Structural information of gas phase complexes of poly(ethylene glycol) (PEG) cationized by one or two different alkali metal ions is inferred from MS and MS/MS experiments performed with an electrospray quadrupole ion trap mass spectrometer. The rationale for selecting PEG was that its sites for cation binding are non-selective with respect to the repeating monomeric unit of the polymer, but there is selectivity with respect to the formation of an inner coordination sphere specific to each metal ion. The dissociation of [M 1 ϩ M 2 ϩ (EO23)], where EO23 ϭ linear polymer of ethylene oxide, 23 units in length, resulted in loss of one of the alkali metal ions, with preference for loss of the larger cation, with no fragmentation of the PEG backbone for Na, K, Rb, and Cs. Li was not examined in this portion of the study. The selectivity for loss of the larger alkali metal ion was [Na -20]. Factors involved in the solution phase molecular recognition are solvation enthalpy and entropy for both species and the number of atoms from the ligand that are involved with the binding of the metal ion, plus any conformational change of the ligand between its unbound and bound forms [21]. The intrinsic properties of the ion-dipole bonds in these non-covalent complexes are being deduced using a variety of gas phase techniques, including the equilibrium method [22], threshold CID [23][24][25][26], and ion chromatography [27,28].The long chain podands have received comparatively less attention by mass spectrometry [29,30], yet their industrial usage is immense, ranging from the fabrication of materials with varied properties to their use as phase transfer catalysts in industrial processes [31]. Long chain podands are also used as agents for improving bio-compatibility in immunological applications [32][33][34] and as ion channel models [35]. Improved knowledge of the structures formed when long chain podands coordinate with alkali metal ions, and particularly with the ubiquitous Na ϩ and K ϩ , can be of use in improving the applications of these materials.Here we report the uni-molecular dissociation of modest length podands (PEG, M.W. avg ϭ 1000) that were doubly cationized by different alkali metal ions. These complexes are abbreviated as [M 1 ϩ M 2 ϩ (EOx)], where M 1 ϩ and M 2 ϩ are different alkali metal ions, EO ϭ ethylene oxide unit, and x ϭ degree of polymerization. This study of the doubly cationized PEG has permitted an in situ measure of the relative binding strength of the cumulative ion-dipole bonds when a single PEG molecule coordinates to the s-orbital of two different alkali metal ions.The dissociation of these complexes inside a quadrupole ion trap is a form of the kinetic method [36,37]. In this variation, rather than sandwiching a single cation between two ligands, a single ligand that was large enough to bind two different metal ions was

The complexation reactions between the alkaline earth metal ions and EDTA were studied by electrospray mass spectrometry to measure the change in concentration of the metal ion-EDTA complex (MY(2)(-)) in the gas phase relative to the solution-phase equilibrium concentration. This work focused on the solution pH range from 4 to 7 where there exists free metal ions in solution at equilibrium. The equilibrium shift, measured through quantitation of the increased abundance of the MY(2)(-) species in the gas phase, was largest for barium and smallest for magnesium. The cause of the net equilibrium shift of the MY(2)(-) species is the combined effect of an electrolytic increase in pH within the capillary plus an additional shift within the evaporating droplets. In a thin diffusion-limited layer created by the products of electrolysis mixing with the bulk solution at the ES capillary tip, the labile species reequilibrate at a new, higher pH. In the evaporating droplets, the formation of new labile species due to increased solute concentrations is kinetically controlled because the ion residence time in the droplet prior to desorption is only ∼5 μs. These results are briefly discussed with respect to the potential for utilizing electrospray mass spectrometry for kinetically labile equilibrium studies.

Preliminary results for the analytical use of electrospray mass spectrometry (ES-MS) for elemental analysis are presented. Spectra with the declustered plus-one ion (M+) as the dominant species have been measured for the alkali metals (Li, Na, K, Rb, and Cs) and for several transition metals (Cr, Mn, Fe, Co, Ni, Cu, Zn, Ag, and Cd) in both aqueous and methanol solvents. A number of background ions are also observed including MH+, MO+, MOH+, and MOH(H2O)+. For vanadyl sulfate and uranyl nitrate, ES conditions can be adjusted to produce VO+ and UO2+ as the dominant ions in the mass spectra, indicating that direct speciation of inorganic solution components is possible. On the basis of these preliminary results, it appears that electrospray may offer a low-cost and simple generic ion source for elemental mass spectrometry.

Methodology enabling mass spectral analysis of the composition of droplet(s) prepared in an electrodynamic balance (EDB) by matrix-assisted laser desorption/ionization (MALDI) is described. The dc field surrounding the electrodynamic balance was manipulated to eject single droplets at a time from the EDB thereby causing their deposition onto a MALDI sample plate precoated with matrix. When the laser was directed onto the droplet(s) and held stationary, marked gains in the signal-to-noise and signal-to-background ratios were realized with each subsequent mass spectrum due to the suppression of matrix cluster ion formation. Optical microscopy of the plate, after 1024 laser shots were fired at eight droplets that had been deposited one on top of the other, revealed a residual island of droplet matter (area approximately 3.1 x 10(-9) m2) inside the region where the crystalline matrix had been ablated away within the laser spot (area approximately 1.6 x 10(-8) m2). Removing the predried crystalline matrix layer and, instead, adding matrix into the starting solution was found to be a more effective means of suppressing matrix cluster ion formation. The chemical composition of the droplet(s) prepared in the EDB is discussed with regard to sample preconcentration, the images of the laser spot after MALDI, matrix cluster ion suppression, and the possibility for improved quantitation and detection limits by MALDI-TOF-MS.

The extent that a kinetically labile equilibrium reaction was perturbed by passage through the electrospray ion source has been measured. The reaction studied was strontium ion chelation by EDTA (Sr(2+) + Y(4)(-) ⇌ SrY(2)(-)) in 100% aqueous solutions. The forward reaction rate is very fast (10(9) M(-)(1) s(-)(1)) and the reverse rate is very slow (10(0) s(-)(1)) relative to the time scale of the ES process (∼10(-2) s). The SrY(2-) detected with the mass spectrometer were expected to be representative of thermodynamic equilibrium within the droplets, but the position of the equilibrium shifted to the right relative to the solution equilibrium position. Given the current status of understanding of the ion generation process in the electrospray ion source, the degree that the [Sr(2+)] changed due to passage through this ion source was smaller than expected, which is fortuitous with respect to the quantitation of such species. The pH of each calibration set determined the fraction of strontium that was uncomplexed in solution. The equilibrium shift induced by passage through the ion source was constant for solutions at constant pH but differed for solutions at different pH. Decreasing the solution pH generated smaller equilibrium shifts as measured by the change in the [Sr(2+)]. In solutions with free Sr(2+) and excess EDTA at equilibrium, the free [Sr(2+)] decreased by ∼100 and ∼10% in solutions at pH 5.85 and 4.98, respectively. Quantitation of kinetically labile species in complex, multicomponent systems will be straightforward with ES-MS, provided all species involved in the equilibria can be identified and monitored.