Combining liquid chromatography with MALDI mass spectrometry using a heated droplet interface

A novel interfacing technology is described to combine solution-based separation techniques such as liquid chromatography (LC) with matrix-assisted laser desorption ionization (MALDI) mass spectrometry. The interface includes a transfer tube having an inlet and an outlet, the inlet being adapted to accept the LC effluents and the outlet being adapted to form continuously replaced, hanging droplets of the liquid stream, and a MALDI sample plate mounted below the outlet of the transfer tube for collecting the droplets. The liquid stream in the transfer tube is heated to a temperature sufficient to cause partial evaporation of the carrier solvent from the hanging droplets. The droplets are dislodged to the MALDI plate, which is heated to above the boiling point of the carrier solvent to cause further evaporation of the carrier solvent from the collected droplets. It is found that analytes can be fractionated and deposited to a sample spot of 0.8 mm in diameter when a liquid flow rate of up to 50 microL/min and a fractionation interval of 1 min/spot are used. Flow rate of up to 200 microL/min can be used with a deposition sample spot of 2.4 mm in diameter on a commercial MALDI target. This heated droplet interface does not introduce sample loss, and the detection sensitivity of LC/MALDI is similar to that of standard MALDI, i.e., low femtomoles for peptide analysis with a microliter sample deposition. It is compatible with microbore and narrow-bore column separation, thus allowing the injection of a larger amount of sample for separation and analysis, compared to a capillary column LC/MALDI system. The detection dynamic range is shown to be in the order of 10(6) for peptide mixture analysis, which is 4 orders of magnitude greater than standard MALDI. The application of this interface for combining LC with MALDI MS/MS is demonstrated in the proteome analysis of water-soluable protein components of E. coli K12 extracts.