Direct Formation of Silver Cluster Ions from Silver Salts by Laser Desorption/Ionization

Abstract

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) has been used to characterize a wide range of polymers. In a MALDI mass spectrum, the protonated molecule and cation adduct of an analyte give an important information about the molecular ion mass. Some researchers reported that sliver clusters can be effectively produced under MALDI conditions from various silver salts in the presence of various matrices. During the analysis of organic molecules when Ag is used as the cationization agent, it can be recognized that besides the desired adduct ions, [M + Ag], an additional silver cluster ions, Agn is formed. Keki and coworkers studied the matrix effect on the formation of the silver cluster ions under MALDI conditions using reductive polar organic matrices and silver trifluoroacetate (AgTFA) and reported that the matrix greatly influenced the resulting cluster ion abundances. Metal clusters are very important in both theoretical and practical points of view. Investigation of cluster formation in mass spectrometry provides valuable information on the stability and electronic properties of clusters of different size. Studies on the formation of silver cluster ions under MALDI conditions have been performed in the presence of matrices such as 2,5-dihydroxybenzoic acid (DHB), 3,5dimethoxy-4-hydroxycinnamic acid (sinapinic acid, SA), trans-3-indoleacrylic acid (IAA), 2-(4-hydroxyphenylazo)benzoic acid (HABA), all-trans-retinoic acid (RTA), and 1,8,9-trihydroxyanthracene (dithranol, DIT). Formation of silver cluster ions directly from silver salts in the absence of matrix was not studied. In the present work, we investigated the silver cluster ions formed directly from silver salts of silver benzoate (AgBz), AgTFA, silver nitrate (AgNO3), and silver p-toluenesulfonate (AgTS) in the absence of any matrices. Scheme 1 shows their chemical structures. Figures 1, 2, 3, and 4 are the laser desorption/ionization (LDI) mass spectra of AgBz, AgTFA, AgNO3, and AgTS, respectively. Only the mass spectrum of AgBz shows a clear distribution of the silver cluster ions. It is a surprising result that the silver cluster ions are directly formed from AgBz in the absence of any matrix. Macha and coworkers investigated the formation of silver clusters from silver salts such as AgTFA, AgNO3, and silver acetylacetonate [Ag(acac)], but silver clusters were not detected. The cluster ion intensity distribution of AgBz shows an odd-even alternation pattern as shown in Figure 1. In other words, the intensity of the odd-number cluster ions tends to be stronger than that of the neighboring even-number cluster ions, especially in the low-mass range. The mass spectrum also shows a steep decrease of the ion intensities after some cluster ions called magic numbers. Figure 1 shows clear magic numbers of n = 9 and 21 of Agn. The specific ion intensity distribution of the silver cluster ions can be explained with the theoretical models. Since odd-number silver cluster ions have even numbers of valance electrons that become paired in the electronic shell of the formed clusters, they show stronger stabilities and higher ion intensities compared to those of even-number ones which have odd-number of electrons in their electronic shells. In addition, odd-number silver cluster ions tended to lose two silver atoms in their dissociation while even-number clusters lose a single atom. The magic numbers are in good agreement with the jellium model theory. In the jellium model one assumes that the positive ions of the cluster are smeared into a uniform background of spherical shape with density equal to that in the bulk. The electrons respond to this uniform background and fill the successive angular momentum states in accordance with the rules of quantum mechanics. A cluster shows pronounced stability when one of the angular momentum levels is filled. This shell closure occurs for alkali atoms at n = 2, 8, 20, 40, 58, 92, ... Formation of the silver cluster ions has been studied in the presence of matrix. In general, formation of the silver