Determination of DEET in Insect Repellants
Situation:
Areas hard hit by recent typhoons are facing a potential epidemic of disastrous proportions for mosquito borne malaria. DDT is an effective insecticide against malaria however its environmental consequences are severe enough that several countries have banned DDT for many years and have placed severe pressure on third world countries to do the same. DEET (N,N-Diethyl-meta-toluamide) is another highly effective insecticide against malaria with far less severe environmental consequences. Because of the impending malaria crisis a lot of DEET must be made and FAST! DEET, as an insecticide, is applied to the skin in either a lotion or a spray. Because of the close proximity DEET has with the human body, it must be sufficiently pure, and of known composition to prevent exposure of humans to chemicals of unknown composition or toxicity from the synthetic byproducts in the manufacture of DEET. You have been hired by the World Health Organization to develop an efficient analytical method to assess the purity of insect repellant containing DEET and to be able to quantitate the amount of DEET in a number of insect formulations.
Pre-lab:
Read sections 26D (pp. 775-781), 27B1-3 (pp.789-792), and the flame ionization section (pp.793-794) prior to coming to lab.
What is the boiling point of DEET and DBP (dibutyl phthalate)? (DBP will be used as your internal standard in this experiment.)
What is the purpose of an internal standard and how is it used?
What is the equation to calculate resolution between two components on a chromatogram?
Experimental:
The method of choice for this determination is gas chromatography with flame ionization detection because of the somewhat volatile nature of DEET and because of the high carbon content of DEET or gas chromatography with mass spectral detection. You will be using GC/FID. In addition to DEET of known purity, you will also need a solution of DBP (dibutyl phthalate) to use as an internal standard. Both the standards and the samples will be diluted into acetone as the solvent.
Make up a 100mL solution of 5% (wt./vol) DBP dissolved in acetone.
Weigh 0.5g (to the fourth decimal) of DEET into a 100mL volumetric flask. To this, pipet 10mL of the 5% DBP solution. This will be your standard solution. You will use this solution to develop your analytical GC method.
Become familiar with the operation of the GC from the instructor’s introduction. Using the boiling points of DEET and DBP as a starting point, develop a GC method that will separate DEET from DBP, (and both from acetone) in less than 20 minutes. Your method should have at least one temperature ramp and should provide a resolution between DEET and DBP of at least 1.5. Do not take the temperature of the column higher than 280oC. The injector temperature should be set at 200oC and the detector temperature at 300oC. Try out several methods until you get one that meets all of the above criteria.
Once you have developed a method for analyzing DEET, validate the method by measuring DEET in a sample of known composition (be sure to add internal standard), and a real insect repellant.
Lab reports should be written with your lab partner. Each individual should submit a contribution form. The reports should be written up in the ACS format, suggested length, not including figures should be 2-3 times number of weeks spent on the lab in pages. Be sure to include references. Lab reports will be due one week after completion of the lab.
In your lab report include the conditions ALL of your chromatograms were run under and the reasoning you went through to decide what your next set of conditions should be. Also include calculations of resolution for your standard chromatogram and the insect repellant sample. Calculations of the exact amount of DEET in the insect repellant as well as a comparison to the known quantity of DEET in the sample should also be done. Finally, in your report, answer the following questions:
What was the column composition? (You may have to consult a column catalog to fully answer this question, see your instructor to obtain such a catalog.)
What other types of GC columns do you think would also have worked?
Would you think you would have gotten a higher or lower detection limit with GC/MS? Why? Would it have mattered in this experiment? Compare and contrast FID and MS detectors.
In this experiment you used a capillary column, compare and contrast a capillary column with the cheaper packed GC columns.
What was the mobile phase used in this experiment? Why isn’t mobile phase composition as important in gas chromatography as it is in liquid chromatography?
Why was DBP chosen as the internal standard to use in this experiment?
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