The use of matrix-precoated targets for the MALDI analysis of peptides and proteins has been investigated by several research groups. It is easy to realize the advantages of a sample preparation method reduced to the straightforward addition of a single drop of undiluted sample to a precoated target spot. Such a method would not only be faster and more sensitive than the ones described before, but it would also offer the opportunity to directly interface the MALDI sample preparation to the output of LC and CE columns.
An early effort by Kochling and Biemann [References, 15 ], described the use of a pneumatic sprayer to fast-evaporate a thin matrix-only layer on a MALDI target. The microcrystalline films were very stable and long- lived and provided adequate MALDI spectra for peptides and small proteins.
Most other efforts have focused on the development of thin layer matrix-precoated membranes. Particular attention has been dedicated to the choice of membrane material. Some of the options that have been tested (with varying results) include: nylon, PVDF, nitrocellulose, anion- and cation- modified cellulose and regenerated cellulose. Particularly encouraging results, in terms of sensitivity and quality of spectra, were obtained by Zhang and Caprioli for regenerated cellulose dialysis membrane [References, 27 ]. Their membrane precoating procedure provided results comparable to dried-droplet method for peptides and small proteins under 25 KDa. Heavier proteins (>25KDa) gave poorer results, presumably due to the limited amount of matrix available in the precoated membranes and or the inability to form protein doped microcrystals.
Preston, Murray and Russell observed [References, 30 ] that using nitrocellulose in a sample preparation for MALDI TOF MS of peptides can increase ion yields. Mass spectrometry and optical microscopy results suggest that the nitrocellulose addition modifies the crystallization of the matrix-analyte solution to allow more even coverage over the sample surface.
Hutchens and Yip developed a sample preparation technique they called Surface-Enhanced Neat Desorption (SEND) in which energy-absorbing-molecules were bound to substrates to provide chemically modified surfaces capable of desorbing “neat” analyte ions [References, 29 ]. The results were very encouraging, but the technique was never mainstreamed into the general MALDI methodology.