Journal of Chromatography A, 1166 (2007) 181–190 A solution for isomerization of pyrethroid insecticides in gas chromatography Jing You 1 , Michael J. Lydy ∗ Fisheries and Illinois Aquaculture Center & Department of Zoology, 171 Life Science II, Southern Illinois University, Carbondale, IL 62901, USA Received 4 June 2007; received in revised form 29 July 2007; accepted 2 August 2007 Available online 9 August 2007 Abstract Isomerization of pyrethroid insecticides was observed during extraction and gas chromatography (GC) analysis. An improvement in sensitivity was noted for pyrethroids in sediment extracts in comparison to pure solvent. Stability of pyrethroids using different solvents and analyte additives were investigated, and GC injection conditions were optimized. Polar solvents enhanced pyrethroid isomerization, while hexane was the best choice as an analytical solvent. Acetic acid was used successfully as an isomer-stabilizing agent for GC analysis of pyrethroids. Acidified (0.1% acetic acid) hexane prevented pyrethroid isomerization, increased peak intensity up to 1.9 times, and calibration curve linearity (relative standard deviation for response factors) 0.8–12.5 times compared to hexane alone. © 2007 Elsevier B.V. All rights reserved. Keywords: Matrix effect; Pyrethroid isomerization; Sediment extract; Gas chromatography 1. Introduction A shift from the use of organophosphate insecticides to pyrethroid insecticides poses a potential risk to sediment- dwelling species. This is because sediments act as a “sink” for pyrethroids, and because pyrethroids are highly toxic to ben- thic invertebrates [1]. Previous studies have shown a strong link between pyrethroid use and benthic invertebrate toxicity from both agricultural and urban areas in California [2–4]. Therefore, it is highly desirable to monitor these contaminants in sediment at environmentally relevant concentrations (ppb level). How- ever, accurate quantification of pyrethroids historically has been problematic due to their low analytical sensitivity and poten- tial for isomerization during the extraction and quantification process. Therefore a solution for controlling pyrethroid isomer- ization is needed to avoid quantification errors during analysis. Pyrethroids are grouped into two types based on the pres- ence of a cyano-group in the molecule (Fig. 1). Pyrethroids are ∗ Corresponding author. Tel.: +1 618 453 4091; fax: +1 618 453 6095. E-mail address: mlydy@siu.edu (M.J. Lydy). 1 Present address: Department of Biochemistry, Chemistry & Physics, Uni- versity of Central Missouri, Warrensburg, MO 64093, USA. neurotoxicants and the two types of pyrethroids have slightly different modes of toxic action. Type I pyrethroids demon- strate T-syndrome effects (tremors), while type II pyrethroids show CS-syndrome effects (choreoathetosis and salivation) [5]. Pyrethroid molecules typically contain 2-3 asymmetric carbon atoms (chiral centers), and therefore have two to four diasteroi- someric pairs of enantiomers. Type I pyrethroids do not contain a cyano-group, and only have two chiral centers on their cyclo- propyl ring, resulting in two possible peaks when separated on a non-chiral GC column. Type II pyrethroids generally contain two chiral centers on the cyclopropyl ring and an additional chiral center on the -carbon connected to the cyano-group, so four diasteroisomers are potentially observed. One exception is the type II pyrethroid esfenvalerate, which does not possess a cyclopropyl ring in its molecule and only has two diasteroi- somers (Fig. 1). Due to the variation noted in the insecticidal activity among pyrethroid isomers, isomer-enriched pyrethroids have been manufactured for better insect control (Table 1). Isomerization has been reported for pyrethroids as the result of exposure to polar solvents [6–8], heat [9], and light [6,10]. While both types of pyrethroids undergo light and heat induced cis–trans isomerization, type II pyrethroids are more susceptible to exposure to polar solvents where -proton exchanges can occur [6]. 0021-9673/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2007.08.014