Article pubs.acs.org/ac
Silver Coating for High-Mass-Accuracy Imaging Mass Spectrometry of Fingerprints on Nanostructured Silicon Taryn M. Guinan,⊥,† Ove J. R. Gustafsson,*,⊥,†,‡ Gordon McPhee,§ Hilton Kobus,∥ and Nicolas H. Voelcker*,†,‡ †
Mawson Institute and ‡ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, University of South Australia, General Post Office Box 2471, Adelaide, South Australia 5001, Australia § Nextcell Pty Ltd, Cooperative Research Centre for Cell Therapy Manufacturing, University of South Australia, Mawson Lakes, South Australia 5095, Australia ∥ School of Chemical and Physical Sciences, Flinders University, General Post Office Box 2100, Adelaide, South Australia 5001, Australia S Supporting Information *
ABSTRACT: Nanostructure imaging mass spectrometry (NIMS) using porous silicon (pSi) is a key technique for molecular imaging of exogenous and endogenous low molecular weight compounds from fingerprints. However, high-mass-accuracy NIMS can be difficult to achieve as time-of-flight (ToF) mass analyzers, which dominate the field, cannot sufficiently compensate for shifts in measured m/z values. Here, we show internal recalibration using a thin layer of silver (Ag) sputter-coated onto functionalized pSi substrates. NIMS peaks for several previously reported fingerprint components were selected and mass accuracy was compared to theoretical values. Mass accuracy was improved by more than an order of magnitude in several cases. This straightforward method should form part of the standard guidelines for NIMS studies for spatial characterization of small molecules.
use delayed extraction and reflector fields to achieve resolutions in the range 10−50 000 and mass accuracies in the 1−5 ppm range for measurements on solid sample supports. However, ToF systems cannot compensate for variations in sample height (distance from sample surface to detector), and as such, mass accuracy is dependent on both uniform sample height and the morphology of the overlaid matrix preparation, in addition to factors that include stochastic peak formation and surface charging during desorption.13 Imaging mass spectrometry (IMS) presents a unique challenge, with respect to sample height heterogeneity, in that the analyzed sample (tissue section, deposited fingerprint) introduces (i) a systematic height change at the sample−surface interface and (ii) variable intrasample height.13 Recently, Gustafsson et al.13 demonstrated the use of internal calibrants for tryptic peptide MALDI imaging on ovarian cancer tissue sections. Via this approach, m/z accuracy could be increased by more than 1 order of magnitude to less than 20 ppm. Subsequently, peaks tentatively identified from calibrated imaging spectra were definitively matched to peptide sequences by a combination of in situ tandem mass spectrometry (MS/MS) and liquid chromatography tandem mass spectrometry (LC/MS/MS).13
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atrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) is a widely used analytical tool for peptide and protein analysis. MALDI requires the cocrystallization of matrix and analyte on a conductive target. In this system, the matrix is the primary energy absorber and facilitates soft ionization of large biomolecules.1 However, this approach is not ideal for the analysis of small molecules since the matrix introduces numerous peaks in the low mass range (
