Direct introduction MALDI FTICR MS based on dried droplet deposition applied to non-targeted metabolomics on Pisum Sativum root exudates

Non-targeted metabolomic approaches based on direct introduction (DI) through a soft ionization source are nowadays used for large-scale analysis and wide cover-up of metabolites in complex matrices. When coupled with ultra-high-resolution Fourier-Transform ion cyclotron resonance (FTICR MS), DI is generally performed through electrospray (ESI), which, despite the great analytical throughput, can suffer of matrix effects due to residual salts or charge competitors. In alternative, matrix assisted laser desorption ionization (MALDI) coupled with FTICR MS offers relatively high salt tolerance but it is mainly used for imaging of small molecule within biological tissues. In this study, we report a systematic evaluation on the performance of direct introduction ESI and MALDI coupled with FTICR MS applied to the analysis of root exudates (RE), a complex mixture of metabolites released from plant root tips and containing a relatively high salt concentration. Classic dried droplet deposition followed by screening of best matrices and ratio allowed the selection of high ranked conditions for non-targeted metabolomics on RE. Optimization of MALDI parameters led to improved reproducibility and precision. A RE desalted sample was used for comparison on ionization efficiency of the two sources and ion enhancement at high salinity was highlighted in MALDI by spiking desalted solution with inorganic salts. Application of a true lyophilized RE sample exhibited the complementarity of the two sources and the ability of MALDI in the detection of undisclosed metabolites suffering of matrix effects in ESI mode. [Display omitted] •Classic non-targeted metabolomics on Pisum sativum root exudates treated with SPE and lyophilization obtained via DDA-MS and direct introduction ESI-FTICR MS gave a first metabolite description.•Direct introduction MALDI-FTICR MS under optimized conditions of selective accumulation and best sample:matrix ratio for different matrices enabled improved precision and high metabolite coverage, increasing the number of detected unique features compared to ESI-FTICR MS.•Investigation of matrix effects under increased salt concentration showed the presence of ion suppression in ESI-FTICR MS and ion enhancement in MALDI-FTICR MS.•Application of the optimized MALDI-FTICR MS based on dried droplet deposition on lyophilized root exudate samples highlighted high complementarity with ESI-FTICR MS for a non-targeted metabolomic approach for root exudate analysis at