Coupling LC with MALDI-MS

Several attempts have been made to combine LC online with MALDI [53]. In one approach, a continuous-flow probe, similar to that used for CF-FAB, has shown success in introducing LC effluents directly into the MALDI source [54,55]. The matrix solution, composed of 3-nitrobenzoyl alcohol, 0.1% TFA, 1-propanol, and ethylene glycol (3 3 5 9 by volume), is mixed postcolumn with the LC flow. A UV laser beam is focused onto the tip of the probe to ionize the emerging LC effluent. 

A pulsed sample introduction (PSI) interface and laser-induced multiphoton ionization have been also used to couple LC with TOF instruments [56]. The interface consists of a heated capillary for aerosol generation and a high-temperature pulsed nozzle for sample vaporization. The LC effluent enters the heated capillary at a flow rate between 0.5 and 1.6 mLmin A solenoid allows the sample vapor to enter the mass spectrometer in a pulse form. 

Another design is based on the concept of aerosol formation [53]. In this interface, LC effluent premixed with a matrix solution flows through a stainless-steel tube into a pneumatic nebulizer and is sprayed directly into the ion source of a TOP mass spectrometer. Following the removal of the solvent, the spray is bombarded with a beam of photons from a UV laser. 

A rotating-wheel interface has been devised to couple LC and CE with MS [57]. The LC effluent premixed with the MALDI matrix flows through a fused-silica capillary at the rate of 100 to 400 nLmin . The tip of the separation capillary is in contact with the rotating wheel. The solution deposited is dried rapidly in vacuum of the MALDI source, leaving a narrow trace of cocrystallized analyte-matrix mixture. The wheel is transported to the repeller, where laser desorption-ionization of the analyte-matrix mixture occurs. A similar system that uses a rotating ball coated with matrix for deposition of a single drop is also available for online LC/MALDI-MS [58]. The droplet deposition is controlled by a piezoelectric-actuated droplet generator. 

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