GC-ECD of Chlorinated Pesticides
The analysis of pesticides is of great importance for environmental, agricultural and food analysis. Separation of some common chlorinated pesticides is possible by gas chromatography. An Electron Capture Detector (ECD) is particularly sensitive and selective for these types of molecules. A mixture of pesticides has been purchased and included the following chromatograph obtained with a Flame Ionization Detector (FID).
The aims of this experiment are:
i. To provide hands-on experience of using a Gas Chromatography system.
ii. To investigate the instrumental parameters in order to achieve the optimum separation of a mixture.
- To estimate the detection limits of this method.
Chlorinated pesticides can present a health risk. Please take all appropriate precautions. Wear gloves and safety glasses, clean all spills and keep vials capped whenever possible.
ECD is a more sensitive detector than FID for halogen containing compounds, therefore, a diluted solution will be used containing:
1 ppm of Lindane, Heptachlor and Aldrin and 4 ppm of Dieldrin, Endrin and DDT. Please be carefully not to lose or contaminate this solution as the amount of pesticide mixture is very limited.
- Open the main nitrogen gas valve, the gas pressure on the front of the instrument should read about 10 psi. Activate method 1 on the GC. You can alter this method using the "build/ configure" button.
- Set up the integrator. Set "PW=3", "PT=512" and start with a high attenuation of 256 (you can lower this later if necessary). Press the level button and wait until to comes down to around 1000. Press the "PT eval" button. You will have to wait a while for a number to appear which ideally should be below 500 before starting. This part may take a little time to stabilize.
- Set up the GC-ECD for pesticides using the program described in the data sheet. It may be necessary to vary the attenuation in order to get peaks that are large enough to see easily but small enough to fit on the page. Use this program to determine the adjusted retention times for all six pesticides. Remember that the conditions and, therefore retention times, will be different than the data sheet.
- Vary the column program in order to give good resolution. Try and get all the peaks to reach the baseline (as in the chromatogram above) in the shortest possible run time.
- Measure the gas flow rate using a bubble meter. What is the effect of increasing and decreasing the gas flow?
- By successive dilutions of the pesticide solution with methanol, estimate the lowest concentration of the pesticides that can be reliably seen above the noise level using the optimum conditions. This concentration is approximately equal to the detection limit for the instrument.