Analysis of Organochlorine Pesticides in a Soil Sample by a Modified QuEChERS Approach Using Ammonium Formate

The QuEChERS method using ammonium formate is a successful alternative to the original, as well as the two official versions for extracting organochlorine pesticides from soil samples. The addition of just 0.5 grams of ammonium formate per gram of sample is sufficient to induce phase separation between the water and acetonitrile layers and achieve good analytical performance. To begin the sample preparation, weigh 10 grams of the soil sample in a 50-milliliter centrifuge tube.

Add 50 microliters of the internal standard, or IS, solution, having a concentration of 20 milligrams per liter, to the soil sample to obtain 100 micrograms of IS per kilogram of soil. Add 10 milliliters of water, and shake the tube using an automated shaker at 10 g for five minutes. Then, add 10 milliliters of acetonitrile, and shake the tube at 10 g for five minutes.

Next, add five grams of ammonium formate, and shake the tube vigorously by hand for one minute before centrifuging it at 1800 g for five minutes. For cleanup, by dispersive solid-phase extraction, transfer one milliliter of the acetonitrile extract into the centrifuge tube containing anhydrous magnesium sulfate, primary secondary amine, and octadecylsilane. Vortex the tube for 30 seconds, and centrifuge it at 1800 g for five minutes.

After centrifugation, transfer 200 microliters of the extract to an appropriately labeled autosampler vial, fitted with a 300-milliliter fused insert, and perform an instrumental analysis using a GC/MS system. Inject 1.5 microliters of the sample in a GC/MS system with a single quadrupole mass spectrometer using an autosampler at 280 degrees Celsius in the splitless mode. The matrix effect distributions for the organochlorine pesticides, or OCPs, studied showed both positive matrix effect percentages corresponding to a signal enhancement, as well as negative percentages corresponding to signal suppression.

Values ranging between minus 20 to 20%indicated a soft matrix effect, while those between minus 20 to minus 50%and between 20 to 50%corresponded to a medium matrix effect. A strong matrix effect was indicated by percentage values higher than 50 or lower than minus 50. Evaluating the overall recovery for all the pesticides at different spiking levels revealed that most of the studied OCPs had average recovery percentages of 70 to 120%and relative standard deviation, or RSD, values lower than 20%However, heptachlor, endrin, and beta-endosulfan possessed slightly higher average recoveries.

This alternative may be used for the analysis of pesticides by LC-MS, which would be even more interesting since ammonium formate is an aid for ionization in positive electrospray ionization.