Before the analysis of a working standard or sample, the VOA vial should be
hand shaken for 10 to 15 seconds. Cohesive materials, such as silts and clays, do
not break apart rapidly upon shaking and may require more than 15 seconds for
complete dispersion. The vial is then visually checked for both the complete dis-
persion of the sample matrix and for particles adhering to the aluminum foil cap
liner (knock large particles off the aluminum foil if present). Then the inlet tube of
the PID is pushed through the foil liner, to a set position about 3 cm below the rim.
A maximum response will be achieved within 2 to 3 seconds of punching through
the foil liner. The maximum response for each sample screened and for the analysis
of each working standard should be recorded.
DAILY OPERATING PROCEDURE FOR VOC SCREENING
The PID should be initially calibrated with a cylinder of standard gas (e.g., 100
ppm of isobutylene) at the beginning of each day. This task can be performed prior
to going to the sampling location. However, the analysis of both site-specific work-
ing standards and the screening of a sampling location should be performed under
the same conditions, thereby normalizing meteorological influences on the perfor-
mance of the PID. Site-specific working standards should be prepared daily, and in
sufficient quantity to satisfy the study's objectives. At a minimum, one working
standard should be analyzed for every hour of site activity.
Collection of samples for VOC analysis should always be the first operation per-
formed after a surface to be sampled has been exposed to the atmosphere. This
includes both samples for screening and for laboratory analysis. To establish how
to handle and prepare the discrete sample for laboratory analysis (low, high, or
both procedures), a total VOC screening analysis should be performed at each sam-
pling location. Therefore, before opening a split spoon, scraping a fresh surface on
a pit wall, removing surface vegetation and the appropriate amount of top soil for
a surface grid location, or removing the first several inches of some other type of
waste material, the PID of choice should be operating. Furthermore, if a working
standard is being utilized to verify performance of the PID for the sampling loca-
tion, the analysis of a working standard should be completed before exposing a
fresh sampling surface.
Once a fresh surface has been exposed, a sample should be quickly obtained,
transferred to a screening VOA vial, dispersed, and analyzed. If the maximum re-
sponse is greater than the working standard (or the running average), the sample
or samples taken for laboratory analysis should be prepared using the high-level
procedure (i.e., MeOH extraction). If the maximum response is below the working
standard, the laboratory sample(s) should be prepared using a low-level proce-
dure. The total elapsed time between exposing a fresh surface, screening a sample
and obtaining samples for laboratory analysis should be less that 2 minutes. As a
precaution against false positive and false negative screening estimates relative to
the decision point, the working standard response should have samples prepared
by both high and low level procedures locations where screening results are be-
tween 0.5 and 2.
METHOD LIMITATIONS
For this method of sample location screening to work, the VOC(s) of interest
must be detectable by photoionization. If more than one analyte is of interest, and
there are large discrepancies (greater than a factor of two) in photoionization po-
tentials, then the range around the decision point where samples are prepared by
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