Ion Enhancement Research Articles

Thermal degradation of β-carotene studied by ion mobility atmospheric solid analysis probe mass spectrometry: full product pattern and selective ionization enhancement.

Atmospheric solid analysis probe mass spectrometry has the capability of capturing full product patterns simultaneously including both volatile and semi-volatile compounds produced at elevated temperatures. Real-time low-energy collision-induced fragmentation combined with ion mobility separations enables rapid identification of the chemical structures of products. We present here for the first time the recognition of full product patterns resulting from the thermal degradation of β-carotene at temperatures up to 600 °C. Solvent vapor-induced ionization enhancement is observed, which reveals parallel thermal dissociation processes that lead to even- and odd-numbered mass products. The drift-time distributions of high mass products, along with β-carotene, were monitored with temperature, showing multiple conformations that are associated with the presence of two β-rings. Products of masses 346/347, however, show a single conformation distribution, which indicates the separation of two β-rings resulting from the direct bond scission at the polyene hydrocarbon chain. The thermal degradation pathways are evaluated and discussed.

Improved Steroids Detection and Evidence for Their Regiospecific Decompositions Using Anion Attachment Mass Spectrometry

Nonpolar anabolic steroids are doping agents that typically do not provide strong signals by electrospray ionization-mass spectrometry (ESI-MS) owing especially to the low polarity of the functional groups present. We have investigated the addition of anions, in ammonium salt form, to anabolic steroid samples as ionization enhancers and have confirmed that lower instrumental limits of detection (as low as 10 ng/mL for fluoxymesterone-M) are obtained by fluoride anion attachment mass spectrometry, as compared to ESI(+)/(-) or atmospheric pressure photoionization (APPI)(+). Moreover, collision-induced decomposition (CID) spectra of precursor fluoride adducts of the bifunctional steroid "reduced pregnenolone" (containing two hydroxyl groups) and its d4-analogue provide evidence of regiospecific decompositions after attachment of fluoride anion to a specific hydroxyl group of the steroid. This type of charting of specific CID reaction pathways can offer value to selected reaction monitoring experiments (SRM) as it may result in a gain in selectivity in detection as well as in improvements in quantification.

Helicity sensitive enhancement of strong-field ionization in circularly polarized laser fields.

We investigate the strong-field ionization from p± orbitals driven by circularly polarized laser fields by solving the two-dimensional time-dependent Schrödinger equation in polar coordinates with the Lagrange mesh technique. Enhancement of ionization is found in the deep multiphoton ionization regime when the helicity of the laser field is opposite to that of the p electron, while this enhancement is suppressed when the helicities are the same. It is found that the enhancement of ionization is attributed to the multiphoton resonant excitation. The helicity sensitivity of the resonant enhancement is related to the different excitation-ionization channels in left and right circularly polarized laser fields.

Selective enhancement of resonant multiphoton ionization with strong laser fields

High-resolution photoelectron momentum distributions of Xe atoms ionized by 800-nm linearly polarized laser fields have been traced at intensities from 1.1*1013 to 3.5*1013W/cm2 using velocity-map imaging techniques. At certain laser intensities, the momentum spectrum exhibits a distinct double-ring structure for low-order above-threshold ionization, which appears to be absent at lower or higher laser intensities. By investigating the intensity-resolved photoelectron energy spectrum, we find that this double-ring structure originates from resonant multiphoton ionization involving multiple Rydberg states of atoms. Varying the laser intensity, we can selectively enhance the resonant multiphoton ionization through certain atomic Rydberg states. The photoelectron angular distributions of multiphoton resonance are also investigated for the low-order above-threshold ionization.

Migration of surface excitations in highly-excited nanosystems probed by intense resonant XUV radiation

Ionization dynamics of resonantly excited helium nanodroplets have been studied by intense XUV light. By doping the nanodroplets with atoms that either attach to the surface or submerge into the center of the droplet, one can study the dynamics of excitation and ionization through the droplet. When resonantly exciting the droplet, we observe a strong ionization enhancement for atoms attached to the surface. On the other hand, atoms embedded inside the nanodroplet are less efficiently ionized. We attribute this effect to an ultrafast energy transfer to the surface of the droplet and subsequent Penning ionization of the surface-bound dopant.

Restricted access supramolecular solvents for removal of matrix-induced ionization effects in mass spectrometry: Application to the determination of Fusarium toxins in cereals

Ion suppression/enhancement caused by matrix effects continues being a major concern in liquid chromatography–mass spectrometry (LC–MS). This research explores the ability of a supramolecular solvent (SUPRAS) made up of inverted hexagonal aggregates of oleic acid to behave as a liquid with restricted access properties (SUPRAS-RAM). Fusarium toxins in cereals were extracted with the oleic acid-based SUPRAS-RAM prior to their quantification by LC-electrospray ionization (ESI)-ion trap-MS (LC-ESI-IT-MS) in order to investigate the capability of this solvent to remove or reduce ionization suppression and/or enhancement in the analysis of complex samples by MS. The method involved the vortex-shaking of 300mg of cereal with 600μL of the SUPRAS-RAM for 15min, centrifugation for separation of the supernatant and quantitation by LC-ESI-IT-MS. Macromolecules such as proteins and carbohydrates were excluded from extraction by chemical and physical mechanisms. Extraction of analytes and sample clean-up were thus carried out in a single step. No evaporation of the extracts was needed. Method detection limits for the legislated Fusarium toxins [i.e. deoxynivalenol (DON), zearalenone (ZEA) and fumonisins B1 (FB1) and B2 (FB2)] were 15μgkg−1 for DON and ZEA and 8μgkg−1 for fumonisins. These values were far below the maximum levels set by the European Commission for these toxins in foodstuffs. The method was successfully applied to the determination of these toxins in wheat and maize harvested in the South of Spain. No contamination of Fusarium toxins was found in samples at detectable levels. Recoveries in spiked samples were in the range 87–105%, with relative standard deviations between 1 and 7%. The use of the oleic acid-based SUPRAS-RAM effectively removed matrix interferences and allowed reliable quantitation of Fusarium toxins in cereals using solvent-based calibration.

Theoretical study of Jesse effect in tritium measurements using ionization chambers

Jesse effect caused by impurities in helium might enhance the output signal significantly in tritium measurements with ionization chamber, which will lead to overestimation of tritium concentration in experiments. A theoretical method was proposed to evaluate Jesse effect quantitatively. Results indicate that besides Penning ionization, sub-excitation electrons also place very important influence on ionization enhancement by Jesse effect. An experiential expression about the relationship between enhancement factor and impurity concentration was established, in which second order of it fits experimental results very well. Theoretical calculation method in this paper is also applicable to evaluate Jesse effect in other kinds of mixtures besides hydrogen as impurities in helium. In addition, Jesse effects about tritium molecules as impurities have also been investigated.

Spherical-harmonics expansion method for density-matrix simulations of quantum electron dynamics in continuum states

Spherical-harmonics expansion method is proposed to solve the quantum time-evolution equations for density matrices numerically in momentum space. This method facilitates efficient real-time simulations of quantum electron dynamics in continuum states through extension of our previous formalism developed for discrete states. Numerical accuracy and efficiency are demonstrated through two examples: (i) multiphoton ionization of a one-electron atom in intense laser field, and (ii) real-time dynamics of plasma oscillations in electron liquids. In case (i), coupled dynamics of density matrices for bound and continuum states reveals an enhancement of multiphoton ionization over the Keldysh approximation through resonant intermediate states. © 2015 Wiley Periodicals, Inc.

Quantitative fingerprinting of O-linked glycans released from proteins using isotopic coded labeling with deuterated 1-phenyl-3-methyl-5-pyrazolone

Investigation of oligosaccharides attached to proteins as post-translational modification remains an important research field in the area of glycoproteomics as well as in biotechnology. The development of new tools for qualitative and quantitative analysis of glycans has gained high importance in recent years. This is particularly true with O-glycans for which quantitative data are still underrepresented in literature. This fact is probably due to the absence of an enzyme for general release of O-linked saccharides from glycoproteins and due to their low ionization yield in mass spectrometry (MS).In this paper, a method is established aimed at improved qualitative and quantitative analysis of mucin-type O-glycans. A chemical reaction combining release and derivatization of O-glycans in one step is combined here with mass spectrometric quantification. For the purpose of improved quantitative analysis, stable-isotope coded labeling by d0/d5 1-phenyl-3-methyl-5-pyrazolidone (PMP) was performed. The “heavy”-version of this label, penta-deutero (d5)-PMP, was synthesized for this purpose. Beneath improving the reproducibility of quantitation, PMP derivatization contributed to an enhancement of ionization yields in MS. By introducing an internal standard (e.g. GlcNAc3) the reproducibility for quantification can be improved. For higher abundant O-glycans a mean coefficient of variation (CV) less than 6% could be attained, for very low abundant CV values between 15 and 20%. For the determination of O-glycan profiles in mixtures, a HPLC separation was combined with a high resolution Qq-oaTOF instrument. RP-type stationary phases were successful in separating glycan species including some of isomeric ones. This separation step was particularly useful for removing of salts avoiding so the presence of various sodium clusters in the MS spectrum.

Ionospheric modifications in high frequency heating experiments

Featured observations in high-frequency (HF) heating experiments conducted at Arecibo, EISCAT, and high frequency active auroral research program are discussed. These phenomena appearing in the F region of the ionosphere include high-frequency heater enhanced plasma lines, airglow enhancement, energetic electron flux, artificial ionization layers, artificial spread-F, ionization enhancement, artificial cusp, wideband absorption, short-scale (meters) density irregularities, and stimulated electromagnetic emissions, which were observed when the O-mode HF heater waves with frequencies below foF2 were applied. The implication and associated physical mechanism of each observation are discussed and explained. It is shown that these phenomena caused by the HF heating are all ascribed directly or indirectly to the excitation of parametric instabilities which instigate anomalous heating. Formulation and analysis of parametric instabilities are presented. The results show that oscillating two stream instability and parametric decay instability can be excited by the O-mode HF heater waves, transmitted from all three heating facilities, in the regions near the HF reflection height and near the upper hybrid resonance layer. The excited Langmuir waves, upper hybrid waves, ion acoustic waves, lower hybrid waves, and field-aligned density irregularities set off subsequent wave-wave and wave-electron interactions, giving rise to the observed phenomena.

A sensitive method for digoxin determination using formate-adduct ion based on the effect of ionization enhancement in liquid chromatograph-mass spectrometer.

A sensitive and rapid method based on formate-adduct ion detection was developed and fully validated for digoxin determination in rat plasma. For LC/MS/MS detection with formate-adducts as precursor ions, transitions of m/z 825.5→779.9 for digoxin and m/z 809.5→763.4 for the internal standard (digitoxin) were monitored in negative mode. To investigate the impact of formic acid on the mass response and method sensitivity, a formic acid concentration range of 0-0.1% (0, 0.0005%, 0.002%, 0.01%, 0.1%, v/v) was evaluated. A concentration of 0.002% gave the highest sensitivity, which was 16- to 18-fold higher than deprotonated ions, and was designated as the contribution giving the strongest ionization enhancement and adduction. A number of parameters were then varied in order to optimize the method, and a limit of quantitation (LOQ) at 0.2 ng/mL was reached with an injection volume of 5 μL, a total run time of 3 min, and 0.1 mL of rat plasma. A calibration curve was plotted over the range 0.2-50 ng/mL (R(2)=0.9998), and the method was successfully applied to study pharmacokinetics in rat following a single oral administration of digoxin (0.05 mg/kg). Four additional steroid saponins (digitoxin, deslanoside, ginsenoside Rg1 and Rb1) were investigated to assess the impact of formic acid on the mass response of steroid saponins. Compounds with a conjugated lactonic ring in their structures such as digoxin, digitoxin and deslanoside tended to form stable formate-adduct ions more easily. The LC/MS/MS method developed here is therefore well suited for the quantification of steroid saponins that are difficult to deprotonate using other MS approaches.

Five-photon sequential double ionization of He in intense extreme-ultraviolet free-electron laser fields

A multiphoton ionization process of He in intense extreme ultraviolet free-electron laser fields is investigated in a photon energy range of $h\mathit{v}=20.1\ensuremath{-}20.8\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ using shot-by-shot photoelectron spectroscopy. A photoelectron peak resulting from sequential double ionization of He due to five-photon absorption is observed around $h\mathit{v}=20.4\phantom{\rule{0.16em}{0ex}}$. Shot-by-shot analysis of photoelectron spectra shows that two-photon resonance to the $\mathrm{H}{\mathrm{e}}^{+}(2s{\phantom{\rule{0.16em}{0ex}}}^{2}S)$ state from the ${\mathrm{He}}^{+}$ ground state is responsible for the substantial enhancement of five-photon double ionization. Effective cross sections for two-photon absorption between the $1s$ and $2s$ states of ${\mathrm{He}}^{+}$ and for one-photon absorption from the ${\mathrm{He}}^{+}$ $2s$ state are obtained.

High-precision (143)Nd/(144)Nd ratios from NdO(+) data corrected with in-run measured oxygen isotope ratios.

The NdO(+) technique has been considerably refined in recent years for high-precision measurement of Nd isotope ratios in low-level samples (1-5 ng Nd). As oxygen isotopic compositions may vary significantly with experimental conditions such as filament material, ionization enhancer and the ambient oxygen in the ion source, great "care" should be taken for using correct oxygen isotopic compositions to do the isobaric oxide corrections for the "conventional" NdO(+) method. Our method presented here for NdO(+) data reduction and PrO(+) interference corrections uses the oxygen isotope composition determined in each cycle of the NdO(+) measurements. For that purpose, we measured the small ion signals of (150)Nd(17)O(+) and (150)Nd(18)O(+) with amplifiers equipped with 10(12) Ω feedback resistors, and those of Nd(16)O(+) ion beams with 10(11) Ω amplifiers. Using 10(12) Ω amplifiers facilitates a precise measurement of the very small (150)Nd(17)O(+) and (150)Nd(18)O(+) ion signals and calculation of highly accurate and precise (143)Nd/(144)Nd isotope ratios. The (143)Nd/(144)Nd ratios for JNdi-1 standards and several whole-rock reference materials determined with the method on 4 ng of Nd loads are consistent with previously reported values within analytical error, with internal and external precision (2 RSE and 2 RSD) of better than 20 and 30 ppm, respectively.

The unexpected formation of [M - H]+ species during MALDI and dopant-free APPI MS analysis of novel antineoplastic curcumin analogues.

Unusual ionization behavior was observed with novel antineoplastic curcumin analogues during the positive ion mode of matrix-assisted laser desorption ionization (MALDI) and dopant-free atmospheric pressure photoionization (APPI). The tested compounds produced an unusual significant peak designated as [M - H](+) ion along with the expected [M + H](+) species. In contrast, electrospray ionization, atmospheric pressure chemical ionization and the dopant-mediated APPI (dopant-APPI) showed only the expected [M + H](+) peak. The [M - H](+) ion was detected with all evaluated curcumin analogues including phosphoramidates, secondary amines, amides and mixed amines/amides. Our experiments revealed that photon energy triggers the ionization of the curcumin analogues even in the absence of any ionization enhancer such as matrix, solvent or dopant. The possible mechanisms for the formation of both [M - H](+) and [M + H](+) ions are discussed in this paper. In particular, three proposed mechanisms for the formation of [M - H](+) were evaluated. The first mechanism involves the loss of H2 from the protonated [M + H](+) species. The other two mechanisms include hydrogen transfer from the analyte radical cation or hydride abstraction from the neutral analyte molecule.

Resonance-like enhancement in high-order above-threshold ionization of formic acid

We have investigated the high-order above-threshold ionization (HATI) spectrum of formic acid by means of high-resolution electron spectroscopy using 800-nm femtosecond laser light. The resonance-like enhancement structures in the HATI spectrum are shown to be experimentally reproducible and three enhancement-suppression patterns are observed in different energy regimes with increasing laser intensity. We simulated these patterns successfully by computing the exact solution of time-dependent Schr\"odinger equation based on the simple hydrogen-like atom, subject to a time-dependent high-intensity laser pulse. The agreement between experiment and theory shows that ``channel closing'' effects play an important role in the observed resonance-like enhancements (RLEs). The existence of the RLEs for formic acid, which lacks any symmetry, shows that molecular symmetry does not affect the RLEs.

Investigations Into the Interactions of a MALDI Matrix with Organic Thin Films Using C60+ SIMS Depth Profiling.

Molecular depth profiling of multilayer organic films is now an established protocol for cluster secondary ion mass spectrometry (SIMS). This unique capability is exploited here to study the ionization mechanism associated with matrix-enhanced SIMS and possibly matrix assisted laser desorption/ionization (MALDI). Successful depth profiling experiments were performed on model bi-layer systems using 2,5-dihydroxybenzoic acid (DHB) as the matrix with dipalmitoylphosphatidylcholine (DPPC) or phenylalanine (PHE). The interaction between the matrix and organic analyte is monitored at the interface of the films. Tri-layer films with D2O as a thin-film sandwiched between the matrix and organic layers are also investigated to determine what role, if any, water plays during ionization. The results show successful depth profiles when taken at 90K. Mixing is observed at the interfaces of the films due to primary ion bombardment, but this mixing does not recreate the conditions necessary for ionization enhancement.

Enhanced one-photon double ionization of atoms and molecules in an environment of different species.

The correlated nature of electronic states in atoms and molecules is manifested in the simultaneous emission of two electrons after absorption of a single photon close to the respective threshold. Numerous observations in atoms and small molecules demonstrate that the double ionization efficiency close to threshold is rather small. In this Letter we show that this efficiency can be dramatically enhanced in the environment. To be specific, we concentrate on the case where the species in question has one or several He atoms as neighbors. The enhancement is achieved by an indirect process, where a He atom of the environment absorbs a photon and the resulting He(+) cation is neutralized fast by a process known as electron transfer mediated decay, producing thereby doubly ionized species. The enhancement of the double ionization is demonstrated in detail for the example of the Mg · He cluster. We show that the double ionization cross section of Mg becomes 3 orders of magnitude larger than the respective cross section of the isolated Mg atom. The impact of more neighbors is discussed and the extension to other species and environments is addressed.

Ultranarrow ionization resonances in a quantum dot under broadband excitation

Semiconductor quantum dots driven by the broadband radiation fields of nearby quantum point contacts provide an exciting new setting for probing dynamics in driven quantum systems at the nanoscale. We report on real-time charge-sensing measurements of the dot occupation, which reveal sharp resonances in the ionization rate as a function of gate voltage and applied magnetic field. Despite the broadband nature of excitation, the resonance widths are much smaller than the scale of thermal broadening. We show that such resonant enhancement of ionization is not accounted for by conventional approaches relying on elastic scattering processes, but can be explained via a mechanism based on a bottleneck process that is relieved near excited state level crossings. The experiment thus reveals a new regime of a strongly driven quantum dynamics in few-electron systems. The theoretical results are in good agreement with observations.

A low-power magnetic-field-assisted plasma jet generated by dielectric-barrier discharge enhanced direct-current glow discharge at atmospheric pressure

A magnetic field is introduced to the dielectric-barrier discharge enhanced direct-current glow discharge for efficient plasma generation, with the discharge power of 2.7 W and total energy consumption reduced to 34% of the original. By spatially examining the emission spectra and plasma temperature, it is found that their peaks shift from edges to the center and the negative and anode glows merge into the positive column and disappear, accompanied by improvement of uniformity and chemical activity of the enlarged plasma. This lies in the enhancement of ionization in the curved and lengthened electron path and the dispersion of discharge domains.

Ionization cross sections and rate coefficients for the ground state of AsI-like ions

Detailed level-by-level calculations of cross sections and rate coefficients for electron impact direct ionization and excitation-autoionization of AsI-like ions in the 3d 104s 24p 3 (4S3/2) ground state have been performed. The cross sections are presented in the energy range near the threshold for the five ions Mo9+, Xe21+, Pr26+, Dy33+ and W41+. The rate coefficients are given for all the ions from Sr5+ to U59+ in the AsI sequence at the seven electron temperatures (k T e = 0.1E I ,0.3E I ,0.5E I ,0.7E I ,E I ,2E I and 10E I , where E I is the first ionization energy). The calculations include the contribution of direct ionization (DI) calculated using the Lotz formula approximation and the contributions of excitation-autoionization (EA) computed in the framework of the Distorted Wave (DW) approximation for the 4s − nl, 3d − nl and 3p − nl resonant inner-shell excitations. The ionization enhancement due to the EA channels is presented as a function of Z along the AsI isoelectronic sequence. The present results show the great importance of the EA processes; an ionization enhancement factor of up to 6.5 is predicted for instance for Dy33+ (Z = 66) at electron temperature of coronal equilibrium maximum abundance.