Sample Contamination
by Tim Loftus
Currently
there is an effort by many regulatory agencies to lower environmental
pollutant limits. Most of us are seeing this directly with our new
NPDES permits. Copper and lead limits, for example, are commonly
in the part per billion ranges. Since some of these limits are so low,
even small amounts of contamination can lead to highly inaccurate
results. It’s important to realize that good sample results don’t
start with calibrated instruments or excellent laboratory technique
(although these are important). Good sample results start with good
sampling, and one of the major components of good sampling is clean
equipment.
Parts per million
(ppm) and parts per billion (ppb) are common units of measurement in
the wastewater field. Now with more accurate analyzers we are starting
to hear about parts per trillion (ppt), especially with organics and
mercury analyses. To put the ppm, ppb, and ppt into perspective, think
of these in terms of time. A million seconds is about eleven days. To
accurately measure wastewater with a copper value of 1 ppm is like
accurately picking one specific second out of eleven days. A billion
seconds is equal to about 32 years. Our NPDES permit requires that we
discharge less than 12 ppb copper. Again, putting this in terms of
time, we must accurately measure better than a 12-second block of time
within a span of 32 years. For those of you lucky enough to deal with
environmental mercury samples in the ppt range, it takes about 32,000
years to get one trillion seconds. The point is if you’re sampling for
parameters at such low levels, especially in the ppb and ppt range, it
is extremely easy to contaminate a sample to the level where you may
violate your NPDES permit (or other) limit.
Space constraints do not permit
details of every situation, but in general clean all sample tubes,
collection containers, and collection equipment (funnels, filtering
apparatus, etc.) according to the recommendations outlined in the test
procedures. Most cleaning procedures are test specific and
involve washing with soap and water, followed by an acid rinse, then
with deionized water. Sometimes an organic solvent is used as a rinse
depending on the test. Cleaning procedures can be found in Standard
Methods for the Examination of Water and Wastewater, EPA
Methods for Chemical Analysis of Water and Wastes (EPA-600
/4-79-020), and the test guidelines and methods outlined in 40 CFR
136. These cleaning recommendations are adequate for most sampling
events. There is, however, an additional level of “clean sampling” not
mentioned in these publications. It involves body suits and a number
of special procedures and is used mostly for sampling ultra low levels
of mercury. These techniques are not covered here since few of
us need to sample at this level - yet.
Oftentimes, too, preservatives can add contamination to a
sample. Acids used for metal preservation can have trace levels of the
analyte in question. Always use the highest quality preservatives when
sampling for trace analytes.
Sample contamination can also be
avoided using just common sense. For example, don’t sample for VOCs
when the grass is being cut nearby, or sample for metals using a Mason
jar with a metal cap.
With environmental limits becoming
more stringent, it’s important to keep samples free from contamination
so you don’t mistakenly report a false violation. Violations, as we
all know, creates more work for us. Like the ppb/seconds analogy,
clean sample containers, clean equipment, contamination-free
preservatives, and a little common sense will save you time - in more
ways than one.
As usual, check your federal, state
and local regulations. You may have additional sampling regulations
that you must meet.
If you have any questions, suggestions, or
comments, please contact Tim Loftus at (508) 949-3865 (timloftus@email.msn.com).