Stability Monitoring of Pesticide Residues in a Chinese Cabbage Certified Reference Material

Abstract

Certified reference materials (CRMs) with values traceable to the International System of Units (SI) play a key role for harmonization of measurement results among laboratories. CRMs can be used for validation/verification of analytical methods employed in individual laboratories and for quality assurance or testing proficiency of laboratories. 1-3 Therefore, national metrology institutes (NMIs) of developed countries have been producing CRMs of various types, which are similar to samples to be analyzed, in respect to matrix types, analytes, and their concentrations. Our laboratory, as the NMI of Korea, also has been developing CRMs for environmental, food, clinical, and material analysis. Unfortunately, (potential) users often face a problem that CRMs matching to their samples are not available. In the organic analysis field, the problem is, in most of case, due to instability of target analytes in CRMs as many organic compounds of interest are air-sensitive, light-sensitive, degrading thermally or in acidic or akalic environment, and/or evaporating out of matrix during long-term storage. 4, 5 Therefore, ensuring stability is a key factor for successful production of CRMs for organic analysis. Obtaining reliable analytical results for pesticide residues in foods and agricultural products is a very important issue for food safety. However, it is well known that measurement results of pesticides, in many cases, show a strong dependence on extraction methods and sample clean-up methods employed for the analysis. 6-9 In this respect, using appropriate CRMs is a secure way for a laboratory to ensure the quality of its measurement results through internal verification of its analytical procedures. However, availability of suitable matrix specific CRMs containing relevant pesticides are limited. Those limited number of CRMs available from NMIs world wide are mostly for the analysis of organochlorine pesticides in food samples as those compounds have been known to be relatively stable compared to other types of pesticides. 10,11 As the agricultural use of most organochlorine pesticides was prohibited internationally, current food safety issues are focused on organophosphorus pesticides, carbamates, and pyrethroids, and the need of food CRMs for the analysis of those pesticides has been increasing. However, the inherent instability of those pesticides in food matrices has been an obstacle for developing relevant CRMs. 4 At the beginning of this study, no food CRMs for the analysis of those pesticides was available, up to our knowledge. Therefore, our laboratory started a project to develop food CRMs for the analysis of organophosphorus pesticides, with anticipation to understand and overcome technical difficulties for ensuring stability of those pesticides in food matrix. We prepared a batch of a candidate CRM for the analysis of pesticide residues in Chinese cabbage. Chinese cabbage was chosen as it represents commonly consumed leafy vegetables in Korea. Target analytes were two organophosphorus pesticides (diazinon and chlorpyrifos) and two organochlorine pesticides (α- and β-endosulfans). The two organochlorine pesticides are occasionally detected above regulatory limits from vegetables produced in or imported into Korea though the agricultural use of those pesticides is prohibited. Diazinon was chosen as it is one of the most volatile organophosphorus pesticides and it is expected that its volatility may cause instability in the CRM. After initial certification of the material, its stability in various storage temperatures was monitored for three years to test whether the candidate CRM has adequate long-term stability at shelf storage condition and short-term stability at conditions for transportation to and conventional handling at users’ sites. In this paper, we report the stability test results of the four pesticides in the CRM.