Analysis of TNT and RDX in groundwater


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LIMIT OF DETECTION: TNT 1 ppb (mg/L), RDX: 5 ppb (mg/L)

This application describes a field screening method for determining TNT and RDX concentrations in water. The method uses solid phase extraction to remove and preconcentrate the analytes from water. In the method, a 2 liter (L) water sample is passed through a stack of two membranes to preconcentrate TNT on the top disk and RDX on the bottom disk. Acetone is used to elute RDX from the bottom disk and a chemical reaction is induced that causes a color change indicative of RDX in the solution. The RDX concentration is estimated from the absorbance at 510 nanometers (nm) on a Hach DR2000/2010 spectrophotometer. Next, the top disk is eluted with acetone and a different chemical reaction is induced causing a color change indicative of TNT. The TNT concentration is estimated from the absorbance at 540 nm on the Hach DR2000/2010.

Extraction and analysis should be performed in a well ventilated area. Laboratory technicians should wear chemical resistant gloves and safety glasses. Use gloves during the entire procedure.

TNT EnSys Soil Test System and RDX EnSys Soil Test System. EmporeÒ SDVB 47 mm membranes (2 per sample). Alumina-A filter (Alltech item #210094). Reagents: acetone, distilled/deionized water Equipment: 47 mm filter flask apparatus, (includes graduated funnel, filter holder/support, clamp, stopper), vacuum pump, aluminum foil, tweezers, 25x200 mm glass test tubes, timer (minutes/seconds)

1. Use tweezers to place two Empore extraction membranes centered on the lower portion of the filter apparatus, cover squarely with the upper portion of the filter apparatus and clamp securely. Do not touch the membranes with your hands. A glass fiber filter may also be used to remove particulate
2. Slowly add 30 mL acetone to the stack and allow it to soak for 10 minutes.
3. Slowly apply vacuum to the filter flask apparatus until there is minimum dripping of acetone (evidence that both filters are completely saturated). Shut off vacuum, add 10 mL of D. I. water. Let set for 10 min or until about 1/4 inch of liquid remains, whichever occurs first. Warning, the next two steps go quickly so have materials measured and in place before starting.
4. Fill the reservoir with sample before the fluid level reaches 1/4 inch. Reapply vacuum slightly. The sample may be filtered at a rate of 10 to 100 mL/min. 5. Continue filling the reservoir until 2 L of sample has penetrated the membrane. Do not allow the fluid level to reach below 1/4 inch until all the sample has been passed through.
6. Add 10 mL of DI water to the reservoir just before the last of the sample penetrates into the membrane. This will wash out the nitrate interferences. 7. Continue to apply vacuum for ~2 minutes after the last of the sample in order to remove excess water.
8. Remove the upper portion of the filter apparatus from the filter stack and discard the glass fiber filter
9. Remove both the disks and set them face up on a clean piece of aluminum foil marked “T” for top disk and “B” for bottom disk, these will be used later for your TNT and RDX extracts.
10. Reassemble the filter apparatus and rinse first with DI water and second with acetone.
11. Disassemble the filter apparatus and pour the water from the 2,000 ml Pyrex flask into a waste container.

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