Capillary liquid chromatography with off-line mid-IR and Raman micro-spectroscopic detection: analysis of chlorinated pesticides at ppb levels

A flow-through microdispenser was used as a solvent elimination interface, allowing vibrational spectroscopic detection in capillary liquid chromatography in addition to standard UV detection. Using a flow-through microdispenser, robust and stable deposition of picoliter-sized droplets on a CaF2 plate window was achieved. The CaF2 window was placed on a thermostated sample holder (80 degrees C) mounted on a computerized x, y stage for achieving fast solvent evaporation and enabling recording of the chromatogram as a trace of deposited material. The dried residues that were formed had diameters of a few tens of micrometers and were analysed by mid-IR and Raman micro-spectroscopy. Conditions were optimized for high sensitivity of measurement and maintaining chromatographic resolution during the deposition step. Due to the destruction-free character of Raman and FTIR spectroscopy, these techniques could be applied sequentially to interrogate the same deposits. To test the usefulness of the methodology for environmental analysis, the determination and unambiguous identification of chlorinated pesticides (chlortoluron, diuron, atrazine, and terbuthylazine) in river water was used as an example, obtaining limits of identification of 2 ng analyte on-column and precision of approximately 10% RSD. The application of the developed method to spiked real river samples demonstrated the identification power of the proposed method as, in addition to the four previously studied pesticides, two additional pesticides (simazine and isoproturon) could also be detected and identified.