Optimisation of the quantitative determination of chlormequat by matrix-assisted laser desorption/ionisation mass...

The plant growth regulator chlormequat, an involatile quaternary ammonium salt, has been quantified by matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOFMS). Restrictions for quantitative MALDI-TOFMS analysis, such as irreproducible crystallisation and unsatisfactory laser stability, have been overcome by the application of two synthesised isotopically labelled standards and the optimisation of the measurement protocol. Data acquisition at constant laser power was compared to data acquisition at approximately constant ion abundance of the relevant ions (analyte and internal standards). Data acquisition at constant ion abundance performed better and enabled a high number of consecutive firings to the same sample deposition area. Furthermore an increased sample-to-sample repeatability and a high reproducibility over several weeks without re-calibration have been attained by this method. Linearity over three orders of magnitude (0.05 to 30 ng/microL chlormequat), with a correlation coefficient of 0.9997, was achieved using [13C3]-chlormequat as internal standard. Limit of detection and limit of determination were determined to be in the low pg/microL range for pure standard solutions. Thin-layer chromatography was applied for the removal of high amounts of choline, which is often present in plant tissue extracts and can adversely affect the ionisation and detection of chlormequat by MALDI-TOFMS. The use of two internal standards ([13C3]- and [2H9]-chlormequat) enabled direct quantification and simultaneous control of the recovery.