This module presents concepts related to structure/solubility relationships of organic compounds, risk assessment, gas chromatography, biodegredation, bioaccumulation, and organic extraction techniques. At the end of the module, students use the data collected by the class to write up a full report on the risks and benefits of pesticides in the food supply. The full module can be carried out in four weeks, which includes one laboratory period for a debate on the use of pesticides in the food supply. Alternatively the instructor may choose to shorten the module by eliminating the debate or lengthen it by having students optimize the GC method of analysis.
This instructor's manual is provided to help someone new to pesticide analysis get started with this module. The following information is provided:
At the beginning of the module, it is important for you, the instructor, to present your students with a guide to what they should expect to have learned by the end of the module. This helps students gauge their progress and clarifies the important concepts covered by the module. By the end of this module, General and Organic Chemistry students should have gained the following:
Upper-Level Analytical students should, in addition, have gained the following:
This module can be used at a variety of levels throughout the undergraduate chemistry curriculum
This module has been tested for seven semesters in a first semester general chemistry course at the University of California, Berkeley in special laboratory sections focusing on environmental chemistry. The experiments have been carried out successfully by students with a wide range of abilities, not just honors chemistry students. The rationale for teaching it in the first semester of freshman chemistry is based on the fact that it provides a broad range of students with evidence of the utility and importance of chemistry in our society. The major disadvantage of doing the module this early in the curriculum is students have had less exposure to some of the concepts and techniques presented. Equipment limitations might make it difficult to do this module at institutions with large (>350 students) general chemistry classes. Creative scheduling and/or use of more than one GC could circumvent this issue. In general, the response of freshmen to this module has been overwhelmingly positive.
The module could also be used for an organic laboratory course, and is arguably better placed in a course where structure/solubility relationships, extraction techniques, and gas chromatography are more traditionally covered.
Week #1: Organic solubilities and risk assessment
Week #2: Extraction of pesticide residues from produce
Week #3: Data interpretation, data entry, and debate preparation
Week #4: Pesticide debate
This module also works well in an upper level instrumental methods of analysis course and has been tested in this course three times at the University of California, Berkeley. With more advanced students, instructors are able to delve deeper into the theory and practice of gas chromatography. Students could determine response ratios and retention times for different pesticides and design their own temperature program that permits separation of all components of a pesticide mixture. They could then test the extraction efficiency of selected pesticides by processing a spiked sample. More time would need to be allotted for method development, but other parts of the experiment could be trimmed if desired.
Week #1: Determination of retention times and response ratios of selected pesticides.
Week #2: Development of an appropriate temperature program for optimum separation and quantification of pesticide residues.
Week #3: Extraction of pesticide residues from produce to determine extraction efficiency for selected pesticides.
Week #4: Data interpretation, risk assessment.
All materials and equipment not normally found in a chemistry stockroom are listed in the Stockroom Preparations sections with a source, catalog number, and approximate price. It is assumed that students have access to balances and the usual collection of flasks, beakers, clamps, buret, etc. in their lab locker.
Analysis of organochlorine pesticide residues at trace levels requires a gas chromatograph with an electron capture detector. Alternatively, a mass spectrometric detector using single ion monitoring (SIM) techniques can be used with similar results. All development work was carried out using a capillary column, as recommended in the Calfornia Department of Food and Agriculture method. An autosampler is highly recommended, particularly if class size is large. If an autosampler is available, up to 175 samples can be run in a week. If students work in pairs, a total of 350 students could easily do the experiment. Creative scheduling, where students rotate through several modules during the semester, would allow even larger classes to carry out the module. Alternatively, more than one GC could be used.
If a GC with an ECD/MSD is not available at your institution and you would like to do this module, it might be worth considering the submission of a proposal to the National Science Foundation's Course Curriculum and Laboratory Improvement (CCLI) program (http://www.nsf.gov). Perkin-Elmer and Hewlett-Packard make excellent capillary instruments. Autosamplers and ECDs can usually be retrofitted onto existing instruments. The typical cost of an autosampler is ~$9,000 and an ECD is ~$2,000.
If you wish to carry out the supercritical fluid extraction process, you will need a supercritical fluid extraction system. An easy-to-use commercial system is the Hewlett-Packard 7690T, available for ~$50-60K, depending on options. Be aware that with all SFE systems, you will likely need to spend a great deal of maintenance time to keep the instrument running well.