Oil-assisted sample preparation: a simple method for analysis of solid samples using matrix-assisted laser desorption/ionization mass spectrometry.

Common mass spectrometric techniques, e.g., electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI), require samples to be soluble in suitable solvents. Samples with solubility problems have difficulties for their mass spectrometric characterization. In this paper, an oil-assisted sample preparation (OASP) method was introduced for the analysis of solid samples using MALDI-MS. The novel method involves the use of a droplet of oil (i.e., paraffin oil) as the mixing and loading media for solid analyte and solid matrix. Using this method, rapid on-target sample preparation can be easily achieved, and only a transferable minimal amount of analyte and matrix is required. This method was demonstrated to be applicable for a wide range of analytes, including poorly soluble organic compounds, polymers, organometallic compounds, membrane peptides, and biological solid samples. The novel method can also be used for the analysis of "wet" and solution samples. The limit of detection of the OASP MALDI-MS was determined to be 1 ng with reserpine.