Study of operational parameters on the desorption/ionization and fragmentation of benzylpyridinium ions using gold-capped silicon nanopillars in SALDI-MS

Introduction

The use of nanostructured substrates as assisting materials in surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) has gained increasing attention, especially for analyzing small molecules. Indeed, nanosubstrates usually led to an increased signal reproducibility and a cleaner chemical background in the low m/z range compared to MALDI matrices. However, the understanding of the SALDI process remains incomplete. The impact of the operational parameters on the analyte desorption/ionization (D/I) and fragmentation is of crucial importance in fundamental research and analytical applications. In this study, the survival yield (SY) and D/I of benzylpyridinium (BP) “thermometer” ions were determined during SALDI-MS experiments using a novel nanosubstrate composed of gold-capped silicon nanopillars as a function of several operational parameters.

Method

Mass spectra of benzylpyridinium ions (p-CH3-BP, 10-4 M in water/acetonitrile 1:1) were acquired between m/z 40 and 260 in reflectron positive ionization mode using a Bruker RapifleX mass spectrometer equipped with a 355-nm SmartBeam3D laser. The influence of several parameters, such as the laser power, footprint and pulse frequency, the number of laser shots, and the duration of the pulsed delay extraction was evaluated. The desorption/ionization (D/I) and survival yields (SY) were calculated for all mass spectra. The D/I is the summation of the intensity of the parent and fragment BP ions.

D/I=I_Parent+I_Fragment

The SY were determined from the relative intensities of the parent and fragment ions.

SY=I_Parent/(I_Parent+I_Fragment )

Preliminary data

Gold-capped silicon nanopillar arrays, which are originally designed for Surface-Enhanced Raman Spectroscopy, were tested in SALDI-MS in the perspective of further multimodal analyses. The D/I and SY of BP thermometer ions, and the influence of the operational parameters were studied using these nanosubstrates as assisting materials. In addition to the laser power whose impact has been widely studied using different types of substrates, several other parameters have been tested here: the laser footprint, the laser pulse frequency, the number of laser shots, and the duration of the pulsed delay extraction. Our study suggests that the laser footprint and the pulsed delay extraction have a significant impact both on the D/I and the SY of BP ions, while the number of accumulated laser shots only has an impact on the D/I. On the contrary, the laser shot frequency does not seem to have an impact neither on the D/I nor on the SY. The integrity of the nanosubstrate after the SALDI-MS analyses was also investigated by scanning electron microscopy. This study enables to better understand the fundamental processes underlying SALDI, and to determine the optimal conditions for efficient desorption/ionization of either the BP parent or the fragment by controlling the fragmentation and the nanosubstrate destruction. Indeed, the degradation of the nanosubstrate leads to the formation of interferent ions in the low m/z range. Besides this physicochemical part of the study, the analytical performance of the nanosubstrates was also evaluated. We demonstrated that under optimal conditions no interferent ion was produced alongside the analyte ions in the studied m/z range. On the contrary, the mass spectra acquired with MALDI matrices and gold nanoparticles (AuNPs) were characterized by more background peaks. The nanosubstrates also offer better reproducibility of the signal intensity compared to CHCA and AuNPs, which opens the way for quantification studies.

Novel aspects

Evaluation of the analytical performance of a SERS nanosubstrate in SALDI-MS and study of the influence of the operational parameters.