Paul Kebarle

A brief overview of the present status of the mechanisms involved in electrospray mass spectrometry

A brief account of the mechanisms by which ions in solution are converted to ions in the gas phase is given on the basis of information available in the literature and the four companion articles on electrospray mass spectrometry (ESMS) in this issue. The following stages/phenomena are described: (a) production of the charged droplets at the ES capillary tip; (b) evolution of the charged droplets due to solvent evaporation and droplet fission caused by Coulombic repulsion of the charges on the droplets; production of the gas phase ion from very small charged droplets by the charge residue model (CRM) or the ion evaporation method (IEM); (c) dependence of the sensitivity in ESMS on the chemical nature of the analyte and its concentration as well as on the concentration of other electrolytes that are present in the solution; qualitative predictions on the sensitivity of the analyte based on the surface activity of the analyte ions; (d) relationship between ions produced in the gas phase and original ions present in the solution; and (e) globular proteins. Much of the information presented in (a)-(e) has been available for some time in the literature. However some significant advances are relatively recent. Recent results by de la Mora and co-workers, including their contribution in this Special Feature, provide very strong evidence that small ions (in distinction from macroions such as bio-macroions) are produced by IEM. On the other hand, macroions and particularly the polyprotonated globular proteins are produced by CRM. Also noteworthy is the development of an equation by Enke with which the observed relative ion signal intensities of the gas-phase ions produced can be predicted on the basis of the ion concentration in solution over a wide concentration range. The recognition that the sensitivity of organic analyte ions can be qualitatively predicted on the basis of the hydrophilicity or hydrophobicity of the part of the molecule that is not part of the charged (ionic) group and affects the surface activity of the ionic species is also noteworthy and a very useful relatively recent development.

Electrospray: from ions in solution to ions in the gas phase, what we know now.

There is an advantage for users of electrospray and nanospray mass spectrometry to have an understanding of the processes involved in the conversion of the ions present in the solution to ions in the gas phase. The following processes are considered: Creation of charge droplets at the capillary tip; Electrical potentials required and possibility of gas discharges; Evolution of charged droplets, due to solvent evaporation and Coulomb explosions, to very small droplets that are the precursors of the gas phase ions; Production of gas phase ions from these droplets via the Ion Evaporation and Charge residue models; Analytical uses of ESIMS of small ions, qualitative and quantitative analysis; Effects of the ESI mechanism on the analysis of proteins and protein complexes; Determination of stability constants of protein complexes; Role of additives such as ammonium acetate on the observed mass spectra.

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 Reactions in Pure Nitrogen and Nitrogen Containing Traces of Water at Total Pressures 0.5–4 torr. Kinetics of Clustering Reactions Forming H+(H2O)n

The ion–molecule reactions in pure nitrogen and nitrogen containing traces of water were studied with a pulsed electron‐beam mass spectrometer having a field‐free high‐pressure source. The reaction N2++2N2 = N4++N2 occurring in pure nitrogen was found to have a third‐order rate constant k = 8 × 10−29cc2molecule−2·sec−1 at 300°K and a negative temperature coefficient corresponding to an “activation energy” of − 2 kcal/mole at pressures up to 3.5 torr. The results for the reaction N++N2→N3+, investigated under the same conditions, indicated either third‐order dependence with k = 5 × 10−29cc2molecule−2·sec−1 and energy of activation − 1 kcal/mole or second‐order dependence with k = 1.3 × 10−12cc molecule−1·sec−1 with no temperature coefficient. The reaction mechanism in nitrogen in the torricelli range containing water vapor in the millitorr range was found to proceed by the following reaction sequence: N2+→N4+→H2O+→H3O+→H+(H2O)2→H+(H2O)n. The rate constants for all reactions were determined. The clustering ...

On the mechanisms by which the charged droplets produced by electrospray lead to gas phase ions

Abstract A brief description of the process leading to the formation of the very small droplets which ultimately produce gas phase ions is followed by a discussion of the ion evaporation model (IEM) and the charge residue model (CRM). The IEM developed for small ions by Iribarne and Thomson is a well developed model which provides quantitative predictions for the rates of evaporation of ions. The CRM attributed to Dole and extended by Rollgen does not provide a detailed consideration as to how the ‘final’ droplets containing only one ion are formed. Several experimental tests of IEM and CRM are discussed: The rates of formation of gas phase alkali ions M + (Li + , Na + , K + , Cs + ) determined from mass spectra, are compared with predicted rates by the Iribarne and Thomson equation. Unfortunately, the available thermochemical data required for the evaluation of the theoretical rates are of insufficient accuracy. Therefore, the theoretically predicted rates exhibit a large scatter. However, also, the experimentally determined relative rates on the basis of mass spectra are not in agreement. Relative rates for the alkali ions from this laboratory predict nearly equal rates for the alkali ions while data by Leize et al. predict an increase from Li + to Cs + . Both sets of data are compatible with the theoretical predictions of the IEM equation. The CRM, when extended to include the effect of surface activity of ions, is compatible with the results of nearly equal rates, but not compatible with the increasing rates of Leize et al . It is very desirable to establish in future work as to which set of experimental data is correct. If the increasing rates of Leize et al. are found to be true, a strong argument in favor of the IEM will be provided. Mass spectrometric observations of the intensities of the ions Na + and Na(NaCl) n + obtained from aqueous solutions of NaCl at different concentrations are compared with ion distributions expected on the basis of IEM and CRM. In general, larger intensities of Na + relative to Na(NaCl) n + would be expected on the basis of IEM. The experimental data are found to correspond much more closely to the predictions of IEM. However, various assumptions had to be made in order to be able to make the predictions. Most important of these is the history of the droplets as they undergo Rayleigh fission, and in particular, the size, charge and number of offspring droplets. Since accurate values for these quantities are not available, the conclusions in favor of IEM are not definitive. The final stages of CRM where droplets have radii of a few nanometers and several charges and solute molecules cannot be treated with the Rayleigh equation. Loss of single charges (ions) is possible in this stage. If this is the case, the last stage of CRM will be IEM-like. Such a stage will lead to a blurring of the distinction between IEM and CRM, for small ions. Fernandez de la Mora has provided very strong evidence that globular, not denatured, proteins are produced by CRM. More open, multiply-charged macroions could be produced by either IEM or CRM. The ions produced in the gas phase are not necessarily those present in the solution. Stable ions like the alkali ions are transferred without change to the gas phase. However, protonated bases may undergo changes as a consequence of the different basicity orders in solution and in the gas phase and other processes.

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.

Mechanism and Rate Constants of Ion–Molecule Reactions Leading to Formation of H+(H2O)n in Moist Oxygen and Air

The formation of H+(H2O)n clusters in moist oxygen is of interest since this process must occur in the troposphere and has been observed to occur in the D region of the ionosphere. The reaction mechanism leading from O2+ to H+(H2O)n was investigated with a pulsed electron‐beam high‐pressure mass spectrometer. At oxygen pressures in the torr range and for [O2]/[H2O]>102 the major reaction proceeds by the sequence O2+→O4+→O2+·H2O→H3O+·OH→H+(H2O)2→H+(H2O)n. The rate constants for all major steps were determined. Rate constants for parallel reactions which may become important at different conditions were also determined.

Hydration Energies and Entropies for Mg2+, Ca2+, Sr2+, and Ba2+ from Gas-Phase Ion−Water Molecule Equilibria Determinations

The sequential enthalpies , free energies , and entropies for the hydration reaction M(H2O)n-12+ + H2O = M(H2O)n2+ were determined in the gas phase for M = Mg, Ca, Sr, Ba. The gas-phase ion hydrates were produced by electrospray, and the hydration equilibria were determined in a reaction chamber attached to a mass spectrometer. The exothermicities of the (n − 1, n) reactions at low n (n = 1 to n = 5) are very high, and the corresponding equilibria require very high temperatures and could not be determined. For these low n, good theoretical results are available for Mg2+ and Ca2+ (Siegbahn and co-workers). A combination of the theoretical data with the experimental results (from n = 6 to n = 14) provides information on the inner and outer hydration shell structure and energetics of the hydrates. Very good agreement is observed between the theoretical and experimental energies where they overlap. For Mg, Ca, and Sr the first six molecules go into the inner shell while the seventh and higher molecules go int...

Features of the ESI mechanism that affect the observation of multiply charged noncovalent protein complexes and the determination of the association constant by the titration method

Several factors, attributable to the ESIMS mechanism, that can affect the assumptions of the titration method are examined: (1) The assumption that the concentrations in solution of the protein P, the ligand L, and the complex PL are proportional to the respective ion intensities observed with ESIMS, is examined with experiments in which ion intensities of two non-interacting proteins are compared with the respective concentrations. The intensities are found to be approximately proportional to the concentrations. The proportionality factors are found to increase as the mass of the protein is decreased. Very small proteins have much higher intensities. The results suggest that it is preferable to use only the intensity ratio of PL and P, whose masses are very close to each other when L is small, to determine the association constant KA in solution. (2) From the charge residue model (CRM) one expects that the solution will experience a very large increase of concentration due to evaporation of the precursor droplets, before the proteins P and PL are produced in the gas phase. This can shift the equilibrium in the droplets: P + L = PL, towards PL. Analysis of the droplet evaporation history shows that such a shift is not likely, because the time of droplet evolution is very short, only several μs, and the equilibrium relaxation time is much longer. (3) The droplet history shows that unreacted P and L can be often present together in the same droplet. On complete evaporation of such droplets L will land on P leading to PL and this effect will lead to values of KA that are too high. However, it is argued that mostly accidental, weakly bonded, complexes will form and these will dissociate in the clean up stages (heated transfer capillary and CAD region). Thus only very small errors are expected due to this cause. (4) Some PL complexes may have bonding that is too weak in the gas phase even though they have KA values in solution that predict high solution PL yields. In this case the PL complexes may decompose in the clean up stages and not be observed with sufficient intensity in the mass spectrum. This will lead to KA values that are too low. The effect is expected for complexes that involve significant hydrophobic interaction that leads to high stability of the complex in solution but low stability in the gas phase. The titration method is not suited for such systems.

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.