Helium Carrier Gas Research Articles

Development of a pulsed uniform supersonic gas expansion system based on an aerodynamic chopper for gas phase reaction kinetic studies at ultra-low temperatures.

A detailed description of a new pulsed supersonic uniform gas expansion system is presented together with the experimental validation of the setup by applying the CRESU (French acronym for Cinétique de Réaction en Ecoulement Supersonique Uniforme or Reaction Kinetics in a Uniform Supersonic Flow) technique to the gas-phase reaction of OH radicals with 1-butene at ca. 23 K and 0.63 millibars of helium (carrier gas). The carrier gas flow, containing negligible mixing ratios of OH-precursor and 1-butene, is expanded from a high pressure reservoir (337 millibars) to a low pressure region (0.63 millibars) through a convergent-divergent nozzle (Laval type). The novelty of this experimental setup is that the uniform supersonic flow is pulsed by means of a Teflon-coated aerodynamic chopper provided with two symmetrical apertures. Under these operational conditions, the designed Laval nozzle achieves a temperature of (22.4 ± 1.4) K in the gas jet. The spatial characterization of the temperature and the total gas density within the pulsed uniform supersonic flow has also been performed by both aerodynamical and spectroscopic methods. The gas consumption with this technique is considerably reduced with respect to a continuous CRESU system. The kinetics of the OH+1-butene reaction was investigated by the pulsed laser photolysis/laser induced fluorescence technique. The rotation speed of the disk is temporally synchronized with the exit of the photolysis and the probe lasers. The rate coefficient (k(OH)) for the reaction under investigation was then obtained and compared with the only available data at this temperature.

CO2–CH4 conversion and syngas formation at atmospheric pressure using a multi-electrode dielectric barrier discharge

The conversion of CO2 and CH4 into value-added chemicals is studied in a new geometry of a dielectric barrier discharge (DBD) with multi-electrodes, dedicated to the treatment of high gas flow rates. Gas chromatography is used to define the CO2 and CH4 conversion as well as the yields of the products of decomposition (CO, O2 and H2) and of recombination (C2H4, C2H6 and CH2O). The influence of three parameters is investigated on the conversion: the CO2 and CH4 flow rates, the plasma power and the nature of the carrier gas (argon or helium). The energy efficiency of the CO2 conversion is estimated and compared with those of similar atmospheric plasma sources. Our DBD reactor shows a good compromise between a good energy efficiency and the treatment of a large CO2 flow rate.

Study of Volatiles in Lysimachia parvifolia Flower Using HS-SPME-GC-MS

Lysimachia parvifolia Hemsl. belongs to the Primulaceae family and is widely distributed in Yunnan, Guizhou, Hunan, Jiangxi, and Zhejiang Provinces in China. It is used as traditional chinese medicine to promote blood circulation clear away heat and toxic material, to relieve cough and asthma, to promote diuresis, and to expel stone [1]. Phytochemical research has shown that the main compounds in plants of Lysimachia genus are flavonoids and triterpenoid saponins. L. pentapetala, L. clethroides, and L. paridiformis contain essential oil [2–4]. Pharmacological investigations have shown that plants of Lysimachia have antitumor, bacteriostatic, anthelminthic, and antioxidation activities [5–8]. No research has so far been conducted concerning the chemical composition and pharmacological activity of L. parvifolia. In order to identify the volatiles in the flowers of L. parvifolia, we report on their essential oils using the HS-SPME technique, which was subsequently supplemented by GC-MS. The air-dried L. parvifolia flowers were collected in Raoren Mountain of Guizhou Province, China, in July 2010. The sample was identified by Prof. Zhiyou Guo (Qiannan Normal College for Nationalities). A voucher specimen was deposited in the Huanghe Science and Technology College (No. 20100729). Volatile organic compounds were extracted by a manual SPME holder together with 5 mL vials and PDMS-DVB fibers purchased from Supelco Inc. (Bellefonte, USA). The powder of L. parvifolia flowers, about 0.7 g, was placed in vials (5 mL); then the SPME fiber was exposed to the upper space of the sealed vial for 30 min at 50 C to adsorb the analytes. After that, the fiber was withdrawn and directly inserted into the GC-MS inlet for desorption of the volatiles for 1 min. Volatile constituents were analyzed by HS-SPME-GC-MS using an Agilent 6890 N gas chromatograph equipped with an DB-5 MS capillary column (5% phenylmethylsiloxane, 30 m 0.25 mm, film thickness 0.10 m, Agilent Technologies, USA) and coupled with a 5975 B mass selective detector spectrometer from the same company. The front inlet was kept at 250 C in the split-less mode. The temperature program was as below: initial column temperature 50 C, held for 2 min, then programmed to 120 C at a rate of 4 C/min and held for 2 min and finally programmed to 230 C at a rate of 6 C/min and held at 230 C for 10 min. As a carrier gas helium at 1.0 mL·min–1 was used. The MS detector was used in the EI mode with an ionization voltage of 1555 eV. The ion source temperature was at 230 C. The transfer line was at 150 C. The spectra were collected at 3 scans/s over the mass range (m/z) 30–400. Retention indices were calculated by using the retention times of n-alkanes that were injected at the same chromatographic conditions [9]. The volatile constituents (Table 1) were identified by comparison of their linear retention indices (relative to C8–C26 alkanes on the DB-5MS column) and their mass spectra with those of authentic standards, as well as those from RTLPEST 3.L and NIST 05.L. The percentage composition of the volatiles was computed from GC peak area normalization without any corrections [10, 11]. The volatiles in the flower of L. parvifolia were presented in Table. Twenty eight compounds were identified, which comprised 93.1% of the volatile fraction. Table 1 showed that the main volatiles in the flower of L. parvifolia were limonene (35.9%), -citral (17.6%) and -citral (15.0%). Limonene is a kind of natural function monoterpene widely used as flavor additive in food. Recently study showed that limonene has a variety of physiological function, especially royal bacteriostasis, antisepticize and anticancer activity [12, 13]. Citral could be used as food flavor, also had the effects of anti-asthmatic and antisepsis. The most widely research was about its effect of inhibition of aspergillus flavus growth [14, 15].

Vibronic spectrum of jet-cooled 2-chloro-5-fluorobenzyl radical: Assignments and substituent effect

We report spectroscopic constants of the jet-cooled 2-chloro-5-fluorobenzyl radical which was generated by corona discharge of precursor 2-chloro-5-fluorotoluene seeded in a large amount of helium carrier gas using a pinhole-type glass nozzle. The visible vibronic emission spectrum was recorded with a long-path monochromator. From an analysis of the spectrum observed, the electronic energy of the D1→D0 transition and vibrational mode frequencies in the D0 state were determined for the first time by comparing its vibronic data with those of a precursor and ab initio calculation. In addition, the substituent effect on electronic transition energy has been discussed.

Spectroscopic observation of jet-cooled 2,5-dichlorobenzyl radical generated by corona discharge

Vibronically excited but jet-cooled 2,5-dichlorobenzyl radical was generated from 2,5-dichlorotoluene precursor in a large excess of helium carrier gas, from which the visible vibronic emission spectrum was recorded. From an analysis of the spectrum observed, it was found that the origin band shows larger shift to red than those expected from mono-substitutions, which has been discussed in terms of orientation of substituents. Also, the electronic energy of the D1→D0 transition and vibrational mode frequencies at the ground electronic state of the 2,5-dichlorobenzyl radical were determined in comparison with the known vibrational data of precursor and ab initio calculations.

Radicals generated from 2-chloro-5-fluorotoluene by corona discharge

The generation of molecular radicals in corona discharge was investigated spectroscopically by varying the experimental conditions applied to a substituted toluene precursor. Vibronic emission spectra were observed from the corona discharge of 2-chloro-5-fluorotoluene seeded in a large amount of carrier gas helium. From an analysis of emission spectra observed, it was confirmed that bond dissociation energy plays a key role in radical formation. The possible pathway for the formation of benzyl-type radicals is proposed to explain the observation.

Essential Oil of Carpolobia lutea

Carpolobia lutea G. Don (Polygalaceae) is a shrub or small tree native to West and Central tropical Africa [1]. It is known by many common names such as cattle stick (English), Abekpok Ibuhu (Eket), Ikpafum, Ndiyan, Nyayanga (Ibibio), Agba or Angalagala (Igbo), and Egbo oshunshun (Yoruba). Herbalists from these tribes use the root decoction in locally-made alcohol as an aphrodisiac and also for the treatment of genitourinary infections, gingivitis, and waist pains. The root decoction is also said to be useful in the treatment of internal heat. The plant Carpolobia lutea has been reported to possess anti-inflammatory, anti-arthritic [2], antimicrobial [3, 4], antidiarrheal [5], anti-ulcerogenic [5, 6], anti-nociceptive [7], antimalarial [8], and analgesic properties [9]. The plant is known to contain triterpene saponins [10] and glycosides [11]. No report on its oil constituents could be found in the literature. Leaves of C. lutea (herbarium number: FHI 109066) used for this study were collected in Igbesa, Ogun State (Southwest) and Minna, Niger State (North central), Nigeria, in August 2011. The air-dried samples (300 g each) were hydrodistilled in a Clevenger-type apparatus [12] for 3 h to afford oils with yield 0.06% v/w and 0.10% v/w, respectively, for Igbesa and Minna samples. Both oils were greenish yellow in coloration. GC analysis was performed on HP-5890 series II instrument equipped with HP-wax and HP-5 capillary columns (both 30 m 0.25 mm, 0.25 m film thickness). Analytical conditions: temperature program; 60 C for 10 min, rising from 5 C·min–1 to 220 C; injector and detector temperature 250 C; carrier gas nitrogen (2 mL/min); detector, FID; split ratio, 1:30; volume injected 0.5 L. Gas Chromatography-electron ionization mass spectroscopy (GC-EI-MS) was performed with a Varian CP-3800 gas chromatography equipped with an HP-5 capillary column and a Varian Saturn 2000 ion trap mass detector. Analytical conditions: injector and transfer line temperature 220 C and 240 C, respectively; oven temperature programmed from 60 C to 244 C at 3 C·min–1; carrier gas helium at a flow rate of 1 mL/min; volume injected 0.2 L (10% hexane solution); split ratio 1:30. Mass spectra were recorded at 70 eV. The acquisition mass range was m/z 30–300 at a scan rate of 1 scan·s–1. Identification of the constituents (Table 1) was based on comparison of the linear retention indices with those of authentic samples, and by the use of a self-constructed spectral library built up from pure substances and components of known oils and MS literature data [13, 14]. Moreover, the molecular weights of all the identified substances were confirmed by gas chromatography-chemical ionization mass spectrometry using methanol as CI ionizing gas. Although the composition of both oils of C. lutea looks similar, it is evident that some quantitative differences could be seen among them. The ubiquitous terpenoids were mostly represented in the oils. The major oil contents were hexahydrofarnesyl acetone (22.5 and 20.0%), (E)-geranyl acetone (11.8 and 11.5%), (E)-2-decenal (4.5 and 4.3%), farnesyl acetone (5.8 and 4.8%), germacrene B (3.6 and 5.4%), and -calacorene (2.7 and 2.5%). The results indicate that there seems to be homogeneity in the volatile oils of C. lutea grown in Nigeria. The essential oils from other plant species around the world have been the subject of literature discussions [15, 16].

Strong effect of carrier gas species on particle velocity during cold spray processes

This study is a comprehensive investigation into the effect that the carrier gas species has on particle velocity during cold spray processes. Numerical simulations were the main method adopted to achieve this objective. A computational fluid dynamics (CFD) model was developed to predict the gas flow and particle velocity both inside and outside the nozzle. This model was experimentally validated and agreed well with the measured data. The carrier gas species was an important factor of the particle velocity. For an air propellant gas, using helium as the carrier gas increases the particle velocity due to the low average molar mass of the mixed propellant gas. Similarly, increasing the helium carrier gas pressure increases the molar fraction of helium in the mixed propellant gas and, thus, the particle velocity.

Development of a plasma driven permeation experiment for TPE

Experiments on retention of hydrogen isotopes (including tritium) at temperatures less than 800°C have been carried out in the Tritium Plasma Experiment (TPE) at Idaho National Laboratory [1,2]. To provide a direct measurement of plasma driven permeation in plasma facing materials at temperatures reaching 1000°C, a new TPE membrane holder has been built to hold test specimens (≤1mm in thickness) at high temperature while measuring tritium permeating through the membrane from the plasma facing side. This measurement is accomplished by employing a carrier gas that transports the permeating tritium from the backside of the membrane to ion chambers giving a direct measurement of the plasma driven tritium permeation rate. Isolation of the membrane cooling and sweep gases from TPE's vacuum chamber has been demonstrated by sealing tests performed up to 1000°C of a membrane holder design that provides easy change out of membrane specimens between tests. Simulations of the helium carrier gas which transports tritium to the ion chamber indicate a very small pressure drop (∼700Pa) with good flow uniformity (at 1000sccm). Thermal transport simulations indicate that temperatures up to 1000°C are expected at the highest TPE fluxes.

Real-time measurements of nitrosamines in air

The ion-molecule chemistry of the three common reagent ions used in selected ion flow tube mass spectrometry (SIFT-MS), H3O+, NO+ and O2+, has been extended to include reactions with seven nitrosamines. Theoretical calculations enabled the dipole moments and polarizabilities of the nitrosamines studied to be found and from these, the proton transfer rate coefficients have been calculated. In all cases rapid proton transfer from H3O+ occurred, and charge exchange and dissociative charge exchange were common processes for NO+ and O2+. For nitrosamines such as N-nitrosodibutylamine, noticeable fragmentation occurred. When the helium carrier gas in the flow tube was changed to nitrogen, the technology still gave good results for monitoring nitrosamines but the product channel ratios for some nitrosamines were different for nitrogen than for helium. The new kinetic data obtained in this study was utilized using a commercial SIFT-MS instrument to monitor the production of N-nitrosodimethylamine and N-nitrosodiethylamine from heated salami.

Experimental device for dioxin vapour generation: A steady and adjustable source of gaseous 2,3-DCDD

PCDD/Fs are Persistent Organic Pollutants (POPs), generated by incomplete combustion of carbonaceous and chlorinated compounds. Because of their toxicity, they have to be monitored at emission, from stationary sources like waste incinerator. In the framework of the development of an on-line analyser, the laboratory prototype requires a gaseous dioxin source. We detail the principles of a dynamic generator based on the sublimation of solid sample and on the diffusion method. Such a device has to supply a polluted flow at steady and adjustable concentration of dioxin.The dioxin generation rate is determined by the weight loss of the solid sample. It is constant during time for temperatures ranging between 75 and 100°C and for generation flow rates greater than or equal to 50mLmin−1. Variation of generation rate with temperature is predictable. With the increasing of temperature, emitted concentration increases while the increase of the inlet pressure decreases the concentration. Helium carrier gas leads to a higher generation rate than with N2 in agreement with mass and molecular volume ratio. Thus, the outlet concentration and flow rate can be settled independently in the range of 3–100ppm and 50–200mLmin−1.

Comparative chromatography–mass spectrometry studies on the antiretroviral drug nevirapine—Analytical performance characteristics in human plasma determination

A contrast between the analytical performance characteristics using gas chromatography-mass spectrometry (GC-MS) liquid chromatography-mass spectrometry (LC-MS) and liquid chromatography-ultraviolet (LC-UV) detection for the determination of the antiretroviral drug (ARV) nevirapine (NVP) in fortified human plasma after QuEChERS extraction has been made. Analytical performance characteristics, i.e. linearities, instrument detection limits (IDLs), limits of quantitation (LOQs), method detection limits (MDLs), % mean recoveries and the corresponding relative standard deviations (%RSDs) were estimated using techniques above. Using GC-MS, the correlation coefficients (r(2)) were ≥0.990, which were deemed acceptable linearities. The MDLs ranged between 11.1-29.8μg/L and 13.7-36.0μg/L using helium and hydrogen carrier gases respectively. The LOQs ranged between 16.5-66.7μg/L and 28.4-98.7μg/L using helium and hydrogen carrier gases respectively with a % mean recovery of 83% and %RSD of 4.6%. Using LC-MS and LC-UV, the correlation coefficients (r(2)) were ≥0.990. The MDLs were ranged between 3.14 and 47.1μg/L. The LOQs ranged between 2.85 and 90.0μg/L respectively. The MDLs using GC-MS, LC-MS and LC-UV were below the therapeutic range for NVP in human plasma is considered to be between 2300μg/L (Cmin) and 8000μg/L (Cmax). This study also demonstrated that helium can be substituted with hydrogen which is relatively cheaper and easily obtainable even by use of a generator.

A New Gas Chromatography Method for Quality Control of Methylsulfonylmethane Content in Multicomponent Dietary Supplements

This paper reports the first gas chromatography method with flame ionization detection (GC–FID) for rapid assay of the quality control of methylsulfonylmethane (MSM) content in multicomponent dietary supplements. The proposed method was completely validated and used for quality control of multicomponent dietary supplements available in the local market. The study showed that the content of MSM varied in different commercial samples (77.7–112.3 % of the declared amount) and was dependent on different qualities of the dietary supplement producer. The developed method was based on the rapid extraction of MSM into acetone using ultrasound bath (5 min) and on the subsequent quantification of the MSM by gas chromatography. The used GC conditions were as follows: fused silica cyanopropylphenylmethylpolysiloxane capillary column Alltech ATTM-624 (30 m × 0.32 mm i.d., 1.8-μm film thickness), injection mode split of 1:10, sample volume of 1.0 μL, injector temperature of 190 °C, detector temperature of 190 °C, column temperature held at 190 °C for whole analysis time, and helium carrier gas flow rate of 0.7 mL min−1. Nitrobenzene was chosen as internal standard for precise and accurate quantification of MSM in dietary supplements. Method precision evaluated as relative standard deviation was 1.7 %, and method accuracy evaluated as recovery was in the range of 102.5–106.2 %. The method required no special sample pretreatment, so it showed to be simple, robust, rapid (short analysis time of less than 4 min), and suitable for routine analysis of MSM in food quality control laboratories.

Atmospheric pressure, nonthermal plasma inactivation of MS2 bacteriophage: effect of oxygen concentration on virucidal activity

The main aim of this study was to determine the virucidal inactivation efficacy of an in-house-designed atmospheric pressure, nonthermal plasma jet operated at varying helium/oxygen feed gas concentrations against MS2 bacteriophage, widely employed as a convenient surrogate for human norovirus. The effect of variation of percentage oxygen concentration in the helium (He) carrier gas was studied and found to positively correlate with MS2 inactivation rate, indicating a role for reactive oxygen species (ROS) in viral inactivation. The inactivation rate constant increased with increasing oxygen concentrations up to 0·75% O2 . 3 log10 (99·9%) reductions in MS2 viability were achieved after 3min of exposure to the plasma source operated in a helium/oxygen (99·25%:0·75%) gas mixture, with >7 log10 reduction after 9min exposure. Atmospheric pressure, nonthermal plasmas may have utility in the rapid disinfection of virally contaminated surfaces for infection control applications. The atmospheric pressure, nonthermal plasma jet employed in this study exhibits rapid virucidal activity against a norovirus surrogate virus, the MS2 bacteriophage, which is superior to previously published inactivation rates for chemical disinfectants.

Precise and Accurate In Situ Determination of Lead Isotope Ratios in NIST, USGS, MPI‐DING and CGSG Glass Reference Materials using Femtosecond Laser Ablation MC‐ICP‐MS

Lead isotope ratio data were obtained with good precision and accuracy using a 266 nm femtosecond laser ablation (fLA) system connected to a multi‐collector ICP‐MS (MC‐ICP‐MS) and through careful control of analytical procedures. The mass fractionation coefficient induced by 266 nm femtosecond laser ablation was approximately 28% lower than that by 193 nm excimer laser ablation (eLA) with helium carrier gas. The exponential law correction method for Tl normalisation with optimum adjusted Tl ratio was utilised to obtain Pb isotopic data with good precision and accuracy. The Pb isotopic ratios of the glass reference materials NIST SRM 610, 612, 614; USGS BHVO‐2G, BCR‐2G, GSD‐1G, BIR‐1G; and MPI‐DING GOR132‐G, KL2‐G, T1‐G, StHs60/80‐G, ATHO‐G and ML3B‐G were determined using fLA‐MC‐ICP‐MS. The measured Pb isotopic ratios were in good agreement with the reference or published values within 2s measurement uncertainties. We also present the first high‐precision Pb isotopic data for GSE‐1G, GSC‐1G, GSA‐1G and CGSG‐1, CGSG‐2, CGSG‐4 and CGSG‐5 glass reference materials obtained using the femtosecond laser ablation MC‐ICP‐MS analysis technique.

An automated GC-C-GC-IRMS setup to measure palaeoatmospheric δ<sup>13</sup>C-CH<sub>4</sub>, δ<sup>15</sup>N-N<sub>2</sub>O and δ<sup>18</sup>O-N<sub>2</sub>O in one ice core sample

Abstract. Air bubbles in ice core samples represent the only opportunity to study the mixing ratio and isotopic variability of palaeoatmospheric CH4 and N2O. The highest possible precision in isotope measurements is required to maximize the resolving power for CH4 and N2O sink and source reconstructions. We present a new setup to measure δ13C-CH4, δ15N-N2O and δ18O-N2O isotope ratios in one ice core sample and with one single IRMS instrument, with a precision of 0.09, 0.6 and 0.7‰, respectively, as determined on 0.6–1.6 nmol CH4 and 0.25–0.6 nmol N2O. The isotope ratios are referenced to the VPDB scale (δ13C-CH4), the N2-air scale (δ15N-N2O) and the VSMOW scale (δ18O-N2O). Ice core samples of 200–500 g are melted while the air is constantly extracted to minimize gas dissolution. A helium carrier gas flow transports the sample through the analytical system. We introduce a new gold catalyst to oxidize CO to CO2 in the air sample. CH4 and N2O are then separated from N2, O2, Ar and CO2 before they get pre-concentrated and separated by gas chromatography. A combustion unit is required for δ13C-CH4 analysis, which is equipped with a constant oxygen supply as well as a post-combustion trap and a post-combustion GC column (GC-C-GC-IRMS). The post-combustion trap and the second GC column in the GC-C-GC-IRMS combination prevent Kr and N2O interferences during the isotopic analysis of CH4-derived CO2. These steps increase the time for δ13C-CH4 measurements, which is used to measure δ15N-N2O and δ18O-N2O first and then δ13C-CH4. The analytical time is adjusted to ensure stable conditions in the ion source before each sample gas enters the IRMS, thereby improving the precision achieved for measurements of CH4 and N2O on the same IRMS. The precision of our measurements is comparable to or better than that of recently published systems. Our setup is calibrated by analysing multiple reference gases that were injected over bubble-free ice samples. We show that our measurements of δ13C-CH4 in ice core samples are generally in good agreement with previously published data after the latter have been corrected for krypton interferences.

Chemical fingerprinting of Su-He-Xiang-Wan and attribution of major characteristic peaks for its quality control by GC-MS

A simple and facile gas chromatography-mass spectrometer (GC-MS) fingerprint of Su-He-Xiang-Wan (SHXW) was developed, the similarity analysis was conducted, and attribution of the major characteristic peaks was identified for SHXW quality control. GC-MS analysis was performed on a QP2010 instrument (Shimadzu, Japan) equipped with a capillary column of RTX-5MS. The column temperature was initiated at 50 °C, held for 5 min, increased at the rate of 3 °C/min to 120 °C, held for 2 min, and then increased at the rate of 4 °C/min to 220 °C, held for 10 min. Helium carrier gas was used at a constant flow rate of 1.3 mL/min. Mass conditions were ionization voltage, 70 eV; injector temperature, 250 °C; ion source temperature, 250 °C; splitting ratio, 30:1; full scan mode in the 40–500 Da mass ranges with rate of 0.2 s per scan. Attribution of the major characteristic peaks was identified for SHXW by comparing the chemical standards, references of Chinese herbal medicines and the negative controls of prescription samples (NC) of SHXW. With the help of the temperature-programmed retention indices (PTRIs) used together with mass spectra and chemical standards, 25 major characteristic peaks have been identified. Nine volatile medicinal materials were identified in the prescription of SHXW by attributing to the 27 major characteristic peaks. The results demonstrate that the proposed method is a powerful approach to quality control of complex herbal medicines.

A soil water distillation technique using He-purging for stable isotope analysis

Summary A new method of soil water extraction for oxygen and hydrogen isotopic analysis has been developed; this method uses a helium flow system as an alternative to the conventional vacuum extraction method. The method significantly increases the efficiency of sample preparation and simplifies the extraction. During the water distillation, a helium carrier gas transfers water vaporized at 95 °C from the soil sample to a cold trap at liquid nitrogen temperature. An extraction time of 180 min is used to distill the water from the fine-grained soil completely. The proposed He-purging distillation technique makes it possible to distill approximately a dozen samples simultaneously. The method was tested using liquid water samples and clayey soil samples hydrated with water of known isotopic composition. The standard deviations for these tests were 0.08‰ for δ18O and 0.7‰ for δD. An intercomparison test was conducted for the helium and vacuum extraction methods using natural soil samples. The correlation coefficients between the methods were 0.9926 and 0.9939 for δD and δ18O, respectively. The proposed He-purging distillation method can achieve high precision for clayey soil samples with low water content and has the potential to provide adequate isotopic data in hydrological and ecological studies. The method is relatively fast, efficient, and inexpensive. We also recommend using the method to distill salt solutions (sea water, mineralized water) before determining the δD and δ18O values using a chromium or carbon reduction method to avoid “salt effects”.

Spectroscopic Identification of Isomeric Jet-Cooled Benzyl-Type Radicals Generated from 4-Fluoro-o-xylene

Vibronically excited but jet-cooled benzyl-type radicals were generated by the corona discharge of precursor 4-fluoro-o-xylene seeded in a large amount of inert carrier gas helium using a pinhole-type glass nozzle, from which the vibronic emission spectra were observed with a long-path monochromator in the visible region. From an analysis of the spectra observed, we identified the existence of two isomeric benzyl-type radicals, 2-methyl-4-fluorobenzyl and 2-methyl-5-fluorobenzyl radicals in corona discharge, and determined the electronic energy of the D1 → D0 transition and several vibrational mode frequencies in the D0 state of both isomers for the first time.

Efficient x-ray line production from laser excited CF2Cl2 clusters. Mixed cluster formation and control of the x-ray line yield

We report on effective x-ray generation under excitation of clusters of polyatomic CF2Cl2 molecules by femtosecond laser radiation of moderate intensity I ≈ 5 × 1015 W cm−2 (E ≈ 5 mJ). The maximal laser energy conversion efficiency to chlorine x-ray lines (E = 2.6–2.8 keV) reached 2 × 10−5 when the CF2Cl2 clusters were formed in the presence of the carrier gas helium. It was demonstrated that the spectrum of laser-induced x-rays can be used to detect mixed clusters, appearing in a 3% mixture of CF2Cl2 with Ar. We proposed adding a light thermalizing gas to the mixture, i.e., using a ternary mixture CF2Cl2–Ar–He, to control the fraction of components that the mixed clusters are composed of. We have given the first demonstration of a dual-energy x-ray source with a tunable line amplitude that is controlled by the helium fraction in the mixture.