and Technology (NIST). A gas burette serving
as an airflow measuring device is a primary standard; a rotometer (which
must be calibrated) is a secondary standard. All dynamic sampling instru-
ments, particularly gas sampling pumps and tube systems, should be cali-
brated frequently. In common practice, calibration of real-time, direct-read
instruments is conducted by using calibration gases that have been prepared
to provide sample concentrations within the measuring range of the instru-
ment. Single- and multiple-point calibrations are conducted depending on
the user; multipoint calibrations are preferred.
ii. Blank/replicate samples, etc. I n collecting samples onto a
medium, it is essential that field or media blanks be used. A field blank is a
media sample taken to the field, opened, then closed and returned to the
laboratory where it is analyzed. Media blanks are samples of liquid media
prepared at the same time as samples used in the field or the same lot of
solid media. Both are used to adjust environmental sample concentrations
for contaminant levels present in unexposed media. Field blanks are used
to determine whether contamination occurred as a consequence of sample
