Tissue surface properties jeopardize quantitative DESI imaging of organic acids in grapevine stem

Introduction: DESI imaging has recently gain popularity as a tool to assess spatially resolved biological processes and to assist biomarker identification over unmodified sample surfaces, but how surface properties affect the output of DESI imaging experiments has not been investigated to date.   We addressed this issue in a series of experiments which studied the distribution of small organic acids in grapevine stems. In our investigation, we compared the spatial distribution of endogenous and xenobiotic compounds obtained by DESI with the one resulting from the conventional analysis of the sections. The specific effects of the surface properties on the DESI detection of this class of compounds was also investigated by DESI profiling on different PTFE surfaces. Methods: DESI imaging of endogenous and xenobiotic organic acids in grapevine stem was performed using a Thermo-Fisher Scientific LTQ Orbitrap XL mass spectrometer equipped with an OmniSprayTM ion source under negative ion mode with spatial resolution of 200 μm. Ion chromatography and direct infusion-MS were used to quantify the endogenous and xenobiotic organic acids in grapevine stems, respectively. DESI profiling of a mixture of organic acid standards was done with the same instrument on 3 PTFE surfaces with different pore size and porosity. Results: DESI imaging showed that the distribution of malic (endogenous), glutaric (xenobiotic) and adipic (xenobiotic) acid were significantly different between pith and out pith region. This specific distribution was not confirmed by IC and direct infusion, which indicated a rather uniform distribution over the tissue section.  DESI profiling results on the PTFE surfaces suggest that the local physical properties of the tissue surfaces strongly affect the ionization process as well as their relative quantitative detection. Conclusions: Different surface properties within a structurally/biologically heterogeneous tissue can affect the quantitative detection of analytes resulting in MS images misrepresenting the true distribution of the analytes.   Novel Aspect: As in the case of MALDI, the outcomes of DESI imaging experiments could be affected by the local properties of the tissue sample. Experimental results, then, have to be carefully validated.