Matrix-assisted laser desorption/ionization time-of-flight mass spectrometric imaging of synthetic polymer sample spots prepared using ionic liquid matrices Stefan J. Gabriel 1 , Dietmar Pfeifer 1 , Clemens Schwarzinger 2 , Ulrich Panne 1,3 and Steffen M. Weidner 1 * 1 Federal Institute for Materials Research and Testing (BAM), Richard-Willstätter-Strasse 11, D-12489 Berlin, Germany 2 Johannes Kepler Universität Linz, Altenbergerstr. 69, A-4040 Linz, Germany 3 Humboldt-Universität zu Berlin, Department of Chemistry, Brook-Taylor-Str. 2, 12489 Berlin, Germany RATIONALE: Polymer sample spots for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) prepared by the dried-droplet method often reveal ring formation accompanied by possible segregation of matrix and sample molecules as well as of the polymer homologs itself. Since the majority of sample spots are prepared by this simple and fast method, a matrix or sample preparation method that excludes such segregation has to be found. METHODS: Three different ionic liquid matrices based on conventionally used aromatic compounds for MALDI-TOF MS were prepared. The formation of ionic liquids was proven by 1 H NMR spectroscopy. MALDI-Imaging mass spectrometry was applied to monitor the homogeneity. RESULTS: Our results show a superior sample spot homogeneity using ionic liquid matrices. Spots could be sampled several times without visible differences in the mass spectra. A frequently observed loss of matrix in the mass spectrometer vacuum was not observed. The necessary laser irradiance was reduced, which resulted in less polymer fragmentation. CONCLUSIONS: Ionic liquid matrices can be used to overcome segregation, a typical drawback of conventional MALDI dried-droplet preparations. Homogeneous sample spots are easy to prepare, stable in the MS vacuum and, thereby, improve the reproducibility of MALDI. Copyright © 2014 John Wiley & Sons, Ltd. When matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) was introduced in the late 1980s, it rapidly became an extremely useful tool for characterization of biological and synthetic polymers. [1–3] A simple sample preparation, fast analysis times, the variety of matrices, low sample consumption, and especially the formation of singly charged ions, are only a few advantages of the MALDI technique. [4] However, it was soon revealed that the quality of MALDI spectra depends heavily on the sample preparation. [5] An ideal matrix will form a homogenous sample surface, has a low vapor pressure and must absorb the laser UV light. [6] Most current matrices comprise low molecular mass, aromatic compounds. Frequently used sample preparation techniques are the so-called dried-droplet methods, matrix sublimation, [7] solvent-free preparation (mill, mortar, etc.), [8–10] thin-layer sample preparation, [11] micro-dispensing, [12] electrospraying, [13] and sandwich techniques. [14] Many of these techniques are not very suitable due to their complex or time-consuming pretreatment (e.g. grinding, sublimation), lower absorptivity at the laser wavelength or higher irradiance needed for the ionization. Because of its simplicity, the most common sample preparation technique is still the dried-droplet method, where the dissolved sample is mixed with a dissolved matrix (and salt) and, a few hundred nanoliters are dropped onto the sample target. The major drawback of this technique is the formation of so-called ’coffee rings’, whose formation has been investigated by several authors. [15–18] Deegan et al. stated that capillary flow is responsible for the formation of ring stains. Due to a faster evaporation at the rim of the droplet, a flow from inside the droplet occurs, which carries the dispersed or dissolved material to the edge. [16] Bhardwaj et al. described additional Marangoni flow loops, which will carry the analyte molecules back to the middle of the droplet. [15,19] Moreover, the formation of coffee rings is affected by the temperature, as shown by Hu et al., who used MALDI-MS Imaging (MSI) and fluorescence spectroscopy for their investigation. [18] MALDI-MSI, initially introduced by Caprioli et al., has been extremely useful for determining the local concentration of proteins, peptides, lipids, and metabolites in biological tissue. [20–24] Using this technique, MALDI spectra are recorded by a lateral scanning of the sample surface. Selected ions are finally used to create ion intensity plots. Recently, first applications for the analysis of synthetic polymers have * Correspondence to: S. M. Weidner, Federal Institute for Materials Research and Testing (BAM), Richard-Willstätter- Strasse 11, D-12489 Berlin, Germany. E-mail: Steffen.Weidner@bam.de Copyright © 2014 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2014, 28, 489–498 Research Article Received: 8 October 2013 Revised: 17 December 2013 Accepted: 17 December 2013 Published online in Wiley Online Library Rapid Commun. Mass Spectrom. 2014, 28, 489–498 (wileyonlinelibrary.com) DOI: 10.1002/rcm.6810 489