Spectrofluorimetric Determination of Pesticide Residue Mixtures.

Table of contents

I. Researching task
II. Necessary instrument
III. Instruent Searching
IV. Instruments list
V. Selecting of two most useful instruments
VI. Specification comparison
VII. Conclusion
VIII. Group Dynamics
IX. Reference

I. Researching task

Benomyl (1), a major product of our company, is widely used as a systemic fungicide to control a wide range of fungi affecting field crops[1]. In the soil and plant, benomyl decomposes to the main fungitoxic metabolite carbendazime (MBC, 2)[2].

Many benomyl analogs have similar fungitoxic effects. Our research is concerned with the modification of benomyl to get more effective but lower residue fungicide. In order to achieve that goal one needs to develop a fast, convenient and economical analytical method to quantify and qualitate the pesticide residue in the plant. Therefore, before new compounds could be synthesized we first use benomyl and its metabolite to test our proposal.

II. Necessary instrument

In order to determine the concentration of the benomyl and MBC residue in plant, fluorescence spectroscopy would be the most effective method, as these two compounds are highly conjugated. UV spectroscopy also could be used. However, the UV sensitivity is not high enough for our determination of the pesticide residues[3]. IR has even lower sensitivity and is not suitable for mixtures. Finally, MS is too expensive and strict maintenance is required. All of these factors pointed us in the direction of fluorescence spectroscopy.

The advantages of fluorescence spectroscopy are: (a) Excellent sensitivity, fluorometric method can detect concentration as low as 0.1 ppb, a sensitivity 1000 times greater than that of most spectrophotometric methods. (b) Good specificity, only 10% of all absorbing compounds will emit radiation via fluorescence. (c) A larger linear range. It is not unusual to encounter six to seven orders of magnitude linearity.

III. Instruent Searching

We screened the market to find the best spectrofluorometers. Our main source was the Lab Guide of Analytical Chemistry. This gave us a list of websites of the companies which manufacture fluorescence spectroscopy. We also obtained instrument model and manufacturer from the experimental section of the current Applied Spectrum Journals.

IV. Instruments list

The table below describes a list of the five companies and the models of fluorescence spectrometers.

V. Selecting of two most useful instruments

Benomyl and MBC have similar structures, therefore the two compounds present overlapping fluorescence spectra which cause a problem for their simultaneous determination. Further, when fluorescence spectroscopy is used as a detection technique in residue analysis, many interfering fluorescence substance from co-extractives affect the sensitivity and reproducibility. We thought of two solution for this problem. One is to use fluorescence spectrometer as a detector for HPLC[5,6].. Second solution is the synchronous derivative technique which has been developed in recent years[7].

We took into consideration of Shimadzu RF-5301PC and Pekin-Elmer LS-50B models as they can fulfill the requirement of our research project. LS-50B has LC flow cell. We can replace the standard cell holder with an LC flow cell accessory and use HPLC to separate the mixture. Both of these two spectrophotometers can handle with synchronous experiments. More importantly, both of LS-50B and RF-5301 PC are controlled by computer. The other three fluorescence spectrometers are designed for routine experiments, it is not computer-controlled. Since we need to process a large amount of data in our research, the computerization of instrument is vital. This led us to the consideration of the LS-50B and RF-5301 PC, over the other three.

VI. Specification comparison

VII. Conclusion

We get the quotation from these two companies. Table 3 is the system and accessory prices.

Table 3. Quotation from Perkin-Elmer and Shimadzu (US$) LS-50B RF-5301 PC Main system 29,000 21,499 Computer price included 1,600 Software price included 980 LC flow cell 1140.00 N/A Total 30,140 24,079 The price difference is not significant. We hope we can get the separation signal by synchronous scanning technique because that means one sample can be finished in several minutes. In case this method does not work, we still need HPLC to separate the sample. HPLC is ready in this research lab. LS-50B has these two function and only synchronous scanning is available in RF-5301 PC. We preferred LS-50B also because of its high sensitivity and accuracy. Fortunately, the price of LS-50B is only 25% higher than that of RF-5301 PC. So far, after careful consideration, we decide to order Perkin-Elmer LS-50B fluorescence spectroscopy.

VIII. Group Dynamics

Working in a culturally diverse group consisting of two Chinese students and a Sri-lankan led to an understanding of each person's culture. Before, the project assignment we were mere acquaintances and working on the project together led us to mold a friendship. We divided the group project into three parts according to each person's best skills. This increased the efficiency of the group as time management being a graduate student is a big concern. We met every Wednesday for one hour to discuss the progress of our project. After each person's project report the meeting usually ended in a discussion of our research projects. As three of us are working under the direction of three different research advisors, this gave insight into the research areas of our group members. When one of our group members came up with a problem, we immediately met that evening and made a joint effort to remedy the problem. This increased our problem solving abilities and also enabled us to compromise. Further, we were able to give, as well as, receive constructive criticism. To sum up working in a group was a positive experience.

The biggest problem we encountered was trying to obtain the prices for the parts of the instrument, as the catalogs which the companies sent us were without price specifications. Further, not being native speakers of English also caused few problems in communication and writing. However, working as a group we were able to overcome these and we would like to participate in another project.

IX. Reference

(1) Pesticide Manual, British Crop Protection Counsil, 7th ed.; Worthing, C. R., Ed.; British Corpprotecction Counsil; Croydon, England, 1983, pp 89 and 523.

(2) Chiba, M.; Cherniak, E. A. J. Agric. Food Chem. 1978 26, 573.

(3) Chiba, M.; J. Agric. Food. Chem. 1977 25, 368.

(4) Guilbault, G., General aspect of luminescence spectroscopy in Practical Fluorescence (Ed. by Guillbault, G.) pp1 - pp41, 1990 Marcel Dekker,Inc.

(5) Tafuri,, F.; Marucchini, C.; Patumi, M.; Bushinelli,,, J. A. J Agric. Food Chem. 1980, 28, 1150.

(6)Kitada, Y.; Sasaki, M.; Taniawa, K. J. Assoc. Of. Amnl. Chem. 1982, 65, 1302.

(7)Miller, J. N.; ?Ahad, T. A.; Fell, A. F. Proc. Anal. Div. Chem. Soc. 1982, 19, 37.