Laser-Induced Thermal Desorption Facilitates Postsource Decay of Peptide Ions

Abstract

We investigated the thermal mechanism involved in laser desorption/ionization (LDI) of thermally labile molecules from the flat surfaces of amorphous Si (a-Si) and crystalline Si (c-Si). a-Si was selected for this study because of its thermal property, such as low thermal conductivity; thus, it was predicted to be highly susceptible to laser-induced surface heating. By virtue of lack of surface nanostructures, the flat surfaces offer a simple model system to focus on the thermal mechanism, avoiding other effects, including possible non-thermal contributions that can arise from the physical existence of surface nanostructures. For the energetics study, the internal energies of substituted benzylpyridinium ions produced by LDI on the bare and coated surfaces of a-Si and c-Si were obtained using the survival yield method. The results, including LDI thresholds, ion yields, and internal energies all suggested that the LDI mechanism would be indeed thermal, which is most likely promoted by thermal desorption caused by laser-induced surface heating. In addition, the LDI process driven by laser-induced thermal desorption (LITD) was also found to be capable of depositing an excessive internal energy in resulting LDI ions, which underwent a dissociation. It exhibited the essentially same features as in postsource decay (PSD) in MALDI-TOF/TOF mass spectrometry. We report that the LDI process by LITD offers not only a way of intact ionization but also a facile means for PSD of peptide ions, which this work demonstrates is well suited to peptide sequencing using TOF/TOF mass spectrometry.

Commercial Silicon-on-Insulator (SOI) Wafers as a Versatile Substrate for Laser Desorption/Ionization Mass Spectrometry

“…Such rapid surface heating is known to promote the intact desorption of thermally labile molecules from the early field desorption mass spectrometry [12]. Very recently, the thermal aspect involved in the LDI process has been further supported by pronounced LDI of peptides from the flat surfaces of a-Si and WSi x [7,8]. The thermal properties of a-Si and WSi x , such as a very low thermal conductivity (K) and a very high heat capacity (C p ), respectively, make the surfaces to be amenable to laser-induced surface heating, which may replace the needs of the presence of surface nanostructures for LDI of thermally labile molecules from the surfaces.…”

“…; t is the laser pulse width) of the layer material [7]. In order to verify this postulation, LDI on SOI wafers was explored.…”

“…For example, it was recently found that the flat surfaces of amorphous Si (a-Si) [7] and tungsten silicide (WSi x ) [8] yielded pronounced LDI of peptide molecules without assistance of surface nanostructures. As LDI on flat surfaces avoids the utilization of nanostructures that necessitates well-controlled surface fabrication, the use of flat surfaces makes it simpler, easier, and more reproducible to prepare uniform target surfaces.…”

Rapid Surface Heating Promotes Laser Desorption Ionization of Thermally Labile Molecules from Surfaces

“…It should be noted that LDI from flat c-Si surfaces has rarely been reported. A simulation with a heat diffusion equation 25,26 predicted that, upon irradiation of laser pulses with a pulse width of 7 nsec and laser power of 6 MW/cm 2 , the surface temperature of a-Si can increase to peak temperature as high as 1400 K at around 9 nsec, while the surface temperature of c-Si can only reach 300 K. The theoretical estimates show that the very low thermal conductivity of a-Si can result in a rapid increase of the surface temperature upon pulsed laser shining, and thus may promote LDI of thermally labile molecules from the surfaces.…”

Observation of Peptide-Ion Generation by Laser-Induced Surface Heating from Tungsten Silicide Surfaces

“…surfaces without nanostructures, which include amorphous Si (a-Si) and commercial Al foils. 11,12 Obviously, flat surfaces for LDI have certain advantages. As it does not require fabrication of complex nanostructures on surfaces, it is easier to prepare in a large scale and even often commercilly available.…”