Nanophotonic Ionization for Ultratrace and Single-Cell Analysis by Mass Spectrometry

Recent mechanistic studies have indicated that at subwavelength post diameters and selected aspect ratios nanopost arrays (NAPA) exhibit ion yield resonances (Walker, B. N.; Stolee, J. A.; Pickel, D. L.; Retterer, S. T.; Vertes, A. J. Phys. Chem. C 2010, 114, 4835-4840). In this contribution we explore the analytical utility of these optimized structures as matrix-free platforms for laser desorption ionization mass spectrometry (LDI-MS). Using NAPA, we show that high ionization efficiencies enable the detection of ultratrace amounts of analytes (e.g., similar to 800 zmol of verapamil) with a dynamic range spanning up to 4 orders of magnitude. Due to the clean nanofabrication process and the lack of matrix material, minimal background interferences are present in the low-mass range. We demonstrate that LDI from NAPA ionizes a broad class of small molecules including pharmaceuticals, natural products, metabolites, and explosives. Quantitation of resveratrol in red wine samples shows that the analysis of targeted analytes in complex mixtures is feasible with minimal sample preparation using NAPA-based LDI. We also describe how multiple metabolite species can be directly detected in single yeast cells deposited on the NAPA chip. Twenty-four metabolites, or 4% of the yeast metabolome, were identified in the single-cell spectra.