OMS Letters
Dear Sir
Are Proton Transfer Reactions of Excited States Involved in UV Laser Desorption Ionization?
Matrix-assisted-pulsed-UV laser desorption ionization (MALD) has gained considerable acceptance as an ionization method for large biomolecules, especially peptides and small proteins.' In addition, Cotter and co-workers have demonstrated that other classes of biomolecules can be successfully analyzed by MALD.' Recent studies have shown that MALD can be used to ionize compounds having molecular masses of several hundred thousand d a l t o n ~, ~ and because the ionization process is non-discriminatory MALD can be used for analysis of peptide/protein mixtures? Also, it has been suggested that MALD used in combination with tandem mass spectrometry may be suitable for structural characterization of trace levels (femtomole and sub-femtomole) of low moiecular mass (500-5000 daltons) pep tide^.^
The objective of this paper is to examine the types of compounds that are useful for MALD studies of peptides. The studies were performed on model peptides ranging in molecular mass from 1000-5000 daltons. Whenever we refer to MALD ionization it should be understood that under the experimental conditions employed ions of the analyte are not observed in the TOF mass spectrum unless a suitable matrix is added to the sample. The results presented here were obtained on a Kratos (A.E.I.) MS-902s mass spectrometer that has been converted to a time-of-flight instrument, i.e. the magnet was removed and a 1-metre flight tube was added to the back of the electrostatic energy analyzer.6 The ions were formed by a 3 nanosecond laser pulse from a Laser Science, Inc. (model VSL 337; 120 pJ peak power; 20 Hz max. repetition rate) pulsed N, laser. The laser beam is focused onto the sample probe by using a 15 cm confocal lens to a spot size of -0.1 mm diameter. The ions formed by MALD are accelerated to a kinetic energy of 6 keV by using a modified field-ionization/ field-desorption ion source. The ions are energy selected by the electrostatic energy analyzer and detected by a 16-stage electron multiplier assembly. The TOF electronics consist of a LeCroy (model TR8828D 200 MHz) transient recorder that is interfaced to an IBM PC/AT computer. The samples were prepared by dissolving the peptide (obtained from Sigma Chemical Co.) in methanol (-1 pg/pL). The sample and matrix (methanol solution of 10-70 pg/pL) were deposited onto the probe tip from solution and air dried to remove the solvent.
The typical matrix for MALD is an aromatic compound that contains a carboxylic acid functional group, e.g. nicotinic acid,la caffeic acid (4-hydroxycinnamic acid), ferulic acid (3- methoxy-4-hydroxycinnamic acid), sinapic acid (3,5dimethoxy-4-hydroxycinnamic acid),' etc. These matrices all