Determination of acidic herbicides using liquid chromatography with pneumatically assisted electrospray ionization mass spectrometric and tandem mass spectrometric detection.

Liquid chromatography-pneumatically assisted electrospray mass spectrometry with negative ionization has been used for the determination of acidic herbicides in ground water. Eighteen pesticides or pesticide degradation products belonging to several different groups of acidic herbicides (phenoxy acids, sulfonylureas, phenols, etc.) were covered in the study. Optimization of electrospray inlet conditions is described as well as results from investigations of the linearity of the detector response. Conditions for tandem mass spectrometry (MS-MS) detection of characteristic daughter ions formed by collision-induced dissociation (CID) of the parent ion are described and a comparison of obtainable instrument detection limits by single MS and MS-MS was made. Detection limits using MS in the selected ion monitoring (SIM) mode were generally in the order of 1 microgram/l or below, whereas detection limits were three-four times higher using MS-MS detection. A principle of analysis is proposed based on single quadrupole MS as a method for quantitative determination followed by verification of positive findings by CID MS-MS. Application of the method for detecting acidic herbicides residues in a "real-world" ground water sample is demonstrated.