Fred L. King

Surface-induced dissociation in tandem quadrupole mass spectrometers: A comparison of three designs

Three different devices that-can be used for surface-induced dissociation (SID) m tandem quadrupole instruments are compared here. The designs were compared by examining the fragmentation of several compounds including benzene, W(CO)6, and (CH3)4N+. These studies show that SID can be readily implemented on a variety of tandem quadrupoIe instruments and that the spectra obtained with the in-line and 90° instruments are similar. Evidence is presented that confirms that high average internal energies and narrow distributions of internal energy are available by this technique. Efficiencies for fragmentation of odd-electron ions are on the order of those previously reported by others. The overall SID efficiency for even-electron ions is higher than that for odd-electron ions of similar structure.

Spectral, spatial and temporal characteristics of a millisecond pulsed glow discharge: metastable argon atom production

Abstract Time resolved atomic emission, atomic absorbance, and laser-induced atomic fluorescence measurements of a millisecond pulsed glow discharge, made perpendicular to the insertion probe, provide temporal profiles of 1s 5 ( 3 P 2 ) and 1s 3 ( 3 P 0 ) metastable argon atom populations. Acquisition of these profiles at different spatial positions in the plasma provides data from which two-dimensional spatial plots of relative populations are constructed. Each map, the result of 368 individual pulse profiles, provides insight into the production of metastable argon atoms as a function of time and position within the plasma. During power application, intensities plateau after 3 ms as the plasma reaches a steady state condition. Metastable argon atoms are most abundant 1–2 mm above the cathode surface during this time. Excitation mechanisms such as electron excitation and fast atom/ion impact appear to dominate in this temporal regime. In contrast, argon ion–electron recombination dominates metastable formation after pulse termination. The relative population maximum for metastable argon atoms in the afterpeak shifts to 5–9 mm above the cathode surface. This shift should impact signals for analyte species generated by Penning processes in the plasma. Absorption and fluorescence measurements of the 3 P 2 (11.55 eV) and the 3 P 0 (11.72 eV) metastable argon atom states indicate possible differences in the populations of these two states between the plateau and afterpeak time regimes.

Direct Determination of the Primary Binding Site of Cisplatin on Cytochrome c by Mass Spectrometry

Protein—cisplatin interactions lie at the heart of both the effectiveness of cisplatin as a therapeutic agent and side effects associated with cisplatin treatment. Because a greater understanding of the protein—cisplatin interactions at the molecular level can inform the design of cisplatin-like agents for future use, mass spectrometric determination of the binding site of cisplatin on a model protein, cytochrome c, was undertaken in this paper. The monoadduct cytochrome c—Pt(NH3)2(H2O) is found to be the primary adduct produced by the cytochrome c—cisplatin interactions under native conditions. To locate the primary binding site of cisplatin, both free cytochrome c and the cytochrome c adducts underwent trypsin digestion, followed by Fourier transform mass spectrometry (FT-MS) to identify unique fragments in the adduct digest. Four such fragments were found in the adduct digest. Tandem mass spectrometry (MS/MS and MS3 indicates that two fragments are Pt(NH3)2(H2O) bound peptides (Gly56-Glu104 and Asn54-Glu104) with one water associated at the peptide bond Lys79∼Met80, and the other two fragments are heme containing peptides (acety1-Gly1-Lys53 and acety1-Gly1-Lys55). The product-ion spectra of the four fragments reveal that Met65 is the primary binding site of cisplatin on cytochrome c.

Spectral, spatial and temporal characterization of a millisecond pulsed glow discharge: copper analyte emission and ionization

Abstract Two-dimensional maps of the spatial distributions of excited and ionized sputtered copper atoms are presented for a millisecond pulsed argon glow discharge. These maps demonstrate the temporal as well as spatial dependence of different excitation and ionization processes over the pulse cycle. Transitions from the low energy electronic states for the atom, characterized by emission such as that at 324.75 nm (3.82→0.00 eV), dominate the plateau time regime at a distance of 2.5 mm from the cathode surface. These processes originate from the electron excitation of ground state copper atoms. Transitions from high-energy electronic states, such as that characterized by emission at 368.74 nm (7.16→3.82 eV), predominate during the afterpeak time regime at a distance of 5.0–6.0 mm from the cathode surface. This observation is consistent with the relaxation of highly excited copper atoms produced by electron recombination with copper ions during the afterpeak time regime. Analyses of afterpeak and plateau intensities for a series of copper emission lines indicate an electron excitation temperature equivalent to 5.78 eV at 0.8 torr and 1.5 W. Temporal profiles exhibit copper ion emission only during the plateau time regime.

Ion formation processes in the afterpeak time regime of pulsed glow discharge plasmas

The formation of ions following the termination of power in a pulsed glow discharge ion source is investigated. The populations of ionized species containing sputtered atoms M+, M21:, and MAr+ are observed to maximize after the termination of discharge power. Collisions involving sputtered atoms and metastable argon atoms, Penning and associative ionization, are considered to be responsible for the formation of ions in the discharge afterpeak time regime. The domination of these ion formation processes during the afterpeak time regime is supported by the results from investigations of discharge operating parameters, metastable argon atom quenching, and ion kinetic energy distributions.

Characterization of cluster ions produced by the sputtering or direct laser vaporization of group 13 metal (Al, Ga, and In) oxides

The stabilities and reactivities of cluster ions generated from the fast‐atom bombardment (FAB) or the direct laser vaporization (DLV) of the Group 13 metal oxides (Al2O3, Ga2O3, and In2O3) were examined by mass spectrometry. The nascent cluster ion distributions, fragmentations, and reactions were studied. The observed patterns of stability and reactivity were compared with the structures and heats of formation calculated from theoretical studies of aluminum oxide cluster ions using MNDO, Xα, and Born–Mayer pair potentials. The method of production of the metal oxide cluster ions, whether by FAB, DLV, or through the reaction of sputtered bare metal cluster ions with oxygen, had little influence on the abundance distribution observed. In agreement with the known M–O binding energies, a trend of increasing cluster oxidation state was observed in the abundance distributions of the cluster ions for decreasing metal atom z value. Dissociation of the oxide cluster ions occurred through the loss of particularly...

Determination of 40Ca+ in the Presence of 40Ar+: An Illustration of the Utility of Time-Gated Detection in Pulsed Glow Discharge Mass Spectrometry

The glow discharge ionization source operated in the pulsed, or modulated, power mode affords a number of distinct advantages over its steady-state counterpart. It is well-known that pulsed plasma operation permits the application of higher instantaneous powers by allowing time for the sample to cool. This minimizes sample overheating while effecting higher sputtering yields and lower limits of detection. The presence of discrete time regimes affords the added advantage of temporal selectivity. Such selectivity allows the observation of analyte ions during a time regime in which their signal is at a maximum while that of electron ionized background species is declining. Significantly, time regimes are found when no background argon ion signals are observable but analyte ion signals remain. This means that discrimination against isobaric interferences arising from the discharge gas is possible. A prime example of the utility of this advantage arises in the determination of calcium with an argon glow discharge. Both the major argon and calcium isotopes are found at a nominal m/z of 40. Time-gated mass spectrometeric detection during the afterpeak time regime enables the ready determination of (40)Ca(+) in samples at the ppm level. A linear calibration curve is obtained that also demonstrates the elimination of the (40)Ar(+) signal from mass spectra obtained with either a dc or rf glow discharge ion source.

Temporal emission characteristics of millisecond pulsed radiofrequency and direct current glow discharges

Interpretation of optical emission spectra reveals the primary excitation mechanisms for discharge gas, argon and sputtered analyte, copper, species in glow discharge plasmas operated with millisecond pulses of radiofrequency or direct current power. There is negligible difference between the two power sources. During the applied power pulse, plasma processes include ion and atom excitation through electron excitation, asymmetric charge exchange and Penning ionization. Fast ion and atom excitation processes, characterized by monitoring argon emission at 811.5 nm, occur within 2 mm of the cathode surface. Electron excitation, for both discharge gas and sputtered species, maximizes 3 mm from the cathode surface. Asymmetric charge exchange between ground state sputtered atoms and discharge gas ions, characterized by Cu II emission at 224.7 nm, occurs at 5 mm from the cathode surface. Upon power termination, the recombination of ions with thermal electrons yields excited atoms and argon metastable species. At this time, emission monitored at 811.5 nm maximizes 6–7 mm from the cathode surface, corresponding to an increase in the metastable population and, hence, Penning ionization.

Mass-spectrometric characterization of cisplatin binding sites on native and denatured ubiquitin

Because interactions between cisplatin and plasma proteins contribute to drug efficacy and side effects, it is important to understand both the binding sites of cisplatin on the proteins and the formation of protein–cisplatin adducts. Previous results suggest that cisplatin preferentially binds to residues on the protein surface. The present work employed electrospray ionization mass spectrometry (MS) to identify such sites on both native and denatured ubiquitin (Ub). Fourier transform (FT) MS and tandem MS (MS/MS and MS3) enable analysis of Ub–cisplatin adduct digests to locate specific cisplatin binding sites. Results indicate that there are three such binding sites, i.e., M1, T12 and T14, and D32, on native Ub. The intensity of the relevant peaks in the FT-MS spectrum of the native Ub adduct digest demonstrates that residues T12 and T14 comprise the primary cisplatin binding site under the native conditions rather than residue M1 as reported in previous research studies. It is found in the present work, however, that M1 is the primary binding site on denatured Ub. Comparison of cisplatin binding sites on native and denatured Ub in this research demonstrates that the conformation of a protein significantly influences the preference of cisplatin for specific binding sites.

Electrochemically-Induced Reactions of Hexafluorophosphate Anions with Water in Negative Ion Electrospray Mass Spectrometry of Undiluted Ionic Liquids

The influence of water on the observed gas-phase population of negative ions in electrospray mass spectrometry was studied for the undiluted ionic liquid 1,3-butyl-methyl-imidazolium hexafluorophosphate (BMIM+PF6−). During the electrospray process, electrolytic reduction of water enhances the production of tetrafluorophosphate (F4PO−), which undergoes further reactions to produce difluorophosphate (F2PO2−) anions. These anions are observed in addition to the pre-existing hexafluorophosphate anion. The apparent substitution of two fluorine atoms with one oxygen is attributed to a series of reactions initiated by hydrolysis of hexafluorophosphate. This hydrolysis reaction was enhanced by the addition of hydroxide, formed via the hydrolysis of water or through the addition of ammonium hydroxide. The formation of FxPOy− was studied as a function of the electrospray current and solution flow rate. The mass spectral response shows a quantitative logarithmic relationship between ΣFxPOy− signal intensities (adjusted for mole equivalents of H2O required) and the amount of water present, against which the water content could be rapidly assessed. Results were found to be comparable to Karl Fischer titration data.