Substrate-assisted laser desorption of neutral peptide molecules.

The ultraviolet (248-nm) laser desorption of neutral peptide molecules is found to be greatly enhanced by applying a thin layer of the sample (500 monolayers) on top of an ultraviolet-absorbing organic substrate, sinapinic acid. With this sample preparation, peptides as large as gramicidin S are desorbed as intact neutral molecules. The samples are examined with laser desorption/chemical ionization (LD/CI) Fourier transform mass spectrometry. The neutrals desorbed by this method have approximately 1 eV less internal energy than those desorbed directly from a metal film surface. The organic substrate aids the desorption of neutrals when the laser wavelength is not strongly absorbed by the peptide sample (248 nm), but is not effective in aiding the desorption of neutrals when the laser wavelength is strongly absorbed by the sample (193 nm).