Fluorescence imaging of gas-phase molecules produced by matrix-assisted laser desorption

Abstract

In the analysis of large biomolecules, one important aspect is the determination of molecular weight. An excellent tool for determining molecular weights is mass spectrometry (MS).One requirement for MS analysis is that the sample either be itselfin the gas phase or can be converted to the gas phase. However, biomolecules, by nature large and typically containing both polar and nonpolar regions, are not easily converted to the vapor phase. When vaporization does occur, a common problem for molecular weight determination is that the biomolecules fall apart into fragments. Several approaches that have been effective in producing intact biomolecules in the gas phase1 include field desorption,Z 252-Cf plasma desorption,3 SIMS,4 thermo-5 and electrospray,6 and laser desorption (LD).'LD of intact molecules requires that the laser pulse be short. In other words, the heating rate of the sample must be rapid enough to favor vaporization over fragmentation.The energy from fast pulsed lasers has been particularly effective in this regard. However, even with the use of a 10-ns pulse from a frequency-doubled Nd-YAG laser, significant sample destruction has been observed.8 A recent innovation9 is matrix-assisted LD (MALD). This approach involves combining a large excess of a matrix material with the sample species. Typical matrices are fairly volatile or sublime easily and are chosen to have significant absorption at the irradiation wavelength. An overview of the technique1 has recently been published. Current understanding of the process10 is that direct absorption by the sample molecules is minimal-rather, they are swept along with the vaporized matrix material. It is also possible that expansive cooling by the matrix minimizes fragmentation." The detailed dynamics of the process are not well understood, however.One interesting feature of MALD is that efficient generation of parent ions depends critically on the laser energy and the matrix-to-sample ratios1 Conditions that fall outside a very (1) Hillenkamp, F.; Karaa, M.; Beavis, R. C.; Chait, B. T. Anal. Chem. (2) Beckey, H. D.narrow range have been found to be ineffective. A complication that exists in interpreting these MS results is that vaporization and ionization are not separately studied.Conditions that favor one process may not be identical to those required for the other. For example, fragmentation can occur as a result of additional excitation during the ionization step. There has been some indication that the bulk of the vaporized species is in the neutral form.12 If so, secondary ionization after vaporization, appropriately implemented, can conceivably increase the yield of the parent ions.Optical spectroscopic probes based on lasers have been successfully employed for monitoring laser-generated plumes previously, providing spatial and temporal information on atoms,13 diatomics,14 and large molecules.* These are based on absorption in the plume,8J3J4 reflection off the surface,lb and laser-enhanced ionization.16 The distinctive feature is that these probes can ...

Analysis of Carbendazim in Agricultural Samples by Laser Desorption and REMPI Time-of-Flight Mass Spectrometry

“…Laser desorption is a poweful technique to produce both ions and neutrals and has been already used to vaporize all kind of molecules. − Coupling this technique with REMPI-TOF-MS allows the analysis of thermally unstable compounds: the laser pulse communicates a high energy in a very short time (a few nanoseconds), and this favors the vaporization over the thermal fragmentation . After desorption, the molecule is cooled by expansion in the vacuum which again minimizes the fragmentation.…”

Wavelength- and Time-Resolved Luminescence Spectroscopy for Investigation of the Matrix-Assisted Laser Desorption Process

“…The fluorescence maximum is located at 20783 cm -1 (481. 16 Usually, the Stokes shift is defined as the energetic difference between the lowest absorption maximum and highest emission band. 31 The shape of both the absorption and the emission band are determined by Franck-Condon factors.…”

“…Mass spectrometric investigations have been carried out by a number of authors who focused on delayed extraction, 5,6 post-ionization, [7][8][9] photodissociation experiments 10 or laser fluence [11][12][13][14] and time-of-flight studies. 15,16 Spectroscopic investigations have dealt with studies on luminescence phenomena [17][18][19] of matrix samples during interaction with laser radiation and on laser-induced fluorescence (LIF)-imaging of the matrix plumes. 20,21 Some absorption and emission spectra in solution that are necessary for comparison are available from Allwood and co-workers.…”

Laser Desorption Dynamics of Carbendazim on a Glass Surface

“…For the latter, one of the most important applications is the analysis of labile molecules. In this case fast desorption dominates over thermal decomposition making the analysis possible as the fast laser heating of the surface enables the observation of the mass spectrum of the intact parent ion [25][26][27].…”