Ion Enhancement Research Articles

Graphene Oxide Carburization Enhanced Ionization Efficiency for TIMS Isotope Ratio Analysis of Uranium at Trace Level.

Isotope analysis of trace uranium is important in nuclear safeguards and nuclear forensics, which requires the analytical methodologies with high sensitivity, accuracy, and precision. As one of the most powerful techniques in isotopic measurement, thermal ionization mass spectrometry (TIMS) usually suffers from its relatively low sensitivity in ultratrace measurements. To overcome this limitation, we have developed a new filament carburization technique for TIMS, with graphene oxide (GO) as the ionization enhancer. A high and steady ionization efficiency of ∼0.2% for uranium was achieved in single-filament mode, which was 10× the classical double-filament method. With total evaporation (TE) measurements, this method was validated with certified reference materials (CRMs) at the picogram level, and the relative uncertainties for n(235U)/ n(238U) were as low as the ∼1% level. The enhancement mechanism of GO's promoting effect on uranium ionization was attributed to the uniform microstructure facilitating energy transfer and formation of carbides. This approach provides an alternative simple and rapid method for trace uranium isotope analysis with high sensitivity and excellent repeatability. Filament carburization and uranium loading could be accomplished within 10 min. This technique has great advantage in analysis of trace uranium isotope ratios and can be applied in the researches of environmental analysis and nuclear forensics.

Multiresidue Pesticides Analysis of Vegetables in Vietnam by Ultrahigh-Performance Liquid Chromatography in Combination with High-Resolution Mass Spectrometry (UPLC-Orbitrap MS).

An ultrahigh-performance liquid chromatography in combination with high-resolution mass spectrometry Thermo Q-Extractive Focus Orbitrap MS has been introduced for analysis of multiclass pesticides in vegetable samples collected in Hanoi, Vietnam. Multiclass pesticides were separated on the Thermo Hypersil Gold PFP column utilizing a gradient of the mobile phase consisting of 5 mM ammonium formate, 0.1% formic acid in deionized water, and methanol. The target analytes were detected in the full-scan mode on Thermo Scientific Q-Exactive Focus Orbitrap MS for quantitation at the optimum operating conditions. These conditions included, but not limit to, the resolution of 70000 at the full width at half maximum in both positive and negative mode, mass range from 80 to 1000 m/z, and optimized parameters for the heated electrospray ionization source. The identification of the analytes in real samples was based on retention times, mass to charge ratios, mass accuracies, and MS/MS spectra at the confirmation mode with the inclusion list of target analytes. The mass accuracies of target analytes were from −4.14 ppm (dinotefuran) to 1.42 ppm (cinosulfuron) in the neat solvent and from −3.91 ppm (spinosad D) to 1.29 ppm (cinosulfuron) in the matrix-matched solution. Target analytes in the vegetable-based matrix were extracted by the QuEChERS method. Some critical parameters of the analytical method such as linearity, repeatability, limit of detection, and limit of quantitation have been evaluated and implemented. Excellent LOD and LOQ of the developed method were achieved at the range of 0.04–0.85 and 0.13–2.9 μg·kg−1, respectively. Intraday and interday repeatability of the analytical signal (peak area, n=6) of the developed method were below 3% and 10%, correspondingly. The matrix effect, extraction recovery, and overall recovery were fully investigated by spiking experiments. Experimental results demonstrated that the ionization suppression or enhancement was the main contribution on the overall recoveries of target analytes. Finally, the in-house validated method was applied to pesticides screening in vegetables samples in local villages in Hanoi, Vietnam. The concentrations of all target analytes were below limit of quantitation and lower than US-FDA or EU maximum residue levels.

Quantum effect of laser-induced rescattering from the tunneling barrier

Rescattering mediated by intense laser fields is one of the dominant processes in strong-field physics. It was usually described classically or semiclassically. Recently, laser-induced recollisions with the tunneling barriers have attracted great theoretical interest. However, fully distinguishing the ``quantum'' effect of rescattering with the tunneling barrier on photoelectron energy and momentum distributions is very difficult if using the linearly polarized light field. Here we perform the measurement of photoelectron momentum distributions by strong-field ionization of Ar atoms in orthogonally polarized two-color laser fields with comparable intensities. We observe the strong ionization enhancement due to rescattering on two-dimensional momentum distribution, which is in striking disagreement with the predictions by the plain strong-field approximation and the classical-trajectory Monte Carlo simulation. With the help of the analysis on the electron complex trajectories in both sub-barrier and quasicontinuum regions, the enhancement is illustrated to originate from the quantum effect of forward scattering between the tunneling electron and its parent ion. The study shows that the electron probability will be significantly modified when the rescattered electrons excurse in the continuum and the ``classical'' rescattering process needs the quantum description.

Molecular ionization enhancement by charge rearrangement at high X-ray intensity

We simulated the multi-photon multi-ionization dynamics of an iodomethane molecule, CH3I, exposed to ultraintense and ultrashort x-ray pulses. The strong ionization causes electronic charge rearrangement in the molecule that leads to an enhanced total charge.

Construction and testing of an atmospheric-pressure transmission-mode matrix assisted laser desorption ionisation mass spectrometry imaging ion source with plasma ionisation enhancement

Matrix assisted laser desorption ionisation mass spectrometry (MALDI-MS) at atmospheric pressure (AP) is, with a few notable exceptions, overshadowed by its vacuum based forms and AP transmission mode (TM) MALDI-MS lacks the up-take its potential benefits might suggest. The reasons for this are not fully understood and it is clear further development is required to realise the flexibility and power of this ionisation method and geometry. Here we report the build of a new AP-TM-MALDI-MSI ion source with plasma ionisation enhancement. This novel ion source is used to analyse a selection of increasingly complex systems from molecular standards to murine brain tissue sections. Significant enhancement of detected ion intensity is observed in both positive and negative ion mode in all systems, with up to 2000 fold increases observed for a range of tissue endogenous species. The substantial improvements conferred by the plasma enhancement are then employed to demonstrate the acquisition of proof of concept tissue images, with high quality spectra obtained down to 10 × 10 μm pixel size.

Spatial asymmetry in topside ion density and vertical E × B plasma drift velocity within 75°E–95°E

The ion density measured by the Ionospheric Plasma and Electrodynamics Instrument (IPEI) on board the ROCSAT -1 over the 75°E and 95°E meridian at 600km altitude has been utilized to examine the latitudinal and longitudinal distribution within the Indian sector, in particular, the north-south and east-west asymmetries of the equatorial ionization anomaly (EIA). A longitudinal gradient in ion density at 600 km higher towards 95°E develops during the noontime and afternoon hours when the EIA is at its peak. The density gradient persists till evening hours when pre-reversal enhancements occur. The vertical E × B plasma drift velocity measured simultaneously by ROCSAT -1 for the same space-time configuration has also been studied. In addition to diurnal, seasonal and solar activity variations in E × B drift velocity, the longitudinal gradient is also observed. The EIA at the altitude of 600 km peaks at different latitudes and are mostly asymmetric about the magnetic equator. From midnight till 0800 LT, the ion density across the equator is nearly uniform in the equinoxes. But in the solstices, the density exhibits a north-south gradient. In the June solstice, density is higher in the northern hemisphere and decreases gradually towards south. The gradient in density reverses in December solstice. Normally, the EIA peaks within 1200 LT and 1600 LT while around 2000 LT, pre-reversal enhancement of ionization occurs affecting the EIA evening structure. The strength of the EIA also exhibits seasonal, year-to-year and hemispheric variations. The longitudinal asymmetry of drift velocity along 75°E and 95°E longitude sectors is the contributing factor behind the observed longitudinal asymmetry in ion density. Significant positive correlation between the strength of the EIA and E × B drift is observed in both longitudes.

Control of H_{2} Dissociative Ionization in the Nonlinear Regime Using Vacuum Ultraviolet Free-Electron Laser Pulses.

The role of the nuclear degrees of freedom in nonlinear two-photon single ionization of H_{2} molecules interacting with short and intense vacuum ultraviolet pulses is investigated, both experimentally and theoretically, by selecting single resonant vibronic intermediate neutral states. This high selectivity relies on the narrow bandwidth and tunability of the pulses generated at the FERMI free-electron laser. A sustained enhancement of dissociative ionization, which even exceeds nondissociative ionization, is observed and controlled as one selects progressively higher vibronic states. With the help of abinitio calculations for increasing pulse durations, the photoelectron and ion energy spectra obtained with velocity map imaging allow us to identify new photoionization pathways. With pulses of the order of 100fs, the experiment probes a timescale that lies between that of ultrafast dynamical processes and that of steady state excitations.

Nighttime Enhancements in the Midlatitude Ionosphere and Their Relation to the Plasmasphere

AbstractIn situ electron density measurements by the CHAllenging Minisatellite Payload and the Defense Meteorological Satellites Program F17 satellites show that the midlatitude ionization at altitudes of ∼350 and 850 km is enhanced in the late evening. The enhancements increase to maximum around midnight and are clearly observed till early morning as the equatorial ionization decays to minimal level. They appear in the winter hemisphere during June and December solstices and in both hemispheres during equinox. The enhancements are well confined between ±30° and ±50° magnetic latitude, with the magnetic flux tubes of L = 1.3 − 2.4 connecting to the plasmasphere. Furthermore, coincident longitudinal variations exist in both the ionospheric enhancements and the plasmaspheric total electron content, especially during the solstice months. The coincidence may suggest essential plasma transport between the ionosphere and the plasmasphere. These facts support the idea that the plasmasphere provides extra plasma to the midlatitude ionosphere through downward plasma influx along the magnetic field lines to form the nighttime ionization enhancements when the sunlight is absent.

Determination and quantification of arbutin in plants using stable isotope dilution liquid chromatography–mass spectrometry

Arbutin is a very safe whitening agent for human skin. Since it is more expensive than other agents and has a challenging synthesis, novel methods to obtain this valuable agent are needed. In this study, we developed a precise and accurate method to detect and quantify arbutin using stable isotope dilution liquid chromatography–mass spectrometry (LC–MS). One challenge that needed to be overcome was the matrix effect occurring during the LC–MS analysis due to the analyte ionisation enhancement or suppression in the electrospray ionisation source by co-eluting compounds. Notably, arbutin had different matrix effects in the various sample matrices. A solution to this problem was the use of [d4]-arbutin as a stable isotope-labelled internal standard (SIL-IS), as it compensated the matrix effect of arbutin because it was affected by almost the same matrix effect. The validation of the developed method showed excellent linearity (r2 = 1.000), precision (relative standard deviation ≤ 2.5%), accuracy (recovery, 97.42–98.52%), limit of detection (0.03 μg/mL), and limit of quantification (0.1 μg/mL). Finally, the method of arbutin detection was applied to blueberry leaves to compare the precision and accuracy results obtained by performing stable isotope dilution using LC–MS and gas chromatography–mass spectrometry. The method was applied to strawberry leaves and pear peels, indicating that the SIL-IS method can be expected to find application in the arbutin analysis in other plants.

Ionospheric response to an intense solar flare in equatorial and low latitude region

Multi-instrument data recorded at multi-stations are used to study the equatorial and low-latitude ionospheric response to an intense solar flare of class X7 (2B) in the current solar cycle 24 with the peak at 08:05 UT on 09 August 2011. Rapid changes in ionospheric total electron contents (TEC) measured by global positioning system (GPS) showed an enhancement of 2–3 TECU. The very low frequency (VLF) data recorded at a low latitude station Varanasi showed an enhancement of VLF signal amplitude during the solar flare period which is attributed to the sudden enhancement of D-region ionization. Ground based GPS measurements are also validated by analyzing the electron density profiles measured from COSMIC satellite mission. COSMIC-derived electron density profile shows a decrease below F2 peak altitude and increase above F2 peak. The D-region ionospheric perturbation observed during the solar flare could be caused by flare time enhanced level of photo-ionization due to X-ray flux enhancement, whereas for E and F-region ionosphere, enhanced EUV flux causes photo-ionization and hence perturbed the TEC.

Enhancement of nonsequential double ionization in counter-rotating two-color circularly polarized laser fields

Nonsequential double ionization is theoretically investigated in the counter-rotating two-color [frequencies (ω1,ω2)] circularly polarized laser fields with classical ensemble approach. It is found that remarkable ionization enhancement is obtained by adjusting the relative electric field amplitude ratios (γE). The result indicates that there is a broader steerable range of γE to obtain higher ionization enhancement for ω2=3ω1 than that for ω2=2ω1. Through the analysis of trajectories, the recollision picture for ω2=3ω1 is complicated leading to slow change of double ionization, which is benefit to gentle control mechanism. However, the recollision ionization channel is easily distinguishable and sensitive to control double ionization in ω2=2ω1 laser fields and the travel time can be controlled in the order of magnitude of 100 attoseconds.

Resonance-like enhancement in high-order above threshold ionization of atoms and molecules in intense laser fields.

We investigate the high-order above-threshold ionization (HATI) of atoms (Ar and Xe) and molecules (N2 and O2) subjected to strong laser fields by numerically solving time-dependent Schrödinger equation. It is demonstrated that resonance-like enhancement of groups of adjacent peaks in photoelectron spectrum of HATI is observed for Ar, Xe, and N2, while this peculiar phenomenon is absent for O2, which is in agreement with experimental observation [ Phys. Rev. A88, 021401 (2013)]. In addition, analysis indicates that resonance-like enhancement in HATI spectra of atoms and molecules is closely related to excitation of the high-lying excited states.

Suppression in high-order above-threshold ionization: destructive interference from quantum orbits

We experimentally study the above-threshold ionization (ATI) spectra of noble gas argon in an intense laser field and focus on a novel suppression structure in the high-order ATI (HATI) spectra. It is found that, when a well-documented resonancelike enhancement feature appears in the HATI spectra, a significant suppression structure is followed in a higher energy region of the spectra. The observation is well reproduced by a numerical solution of the time-dependent Schrödinger equation. In terms of quantum-orbit theory, the observed suppression structure can be ascribed to the destructive interference from longer quantum orbits. Furthermore, an intrinsic relation between the ionization suppression and the ionization enhancement in the HATI spectra is well established.

Enhancement and Suppression of Ionization in Drug Analysis Using HPLC-MS/MS in Support of Therapeutic Drug Monitoring: A Review of Current Knowledge of Its Minimization and Assessment: Erratum.

High-performance liquid chromatography coupled with tandem mass spectrometry is commonly used for quantitation of analytes in biological matrices, because of the selectivity, sensitivity, and high throughput offered by this technique. However, the presence of both suppression and enhancement of ionization (SEI) by matrix components is an increasingly recognized impediment to accurate results. The existence of SEI indicates that ionization efficiency is a result of the chemical environment seen by both the analyte and internal standard during ion formation. SEI is influenced by the type and the make of ion source used, mobile-phase composition, extent of sample preparation, and the ability to chromatographically separate other compounds that may influence ionization of the analyte and/or internal standard. A comprehensive review of the phenomenon of SEI in high-performance liquid chromatography coupled with tandem mass spectrometry was conducted, and a summary of salient papers relating to therapeutic agents in biological matrices is presented. Suggestions for approaches to minimize, normalize, or assess SEI and its deleterious effect on accuracy and sensitivity, and hence the validity of quantitative results, are provided. Consideration is also given to a strategy to test for SEI, including the number of samples from different sources that are required to adequately test for SEI.

Investigation into the formation of the scrape-off layer density shoulder in JET ITER-like wall L-mode and H-mode plasmas

The low temperature boundary layer plasma (scrape-off layer or SOL) between the hot core and the surrounding vessel determines the level of power loading, erosion and implantation of material surfaces, and thus the viability of tokamak-based fusion as an energy source. This study explores mechanisms affecting the formation of flattened density profiles, so-called ‘density shoulders’, in the low-field side (LFS) SOL, which modify ion and neutral fluxes to surfaces—and subsequent erosion. There is evidence against local enhancement of ionization inducing shoulder formation. We find that increases in SOL parallel resistivity, Λdiv (=[L||νeiΩi]/csΩe), postulated to lead to shoulder growth through changes in SOL turbulence characteristics, correlates with increases in SOL shoulder amplitude, As, but only under a subset of conditions (D2-fuelled L-mode density scans with outer strike point on the horizontal target). Λdiv fails to correlate with As for cases of N2 seeding or during sweeping of the strike point across the horizontal target. The limited correlation of Λdiv and As is also found for H-mode discharges. Thus, while it may be necessary for Λdiv to be above a threshold of ~1 for shoulder formation and/or growth, another mechanism is required. More significantly, we find that in contrast to parallel resistivity, outer divertor recycling, as quantified by the total outer divertor Balmer Dα emission, I–Dα, does scale with As where Λdiv does and even where Λdiv does not. Divertor recycling could lead to SOL density shoulder formation through: (a) reducing the parallel to the field flow (loss) of ions out of the SOL to the divertor; and (b) changes in radial electric fields which lead to E × B poloidal flows as well as potentially affecting SOL turbulence birth characteristics. Thus, changes in divertor recycling may be the sole process involved in bringing about SOL density shoulders or it may be that it acts in tandem with parallel resistivity.

Advances in Space Weather Data Interpretation and Simulations

This editorial highlights recent data and model results for Space Weather readers. One of the ways that American Geophysical Union (AGU) features some of the most interesting journal articles is Editors' Highlights. As of late 2017 these are now hosted on the Eos website: https://eos.org/editor-highlights, giving them new prominence. Editors contribute a short summary of the highlighted article in language that will appeal to general Eos readers and include a “teaser” line and visually attractive figure from the manuscript. Doing so helps to share scientific content beyond a journal's readership to a broader audience. However, occasionally a situation arises, wherein an Editor sees a manuscript worthy of additional notoriety, but the summary simply needs more technical wording than might be appropriate for general readers. Below, I am highlighting two such manuscripts that illustrate advances in Space Weather (SWE). Based on analysis of total electron content estimated from Global Positioning System (GPS) and Defense Meteorological Satellite Program spacecraft measurements, Katamzi-Joseph et al. (2017) report the occurrence of ionospheric depletions (plasma bubbles) at anomalously high, European latitudes (42°N geographic) during two intense geomagnetic storms in 2000 and 2001. Somewhat counterintuitively, such depletions may develop in the postsunset sector where plasma density has been previously enhanced. GPS signals often scintillate passing through these bubbles. The authors suggest that conditions supporting rapid upward plasma drift near the equator can extend to the midlatitude ionosphere during storm time, making these regions susceptible to navigation signal scintillation during intense geomagnetic storms. They estimate a maximum apex height of the postsunset plasma depletions as ~4,000 km. The data for the two intense storms show that the evening enhancement of the equatorial ionization anomaly extended to middle-latitude Europe during the time of the plasma bubble occurrence. The authors propose an eastward penetrating electric field-associated with the interplanetary Bz southward turning as a possible mechanism for plasma enhancement and subsequent middle-latitude plasma bubbles. Although the midlatitude ionosphere is generally considered safe from radio signal scintillation effects, this study confirms that there may be no safe zone from space-based signal scintillation during intense storms. (See Mendillo et al., 2018; Wei et al., 2015, and references, within these for previous examples and further impacts.) The Space Weather Prediction Center (SWPC) now runs subsets of the models that comprise the Space Weather Modeling Framework (SWMF). See http://www.swpc.noaa.gov/news/announcing-geospace-model-version-15. Haiducek et al. (2017) simulated a month of geospace response for January 2005 using SWMF with the University of Michigan global magnetohydrodynamic code Block-Adaptive-Tree-Solarwind-Roe-Upwind-Scheme (BATS-R-US) coupled to the Rice Convection Model (RCM) inner magnetosphere and the Ridley Ionosphere Model. The interval included three large, previously simulated, and well-studied geospace storms. The authors launched the simulation three times. The first launch was with a standard 0.25 Earth radii (RE) SWMF resolution, corresponding to the operational version at SWPC. Then a higher (0.125 RE) resolution grid was applied with and without the RCM inner magnetosphere model. Various indices of geomagnetic activity (Kp, Sym-H, and AL) and the cross polar cap potential (CPCP), which is a proxy for geospace interaction with the solar wind, were calculated from the simulation and compared with observations and an empirical model that estimates CPCP. The SWMF model predicted the Sym-H (proxy for ring current) index very well. The Kp geomagnetic index and the CPCP are well predicted during storm conditions, but these indicators are overpredicted during quiet time. The model tends to underpredict the magnitude of the auroral electrojet, AL index. The Generally, the grid resolution has limited influence for the accuracy of the predictions except for auroral activity. However, the use of the inner magnetosphere model is essential. For the CPCP there is little effect from turning off the RCM. The Sym-H, AL, and Kp indexes, however, are very different when the inner magnetosphere model was turned off. This study is a significant step in showing how well one of the new SWPC models performs during storm conditions and in detailing the sensitivity in model driving. Importantly, the authors provide access to the model output for others to use. As an applications journal, SWE content covers data, model description and validation, and aspects of engineering that challenge even our technically savvy readers. So there are times when technical terms and acronyms might be problematic for a short highlight aimed at a wider readership. You can help SWE Editors promote and publicize as much content as possible by providing illustrative graphics with easy-to-follow captions and by writing plain language summaries of your manuscripts. In turn your Editors will work to highlight the best work. No data were created for this editorial.

Channel-closing effects in strong-field ionization by a bicircular field

Channel-closing effects, such as threshold anomalies and resonantlike intensity-dependent enhancements in strong-field ionization by a bicircular laser field are analyzed. A bicircular field consists of two coplanar corotating or counter-rotating circularly polarized fields having different frequencies. For the total detachment rate of a negative ion by a bicircular field we observe threshold anomalies and explain them using the Wigner threshold law and energy and angular momentum conservation. For the corotating bicircular case, these effects are negligible, while for the counter-rotating case they are pronounced and their position depends on the magnetic quantum number of the initial state. For high-order above-threshold ionization of rare-gas atoms by a counter-rotating bicircular laser field we observe very pronounced intensity-dependent enhancements. We find all four types of threshold anomalies known from collision theory. Contrary to the case of linear polarization, channel-closing effects for a bicircular field are visible also in the cutoff region of the electron energy spectrum, which is explained using quantum-orbit theory.

Liquid chromatography–tandem mass spectrometric assay for the quantitation of the novel radiation protective agent and radiation mitigator JP4-039 in murine plasma

JP4-039 radio-protects prior to, and radio-mitigates after ionizing radiation by neutralizing reactive oxygen species. We developed and validated an LC–MS/MS assay for the quantitation of JP4-039 in murine plasma. Methanol protein precipitation of 50μL plasma was followed by isocratic reverse phase chromatography for a 6min run time, and electrospray positive mode ionization mass spectrometric detection. The plasma assay was linear from 1 to 1000ng/mL with appropriate accuracy (97.1–107.6%) and precision (3.7–12.5%CV), and fulfilled FDA guidance criteria. Recovery was 77.2–136.1% with moderate ionization enhancement (10.9–39.5%). Plasma freeze-thaw stability (98.8–104.2%), stability for 13.5 months at −80°C (93.1–105.6%), and stability for 4h at room temperature (94.2–97.6%) were all acceptable. Limited cross-validation to tissue homogenates suggested that these could also be analyzed for JP4-039 accurately. This assay has been directly applied to determine the pharmacokinetics of JP4-039 in C57BL/6 male mice after IV administration of 20mg/kg JP4-039 and will be extended to other studies of this agent.

Improvement of Detection Sensitivity of Triglyceride with Methylamine Formate as Ionization Enhancer in Reversed Phase Liquid Chromatography-Electrospray Ionization Mass Spectrometry

Abstract A novel ionization enhancer, methylamine formate, was proposed for improving the detection sensitivity of triglyceride in edible oil by reversed phase liquid chromatography electrospray ionization mass spectrometry (LC-ESI-MS). The commonly used isopropanol-acetonitrile-methanol-water and isopropanol-acetonitrile were selected as the mobile phase. By using a reversed phase C18 column, and taking the corn oil in isopropyl alcohol as sample solution, we compared methylamine formate with ammonium formate, as ionization enhancers, for their effect on the detection sensitivity of triglyceride by LC-ESI-MS after screening other different ionization enhancers, such as formic acid, acetic acid, ammonium formate, ammonium acetate, butyl formate, dibutylamine formate, triethylamine formate, diethylamine formate, methylamine formate, and ethylamine formate. The result indicated that, using methylamine formate in mobile phase, the mass spectral peak response and the signal to noise ratio of trilinolein component were all about 5 times higher than that using ammonium formate. The effect of the concentration of methylamine formate ionization enhancer, the flow rate of the mobile phase and the flow rate of nebulizing gas on the detection of methylamine formate was investigated. The concentration of triglyceride components in corn oil starting to form aggregates was similar in different mobile phases in the electrospray process according to measurement of the relationship between corn oil concentrations and the total ion chromatogram peak area of triglyceride. In particular, the peak area of trilinolein was linear with its concentration in the range of 7 × 10 −7 −2 × 10 −4 M with the correlation coefficient R 2 = 0.9997, but increased slower in the higher concentration range. According to the experimental data, the mechanism for improvement of detection sensitivity of methylamine formate was suggested as that the addition of methylamine mono-charged ions with hydrophobic groups had lower solvation energy, which made the enriched addition ions easily evaporation from the droplet surface, thus improving the electrospray ionization efficiency. This method provided an effective way to improve the detection sensitivity of triglyceride in edible oil by LC-ESI-MS.

Observation of ionization enhancement in two-color circularly polarized laser fields

When atoms are irradiated by two-color circularly polarized laser fields the resulting strong-field processes are dramatically different than when the same atoms are irradiated by a single-color ultrafast laser. For example, electrons can be driven in complex two-dimensional trajectories before rescattering or circularly polarized high harmonics can be generated, which was once thought impossible. Here, we show that two-color circularly polarized lasers also enable control over the ionization process itself and make a surprising finding: the ionization rate can be enhanced by up to $700%$ simply by switching the relative helicity of the two-color circularly polarized laser field. This enhancement is experimentally observed in helium, argon, and krypton over a wide range of intensity ratios of the two-color field. We use a combination of advanced quantum and fully classical calculations to explain this ionization enhancement as resulting in part due to the increased density of excited states available for resonance-enhanced ionization in counter-rotating fields compared with co-rotating fields. In the future, this effect could be used to probe the excited state manifold of complex molecules.