Analysis of small organics on planar silica surfaces using surface-assisted laser desorption/ionization mass spectrometry

This study demonstrates the feasibility of directly obtaining analyte signals from a planar silica gel surface by using laser desorption mass spectrometry (LDMS). The surface of the planar silica gel was pretreated using a line drawn by a soft carbon pencil, and 0.3 μL of a liquid matrix (15% sucrose/glycerol-methanol 1:1, v/v) was applied to it (width ∼1mm) before sample deposition. The laser was then used to directly irradiate the sample spot on the pencil line (∼2 mm). Carbon powder ablated from the pencil residue is used to couple the laser UV energy into a liquid solution in LDMS. This sample preparation method is related to surface-assisted laser desorption/ionization (SALDI) mass spectrometry, which uses a suspension of μm-sized carbon powder (SALDI solid) mixed with a viscous liquid (SALDI matrix) as the matrix. In the present approach, the ablated carbon powder from the pencil drawing on a planar silica gel surface was used as an alternative to the SALDI solid. The liquid matrix was that used in conventional SALDI matrix systems, and the sample preparation applied herein is simpler than conventional SALDI. The matrix background remains quite low in this approach, which significantly assists the analysis of small organics such as methylephedrine and cytosine. PEG (polyethylene glycol) 400 and PEG 2000 were also used for demonstration purposes.