Michael G. Ikonomou

Studies of alkaline earth and transition metal M++ gas phase ion chemistry

A breakthrough into the hitherto inaccessible alkaline earth and transition metal M++ gas phase ion chemistry is reported. Ions M++(L)n, where M++(Mg++, Ca++, Sr++, Ba++, Mn++, Fe++, Co++, Ni++, and Zn++) and L=H2O could be produced. The hydrate equilibria M++(H2O)n−1+H2O=M++(H2O)n (n−1, n), were determined for Mg++, Ca++, Sr++, Mn++, and Co++. These lead to successive ion–H2O binding energies for high n, i.e., n=8–13 which are in the 15 kcal/mol range. The above hydrates and many other ion–ligand complexes could be produced by transferring the ions from liquid solution into the gas phase by means of electrospray. The ions were detected with a triple quadrupole mass spectrometer. The much stronger inner shell ion–ligand interactions can be studied by collision‐induced dissociation in the triple quadrupole. Single ligand loss gives way to charge reduction at low n. Thus the M++(H2O)n give MOH+(H2O)k+H3O+ at a low n. The n for which reduction occurs decreases as the second ionization energy of M decreases. ...

Ion-molecule clusters involving doubly charged metal ions (M2+)

Abstract Doubly charged metal ion-ligand L clusters, M2+ (L)n, where M2+ = Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, can be produced in the gas phase by electrospray of solutions of chloride, bromide or nitrate salts of M2+ in methanol-water. L may be added to the solution or to the gas phase. The ions produced by electrospray at atmospheric pressure are transferred to an interface chamber containing pure N2. When known amounts of ligand vapour are added, the equilibria M2+ (L)n−1 + L = M2+ (L)n can be determined by sampling the ions escaping from an orifice in the interface chamber with a quadrupole mass spectrometry. Equilibria for Ni2+ (H2O)n are determined. The clusters are with n ≈ 10 and the binding energies are in the 15 kcal mol−1 range. Bonding of the ligands at n Different ligands may accelerate (NH3, pyridine) or retard (DMF, DMSO) the onset of charge reduction. Charge reduction with DMF and DMSO occurs by simple charge transfer Polydentate cage-type ligands protect ions from charge reduction. Thus the only triply charged ion, M3+, so far observed was Co3+ sepulchrate where the ion is coordinated to six nitrogens.

First studies of the gas phase ion chemistry of M3+ metal ion ligands

Abstract Triply charged ion ligand complexes, M(L) 3+ n , were produced in the gas phase by electrospray of solutions of the M 3+ salts and observed with a triple quadrupole mass spectrometer. (M = yttrium, lanthanum, cerium, neodymium or samarium.) Where L was dimethylsulfoxide or dimethylformamide M 3+ resulted. However, H 2 O as L led only to the charge reduced ion MOH(H 2 O) 2+ n . Collision induced dissociation with the triple quadrupole was used to confirm the assignments. The above M have third ionization energies which are, in relative terms, very low: IE III = 19–23 eV. A triply charged complex for the much higher IE III = 33.5 eV (cobalt) could be produced by using the hexadentate, tricyclic ligand sepulchrate.

Electrospray mass spectrometry of methanol and water solutions suppression of electric discharge with SF6 gas.

An equation by D. P. H. Smith predicts the capillary voltage required for the onset of electrospray (ES). For different solvents the voltage increases with the square root of the surface tension. Water requires a potential that is 1.8 times higher than that for methanol. This is verified experimentally. The higher potential required for water leads to ES in the presence of corona electric discharge. For low total ES plus corona currents, the electrosprayed analyte ion intensity is not adversely affected by the presence of discharge. At high total currents, there is a large decrease of analyte sensitivity. The sensitivity decrease is probably due to adverse space charge effect at high currents. The discharge can be suppressed by adding sulfur hexafluoride to the ambient gas. Both sensitivity and signal stability are improved. However, the sensitivity still remains lower by a factor of — 4 relative to that observed with methanol. This is attributed to lower efficiency of gas-phase ion formation from charged water, relative to methanol, droplets.

SIMS spectra of alcohols and the phase explosion model of desorption ionization

Abstract Secondary ion mass spectra were obtained for water and for a series of alcohols at low temperatures by bombarding with 8 keV Xe atoms. Clustering was found to be very extensive for water and methanol but decreased for progressively larger alcohols. The “phase explosion model” of desorption ionization is forwarded in order to explain these results. The phase explosion model is presented using bulk properties of liquids under thermal equilibrium. The role of the collision cascade is to heat the matrix (thermal spike). As the liquid is heated in vacuum, it penetrates deeply into the metastable region and approaches the limit of absolute stability, or the “spinodal”. As a result, the liquid undergoes an irreversible expansion into a gas without the formation of vapor nuclei. This expansion results in a very rapid cooling of the gas. The phase explosion model offers a qualitative explanation for the extent of clustering in different matrices and suggests how thermally labile analyte molecules can survive the desorption process and also how extensive ion-molecule chemistry can occur in FAB.

An ion source with which ions produced by electrospray can be subjected to ion/molecule reactions at intermediate pressures (10–100 Torr). Deprotonation of polyprotonated peptides

Abstract Ions produced by electrospray at 1 atm pressure are transferred by means of a small bore capillary to a cylindrical reaction chamber of ≈ 5 cm length which is attached to the inlet of a triple quadrupole mass spectrometer. The reaction chamber pressure can be maintained between 10 and 100 Torr and reagent gases can be introduced in flow through the chamber. When protonated bases BH+ are produced by electrospray, proton transfer to a stronger reagent gas base such as tributylamine (TBA) can be initiated in the reaction chamber. Also, polyprotonated peptides such as polyprotonated cytochrome C can be partially deprotonated in the presence of TBA. The intensities of the mass-analyzed ions after reactive changes are still very high, up to 5 × 105 counts s−1, and therefore the reaction chamber is very useful for reactive preparation of ions which are to be studied by collision-induced dissociation. The chamber is less well suited for studies of reaction rates and equilibria but the present preliminary design provides information on flow conditions inside the reactor and required changes that would make rate and equilibria studies possible. The question regarding the dependence of the rate constant, k, for proton transfer from multiply protonated peptides on the number of protons present is examined.

Electrospray, Mechanism and Performance

Recent work by Fenn et al. [1–4] has demonstrated the extraordinary potential of electrospray ionization (ES) as an interface for capillary liquid chromatography-mass spectrometry. Henion et al. [5–7] using a modification of the electrospray method in which the nebulization of the liquid effluent is achieved by a turbulent gas (N2) flow, have also described a number of exciting applications of the technique. Smith et al., using electrospray as the interface for capillary electrophoresis have also reported impressive results [8–10]. Particularly exciting have been the reports of the production and mass spectrometric detection of multiply protonated peptides, where the presence of some 10–30 protons leads to a large reduction of the m/z ratio of the ions observed [6].

Real-time analysis of polyaromatic hydrocarbons in flames using Atmospheric Pressure Ionization and tandem mass spectrometry

Abstract The use of Atmospheric Pressure Ionization followed by tandem mass spectrometry (API/MS/MS) for the analysis of flame gases was demonstrated. The hot flame gases from a methane/air laminar diffusion flame were sampled by rapid turbulent mixing with cold nitrogen gas, in a molar ratio of ca. 1:10. After 3 ms the gases underwent an additional dilution by a factor of 20 in synthetic air. The gas mixture was ionized by a corona discharge at atmospheric pressure. Subsequent chemical ionization reactions ionize mainly the polyaromatic hydrocarbons, PAHs. The PAH ions were analyzed in a triple quadrupole mass spectrometer. A sequence of PAH ions started with the perinaphthenyl cation, C 13 H 9 + , and extended up to protonated coronene, C 24 H 13 + , and beyond. That the observed ions were indeed protonated PAH molecules was confirmed by comparing the collision-induced dissociation spectra in the MS/MS mode with those of authentic samples. It is argued that most of the ions originate from PAHs that have substituents attached to the polyaromatic skeleton. The identities of the substituents could, however, not be determined. By rapid turbulent mixing of the flame gases with air, the PAHs were partially oxidized. The high mass region of the API spectrum was then dominated by a sequence of singly oxygenated PAHs.