Minimizing the Coffee Ring Effect: Improving MALDI Dried-Drop Analysis by Sequential Spotting.

In matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) experimentation, dried-drop analyses are valuable for their ease of preparation and the generation of efficient results. They hold value in their ability to assess the ionizability of an analyte or determine whether it will fragment when subjected to the MALDI laser. However, this technique can be limited by the "coffee ring effect", producing heterogeneity in dried drops with "sweet spots" (areas of concentrated analyte) and "dead zones" (areas with little to no analyte). The coffee ring forms when the analyte concentrates on the edge of the MALDI spot, which leads to difficulty in finding consistent signals from a sample. We demonstrate that sequentially spotting dried drops at halved and quartered pipet solution levels reduces the spot diameter and crucially enhances the signal intensity. With this technique, these concentrated areas are less prevalent and there are fewer dead zones across the droplet. This study has its limitations in sample size; therefore, further investigations will be necessary. Although it increases the sample preparation time, sequential spotting makes the MS process more robust, reducing dead zones and obtaining higher signals in this proof-of-concept study.