Determination of the plant growth regulator chlormequat in food by liquid chromatography–electrospray ionisation tandem mass spectrometry

A confirmatory method for the determination of trace levels of chlormequat in a variety of different food matrices was developed. It entails a single clean-up step over a solid-phase cation exchange resin and subsequent liquid chromatography–electrospray ionisation tandem mass spectrometry using a stable isotopically labelled internal standard. Mass spectral acquisition was done in selected reaction monitoring mode, selecting the transitions from both the 35Cl and the 37Cl isotope of chlormequat. Recoveries after extraction and clean-up, determined with radio-labelled chlormequat and averaged over the spiking range (16–65 μg kg−1) in four different commodities, were within 88–96%, with a coefficient of variation better than 8%. The method can be applied to pears, pear juice concentrates, fruit purées, and cereal products, with typical limits of detection for chlormequat estimated at 2–5 μg kg−1. A survey of different food commodities revealed that chlormequat was detectable — albeit at very low levels — in many of the food samples analysed, with the highest concentration recorded in pears purchased in Switzerland and of South African origin (5.5 mg kg−1). Measurements were also conducted on two LC–MS instruments and demonstrate the versatility and robustness of the method and its applicability to instruments of different ion source design.