As recently shown, prestructured MALDI targets equipped with small hydrophilic patches (“anchors”) on a hydrophobic substrate yield a more than 10-fold increase in sensitivity compared to dried droplet preparations on conventional metal targets, even for fully automated sample preparation and data acquisition. Upon solvent evaporation, the applied μl droplets shrink onto the anchors, thereby increasing the analyte concentration.
Solvent composition is crucial, especially for matrices insoluble in water like a-cyano-4-hydroxycinnamic acid (HCCA). The organic solvent evaporates faster than water and matrix crystallization starts when matrix saturation is reached. If there is too much water in the droplet, the matrix precipitates too early, resulting in crystallization outside the anchors.
Even if the matrix is dissolved in a pure organic solvent, the deposited analyte solution often contains too much water. We are presenting a simple on-target recrystallization method that brings the crystals back to the anchors, independent of the initial solvent composition.
This recrystallization step can, optionally, be combined with an on-target washing step which removes buffers from the prepared samples. Two examples with commonly used buffers (ammonium bicarbonate and TrisHCl) will be shown to demonstrate that this step does not affect the overall sensitivity and is at least as efficient as ZipTip C18 purification.