Analysis of dammar resin with MALDI-FT-ICR- MS and APCI-FT-ICR-MS Signe Vahur,* Anu Teearu, Tõiv Haljasorg, Piia Burk, Ivo Leito and Ivari Kaljurand Comprehensive analysis of high-resolution mass spectra of aged natural dammar resin obtained with Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR-MS) using matrix-assisted laser desorption/ionization (MALDI) and atmo- spheric pressure chemical ionization (APCI) is presented. Dammar resin is one of the most important components of painting varnishes. Dammar resin is a terpenoid resin (dominated by triterpenoids) with intrinsically very complex composition. This complexity further increases with aging. Ten different solvents and two-component solvent mixtures were tested for sample preparation. The most suitable solvent mixtures for the MALDI-FT-ICR-MS analysis were dichloromethane-acetone and dichloromethane-ethanol. The obtained MALDI-FTMS mass spectrum contains nine clusters of peaks in the m/z range of 420–2200, and the obtained APCI-FTMS mass spectrum contains three clusters of peaks in the m/z range of 380–910. The peaks in the clusters correspond to the oxygenated derivatives of terpenoids differing by the number of C 15 H 24 units. The clusters, in turn, are composed of subclusters differing by the number of oxygen atoms in the molecules. Thorough analysis and identification of the components (or groups of components) by their accurate m/z ratios was carried out, and molecular formulas (elemental compositions) of all major peaks in the MALDI-FTMS and APCI-FTMS spectra were identified (and groups of possible isomeric compounds were proposed). In the MALDI-FTMS and APCI-FTMS mass spectrum, besides the oxidized C 30 , triterpenoids also peaks corresponding to C 29 and C 31 derivatives of triterpenoids (demethylated and methylated, correspondingly) were detected. MALDI and APCI are complementary ionization sources for the analysis of natural dammar resin. In the MALDI source, preferably polar (extensively oxidized) components of the resin are ionized (mostly as Na + adducts), whereas in the APCI source, preferably nonpolar (hydrocarbon and slightly oxidized) compounds are ionized (by protonation). Either of the two ionization methods, when used alone, gives an incomplete picture of the dammar resin composition. Copyright © 2012 John Wiley & Sons, Ltd. Supporting information can be found in the online version of this article. Keywords: MALDI; APCI; FT-ICR-MS; dammar resin; mass resolution; mass accuracy INTRODUCTION Determining the composition of natural products (e.g. resins) is challenging because of their high complexity. The materials are usually intrinsically complex and have become additionally more complex with aging (oxidizing, polymerizing, etc.). On the other hand, studying aged materials may lead to development of ways to slow down the aging processes. Mass-spectrometric (MS) methods are increasingly taking a cen- tral role in investigations of different materials. Modern ionization methods, matrix-assisted laser desorption/ionization (MALDI), atmospheric pressure chemical ionization (APCI), and electrospray ionization (ESI) can be used for a wide variety of complex materials, including those that are (partly) polymeric, nonvolatile and those that cannot be easily separated by chromatographic methods. [1–5] ESI ionization is suitable, first of all, for compounds and materials soluble in polar solvents and is able to ionize by protonation or deprotonation. APCI ionization [6] also assumes solubility of the compounds or materials under investigation but is able to work in nonpolar solvents and does not require protonation/ deprotonation ability. It is therefore applicable to low-polarity compounds that have low ionization efficiency [7] in the ESI source. MALDI produces ions directly from the solid state and is applicable to such compounds and materials, which are diffi- cult to dissolve. [1] All three ionization methods are relatively soft. They produce the molecular ions (in the form of adducts with H + , Na + or other cations, i.e. [M + H] + , [M + Na] + , etc.) with little (APCI, ESI) or almost no (MALDI) fragmentation. [1][6] A mass spectrum of a natural/biological sample that has been obtained without additional separation of components is usually immensely complex. If the resolution is not sufficient, then peaks of ions with the same nominal m/z ratio will overlap, thereby, seriously reducing the usefulness of the spectrum. High-resolution (HR) MS is able to separate peaks of the same nominal m/z ratio and has a distinct advantage here. This is especially true if, in addition to the high mass resolution, high accuracy in the m/z * Correspondence to: Signe Vahur, Institute of Chemistry, Faculty of Science and Technology, University of Tartu, Ravila 14A, 50411, Tartu, Estonia. E-mail signe. vahur@ut.ee Institute of Chemistry, Faculty of Science and Technology, University of Tartu, Ravila 14A, 50411, Tartu, Estonia J. Mass. Spectrom. 2012, 47, 392–409 Copyright © 2012 John Wiley & Sons, Ltd. Research Article Received: 30 June 2011 Revised: 3 February 2012 Accepted: 4 February 2012 Published online in Wiley Online Library (wileyonlinelibrary.com) DOI 10.1002/jms.2971 392