Metal Probe Microextraction Coupled to Dielectric Barrier Discharge Ionization–Mass Spectrometry for Detecting Drug Residues in Organisms

In this study, a novel method for the direct coupling of metal probe microextraction (MPME) and a dielectric barrier discharge ionization (DBDI) source with mass spectrometry (MS) is reported. Analytes adsorbed on a tungsten needle were directly transferred to the DBDI source via rapid thermal desorption, which resulted in a limit of detection as low as 8 pg/mL. This is in part due to the “active capillary” configuration of the plasma ion source, where the efficiency of ion transfer to the MS is ∼100%. Specialty gases to maintain the plasma and carry analytes to the MS are not required. In contrast to direct one-step ionization of molecular adsorbates, the complete separation of the analyte desorption from the probe and the ionization event in our experimental setup greatly enhanced the sensitivity and detection reproducibility (RSD of 8.3%). We show detection of pyrimethamine, a first-line drug for the treatment and prevention of Plasmodium falciparum malaria all over the world, by this MPME/DBDI/MS method. The detection of drug residues in live fish and paramecium was achieved without the need for any sample pretreatment. The relative concentration of the drug in different organs of the fish was determined. This simple and convenient method has the potential for the analysis of chemicals even in single-cell organisms.