Laser desorption/ionization mass spectrometry on porous silica and alumina for peptide mass fingerprinting

Abstract

We investigated a variant of desorption/ionization on porous silicon (DIOS) mass spectrometry utilizing an aqueous suspension of either porous silica gel or porous alumina (pore size of 60 and 90 Å, respectively). Laser desorption/ionization (LDI) from samples directly deposited on a stainless steel surface without any inorganic substrates was also achieved. Synthetic peptides designed to cover large sequence diversity constituted our model compounds. Sample preparation, including material conditioning, peptide solubilization, and deposition protocol onto standard matrix-assisted laser desorption/ionization (MALDI) probe, as well as ionization source tuning were optimized to perform sensitive reproducible LDI analyses. The addition of either a cationizing agent or an alkali metal scavenger to the sample suspension allowed modification of the ionization output. Comparing hydrophilic silica gel to hydrophobic reversed-phase silica gel as well as increasing material pore size provided further insights into desorption/ionization processes. Furthermore, mixtures of peptides were analyzed to probe the spectral suppression phenomenon when no interfering organic matrix was present. The results gathered from synthetic peptide cocktails indicated that LDI mass spectrometry on silica gel or alumina constitutes a promising complementary method to MALDI in proteomics for peptide mass fingerprinting. [19]. These LDI methods, sometimes referred to as "matrix-free", produce mass spectra that exhibit less matrix ions in the low mass range compared with the chemical noise generated by the organic matrices used in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. Such chemical noise that usually pollutes the low mass range of MALDI mass spectra and that may complicate the detection of small molecules is less pronounced in the DIOS mass spectra.Nevertheless, the drawback of such an approach is in the preparation of the silicon material. To present adequate physical properties [20], silicon surfaces are conditioned via a galvanostatic etching procedure from Si wafer to produce pores with nanometer diameters. Commercially available ready-to-use DIOS-chips can also be mounted on modified MALDI plates [20]. Otherwise, silicon powders (5 to 50 nm particle size) were investigated as a substitute to DIOS chips to make the overall sample preparation simpler and safer [18]. Provided that the appropriate powder preparation was performed (including particle etching by HF, oxidation by HNO 3 , and derivatization of the generated SiOH groups), such silicon nanopowder method was found to exhibit the same characteristics as DIOS mass spectrometry.Porous silicon is a material that presents (1) a large active surface and (2) a strong absorbance in the UV range. Such a form of silicon was thus found particularly suitable for LDI mass spectrometry with irradiation at 337 nm, the pore size and the surface chemical nature being the most important parameters as stated by Siuzdak and coworkers in their early publications on DIOS [20...

Sample preparation for mass spectrometry imaging: Small mistakes can lead to big consequences

“…It is worth noting that the selection of DESI solvent could be considered as similar to the selection of MALDI matrix as manipulation of the chemistry accompanying ambient ionization can be used to optimize chemical analysis and a recent review of the subject has been published [12]. A comprehensive review of matrixfree methods [104] describes methods such as desorptionionization on silicon (DIOS) which is as a matrix-free method for small molecule analysis [75,105], and has also been show to improve peptide coverage in PMF MALDI applications [106]. This has also been proven suitable for MSI, through the transfer of target analytes to the DIOS surface through contact with the tissue of interest [75].…”

Laser desorption ionization mass spectrometry of protein tryptic digests on nanostructured silicon plates

“…In the search for an alternative method to conventional MALDI-MS peptide mass fingerprinting, various 'matrix-free' LDI analyses have been undertaken in the laboratory for peptide detection starting with commercially available inert materials, which were compared to the original method of Siuzdak on porous etched silicon (DIOS-chips). Porous chromatography materials (silica gel, reversed-phase silica gel and alumina, pore size of 60 and 90 Å), carbon powder (scratched pencil lead), silicon-based chips (NALDI™ target) [26,27] and more recently diamond nanowires have been investigated [28]. Among all inert studied substrates, silicon-based nanostructured materials provided the most sensitive peptide detection and were thus selected for further investigations.…”

Nanomaterial-Assisted Laser Desorption Ionization for Mass Spectrometry-Based Biomedical Analysis

“…Other inorganic materials are also used to assist LDI, such as graphite [33], carbon nanomaterials [34][35][36], nanodiamonds [37], porous alumina [38], nano-ZnO [39,40], EuF 3 hollow hexagonal nanodisks [41], silver nanoparticles [42], HgTe nanostructures [43], platinum nanoflowers [44], manganese oxide nanoparticles [45], Fe 3 O 4 particles [46], fullerene-derivatized silica [47] and germanium nanodots [48], among others. Recently, we have shown that quantum dots (QDs) of cadmium selenide and cadmium selenide covered with cadmium sulfide can also be used to assist LDI [49].…”