The well-known elemental quantification method of glow discharge mass spectrometry (GDMS) is explored here for its potential in biomolecule quantification, specifically for the case of glycyl-L-histidyl-L-lysine copper (GHK-Cu) complex. A pulsed glow discharge (GD) coupled with time-of-flight mass spectrometer (TOF MS) is employed to examine analyte behaviors in different segments of a pulse. The Cu-63(+) ion signal observed in the afterpeak regime of the discharge pulse provides a strong and stable signal suitable for quantification of the GHK-Cu complex. GD operating conditions were optimized based on the Cu-63(+) ion signal. Using cesium iodide as an internal standard, the normalized Cu-63(+) ion signal measured from a set of pure standard solutions exhibited a strong correlation to the GHK-Cu concentration. A linear response of Cu-63(+) ion can be obtained from GHK-Cu in 5-40 mu g/mL, with a limit of detection of 3.54 mu g/mL. The good correlation of the elemental signal and the tripeptide concentration demonstrates the capability of GDMS in quantification of GHK-Cu and its potential for application in the quantification of other biomolecules. (C) 2019 Elsevier B.V. All rights reserved.

Although glow discharge mass spectrometry (GDMS) is best known for its utility in trace element analysis, recent work demonstrates that the technique can also provide chemical speciation information. Previous reports outline approaches for the direct speciation of chromium and manganese oxides utilizing GDMS. As that work illustrates, different approaches to speciation are needed depending on the metal oxide under study. The present work extends the application of this technique to the direct speciation of three iron oxide species in solid state samples. The appropriate adjustment of parameters such as sampling distance, temporal regime, discharge gas pressure, pulse frequency, and duty cycle is essential to enable such speciation. For the iron oxides, species specific variations in ratios between Fe+ and FeOH+ signal intensities provide the ability to discriminate between FeO, Fe2O3, and Fe3O4. Iron–oxygen cluster generation correlates with oxygen abundance in the original sample as seen in these comparisons.

Ion intensity profiles, for both discharge gas ((40)Ar(2+), (40)Ar(+), (40)Ar(2)(+)) and sputtered species ((63)Cu(+)), have been measured for a series of parameters including sampling distance, pulse power, discharge gas pressure, pulse width, and duty cycle in a millisecond pulsed direct current (dc) glow discharge plasma using time-gated detection with a time-of-flight (ToF) mass spectrometer. Throughout these experiments constant power was maintained for comparative profile measurements. Intensity profiles for both discharge gas and sputtered material were constructed using the intensity values from a compilation of over 100 mass spectra. Ion signals from analytically important (sputtered) species differed in their response to changes in sampling distance, discharge gas pressure, pulse width, and duty cycle than those of discharge gas species. These intensity profiles provide insight into the effects of discharge conditions on the time-dependent behavior of different ions in the plasma. In addition to using time-gated acquisition, it is possible to further influence ion formation within the plasma through the careful selection of these plasma parameters; in doing so, one can maximize sputtered ion signals while suppressing ion signals from discharge gas species. In this research, we conduct a comparative investigation of ion signal temporal profiles through the variation of discharge parameters to better refine the method and to gain a better understanding of the processes taking place in the discharge.

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.

Mass spectrometric studies of the interactions of cisplatin and transplatin with myoglobin (Mb) provide information concerning interaction kinetics, Mb adduct identity, and cisplatin and transplatin binding sites on Mb. Although the Mb–cisplatin interaction is faster than the Mb–transplatin interaction, monoadducts and diadducts were formed in both the interactions over 30 h. In order to locate the binding sites of cisplatin and transplatin on Mb, digests of free Mb, Mb–cisplatin and Mb–transplatin adducts were subjected to analysis by Fourier transform mass spectrometry (FT-MS). This analysis revealed that two fragment ions, 1313.275+ and 1316.685+, were obtained only from the Mb–cisplatin and Mb–transplatin adduct digests. Tandem mass spectrometry (MS/MS and MS3) of the 1313.275+ and 1316.685+ ions indicate that these ions arise from [Pt(NH3)]2+ and [Pt(NH3)2]2+, respectively, bound to peptide His97-Gly153. The product-ion spectra of the MS/MS and MS3 analyses of the 1313.275+ ion indicate a common binding site of cisplatin and transplatin on His116-His119 residues. The interactions of cisplatin and transplatin with a dipeptide His-Ser and the three dimensional (3D) structure of native Mb suggest that cisplatin and transplatin coordinate to His116 and His119.

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.

Accurate quantification of specific oxidation states of elements present in complex, solid state samples can be difficult because extraction procedures often lead to changes in those oxidation states. Direct approaches to speciation for solid state materials preclude the extraction step and thus enable more accurate quantification of different oxidation states present in the original material. In this work, pulsed glow discharge time-of-flight mass spectrometry provides such direct differentiation between manganese (IV) dioxide and manganese (II) monoxide in a solid state sample. The spatial and temporal characteristics of the glow discharge source are evaluated and optimized for such speciation by careful control of operating parameters, such as supporting gas pressure, operating power, and temporal observation window. Conditions are found that favor the production of the cluster ion, Mn2O3+, characteristic for manganese (IV) dioxide. It is demonstrated here that once this has been accomplished, the two oxides can be differentiated and quantified.

Among the various elemental mass spectrometry techniques, glow discharge mass spectrometry is recognized for its ability to provide direct determination of trace elements present in solid state samples. In the present work, a pulsed glow discharge time-of-flight mass spectrometry method is developed for the direct speciation of chromium in solid state samples. The millisecond pulsed glow discharge operated with radio frequency power is a versatile ion source that provides elemental, structural and molecular information. Careful tuning of the operating parameters yields the plasma chemistry that favors cluster ion formation. Cluster ions unique to specific species permit differentiation between the trivalent and hexavalent forms of chromium, (Cr(III) and (Cr(VI)), respectively, in chromium oxide samples. Specifically, signals at 104 and 120 m/z corresponding to the Cr-2(+) and Cr2O+ cluster ions arise from the presence of Cr(III) in the sample, whereas signal at 100 m/z corresponding to the CrO3+ cluster ion arises from the presence of Cr(VI). The impact of glow discharge operating conditions on the appearance of these characteristic cluster ions is discussed.

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.

This study employed a power perturbation method to examine the energy transfer processes at different locations within the afterpeak regime of a millisecond pulsed glow discharge plasma. Brief power perturbation pulses were applied during the afterpeak regime altering the environment of the collapsing plasma. Responses of several transitions to the power perturbations were measured via atomic emission and absorption spectroscopic methods at various distances from the surface of the cathode. The experimental data provide further insight into the energy transfer processes that occur at different spatial locations and in different temporal regimes of these pulsed glow discharge plasmas. Although the enhancement of the large population of metastable argon atoms is again confirmed, the mechanism responsible for this enhancement remains unclear. The most likely possibility involves some form of ion–electron recombination followed by radiative relaxation of the resulting species. The metastable argon atoms subsequently Penning ionize sputtered copper atoms which then appear to undergo a similar ion–electron recombination process yielding variable degrees of observable afterpeak emission for copper atom transitions. The kinetic information of these processes was approximated from the corresponding relaxation time. The electron thermalization time allowing for recombination with ions was found to be ∼25 μs after the discharge power termination.

A millisecond pulsed glow discharge is used as a versatile ion source for time-gated generation of elemental, structural, and molecular ions. The utility of this ion source for comprehensive chemical analysis of a series of aromatic and halogenated hydrocarbons is illustrated in this manuscript. To highlight the analytical utility of this transient ion source, it was connected to a gas chromatograph for the mass spectrometric determination of mixtures containing benzene, toluene, o-xylene, cymene, tert-butylbenzene, carbon tetrachloride, chloroform, chlorobenzene, tetrachlorethane, and dichlorobenzene. Explicit chemical analysis was accomplished by introducing the GC eluent into a pulsed glow discharge operating at a rate of 100 Hz with a 50% duty cycle. Using three independent digitizers for time-gated acquisition in three separate time regimes, nearly concurrent collection of elemental, structural, and molecular information was accomplished. In general, elemental information was obtained during the first 0.015 ms after the plasma onset; structural information, as ascertained from molecular fragmentation, was obtained during the plateau time regime when the plasma pulse is at a steady state, whereas molecular M(+) and MH(+) ions were obtained during the afterpeak time regime, that is, after the cessation of the plasma power pulse.

Evidence is provided that illustrates quadrupole ion traps can be used to selectively attenuate strongly bound diatomic ions occurring at the same nominal mass as an analyte ion of interest. Dissociation rates for TaO+ (D0 ∼ 750 kJ mol−1) are found to be at least an order of magnitude larger than the loss rate of Au+ due to scattering under "slow heating" resonance excitation conditions at qz = 0.67 and using neon as the bath gas. This rate difference is sufficient for the selective removal of this strongly-bound diatomic ion over the loss of the Au+ at the same mass-to-charge ratio. Other examples of quadrupole ion trap CID for the selective reduction of common plasma-generated species are also evaluated by examining the dissociation of GdO+ in the presence of Yb+, and Cu2+ in the presence of Te+. In each case, a different method of applying the excitation signals is presented, and the attenuation rates for the diatomic species due to CID are substantially larger than scattering losses for the bare metal ions. Evidence is also presented that demonstrates CID can be accomplished in concert with a slow mass analysis scan, thereby providing a means of (1) eliminating polyatomic ions (formed in the plasma or reaction cell) over an extended mass range, (2) recovering metal ion signal from the metal-containing polyatomic ions, and (3) minimizing deleterious secondary reactions of product ions.

This work focuses on the spatial and temporal characteristics of a glow discharge plasma operated with power pulses of 5 ms in duration at 25% duty cycle. Interpretation of emission data provides insight into the nature of the plasma at each instant of a typical pulse cycle and at each position in space. Because the bulk plasma properties affect the distribution of excited energy levels of the sputtered atoms, an improved understanding of the plasma affords the ability to select conditions that enhance analytically important emission lines. Optical emission spectroscopy was used to determine the relative populations of excited states for atoms and ions during the initial breakdown, the steady state and the recombining periods of the discharge pulse cycle. The plasma is highly ionizing in nature at the time of breakdown—with lower excited states being overpopulated—before reaching the steady state, or plateau, period, also ionizing in nature. These behaviors arise from a loss of charged particles and photons to the surroundings that shifts the plasma away from Saha and Boltzmann balances during these periods. The post-pulse period typically displays recombining behavior, characterized by population inversion for selected species—except for regions close to the cathode, where electrons and ions are lost by diffusion and are not available for recombination. The sputtered analyte atom emissions closely mimic those of the plasma bath gas, except that their emissions persevere for longer in the recombining after-peak period than do the discharge gas species.

A series of bromine-containing flame-retardant plastics was used to demonstrate the applicability of pulsed radio frequency glow discharge mass spectrometry to the determination of elements in commercial polymers. The direct analysis of bulk samples produced both atomic and molecular species, allowing elemental identification and molecular characterization. However, the low ion signal intensities hindered quantification. Atomization mechanisms for the plastic samples were studied in detail using scanning electron microscopy and mass spectrometry. Although some thermal desorption was observed, sputter atomization dominated when samples were not subjected to excessive discharge power. The sputter rates of various polymers, as evidenced by sputter weight loss measurements, showed a strong correlation with their ion production capabilities. Sputtering rates were related to the physical and chemical properties inherent to a polymer's composition. Analysis of samples compacted with a silver binder provided intense analyte signals allowing quantitative analysis. Signal stability, measurement accuracy, measurement precision, and detection limits were all assessed.

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.

The addition of N2 to a millisecond-pulsed glow discharge (PGD) allows diagnostic measurements of the PGD but is found to drastically influence the transient signals arising from the argon and sputtered analyte atoms. Penning excitation between metastable argon atoms and ground state nitrogen molecules and charge transfer between argon ions and the added nitrogen reduce the degree of ionization of sputtered atoms during the power-on, plateau, period by a factor of ∼10 (at 1% N2 by vol.). The added nitrogen affects sputtered atom emission signals less at this time because electron excitation dominates the excitation of these species. Upon power termination, afterpeak, the added nitrogen prevents plasma recombination in two major mechanisms: (i) the nitrogen reduces the number of argon ions available for recombination in the afterpeak; and (ii) vibrationally excited states of nitrogen slow the thermalization of electrons thereby decreasing recombination efficiency. The argon ion population contributes significantly to the afterpeak increase in the number of metastable argon atoms. These atoms are essential for the afterpeak ionization of sputtered atoms. Judicious selection of the nitrogen partial pressure can tune the delay time of afterpeak ionization/recombination by up to 200 μs. This could be particularly beneficial for time-resolved optical or mass spectrometric analyses.

The internal energy distributions, P(epsilon), of a millisecond pulsed radio frequency glow discharge plasma were investigated using tungsten hexcarbonyl W(CO)(6) as a "thermometer molecule". Vapor of the probe molecule, W(CO)(6), was introduced into the plasma and subjected to various ionization and excitation processes therein. The resultant molecular and fragment ions were monitored using a Time-of-Flight mass spectrometer. Ion abundance data were utilized in combination with the known energetics of W(CO)(6) to construct the P(epsilon) plots. The P(epsilon) of W(CO)(6) exhibited strong temporal dependence over the pulse cycle: Distinct internal energy distributions were found at the discharge breakdown period (prepeak), the steady state period (plateau), and the post-pulse period (afterpeak). Spatial variation in P(epsilon) was also observed, especially during the plateau regime. The observations suggest that this pulsed glow discharge affords excellent energy tunability that can be used to perform selective ionization and fragmentation for molecular, structural, and elemental information. Parametric studies were performed to evaluate the effects of discharge pressure and operating power on P(epsilon). These studies also provided insight into the correlation of the observed P(epsilon)s with the fundamental ionization and excitation mechanisms in the plasma. The temporal and spatial variations in P(epsilon) were hence attributed to changes in the dominant energy transfer processes at specific times in specific regions of the plasma. These data will be useful in future efforts to optimize the analytical performance of this source for chemical speciation.

Reaction pathways and rate constants of gas-phase uranium and uranium oxide ions with O-2 and H2O have been investigated using a quadrupole ion trap mass spectrometer (QIT-MS). A new reaction pathway is identified for the reaction between U2+ and H2O, which leads to the formation of UO+ via the intermediate UOH2+. Reaction rate constants are determined for several reactions by measuring the reaction rate at different partial pressures of the reagent gas and are found to be in reasonable agreement with the literature. These rate constants include the first known measurement for the reaction of U2+ with H2O (similar to0.4 k(ADO)). New limits on thermochemical values are also provided for certain species. These include DeltaH(f) (UO2+) less than or equal to 1742 kJ mol(-1) and 1614 less than or equal to DeltaHf (UOH2+) less than or equal to 1.818 kJ mol(-1) and are based on the assumption that only exothermic or thermoneutral reactions are possible under the conditions used. This assumption is supported by simulations of the root-mean-square (RMS) ion kinetic energy of stored uranium ions in the QIT. Only a slight increase in the RMS ion kinetic energies, from 0.1 to 0.2 eV, is predicted over the range of trapping conditions studied (0.05 less than or equal to q(z) less than or equal to 0.75) corresponding to a theoretical reaction temperature of similar to384 K. The simulations also compare helium and neon as bath gases and show that the RMS kinetic energies are found to be very similar at long trapping times (>20 ms), although neon establishes steady state conditions in approximately half the time.

Collision-induced dissociation (CID) rates are measured for a suite of lanthanide (plus yttrium) monoxide ions stored in a quadrupole ion trap. Yttrium, neodymium, and gadolinium oxides, having the same nominal dissociation energy (D0 ≈ 735kJmol−1) but different masses, provide an empirical correction of −1.2s−1amu−1 for the measured CID rates. The CID rate correction enables the correlation of bond dissociation energy with CID rate, allowing quantitative determinations of bond dissociation energies for lanthanide metal monoxide ions. For bond dissociation energies in the range of 567–849kJmol−1, mass corrected rates range from 50 to 120s−1. The calibration sensitivity is equivalent to 5kJmol−1s−1, and the measurement precision is ∼25kJmol−1. Bond energy determinations are found to agree with the average values reported for lanthanide oxide ions.

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 1s5 (3P2) and 1s3 (3P0) 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 3P2 (11.55 eV) and the 3P0 (11.72 eV) metastable argon atom states indicate possible differences in the populations of these two states between the plateau and afterpeak time regimes.