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Anal. Chem. 1085, 57, 1766-1768
Fast Atom Bombardment Mass Spectrometric Studies of the Aluminum Chlorideln-Butylpyridinium Chloride Molten Salt Sir: We report here a fast atom bombardment (FAB) mass spectrometric analysis of the aluminum chloride N-n-butylpyridinium chloride molten salt system (AlCl,/BPCl), which is an ionic (ion-paired) liquid formed by mixing its two solid components, AlC13 and BPC1. This system has received considerable attention recently since it is a melt at room temperature in contrast to the more commonly studied molten salt systems of the type A1C13/MX (M is an alkali metal), which exist as melts at elevated temperatures (1150 “C). The analytical methods that have been most commonly used to characterize the species present in molten salt mixtures include potentiometry ( I ) as well as multinuclear NMR (2, 3) and Raman spectrometries (4). The ionic, nonvolatile nature of molten salt samples prohibited their characterization by conventional mass spectrometric methods. Recently, however, Knudsen effusion mass spectrometry has been used to study the compositionof the vapor above the MC1-saturated MA1C14 (M = Na, K) system at elevated temperatures (5). Such studies permit the characterization of atomic and molecular species present only in the vapor phase above the melt, Le., they present little direct information concerning the nature of the condensed phase. We suggest here the use of particle induced desorption (PID) techniques in mass spectrometry which enable “direct” sampling of the condensed phase. Recent experiments suggest that results obtained by using a low-energy (kiloelectronvolt) desorption technique (FAB-MS) reflect the chemistry in the condensed phase sample. Examples include work by Caprioli (6)who demonstrated the ability of FAB to obtain pK, values for acids in good agreement with literature values, and work by Johnstone and Rose who used FAB to examine complexation between metal cations and macrocyclic ligands in an aqueous glycerol solution (7).In the low-energy PID techniques (FAB, SIMS) a fast atomic or ionic beam bombards the anal* which is usudy dispersed in a nonvolatile viscous liquid (e.g., glycerol) frequently giving rise to both protonated molecules and fragment ions (8,9).It has been demonstrated that results obtained from both FAB and SIMS are frequently similar (IO). Derivatization of samples to form “preionized” species (prior to bombardment) has been shown to enhance spectra in several desorption techniques including FAB and SIMS (11). Hence, the AlCl,/BPCl molten salt, due to its ionic nature (ion-pairedliquid), should be ideally suited for analysis by these methods. A unique feature of the studies of molten salts by FAB is that no matrix is required, since the molten salt is a viscous, nonvolatile liquid. Thus, FAB spectra of 1 pL of the molten salt persisted for up to 45 min when subjected to fast atom bombardment. Results for the positive ions obtained from fast atom bombardment of a series of AlCl,/BPCl mixtures varying in composition are presented. The data obtained by FAB-MS are then discussed in light of the results previously obtained by other analytical techniques commonly used to characterize molten salts. EXPERIMENTAL SECTION The molten salt samples were prepared by mixing accurately weighed quantities of purified aluminum chloride and n-butylpyridinium chloride (4). The colorless samples were titrated by weighed additions of n-butylpyridinium chloride to an “acidic“ melt. Every attempt was made to limit the extent of oxidation of the samples. Oxidation is readily apparent by discoloration of the clear liquids. The samples were prepared in a drybox and sealed in vials. The vials were transferred using a desiccator to 0003-2700/85/0357-1766$01.50/0
a nitrogen-filled glovebag (containing Pz05as a moisture indicator and Drierite as a desiccant), which was attached to the ion source of a Varian MAT CH-5 double focusing reversed geometry mass spectrometer. A magnet scan rate of approximately 25 s/decade was used in all experiments. Fast xenon atoms were produced using a Model BllNF fast atom gun (Ion Tech, Ltd., Teddington, U.K.) equipped with a B50 current regulated power supply. High-purity xenon gas (99.995%) was purchased from Matheson (East Rutherford, NJ). Each sample was deposited onto a stainlees steel probe tip and bombarded with a 7.4-keV Xe beam, resulting in the observed FAB mass spectra. Data were collected with a dual PDP 8/e data system, described previously (12). This arrangement dowed for real time data display and enabled averaged scans to be obtained. The mass spectra shown represent the average of 25 scans. In the following discussion, when a single molecular species appears as a cluster of isotopic peaks in the mass spectrum, we will follow the convention of only referring to the lowest m / z value of the cluster. RESULTS AND DISCUSSION The AlCl,/BPCl molten salt system exists as a melt (liquid) at room temperature between the range of approximately 45 to 67 mol % AlCl,, where the melt “acidity” increases with the amount of AlCl, added to the mixture. Solutions containing an excess of the Lewis acid AlCl, (>50 mol %) are referred to as acidic, whereas solutions containing an excess of the Lewis base C1- (
