Direct introduction of biological samples into a LTQ-Orbitrap hybrid mass spectrometer as a tool for fast metabolome analysis.

We report the direct introduction of biological samples into a high-resolution mass spectrometer, the LTQ-Orbitrap, as a fast tool for metabolomic studies. A proof of concept study was performed on yeast cell extracts that were introduced into the mass spectrometer by using flow injection analysis, with an acquisition time of 3 min. Typical mass spectra contained a few thousand m/z signals, 400 of which were found to be analytically relevant (i.e., their intensity was 3-fold higher than that of the background noise and they occurred in at least 60% of the acquisition profiles under identical experimental conditions). The method was validated by studies of the matrix effect, linearity, and intra-assay precision. Accurate mass measurements in the Orbitrap discriminated between isobaric ions and also indicated the elemental composition of the ions of interest with mass errors below 5 ppm, for identification purposes. The proposed structures were then assessed by MSn experiments via the linear ion trap, together with accurate mass determination of the product ions in the Orbitrap analyzer. When applied to the study of cadmium toxicity, the method was as effective as that initially developed by using LC/ESI-MS/MS for a targeted approach. The same metabolic fingerprints were also subjected to multivariate statistical analyses. The results highlighted a reorganization of amino acid metabolism under cadmium conditions in order to increase the biosynthesis of glutathione.