Tech Tips for DMRQA

Article written by Phillip Eleyette, QC Chemist, Analytical Products Group, Inc.

Complete article from the DMRQA 26 APG eNewsletter

Each year during DMRQA we receive technical questions regarding sample handling and preparation techniques. APG has complied the following list of technical tips that may help you with your DRMQA testing. Should you have any additional questions, please call APG at 800.272.4442. There are several new PT samples added to this year's DMRQA Study. Please read and follow all instructions carefully before beginning analysis.

Microbiological

1. Dilutions
: The product range is 20-2400 cfu/100mL. The dilution is based on how many cfu/plate is acceptable. If plated at the following dilution, approximately cfu/plate for 20-2400 range is as follows:

Dilution/100mL	cfu/plate
1 mL	0.0-24
2 mL	0.0-48
5 mL	1.0-120
10 mL	2.0-240
20 mL	4-480
50 mL	10-1200

2. Homogeneity: The sample should be well mixed before each aliquot is removed.

Minerals

1. Dilutions: Dilutions should be made into water. Do not directly mix vials.

Turbidity

1. Preparation: Shake the vial prior to use.

Demand

1. The pH adjustment: The resulting one-liter solution should be adjusted to a pH of 6.5-7.5. Hydrochloric acid and sodium hydroxide are acceptable for the adjustment. If the pH is not properly adjusted, it will result in the failure of any microorganisms to live in the solution, and no BOD will be measured.
2. Initial dilution: The initial dilution is 20 ml to one-liter of laboratory grade water. The diluted sample should be mixed for at least 15 minutes before it is tested.
3. Seeding the PT sample: In the case of the APG PT samples, specific steps have been taken to inhibit bacterial growth in order to provide a stable sample. Effluent from the primary settling tank may be used for seeding. How much seed is used depends on the instructions provided by the seed manufacturer.
4. Determining the amount of PT sample to put in the BOD bottle: The easiest answer is to treat the PT sample in the same manner that you would treat any unknown sample, and do multiple dilutions. Dilutions of 5, 10, 25, 50, 100mL are recommended.

Total Residual Chlorine

1. Sample range: The sample range is 0.5 to 3 mg/L. In some cases, this is above the range of the instrument, and a second dilution may need to be made from the prepared PT sample. Additional dilutions (if necessary) of the PT sample after following the PT sample preparation instructions must be applied to results calculations.

Oil and Grease Method 1664

1. pH adjustment: The sample is acidified to a pH <2 and extracted with n-hexanes by liquid-liquid extraction.
2. Solvent distillation: This step should be accomplished in thirty minutes or less, but it should not be done too quickly.
3. Filter size: The size of the filter should be carefully selected in order to prevent the silica gel from moving through the filter membrane and remaining in the sample during the drying process.

Solids

1. Transfer of solids: Tap the vial to make sure all of the solid material is in the bottom of the vial and not near the cap or opening.
2. Analytical transfer: The first step is to wet the solid material in the vial; this will minimize the loss of material due to dusting. Begin by carefully rinsing the sides of the vial, and then pour the contents into the funnel. Finally, rinse the vial.
3. Homogeneity of the sample: Once the sample has been transferred, we recommend that you stir the solution for fifteen minutes with a stir bar or shake vigorously for at least thirty seconds.