Analytica Chimica Acta 397 (1999) 235–245 Influence of ionization energy on charge-transfer ionization in matrix-assisted laser desorption/ionization mass spectrometry Stephen F. Macha, Tracy D. McCarley, Patrick A. Limbach ∗ Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA Received 7 October 1998; received in revised form 11 January 1999; accepted 21 January 1999 Abstract In this study, non-polar matrices are used in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) to analyze selected non-polar analytes. Our hypothesis is that gas-phase charge-transfer reactions between matrix and analyte are responsible for the generation of analyte radical molecular ions. Following this hypothesis, the ion- ization energies of the matrices and analytes should have a direct influence on the production of radical molecular cations of the analytes. Several non-polar analytes, including ferrocene and ferrocene derivatives, trans-stilbene, triphenylphosphine, 2,2 ′ -methylenebis(6-tert-butyl-4-methylphenol), biphenyl and 1,4-bis(methylthio)benzene were studied using positive-ion mode MALDI-TOFMS. The results of these studies demonstrate that formation of the radical molecular cation depends on the difference in ionization energies between the matrix and the analyte. The propensity for charge-transfer ionization, as opposed to proton-transfer ionization, for these analytes, was confirmed using atmospheric pressure chemical ionization mass spectrometry. Charge-transfer ionization using non-polar matrices in MALDI-MS is a suitable method for the characterization of a number of non-polar, thermally labile analytes. ©1999 Elsevier Science B.V. All rights reserved. Keywords: Charge-transfer; Electron-transfer; Non-polar compounds; Hydrocarbons; Ionization potential 1. Introduction Since the introduction of MALDI-MS by Karas et al. in 1987 [1] and Tanaka et al. in 1988 [2], the primary utilization of this technique has been for the characterization of polar organic and bio-organic molecules. The matrices used for the analysis of such compounds are acidic organic molecules such as 2,5- dihydroxybenzoic acid, -cyano-4-hydroxycinnamic acid, and 3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid). For these types of matrices and an- ∗ Corresponding author. Tel.: +1-225-388-3417; fax: +1-225-388-3458 E-mail address: plimbac@unix1.sncc.lsu.edu (P.A. Limbach) alytes, the predominant mechanism for ionization appears to be either proton-transfer reactions be- tween the analyte and the matrix or cation adduction. Although not nearly as common as reports on the production of protonated or cation-adducted molec- ular ions, there are several reports in the literature on the formation of analyte molecular radical cations during MALDI-MS experiments [3–5]. In addition, laser desorption studies of matrix materials reveal that many compounds that have been used as matrices form M +• ions upon ionization [6,7]. Juhasz and Costello presented the first and most complete description on the production of rad- ical cations using MALDI [3]. They reported the successful use of four different matrices 0003-2670/99/$ – see front matter ©1999 Elsevier Science B.V. All rights reserved. PII:S0003-2670(99)00408-0