As we mentioned in our last issue, this year’s DMR-QA study opened one month earlier
than in past years, so most of you have already received your samples from ERA.
In this article, we offer guidance on analyzing some of the more common DMR-QA analytes.
By implementing these tips for your routine analyses, as well as for DMR-QA, you
will improve your data quality for your regular discharge monitoring. We will also
point out some unique characteristics of DMR-QA PT samples and data reporting that
you should look out for.
With over 20,000 participants in our DMR-QA studies over the past 10 years, ERA
has developed a good understanding for some of the common problems that are encountered
while analyzing these PT samples. Many of the questions we get, and answers we give,
are equally important for the analysis of normal discharge monitoring samples as
they are for the once-a-year DMR-QA samples. These tips should help you improve
all of your routine analyses.
Biochemical Oxygen Demand (BOD)
The fact that BOD analysis has many steps that each require attention to
detail is probably the reason that this is the test we get the most questions about.
Very simply, the purpose of the test is to determine the amount of dissolved oxygen
used by microorganisms to break down the organic matter in a sample. As microorganisms
and larger aquatic species both rely on the dissolved oxygen in any water system,
maintaining tight control over BOD is necessary to having a balanced ecosystem.
As contamination, either from residual chlorine or organic material, can drastically
affect your results, it is critical that every piece of glassware and other apparatus
used in your analysis is scrupulously clean. All of your test equipment should be
well cleaned after each use and then rinsed at least three times with deionized
or otherwise purified water prior to being used again. Disposable BOD bottles and
other apparatus have helped to lessen the possibility of carryover, but it is important
to store and handle these items per the manufacturer’s recommendations. Analyzing
seeded and unseeded water blanks will help you determine if you have any contamination
in your process.
Equally important, but often overlooked, is the requirement to adjust the pH of
samples prior to dilution. This step is important because the test must be conducted
at a pH between 6.5 and 7.5 in order for the results to be accurate. In some municipal
labs, adjusting the pH of “real” samples is never required as they always fall in
the required range. Over time, some of these labs even begin to stop checking the
pH of their samples altogether. While this may not cause a problem when testing
plant effluents, it can be a significant problem when analyzing a DMR-QA sample.
In order to prepare a homogenous, stable proficiency testing sample, ERA preserves
the Demand concentrate with acid. Once you have made the initial dilution into deionized
water, the sample pH will still be slightly acidic. If you do not adjust the pH
of this sample prior to preparing your analytical dilutions, you run a high risk
of failing the sample. The best way to adjust the pH is to use a dilute solution
of sodium hydroxide (~0.2 molar) and add it one drop at a time to the diluted sample,
mixing and then checking the pH after each addition until the pH is between 6.5
and 7.5.
Here are a few additional keys to improving your BOD analyses:
- Make sure that your incubator temperature remains consistent at 20±1°C. The use
of an independent thermometer is highly recommended.
- When analyzing CBOD be cautious about using plant influent as your seed if it contains
a high percentage of nitrifying bacteria. The nitrification inhibitor you add to
the sample will suppress the nitrifying bacteria and you may not be left with enough
carbonaceous bacteria to generate proper oxygen depletion.
pH
While pH is generally considered a simple analysis, it too requires some
attention to detail to ensure consistently accurate results. Don’t let the test’s
seeming simplicity make you take the importance of these details for granted.
It is always better to run calibration, check and field samples at room temperature
rather than relying on automatic temperature compensation (ATC). If you must analyze
samples of different temperatures, make sure that your meter has ATC and that it
is working appropriately.
Use of fresh calibration buffers is recommended. You should never stick your probe
into your bottle of buffer or return buffer to the bottle after it has been used
once. By following the storage recommendations given by the supplier and making
sure to not contaminate the solution, you should be able to use the buffer up until
its expiration date. Using an independent quality control sample will help you identify
bad buffer solutions or a probe or meter that is not operating correctly. Finally,
you should always bracket the concentration of your real samples with your calibration
buffers. By screening unknown samples before analysis you can identify which buffers
to use.
Total Residual Chlorine
Residual chlorine analysis can easily be compromised due to contamination
from chlorine present in tap water as well as the volatility of the analyte. A few
simple steps can minimize these and other sources of error in this test.
- Make sure all glassware is cleaned and rinsed at least three times with deionized
water prior to use.
- Analyze a blank to ensure no contamination from water or glassware is present.
- Samples should be analyzed as soon as possible. Proficiency testing (PT) and quality
control (QC) samples should be freshly diluted just before they are to be analyzed.
- Make sure you are using fresh reagents. Reagent packets (powder pillows) generally
work very well, but you must make sure that the concentration range of the powder
pillow is appropriate for the sample you are testing.
- When diluting PT or QC samples, some glass pipets may not fit into the neck of the
glass ampule. A good solution is to use a clean, gas-tight syringe capable of delivering
1 mL of concentrate.
Total Suspended Solids (TSS)
TSS is a pretty straight-forward test which many laboratories perform.
Here are a few tips to help ensure this analysis is in control.
- Filter an amount of sample to ensure that the resulting residue on your filter is
large enough to be weighed accurately yet small enough to dry efficiently. A range
of 10-200 mg of residue is recommended.
- Always use a 4-place analytical balance when weighing your filters before and after
use.
- You should always filter a blank (deionized water) sample and perform corrective
action if the results are outside of your normal observations.
- Ensure that your pans and filter papers are completely dry before use. They should
be stored in a dessicator after they are dried and before they are used.
- Dry and weigh each sample at least twice to ensure that the filters are completely
dry.
- Dried pans and filters should be allowed to cool in a dessicator prior to weighing.
Oil and Grease
Oil and Grease is a gravimetric analysis with some similarities to TSS.
A few of the same tips and some unique steps apply to this analysis.
As with TSS, it is important to use a 4-place analytical balance for all weighings.
Unlike TSS, where you choose the sample size to filter, the entire oil and grease
sample should always be used to ensure that all of the analyte is captured. You
also need to rinse the bottle and cap three times with solvent to ensure that none
of the sample remains attached to the container.
Drying and weighing at least twice should be performed for the oil and grease analysis
as well, so that you can make sure the sample is completely dry. It is also necessary
to ensure that no water ends up in your collection pans as the dissolved solids
present can result in a high bias.
Coliforms
Like BOD, there are many variables that go into an accurate analysis of coliforms.
Over the years, ERA has answered hundreds of questions about this analysis with
many of the questions focusing on some of the same topics.
Positive Controls - For each new
batch of media and method apparatus, test a positive control to demonstrate that
the media produces the expected reaction to the organism under test. Reference cultures
used for positive controls should come from a national collection, organization,
or manufacturer recognized by an accrediting authority. These cultures may be a
single-use preparation or cultures maintained by procedures that ensure the continued
purity and viability of the organism.
Negative Controls - For each new
batch of media and method apparatus, test a negative control to demonstrate that
the media does not demonstrate a false-positive reaction.
Reference cultures used for negative controls should come from a national collection,
organization, or manufacturer recognized by an accrediting authority. These cultures
may be a single-use preparation or cultures maintained by procedures that ensure
the continued purity and viability of the organism.
Method Evaluation - All methods
in use in the laboratory should be evaluated for their ability to produce acceptable
results prior to first use. For quantitative microbiology methods this can be accomplished
by participation in an approved Proficiency Testing program or the analysis of a
Quality Control sample. .
Hydration of Pellet - The initial
hydration of the gelatin tablet supplied with ERA samples should be performed following
the instructions provided. Please do not deviate from the instructions as the certified
values and acceptance intervals are based on customer data and deviations could
result in an incorrect result.
Once hydration of the pellet is complete, perform your analysis immediately. Waiting
more than 30 minutes to perform your analysis could have an affect on results. Gently
shake the sample prior to taking aliquots for analysis.
Culture Media - Quality of culture
media is critical. Never prepare media from raw ingredients if a source of dehydrated
media is available. If preparing dehydrated media, follow instructions closely.
Always check pH and make any adjustments that are necessary. Always evaluate media
prior to first use. Never use media outside of its expiration date and never use
media that has not been stored according to the manufacturer’s specifications. Confirm
that prepared media and dehydrated media ingredients and proportions match the specifications
for your method.
Dilution and Rinse Water - Only
hydrate the gelatin tablet using the phosphate buffer supplied by ERA. To perform
dilutions and apparatus rinsing, do not use DI or distilled water. ERA recommends
Phosphate Buffer or Peptone water as it is not uncommon for DI or distilled water
to cause inhibitory effects. See Standard Methods 9050C 20th Edition for the preparation
of Buffered water or Peptone water.
Data Reporting
When reporting data for DMR-QA, it is important to understand that EPA separates
the responsibilities for the laboratory and permittee function, even if the same
person is responsible for both areas. It is the permittee function that is ultimately
responsible for collecting all of the analytical data from any in-house or contract
laboratories, compiling the data and sending it into the provider along with the
EPA’s DMR-QA cover sheets.
For 2008, not all states are allowing the use of electronic reporting such as ERA’s
eDATA™ website. If you report your routine monitoring data electronically,
you can report your DMR-QA data electronically as well. If you normally report via
paper, or by re-typing your laboratory data, you should check with your state DMR-QA
coordinator to ensure what their preference is.
For any other questions that you may have, whether about analysis or reporting for
DMR-QA, ERA’s customer service and technical are ready to help at 1-800-372-0122.
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