MALDI-MSI: a fast and reliable method for direct melatonin quantification in biological fluids

Background: This work reports, for the first time, a validated methodology for qualitative and quantitative melatonin analyses directly from a complex matrix, namely rat milk, using matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI), in a useful strategy for fast assessment of this molecular species. Methods: This novel approach uses the imaging feature of a MALDI instrument to create bidimensional images of a selected area. Results for sample concentration are expressed as a function of pixel-by-pixel intensity in grayscale, which are quantified according to pixel intensity through ImageJ open source software. For the extent of this work, two conditions were monitored: lightand dark-obtained milk. Results: Quantification was performed using the selected reaction monitoring (SRM) transitions m/z 233→174 for melatonin. Limit of detection and limit of quantification have presented lower values (picomolar) when compared to other melatonin quantification strategies, indicating that this method presents high sensitivity. Furthermore, this method has a total analysis time of less than 1 min. Conclusions: Melatonin concentrations are significantly higher in samples obtained from dark conditions, consistent with previous results, and thus proving that MALDI-MSI is a suitable, simple, and highly sensitive methodology for direct melatonin analysis from complex biological samples.