Evaluation of a method based on liquid chromatography electrospray mass spectrometry for analyzing carbamate insecticides in fruits and vegetables

The feasibility of using reversed-phase liquid chromatography/mass spectrometry (LC/MS) with an electrospray (ES) interface for measuring traces of N-methylcarbamate insecticides in 10 different types of fruits and vegetables Was evaluated, Twelve carbamates added to vegetable materials were extracted with methanol by the aid of a homogenizer. After filtration, an aliquot of the homogenizate equivalent to 5 g of the vegetable material was suitably diluted with water and passed through a 1-g Carbograph 1 extraction cartridge. Carbamates were eluted by passing through the cartridge 6 mL of a CH2Cl2/CH3OH (80:20 v/v) mixture, After eluate concentration down to 100 mu L, 5 mu L of the final extact was injected into the LC column. Recovery of the analytes was better than 80%, irrespective of the type of vegetable matrix to which the analytes were added. Replacement of CH3OH with CH3CN as organic modifier resulted in a significant decrease of the ion signal for carbamates. The same occurred by adding moderate amounts of HCOOH or NaCl to the LC mobile phase, Varying the skimmer cone voltage produced characteristic fragment ions without significant sensitivity loss. The presence in the electrosprayed solution of a lot of vegetable constituent did not interfere significantly with the ionization process of carbamates. The analysis of a tomato extract spiked with carbamates at the individual level of 5 ng/g of vegetable performed by selected ion monitoring showed that the limit of detection for these analytes could be set at a few hundreds of picograms per gram of vegetable or fruit. Over 1 working day of heavy use of the ES/MS intrumentation, the ion signal intensities for carbamates were found to be unaffected by the rats of contamination of the ES source by vegetable constituents.