DMRQA Tech Tips

by Sherri McManaway, Production Supervisor

It's summer, it's hot outside and it's DMRQA season again! For many of our customers this is a routine summer event, much like forth of July fireworks but without the fanfare.  For some it is the first time participating in the ritual of proficiency testing.

The basic idea of testing the DMRQA (Discharge Monitoring Report Quality Assurance) products is to evaluate how well a laboratory performs their routine analysis for NPDES (National Pollutant Discharge Elimination System).  So if you pass your DMRQAs then the assumption is that your routine tests are performed correctly and accurately.

APG’s Technical Support staff is available for assistance with questions that arise in the preparation and testing of the DMRQA samples.  Anytime clarification is needed we can answer or find the answer to your questions.

DMRQA season is the busiest time of year for our Technical Support team and below are some frequently topics and tips for your success:

Demand- Our most common questions arise from the BOD and CBOD portion of this test.  Proper dilution of the concentrate of 20mLs to 1L, pH adjustment to between 6.5 and 7.5 units, and seeding are keys to acceptable results.  pH adjustment can be accomplished by adjusting the diluted sample with 0.1N NaOH to obtain the correct pH.  Seeding the sample with primary settling effluent or commercially available seed is required to add organisms to the sample.  We would further recommend multiple sample bottles be prepared at various volumes to cover the product range.  5,10,25,50,100 mLs are recommended.

Solids- TSS samples are provided as a mix of powers that are diluted to 1L.  Great care must be taken to ensure no loss of the material occurs when transferring.  Dusting can be prevented by allowing the rinse to slowly flow down the insides of the vial and rinsing the vial completely after pouring into the flask.  Mixing the sample completely and filtering immediately after preparation are steps to better results.  Due to the small weight of material, APG recommends no less than 250mLs of sample be filtered and that a graduated cylinder be used for the measurement of the aliquot.  Rinse the cylinder after use to ensure complete transfer.

Oil and Grease- pH adjustment is required by the method, adjust the diluted sample to <2.  For use with automated solvent extraction, preparation into the sample bottle will reduce loss of material through transfer.

Nitrate, Nitrite, Ammonia, and Orthophosphate Conversions- Many laboratories calibrate with the compounds reported as the complex analyte.  Care should be taken to ensure that the value reported is in the correct form, i.e. Nitrite is reported as Nitrite as N, if you calibrated Nitrite as NO2 you must convert the result to N for the proficiency study.  Listed below are common conversions, conversions can be made by multiplying the complex result by the conversion factor to correct the results.
           
            Nitrite as N-NO2 as N/NO2 as NO2= 0.304
           
            Nitrate as N-NO3 as N/NO3 as NO3=0.226

            Ammonia-NH3 as N/NH3 as NH3=0.822

            Orthophosphate as P-PO4 as P/PO4 as PO4=0.326

Minerals-Both vials of the concentrate are used to dilute the one unknown standard.  Care should be taken to dilute the samples into water and not mix directly.

Trace Metals-Each of the two samples provided should be prepared individually, combining is not recommended.

Nutrients- Each of the two samples provided should be prepared individually, combining is not recommended.

Sample preparation tips-Dilution of samples should be done immediately prior to analysis.  Always confirm the dilution of the sample versus the sample preparation information provided.  Not all dilutions are the same.  Use class A laboratory glassware and laboratory grade water for sample preparation. 

Result interpretation tips-As part of your normal routine quality control process, testing of quality control data and samples analyzed within your instruments' calibration range will ensure that your test was performed properly.  If your results are too high for your instrument then the values might not have the accuracy necessary for you to report acceptable results, i.e. For the Total Residual Chlorine sample the range is 0.5-3.0 mg/L. but your meter only reads to 2.0.  If the sample is above 2.0 and the meter reads above calibration additional dilution of the sample would bring the sample within range.  However any additional dilution must be used to calculate the reported value.  Another issue faced by the laboratories is what to do if the product range and your final results don’t match.  For Total Cyanide as example the product range is 0.1 to 1.0 mg/L.  The assigned value for the DMRQA study 26 was 0.86, well within the sample range; however do to the expected result and the regression calculation that determines the acceptable range customers reporting up to 1.17 mg/L would be deemed acceptable.  Also in study 26 the Total Residual Chlorine the assigned APG value was 0.64 mg/L with in the 0.5-3.0 mg/L sample range.  But due to the sample regression calculation the acceptable range was 0.47 to 0.81.  Thus allowing results below the 0.5 mg/L bottom of the range to be acceptable.  Therefore not all acceptable results will be found within the product range, however careful calibration and quality control should always confirm analysis.

Please contact APG’s Technical Support Team for any further questions or issues that arise.  Good luck and have a great summer!

For more information please contact Sherri McManaway at
s.mcmanaway@apgqa.com.