Photoionization Source Research Articles

Structural Level Characterization of Base Oils Using Advanced Analytical Techniques

Base oils, blended for finished lubricant formulations, are classified by the American Petroleum Institute into five groups, viz., groups I–V. Groups I–III consist of petroleum based hydrocarbons whereas groups IV and V are made of synthetic polymers. In the present study, five base oil samples belonging to groups I and III were extensively characterized using high performance liquid chromatography (HPLC), comprehensive two-dimensional gas chromatography (GC×GC), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) equipped with atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) sources. First, the capabilities and limitations of each analytical technique were evaluated, and then the availed information was combined to reveal compositional details on the base oil samples studied. HPLC showed the overwhelming presence of saturated over aromatic compounds in all five base oils. A similar trend was further corroborated using GC×GC, which yie...

Molecular characteristics of a Chinese coal analyzed using mass spectrometry with various ionization modes

Abstract A Xinjiang subbituminous coal was directly oxidized in aqueous sodium hypochlorite and sequentially extracted with diethyl ether and ethyl acetate. The extracts were esterified with diazomethane and the esterified products were analyzed using a high performance liquid chromatograph/mass spectrometer (HPLC/MS) equipped with atmospheric pressure photoionization (APPI) or electrospray ionization (ESI) ion source. Toluene and a toluene/anisole mixture (vol/vol = 95:5) were used as dopant for APPI-MS. The relative of organic species with molecular mass over 500 u detected by ESI-MS is much higher than that identified by APPI-MS with and without dopant. The addition of dopant greatly enhanced the ionization efficiency and toluene induced better ionization effects compared to the mixture in both ion signal intensity and number of detected species. However, according to the ionization mechanism of APPI, a small portion of analytes could be well ionized under direct APPI mode rather than dopant-assisted mode.

Quantification of tocopherols and tocotrienols in soybean oil by supercritical-fluid chromatography coupled to high-resolution mass spectrometry.

For the most effective analytical strategies, development and validation include optimization of such analytical variables as resolution, detectability, sensitivity, simplicity, cost effectiveness, flexibility, and speed. However, other aspects concerning operator safety and environmental impact are not considered at the same level. The result has been many unintended negative effects of analytical methods developed to investigate different kinds of sample, especially hydrophobic compounds that generate a large amount of chemical waste and have a strong negative environmental impact. In this context, quantification of tocopherols and tocotrienols, i.e. the vitamin E family, is usually achieved by normal-phase liquid chromatography using large volumes of toxic organic solvents, or reversed-phase liquid chromatography using a high percentage of methanol for elution. We propose here a "greener" analytical strategy, including the hyphenation of supercritical-fluid chromatography, using CO2 and ethanol as mobile phase, NH2 as stationary phase, and mass spectrometry for the detection and quantification of vitamin E congeners in soybean oil. An atmospheric-pressure photoionization (APPI) source seemed significantly more sensitive and robust than electrospray or atmospheric-pressure chemical ionization (APCI). This method led to shortened analysis time (less than 5 min) and was revealed to be as sensitive as more traditional approaches, with limits of detection and quantification in the tens of μg L(-1).

Chiral recognition in electron scattering by S- and R-2-butanol

Experiments are described involving the low-energy scattering of electrons from the two optical enantiomers S- and R-2-butanol. Using a synchrotron radiation photoionisation source on the ASTRID storage ring, scattering spectra are reported between a few meV and 140 meV at an electron energy resolution of 1.6 meV. These data show electron dichroism at 35 meV, for scattering into the backward hemisphere, with a cross-section of 202.1 ± 8.0 Å2 (4σ) and 182.8 ± 9.0 Å2 (4σ) for S- and R-, respectively, with the racemic mixture lying at the average value. No dichroic effect could be detected in the integral cross-section within the accuracy of the experiment. A brief qualitative discussion is given of a possible electron dissociative attachment mechanism for the creation of homochirality in space, including a new feature involving feedback through enhancement of electron polarisation through scattering from non-racemic mixtures of enantiomers.

Microplasma discharge vacuum ultraviolet photoionization source for atmospheric pressure ionization mass spectrometry.

In this paper, we demonstrate the first use of an atmospheric pressure microplasma-based vacuum ultraviolet (VUV) photoionization source in atmospheric pressure mass spectrometry applications. The device is a robust, easy-to-operate microhollow cathode discharge (MHCD) that enables generation of VUV photons from Ne and Ne/H(2) gas mixtures. Photons were detected by excitation of a microchannel plate detector and by analysis of diagnostic sample ions using a mass spectrometer. Reactive ions, charged particles, and metastables produced in the discharge were blocked from entering the ionization region by means of a lithium fluoride window, and photoionization was performed in a nitrogen-purged environment. By reducing the output pressure of the MHCD, we observed heightened production of higher-energy photons, making the photoionization source more effective. The initial performance of the MHCD VUV source has been evaluated by ionizing model analytes such as acetone, azulene, benzene, dimethylaniline, and glycine, which were introduced in solid or liquid phase. These molecules represent species with both high and low proton affinities, and ionization energies ranging from 7.12 to 9.7 eV.

An effective approach for coupling direct analysis in real time with atmospheric pressure drift tube ion mobility spectrometry.

Drift tube ion mobility spectrometry (DTIMS) has evolved as a robust analytical platform routinely used for screening small molecules across a broad suite of chemistries ranging from food and pharmaceuticals to explosives and environmental toxins. Most modern atmospheric pressure IM detectors employ corona discharge, photoionization, radioactive, or electrospray ion sources for efficient ion production. Coupling standalone DTIMS with ambient plasma-based techniques, however, has proven to be an exceptional challenge. Device sensitivity with near-ground ambient plasma sources is hindered by poor ion transmission at the source-instrument interface, where ion repulsion is caused by the strong electric field barrier of the high potential ion mobility spectrometry (IMS) inlet. To overcome this shortfall, we introduce a new ion source design incorporating a repeller point electrode used to shape the electric field profile and enable ion transmission from a direct analysis in real time (DART) plasma ion source. Parameter space characterization studies of the DART DTIMS setup were performed to ascertain the optimal configuration for the source assembly favoring ion transport. Preliminary system capabilities for the direct screening of solid pharmaceuticals are briefly demonstrated.

UV photo-ionization based asymmetric field differential ion mobility sensor for trace gas detection

A high-field Asymmetric Waveform Ion Mobility Spectrometry (FA-IMS) with ultra-violet (UV) photo-ionization source is proposed and demonstrated for measurement of trace amounts of volatile organic compound (VOC) gases. Experimental studies performed with the sensor using a high frequency asymmetrical waveform differential field for detection of acetone, hexane and acetone–hexane mixture in trace concentrations are discussed. The detection limit as estimated for a signal-to-noise ratio of 3 is of the order of 100ppb for acetone and hexane. Experimental studies clearly show that selective sensing of a gas in a mixture of gases is also possible by appropriate value for the compensation voltage. Numerical Simulation is also performed using two-dimensional fluid-flow equations to estimate the motion of ions in an electric field. The study also demonstrated influence of parameters such as flow velocity, duty-ratio, on compensation voltage for detection of trace VOC gases with asymmetrical high-frequency electric field. Results based on theoretical study are in agreement with the experimental studies.

Determination ofAr2+andN4+recombination coefficients by subpicosecond multiphoton ionization at 248 nm and microwave interferometry

Rate coefficients (\ensuremath{\alpha}${}_{\mathrm{D}}$) for the dissociative recombination of ${\mathrm{Ar}}_{2}$${}^{+}$ and N${}_{4}$${}^{+}$ in low-temperature plasmas have been determined by multiphoton ionization of the parent gas (Ar or N${}_{2}$) at 248 nm, in combination with microwave interferometry at 9.2 GHz. A subpicosecond, Ti:Al${}_{2}$O${}_{3}$-KrF hybrid laser system generating 40 mJ pulses at 248 nm serves as a photoionization source within one arm of an interferometer having a bandwidth of \ensuremath{\sim}800 MHz, thereby providing an ultrafast (\ensuremath{\delta}-function) plasma channel generator and a noninvasive, microwave probe of the plasma that together decouple the ionization mechanism (and source) from the electron detection process. Comparisons of measurements of the temporally resolved electron density with numerical simulations find \ensuremath{\alpha}${}_{\mathrm{D}}$ to be $(1.2\ensuremath{-}6.0)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ cm${}^{3}$ s${}^{\ensuremath{-}1}$ for ${\mathrm{Ar}}_{2}$${}^{+}$ and $(2\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ cm${}^{3}$ s${}^{\ensuremath{-}1}$ for N${}_{4}$${}^{+}$ for background pressures in the 150--600 Torr Ar and 10--400 Torr N${}_{2}$ intervals, respectively. Both sets of constants are consistent with values reported previously in other ranges of gas pressure. The data and simulations indicate the cross section for four-photon ionization of Ar at 248 nm to be $(5\ifmmode\pm\else\textpm\fi{}3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}118}$ cm${}^{8}$ s${}^{3}$, or approximately 1.4 orders of magnitude lower than the single value in the literature [Uiterwaal et al., Phys. Rev. A 57, 392 (1998)]. Sub-50-ns transients observed in the electron density temporal profile for N${}_{2}$ pressures above \ensuremath{\sim}100 Torr suggest that the rate constant for N${}_{2}(B)$-N${}_{2}(B)$ associative ionization is almost an order of magnitude larger than the currently accepted value.

Note: Hollow cathode lamp with integral, high optical efficiency isolation valve: A modular vacuum ultraviolet source

The design and operating conditions of a hollow cathode discharge lamp for the generation of vacuum ultraviolet radiation, suitable for ultrahigh vacuum (UHV) application, are described in detail. The design is easily constructed, and modular, allowing it to be adapted to different experimental requirements. A thin isolation valve is built into one of the differential pumping stages, isolating the discharge section from the UHV section, both for vacuum safety and to allow lamp maintenance without venting the UHV chamber. The lamp has been used both for ultraviolet photoelectron spectroscopy of surfaces and as a "soft" photoionization source for gas-phase mass spectrometry.

Spectral optical monitoring of a double-peaked emission line AGN Arp 102B

Here we present results of the long-term (1987-2010) optical spectral monitoring of the broad line radio galaxy Arp 102B, a prototype of active galactic nuclei with the double-peaked broad emission lines, usually assumed to be emitted from an accretion disk. To explore the structure of the broad line region (BLR), we analyze the light curves of the broad H\alpha and H\beta lines and the continuum flux. We aim to estimate the dimensions of the broad-line emitting regions and the mass of the central black hole. We use the CCF to find lags between the lines and continuum variations. We investigate in more details the correlation between line and continuum fluxes, moreover we explore periodical variations of the red-to-blue line flux ratio using Lomb-Scargle periodograms. The line and continuum light curves show several flare-like events. The fluxes in lines and in the continuum are not showing a big change (around 20%) during the monitoring period. We found a small correlation between the line and continuum flux variation, that may indicate that variation in lines has weak connection with the variation of the central photoionization source. In spite of a low line-continuum correlation, using several methods, we estimated a time lag for H\beta around 20 days. The correlation between the H\beta and H\alpha flux variation is significantly higher than between lines and continuum. During the monitoring period, the H\beta and H\alpha lines show double-peaked profiles and we found an indication for a periodical oscillation in the red-to-blue flux ratio of the H\alpha line. The estimated mass of the central black hole is \sim 1.1 \times 10^8 M\odot that is in an agreement with the mass estimated from the M-\sigma* relation.

Carbon disulfide as a dopant in photon‐induced chemical ionization mass spectrometry

The addition of a dopant to an Atmospheric Pressure PhotoIonization (APPI) source of a mass spectrometer has been shown to enhance the degree of analyte ionization. A series of different dopants has been successfully utilized; however, there has been very little published on the characteristics of a good dopant. We have proposed carbon disulfide (CS2) as a novel new dopant based on its absorption cross-section for the VUV photon's energy used and its unique gas-phase ion chemistry, notably the fact that it does not contain a proton. The ability of CS2 to enhance the ionization effectiveness of APPI was tested by using a group of compounds that have different proton affinities (PAs) and electron affinities (EAs). These results were compared to results obtained using the commonly used dopants, toluene and anisole. Particular attention was paid to the formation of [M](+) ions relative to [M+H](+) ions. Mass spectra were collected using a Waters Quattro Premier liquid chromatography/tandem mass spectrometry (LC/MS/MS) system equipped with a commercial Photomate™ photoionization source. The results show that CS2 increases the ionization efficiency of most of the analytes studied in this work comparably to toluene and anisole. CS2 promotes both ionization routes of [M](+) and [M+H](+). In addition, due to the higher ionization energy (IE) of CS2 (10.01) compared to the IEs of toluene (8.83) and anisole (8.20), CS2 can enhance the ionization efficiency of analytes that cannot be enhanced with toluene and anisole. We have determined that CS2 is a viable dopant for use in APPI sources. For some analytes, significant [M+H](+) ion signals are observed; therefore, the donated proton must come from either water clusters or solvents. In addition, CS2 promotes the ionization of analytes with low PAs and higher IEs than that of toluene and anisole.

UNVEILING A POPULATION OF GALAXIES HARBORING LOW-MASS BLACK HOLES WITH X-RAYS

We report the discovery of three low-mass black hole candidates residing in the centers of low-mass galaxies at z<0.3 in the Chandra Deep Field - South Survey. These black holes are initially identified as candidate active galactic nuclei based on their X-ray emission in deep Chandra observations. Multi-wavelength observations are used to strengthen our claim that such emission is powered by an accreting supermassive black hole. While the X-ray luminosities are low at L_X ~ 10^40 erg s^-1 (and variable in one case), we argue that they are unlikely to be attributed to star formation based on H\alpha or UV-fluxes. Optical spectroscopy with Keck/DEIMOS and VLT/FORS allows us to (1) measure accurate redshifts, (2) confirm their low stellar host mass, (3) investigate the source(s) of photo-ionization, and (4) estimate extinction. With stellar masses of M* < 3*10^9 M_\sun determined from HST/ACS imaging, the host galaxies are among the lowest mass systems known to host actively accreting black holes. We estimate BH masses M_BH ~ 2*10^5 M_\sun based on scaling relations between BH mass and host properties for more luminous systems. In one case, a broad component of the H\alpha emission-line profile is detected thus providing a virial mass estimate. Black holes in such low-mass galaxies are of considerable interest as the low-redshift analogs to the seeds of the most massive BHs at high redshift which have remained largely elusive to date. Our study highlights the power of deep X-ray surveys to uncover such low-mass systems.

Identification of Conjugated Linoleic Acid (CLA) Isomers by Silver Ion-Liquid Chromatography/In-line Ozonolysis/Mass Spectrometry (Ag+-LC/O3-MS)

A novel method for the identification of conjugated linoleic acid (CLA) isomers has been developed in which silver ion liquid chromatography is coupled to in-line ozonolysis/mass spectrometry (Ag(+)-LC/O3-MS). The mobile phase containing CLA isomers eluting from the Ag(+)-LC column flows through a length of gas-permeable tubing within an ozone rich environment. Ozone penetrating the tubing wall reacts with the conjugated double bonds forming ozonolysis product aldehydes. These, and their corresponding methanol loss fragment ions formed within the atmospheric pressure photoionization (APPI) source, were detected by in-line MS and used for the direct assignment of double bond positions. Assignment of positional isomers is based entirely on the two pairs of diagnostic ions seen in the in-line O3-MS mass spectra. In this way, de novo identification of CLA positional isomers, i.e. without requiring comparison to CLA standards, was achieved. The Ag(+)-LC/O3-MS method was applied to the analysis of CLA isomers in a commercial CLA supplement, milk fat, and the lipid extract from a Lactobacillus plantarum TMW1460 culture. The results demonstrate how Ag(+)-LC/O3-MS can be used for the direct and fast determination of CLA isomers at low concentrations and in complex lipid mixtures.

Characterization of Zhundong subbituminous coal by time-of-flight mass spectrometry equipped with atmospheric pressure photoionization ion source

Zhundong subbituminous coal pretreated with aqueous H2O2 was oxidized by aqueous NaOCl and sequentially extracted with diethyl ether (DEE) and ethyl acetate (EA) to afford DEE- and EA-extractable fractions and residue. Both extracts were esterified with CH2N2 and analyzed using high performance liquid Chromatography/mass spectrometry equipped with electrospray ionization (ESI) or atmospheric pressure photoionization (APPI) ion source. Molecular mass of 60% compounds in the esterified extracts ranged between 200 and 600Da and more than 20% of the esterified extracts had molecular mass over 600Da. Toluene and toluene/anisole (vol./vol.=95/5) were used as dopants for APPI. Low- and non-polar compounds in the esterified extracts were well analyzed using APPI/MS. Compared to toluene/anisole mixture, toluene greatly increased ionization efficiency of APPI. More molecular associations were detected by ESI than by APPI. Heteroatoms were identified in most of the associated species.

The direct determination of double bond positions in lipid mixtures by liquid chromatography/in-line ozonolysis/mass spectrometry

The direct determination of double bond positions in unsaturated lipids using in-line ozonolysis-mass spectrometry (O3-MS) is described. In this experiment, ozone penetrates through the semi-permeable Teflon AF-2400 tubing containing a flow of a solution of fatty acid methyl esters (FAME). Unsaturated FAME are thus oxidized by the ozone and cleaved at the double bond positions. The ozonolysis products then flow directly into the atmospheric pressure photoionization (APPI) source of a mass spectrometer for analysis. Aldehyde products retaining the methyl ester group are indicative of the double bond positions in unsaturated FAME. For the first time, O3-MS is able to couple directly to high performance liquid chromatography (HPLC), making the double bond localization in lipid mixtures possible. The application of LC/O3-MS has been demonstrated for a fat sample from bovine adipose tissue. A total of 9 unsaturated FAME including 6 positional isomers were identified unambiguously, without comparison to standards. The in-line ozonolysis reaction apparatus is applicable to most mass spectrometers without instrumental modification; it is also directly compatible with various LC columns. The LC/O3-MS method described here is thus a practical, versatile and easy to use new approach to the direct determination of double bond positions in lipids, even in complex mixtures.

Dopant-Assisted Negative Photoionization Ion Mobility Spectrometry for Sensitive Detection of Explosives

Ion mobility spectrometry (IMS) is a key trace detection technique for explosives and the development of a simple, stable, and efficient nonradioactive ionization source is highly demanded. A dopant-assisted negative photoionization (DANP) source has been developed for IMS, which uses a commercial VUV krypton lamp to ionize acetone as the source of electrons to produce negative reactant ions in air. With 20 ppm of acetone as the dopant, a stable current of reactant ions of 1.35 nA was achieved. The reactant ions were identified to be CO(3)(-)(H(2)O)(n) (K(0) = 2.44 cm(2) V(-1) s(-1)) by atmospheric pressure time-of-flight mass spectrometry, while the reactant ions in (63)Ni source were O(2)(-)(H(2)O)(n) (K(0) = 2.30 cm(2) V(-1) s(-1)). Finally, its capabilities for detection of common explosives including ammonium nitrate fuel oil (ANFO), 2,4,6-trinitrotoluene (TNT), N-nitrobis(2-hydroxyethyl)amine dinitrate (DINA), and pentaerythritol tetranitrate (PETN) were evaluated, and the limits of detection of 10 pg (ANFO), 80 pg (TNT), and 100 pg (DINA) with a linear range of 2 orders of magnitude were achieved. The time-of-flight mass spectra obtained with use of DANP source clearly indicated that PETN and DINA can be directly ionized by the ion-association reaction of CO(3)(-) to form PETN·CO(3)(-) and DINA·CO(3)(-) adduct ions, which result in good sensitivity for the DANP source. The excellent stability, good sensitivity, and especially the better separation between the reactant and product ion peaks make the DANP a potential nonradioactive ionization source for IMS.

Comparison of the performance of three ion mobility spectrometers for measurement of biogenic amines

The performance of three different types of ion mobility spectrometer (IMS) devices: GDA2 with a radioactive ion source (Airsense, Germany), UV-IMS with a photo-ionization source (G.A.S. Germany) and VG-Test with a corona discharge source (3QBD, Israel) was studied. The gas-phase ion chemistry in the IMS devices affected the species formed and their measured reduced mobility values. The sensitivity and limit of detection for trimethylamine (TMA), putrescine and cadaverine were compared by continuous monitoring of a stream of air with a given concentration of the analyte and by measurement of headspace vapors of TMA in a sealed vial. Preprocessing of the mobility spectra and the effectiveness of multivariate curve resolution techniques (MCR-LASSO) improved the accuracy of the measurements by correcting baseline effects and adjusting for variations in drift time as well as enhancing the signal to noise ratio and deconvolution of the complex data matrix to their pure components. The limit of detection for measurement of the biogenic amines by the three IMS devices was between 0.1 and 1.2ppm (for TMA with the VG-Test and GDA, respectively) and between 0.2 and 0.7ppm for putrescine and cadaverine with all three devices. Considering the uncertainty in the LOD determination there is almost no statistically significant difference between the three devices although they differ in their operating temperature, ionization method, drift tube design and dopant chemistry. This finding may have general implications on the achievable performance of classic IMS devices.

Elimination of matrix effects during analysis of perfluorinated acids in solid samples by liquid chromatography tandem mass spectrometry

Matrix effects can significantly hamper the accuracy and precision of the analysis results of perfluorinated acids (PFAs) in environmental solid samples. Several methods, such as standard addition, isotopically labeled internal standards, clean-up of SPE (solid phase extraction) eluents by dispersive graphitized carbon sorbent and substitution of eletrospray ionization (ESI) source by atmosphere pressure photoionization (APPI) source, were demonstrated for elimination of matrix effects in quantitative analysis of PFAs in solid samples. The results indicate that matrix effects can be effectively eliminated by standard addition, but instrumental analysis time will be multiplied. Isotopically labeled internal standards can effectively negate matrix effects of PFAs with the same perfluorocarbon chain length, but is not valid for the other analytes. Although APPI can eliminate matrix effects for all analytes, it is only suitable for analysis of high pollution levels samples. Clean-up of SPE eluents by dispersive graphitized carbon sorbent not only effectively negate the impact of matrix effect, but also avoid frequent clean of the ESI in order to maintain instrumental sensitivity. Therefore, the best method for elimination of matrix effects is the usage of dispersive graphitized carbon sorbent for clean-up of SPE elution.

Numerical models for the diffuse ionized gas in galaxies

Aims: The aim of this work is to verify whether turbulent magnetic reconnection can provide the additional energy input required to explain the up to now only poorly understood ionization mechanism of the diffuse ionized gas (DIG) in galaxies and its observed emission line spectra. Methods: We use a detailed non-LTE radiative transfer code that does not make use of the usual restrictive gaseous nebula approximations to compute synthetic spectra for gas at low densities. Excitation of the gas is via an additional heating term in the energy balance as well as by photoionization. Numerical values for this heating term are derived from three-dimensional resistive magnetohydrodynamic two-fluid plasma--neutral-gas simulations to compute energy dissipation rates for the DIG under typical conditions. Results: Our simulations show that magnetic reconnection can liberate enough energy to by itself fully or partially ionize the gas. However, synthetic spectra from purely thermally excited gas are incompatible with the observed spectra; a photoionization source must additionally be present to establish the correct (observed) ionization balance in the gas.

Combined corona discharge and UV photoionization source for ion mobility spectrometry

An ion mobility spectrometer is described which is equipped with two non-radioactive ion sources, namely an atmospheric pressure photoionization and a corona discharge ionization source. The two sources cannot only run individually but are additionally capable of operating simultaneously. For photoionization, a UV lamp was mounted parallel to the axis of the ion mobility cell. The corona discharge electrode was mounted perpendicular to the UV radiation. The total ion current from the photoionization source was verified as a function of lamp current, sample flow rate, and drift field. Simultaneous operation of the two ionization sources was investigated by recording ion mobility spectra of selected samples. The design allows one to observe peaks from either the corona discharge or photoionization individually or simultaneously. This makes it possible to accurately compare peaks in the ion mobility spectra from each individual source. Finally, the instrument's capability for discriminating two peaks appearing in approximately identical drift times using each individual ionization source is demonstrated.