Requirements for laser-induced desorption/ionization on submicrometer structures.

Laser-induced and matrix-free desorption/ionization on various submicrometer structures was investigated. First, to examine the effect of surface roughness on ionization, a silicon wafer or stainless steel was scratched with sandpaper. The fluences of a 337-nm nitrogen laser, required for ionization of synthetic polymers and reserpine, were markedly reduced on the scratched stainless steel or silicon as compared to the corresponding untreated surface. Next, arrays of submicrometer grooves, which had been lithographically fabricated on a silicon wafer, yielded protonated angiotensin, and the morphologic orientation demonstrated the positive relation between the laser and groove directions for promoting ionization. The fabricated structure also suggested the submicrometer, but not smaller, or nanometer, structures to be a key factor in direct desorption/ionization on rough surfaces. Finally, submicrometer porous structures of alumina or polyethylene yielded intense molecular ion signals of angiotensin and insulin, in response to direct UV irradiation, when the surface was coated with Au or Pt. The coating provided the additional advantage of prolonged activity for a porous alumina chip, exceeding a month even when the chip was left in the open air. These results indicate that laser-induced desorption/ionization of organic compounds can be implemented on submicrometer structures with an Au- or Pt-coated surface irrespective of the basal materials.