Picoliter sample preparation in MALDI-TOF MS using a micromachined silicon flow-through dispenser

This paper presents a picoliter sample preparation technique utilizing the flow-through principle, allowing on-line coupling of chromatographic systems to be made. The work was performed in order to investigate the characteristics and the physicochemical properties of the sample preparation using typical mobile phase conditions from μ-CLC (column liquid chromatography) separations. The device presented here is a pressure pulse- driven dispenser, formed by two silicon structures processed by conventional micromachining. The pressure pulse is generated in the flow-through channel by a piezoceramic element. Depending on the orifice size, the droplets ejected range between 30 and 200 pL. The maximum ejection frequency is 500 Hz, limited by resonances within the unit. A pyramid-shaped nozzle improves the directivity of the droplets since it reduces the wetting of the orifice front surface area. The risk of particles sticking close to the orifice is also minimized. The analyses of the deposited sample spots were carried out on a matrix-assisted laser desorption/ionization time-of-flight mass spectrometer with delayed extraction. It was possible to detect attomole amounts (159-248 amol) of various proteins (cytochrome c, ribonuclease A, lysozyme, and myoglobin) from a single droplet of matrix:analyte 1:1 (drop volume ≃ 110 pL). Additionally, it was found that sample enrichment could be carried out using multiple depositions on the same spot; i.e., 31 nM of insulin was easily detected when more than four depositions were made on the same spot, while no detection was possible without sample enrichment. Size optimization of the MALDI sample spot gave target zones of 100-500-μm diameter that matched the size of the laser focal point and resulted in a considerably increased sample throughput.