A reader is trying to figure out the fate of a pesticide in water. For the test, he takes a soil sample (e.g., 5 g) and adds 100 mL of an aqueous solution containing 10 ppm of a pesticide. He takes 1 mL aliquots of the liquid phase each hour and analyses for the pesticide until it can no longer be detected. At this point, he doesn’t know whether the pesticide is adsorbed onto the soil or something else has happened to it, such as degradation. He is also concerned that his data may not be valid because each time he removes a 1 mL sample, the total volume of the system is reduced. He asked for some advice on how to figure out what is going on.
First, I suspect that removal of 1 mL aliquots for sampling will have no impact on the result. This is a 1% reduction in volume and I expect that the magnitude of change he is looking for is much greater than this. However, you could mathematically correct for the removal each time it was done. For example, the first aliquot would represent 1/100 of the original solution, the second aliquot would be 1/99, the third 1/98, and so forth as the total solution volume was reduced by successive samplings. Another alternative would be to add a volume of water equal to the aliquot after each aliquot were taken. So remove 1 mL of sample and add back 1 mL of water and the total volume will stay constant.
For sampling, I would suggest either filtering or centrifuging the sample to separate the liquid and solid portion, then the 1 mL sample could be removed. Then remix the solid and liquid and let it interact until the next sample cycle. This would avoid unintentional removal of soil with each aliquot and would simplify sample preparation prior to HPLC analysis.
To figure out where the pesticide ended up, the easiest thing to do would be to analyse the soil and see if it can be recovered from the soil. I’d filter the solution again, then take a sample of the soil (mud) and dry it to remove the water. You’ll want to be careful to use drying conditions that won’t degrade the pesticide, such as air drying at room temperature or vacuum drying. On the other hand, if the pesticide is known to be stable, oven-drying may be fine and will be much faster. Next, weigh an aliquot of the soil and extract it under conditions you know will (or suspect) will remove the pesticide from the soil, for example, with an organic solvent such as ethyl acetate. Analyse this solution and you can calculate how much pesticide was on the original 5 g of soil to calculate the mass balance.
Whenever I start on a project like this, I want to make the first run under conditions that will be easy to see what’s going on. For example, if you normally would expect the pesticide to be present at 10 ppm, but at this concentration, the peaks are quite small, increase the concentration to 100 ppm for the first cycle. And I wouldn’t worry about correcting for volume loss from the whole solution due to aliquotting. On the first pass you want to find out if your experimental strategy will work, what kind of problems will arise, and to get a general feel about what the results will be (e.g., ±10-20% probably is close enough for a first pass). Once you’ve figured out how to run the experiment and what results to expect, you can refine the process to get higher quality data. You certainly don’t want to spend time pre-analysing every nuance and working at trace levels until you know your experiments are likely to work.
This blog article series is produced in collaboration with John Dolan, best known as one of the world’s foremost HPLC troubleshooting authorities. He is also known for his research with Lloyd Snyder, which resulted in more than 100 technical publications and three books. If you have any questions about this article send them to TechTips@sepscience.com
