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

The plant growth regulator chlormequat, an involatile quaternary ammonium s alt, has been quantified by matrix-assisted laser desorption/ionisation tim e-of-flight mass spectrometry (MALDI-TOFMS). Restrictions for quantitative MALDI-TOFMS analysis, such as irreproducible crystallisation and unsatisfac tory laser stability, have been overcome by the application of two synthesi sed isotopically labelled standards and the optimisation of the measurement protocol. Data acquisition at constant laser power was compared to data ac quisition at approximately constant ion abundance of the relevant ions (ana lyte and internal standards). Data acquisition at constant ion abundance pe rformed better and enabled a high number of consecutive firings to the same sample deposition area. Furthermore an increased sample-to-sample repeatab ility 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/muL chlormequat), with a correlation coefficient of 0.9997, was achieved using [C-13(3)]-chlormequat as internal standard. Limit of de tection and limit of determination were determined to be in the low pg/muL range for pure standard solutions. Thin-layer chromatography was applied fo r the removal of high amounts of choline, which is often present in plant t issue extracts and can adversely affect the ionisation and detection of chl ormequat by MALDI-TOFMS. The use of two internal standards ([C-13(3)]- and [H-2(9)]-chlormequat) enabled direct quantification and simultaneous contro l of the recovery. Copyright (C) 2001 John Wiley & Sons, Ltd.