Coupling of single droplet micro-extraction with desorption electrospray ionization-mass spectrometry

Single droplet micro-extraction (SDME) is a powerful preconcentration and purification technique for trace chemical analysis, but the detection of the resulting single droplet extract is often time-consuming with traditional detection methods such as GC/MS or LC/MS. In this study, desorption electrospray ionization-mass spectrometry (DESI-MS) was coupled with SDME to serve as a fast detection method for the first time, demonstrated by the trace analysis of methamphetamine (MA) in aqueous solution and the detection of an organic reaction product from an ionic liquid (IL). In the former application, three-phase liquid SDME was conducted to enrich MA in aqueous solution to an organic solvent and then to back-extract the analyte to a single aqueous droplet. Subsequent DESI-MS analysis can be performed either to a single droplet or multiple droplets in a row. The average enrichment factor obtained for MA was 390-fold. In the latter case, two-phase liquid SDME was conducted to directly extract the product of an organic reaction performed in a room temperature ionic liquid (i.e., the nucleophilic addition of aniline to phenyl isothiocyanate to form N,N′-diphenylthiourea in 1-butyl-3-methyl- imidazolium tetrafluoroborate). The ionization of the resulting droplet extract by DESI allows one to directly examine the reaction product without interference from the ionic liquid. Such an analysis uses a few microliters of an organic solvent, achieving green chemistry. This combined SDME/DESI-MS method is characterized with high preconcentration efficiency, high throughput capability and minimizing organic solvent waste.