Energetics of gramicidin S after UV laser desorption from a ferulic acid matrix

Abstract

Photoionization with coherent 118 nm vacuum ultraviolet light is applied to study kinetic energies of gramicidin S and ferulic acid molecules, laser desorbed from a ferulic acid matrix with 266 nm light. Analysis of the matrix photodecomposition products shows decarboxylation as a main photochemical pathway involved with desorption. Entrainment of the polypeptide by the matrix molecules is found under typical conditions. However, the velocity distributions are not particularly narrow, being at most borderline supersonic.Since the introduction' of matrix-assisted UV laser desorption in 1988 for desorbing and ionizing large organic molecules, the technique has been investigated by many research groups (for a review see Ref.2), but the desorption process itself and the role the matrix molecules play are not yet well understood. The first systematic experimental3 and theoretical4 studies describing dynamics of the desorption process were published only recently. Early data on the kinetic energy of laser desorbed tryptophan ions' and neutrals6 from pure trypotophan have also been available. However, little is known about the photochemical fate of the matrix molecules, which, in most cases to date, are relatively small aromatic organic acids that empirically have been found to be effective for producing mass spectra of polypeptides and proteins.', Their vaporization behaviour has been considered theoretically, but not the role their decomposition may play in de~orption.~ In this work, both the chemical identity of desorbed particles and their kinetic energies were obtained by photoionizing the desorbed neutrals above the surface (post-ionization) prior to time-of-flight (TOF) mass analysis. The kinetic energy is obtained from a separate time-of-flight measurement over the drift region between the surface and the ionizing laser beam; the time distribution is obtained by varying the time delay between desorption and ionizing laser pulses; velocity and kinetic energies are obtained by transforming the arrival time distribution. Single-photon ionization with 118 nm vacuum ultraviolet (VUV) light generally causes minimal fragmentation and is nonselective in the photoionization of organic molecules.' Thus we chose this technique to gain data on the kinetics of a polypeptide desorbing from ferulic acid as a matrix, irradiated with 266 nm light. EXPERIMENTALThe desorption is caused by illuminating the sample with 5 ns pulses of quadrupled Nd:YAG radiation at 266nm using a model GCR-3 laser (Spectra Physics) which was focused with a 300 mm focal-length quartz lens to a spot size of approximately 200 pm diameter, a value derived from burn marks on heat-sensitive paper.Author to whom correspondence should be addressed By angle tuning the doubling crystals and by inserting a stack of quartz glass windows into the beam, the energy per pulse delivered to the sample could be varied between 5 and 200 115.Photoionization is caused by 118 nm, 10.5 eV photons, which are the ninth harmonic of an additional Nd:YAG laser (Quanta-Ray, Model ...

Measurements of mean initial velocities of analyte and matrix ions in infrared matrix-assisted laser desorption ionization mass spectrometry

“…Here, usually the delay time between desorption and postionization laser pulses is varied and velocities are determined from the ion intensity versus delay time distribution [8,21]. Finally, a laser induced fluorescence (LIF) technique has been employed to characterize UV MALDI plume dynamics [27].…”

Coupled Space- and Velocity-Focusing in Time-of-Flight Mass Spectrometry—a Comprehensive Theoretical Investigation

“…Therefore, the optimal s 1 value for heavy ions is inapplicable to light ions because, as reported in the literature, the desorbed materials from the MALDI reaction have roughly equal values for υ [17,22,23]. In such a case, ions that have different masses but are traveling at the same initial velocity result in different E 0 (i.e., higher mass ions exhibit higher E 0 ).…”

“…Assuming the ions have an initial temperature of 1700 K [12][13][14][15][16][17], the ion population with a certain average velocity in one dimensional space can be described by the Maxwell-Boltzmann distribution [16][17][18][19][20][21] as follows:…”

“…Table 1 shows the calculation results obtained for protonated 2,5-dihydroxybenzoic acid (DHB) with a m/z of 155 and a temperature of 1700 K. To mimic the conventional space-focusing condition, Equation 1 is used to obtain an s 0 of 13.5 mm. Notably, the selected temperature is only a rough estimate and must be adjusted according to the experimental conditions [12][13][14][15][16][17].…”

Mechanism of insulin emission in matrix assisted laser ionization

“…Numerous experiments on the velocity measurements of laser desorbed ions and neutrals and theoretical calculations have been reported [3][4][5][6][7][8][9]. Average initial velocities of sample ions vary between 300 and 1000 m/s and between 300 and 1700 m/s for matrix ions.…”