Lipid profiling of meat samples using surface extraction followed by multiple reaction monitoring profiling

Direct sample extraction methods suitable for lipids such as desorption electrospray ionization (DESI), rapid evaporative ionization mass spectrometry (REIMS), and liquid extraction surface analysis (LESA) are combined with ionization or direct sample delivery to a mass spectrometer. Here, we describe three types of off-line direct sample extraction methods and their combination with multiple reaction monitoring (MRM) profiling to survey lipids present on the surface of beef and chicken meat as proof-of-concept samples. A collection of 679 MRMs selected from over 3200 MRMs related to lipids was profiled using a conventional capillary binary pump, autosampler and a triple quadrupole mass spectrometer system. The surface extraction experiments were performed manually by simply dispensing solvent onto the surface of the samples and recovering it immediately (liquid surface extraction - LSE), or by using solvent loaded into a large orifice tip to contact the sample for 5 seconds without dispensing (solvent "touch" extraction - STE). The third approach, surface vaporization extraction (SVE), was based on extracting vapors from the surface of the sample created using the tip of a wood-burning tool, collecting the vapors in a microtube, and diluting them for flow injection. LSE was used to evaluate the effect of different solvent compositions on the number of MRMs detected. Solvent acidification was also evaluated. Compared to the Bligh & Dyer extraction method using homogenized beef samples, LSE provided a consistently lower number of MRMs detected and the most informative solvent for surface extraction was pure acetonitrile. Since meat samples' surfaces are not completely flat and also present natural irregularities, the solvent volume recovered was variable. Therefore, we turned to STE to evaluate whether the lipid composition of beef brown spots was different from the red regions and to probe changes in composition due to one freeze-thaw cycle. Also, the reproducibility of STE between three users was evaluated. The third surface extraction approach, SVE, was applied to study the lipid profiles of vapors collected from raw and cooked meat. Overall, the three surface methods were able to provide informative discrimination for the different aspects assayed.