Arthur T. Blades

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.

Formation, acidity and charge reduction of the hydrates of doubly charged ions M2+ (Be2+, Mg2+, Ca2+, Zn2+)

Abstract There are two methods for producing in the gas phase doubly charged metal ion hydrates, M(H 2 O) n 2+ (or other ion ligand L ML n 2+ complexes). In the clustering method, one starts with the naked ion M 2+ , and in the presence of a third (bath) gas and water vapor, the ion hydrates form by ion-molecule clustering reactions. The second method is based on electrospray with which a spray of aqueous solutions containing the dissolved salts M 2+ + 2X − , leads to gas phase M(H 2 O) n 2+ with a distribution around n ≈ 8. For M, which has a high second ionization energy, IE(M 2+ ), both methods can fail to produce a full range of hydrates with a given n , because of the interference of a charge reduction reaction which involves intramolecular proton transfer. This reaction becomes possible at n = 2; (M(H 2 O) 2 2+ )∗ = MOH + + H 3 O + , and competes with the simple ligand loss: (M(H 2 O) 2 2+ )∗ = M(H 2 O) 2+ + H 2 O. The thermally excited (M(H 2 O) 2 2+ )∗ results in the clustering method by the exothermicity of the forward clustering reaction and in the electrospray method by the thermal declustering required to produce lower n ions. Ab initio calculations are presented for the energies of the above reactions and transition states for Mg 2+ and Ca 2+ . These show that the transition state for the charge reduction reaction is much lower than that for the simple ligand loss at n = 2. However, as n increases, the two transition states move closer together and above a given n = r , simple ligand loss becomes dominant. The capabilities and limitations of the two methods to produce hydrates of a given n is discussed. Experimental results illustrate competing charge reduction and simple H 2 O loss for Be(H 2 O) n 2+ under thermal equilibrium conditions at n ≈ 9. Charge reduction reactions when occurring in the forward clustering direction can be viewed as proton transfer reactions to the incoming H 2 O molecule. These can be generalized by examining the proton affinities of the MOH(H 2 O) n + ions, which are obtained by ab initio calculations. Proton transfer from M(OH) 2 ) n 2+ can be induced not only by H 2 O but also by other bases B. Experimental results for the deprotonation of Zn(OH 2 ) n 2+ , n = 8 or 9, by NH 3 are presented. The charge reduction reactions by which a deprotonated ligand attached to M is formed, can have synthetic utility. Examples are given for the production of methylthiolate complexes which may be useful for modeling ion complexes in which one of the ligands is the deprotonated amino acid residue cysteine.

Negative ion electrospray mass spectrometry of nucleotides: ionization from water solution with SF6 discharge suppression

The total current and selected ion currents from the electrospray ionization (ES1) of 10−5 M solutions of cocaine hydrochloride and deoxycytidine monophosphate (dCMP) monosodium salt in methanol and water solvents were compared in positive and negative ion modes, respectively, without and with SF6, gas as a discharge suppressant. The ESI onset voltages (Von), were the same for the positive and negative ion modes. The Von, for methanol was much lower than that for water and in agreement with the equation of D. P. H. Smith, who attributes the difference to the higher surface tension of water. The onset of electric discharge (Vdis) without SF6, occurred at lower capillary voltages for the negative relative to the positive ion mode for methanol; but Vdis is much higher than Von for methanol, and discharges do not interfere with ESI operation. For water, Von ≈ Vdis in the absence of SF6, in the negative ion mode, and ESI operation is impossible without SF6, discharge suppression. The discharge problem in the positive ion mode is less severe, but SF6, is still very useful. A dynamic range of 10 −7–10−5 M was obtained by selected ion monitoring of [dCMP - H]− at 4.5 and 20 μL/min. flows. Subpicomole detection limits for the nucleotide salt were obtained under these conditions.

Determination of ion-ligand bond energies and ion fragmentation energies of electrospray-produced ions by collision-induced dissociation threshold measurements

Abstract A triple quadrupole mass spectrometer adapted for the determination of collision-induced dissociation (CID) threshold energies of ions produced by electrospray (ES) is described. The ES-produced ions are transferred from 1 atm atm a low pressure source, 10 Torr, where they can be partially declustered and after that thermalized by gas collisions. The ions entering the vacuum of the triple quadrupole are not accelerated in the front end of the apparatus in order to avoid ion/gas molecule collisions which will induce an increase of the internal energy of the ions and broaden the kinetic energy (KE) profile of the ions. Nevertheless, KE broadening is observed. It is demonstrated that it originates in Q1 and is due to the radial d.c. field of the quadrupole. The KE broadening is attributed to ion/gas molecule collisions in Q1. In spite of this effect, threshold energy, E0, determinations can be obtained to an estimated accuracy of approximately 0.2 eV.

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].