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Electrospray ionization mass spectrometry (ESI-MS) was used to examine clusters of protonated amine salt solutions with chloride counter ions in the negative ion mode. These ions have the general formula [(RNH 3 ) x Cl xϩ1 ]Ϫ . Primary amines generate a wide cluster distribution with clusters up to 14 mer for methylamine hydrochloride clusters. Secondary and quaternary amines only generate the monomer ion under identical conditions. Collision induced dissociation (CID) of the cluster ions generates cluster ions of lower m/z with the next lower cluster being the most abundant. . Typically, the electrospray interface attempts to desolvate and remove all counter ion complexes and clusters before the ions enter the mass spectrometer. Cluster ions usually lower analyte signals, complicate mass spectra, and hinder quantification. However, there are times when cluster ions are beneficial in ESI-MS. They can be used to calibrate the m/z scale and to study ionization mechanisms and solvent effects on the ESI process [2][3][4][5][6]. Cluster distributions for species such as Na x Cl xϪ1 ϩ exhibit "magic numbers" and are of interest in their own right [7,8]. If a molecule is poorly ionized in electrospray, counter ions can be added to increase the ionization yield [9,10]. The counter ions can also provide a net charge for a neutral molecule so that it can be detected in a mass spectrometer. The counter ions change the ionization properties of the molecule by changing the charge state of the molecule. If the ion originally has a net positive charge, the addition of counter ions will reduce the charge state of the ion or even convert it to the opposite polarity. If the overall charge of the ion is changed from positive to negative, negative ion mode ESI must be used. Negative ion mode has the added benefit of fewer background ions in the mass spectrum.Clusters can also lock in the charge state of a given ion in solution. Counter ions can interact with the ion of interest, and keep the ion in a specific charge state. Halides are commonly used because of their charge affinity and ability to coordinate around an ion [11,12]. This charge state can be preserved by adjusting the ionization conditions at the electrospray interface.To observe cluster ions, softer ion extraction conditions are sometimes required. This often means a lower capillary temperature and/or lower extraction voltages in the atmospheric sampling region [8].Previous studies in our group have shown that anions such as nitrate can stabilize the oxidation state of highly charged, reactive metal ions in solution [13,14]. In this paper, complex and cluster ions from various amine salts are investigated by ESI-MS. CID spectra show the fragmentation pattern of the cluster ions. Threshold dissociation energies are determined for primary and secondary monomer ions as well as for a methylamine cluster ion. The threshold reactions provide valuable information regarding differences in cluster ion formation and the strength of the interaction between the cations and anions ...

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