Inability of unpacked gooseneck liners to stop the sample liquid after injection with band formation (fast autosampler) into hot GC injectors

With the introduction of the fast autosampler, the rules on how to perform splitless injection should have been revisited: the fast autosampler releases the sample liquid as a band that is no longer nebulized by solvent vapors as in previous injection techniques. In empty gooseneck liners, the sample liquid is shot to the bottom of the injector and jumps around in a largely uncontrolled manner. Visual experiments showed that the liquid is partially rejected. Another portion passes by the column inlet into the column attachment region, while the last part may directly enter the column. The impact on chromatography was investigated by using a mixture of n-alkanes: the higher boiling components passing by the column entrance into the zone of the column attachment were largely lost, i.e., were discriminated against the volatile components. It was concluded that empty gooseneck liners are not suitable for injection by fast autosamplers.     

Design and characterization of poly(dimethylsiloxane)-based valves for interfacing continuous-flow sampling to microchip electrophoresis

This work describes the fabrication and evaluation of a poly(dimethyl)siloxane (PDMS)-based device that enables the discrete injection of a sample plug from a continuous-flow stream into a microchannel for subsequent analysis by electrophoresis. Devices were fabricated by aligning valving and flow channel layers followed by plasma sealing the combined layers onto a glass plate that contained fittings for the introduction of liquid sample and nitrogen gas. The design incorporates a reduced-volume pneumatic valve that actuates (on the order of hundreds of milliseconds) to allow analyte from a continuously flowing sampling channel to be injected into a separation channel for electrophoresis. The injector design was optimized to include a pushback channel to flush away stagnant sample associated with the injector dead volume. The effect of the valve actuation time, the pushback voltage, and the sampling stream flow rate on the performance of the device was characterized. Using the optimized design and an injection frequency of 0.64 Hz showed that the injection process is reproducible (RSD of 1.77%, n = 15). Concentration change experiments using fluorescein as the analyte showed that the device could achieve a lag time as small as 14 s. Finally, to demonstrate the potential uses of this device, the microchip was coupled to a microdialysis probe to monitor a concentration change and sample a fluorescein dye mixture.     

Synthesis of diamond films by pulsed liquid injection chemical vapor deposition using a mixture of acetone and water as precursor

A chemical vapor deposition reactor based on the flash evaporation of an organic liquid precursor was used to grow diamond films on Si substrates. An effective pulsed liquid injection mechanism consisting of an injector, normally used for fuel injection in internal combustion engines, injects micro-doses of the precursor to the evaporation zone at 280 °C and is instantly evaporated. The resulting vapor mixture is transported by a carrier gas to the high-temperature reaction chamber where the diamond nucleates and grows on the substrate surface at temperatures ranging from 750 to 850 °C. The injection frequency, opening time, number of pulses and other injector parameters are controlled by a computer-driven system. The diamond film morphology and structure were characterized by scanning electron microscopy and Raman spectroscopy. The as-deposited diamond films show a ball-shaped morphology with a grain size that varies from 100 to 400 nm, as well as the characteristic diamond Raman band at 1332 cm^− 1. The effects of the experimental parameters and operation principle on the diamond films quality are analyzed and discussed in terms of crystallinity, composition, structure, and morphology.     

Pseudolinear gradient ultrahigh-pressure liquid chromatography using an injection valve assembly

The use of ultrahigh pressures in liquid chromatography (UHPLC) imposes stringent requirements on hardware such as pumps, valves, injectors, connecting tubing, and columns. One of the most difficult components of the UHPLC system to develop has been the sample injector. Static-split injection, which can be performed at pressures up to 6900 bar (100,000 psi), consumes a large sample volume and is very irreproducible. A pressure-balanced injection valve provided better reproducibility, shorter injection time, reduced sample consumption, and greater ease of use; however, it could only withstand pressures up to approximately 1000 bar (15,000 psi). In this study, a new injection valve assembly that can operate at pressures as high as 2070 bar (30,000 psi) was evaluated for UHPLC. This assembly contains six miniature electronically controlled needle valves to provide accurate and precise volumes for introduction into the capillary LC column. It was found that sample volumes as small as several tenths of a nanoliter can be injected, which are comparable to the results obtained from the static-split injector. The reproducibilities of retention time, efficiency, and peak area were investigated, and the results showed that the relative standard deviations of these parameters were small enough for quantitative analyses. Separation experiments using the UHPLC system with this new injection valve assembly showed that this new injector is suitable for both isocratic and gradient operation modes. A newly designed capillary connector was used at a pressure as high as 2070 bar (30,000 psi).     

Analysis of n-alkanes in water samples by means of headspace solvent microextraction and gas chromatography

A simple and efficient headspace solvent microextraction (HSME) was developed for the simultaneous determination of the trace concentrations of some n-alkanes in water samples. Therefore, a microdrop of an organic solvent was extruded from the needle tip of a gas chromatographic syringe to the headspace above the surface of the solution in a sealed vial. Then the volatile organic compounds are extracted and concentrated in the microdrop. Next, the microdrop was retracted into the microsyringe and injected directly into the gas chromatograph. Experimental parameters which control the performance of HSME such as the type of microextraction solvent, organic drop and sample volume, sample stirring rate, sample solution and microsyringe needle temperatures, salt addition and exposure time profiles were investigated and optimized. Finally, the enrichment factor, dynamic linear range (DLR), limit of detection (LOD) and precision of the method were evaluated. Using optimum extraction conditions, good linearity with correlation coefficients in the range of 0.995相似文献   

Analysis of organic compounds by particle beam/hollow cathode atomic emission spectroscopy: determinations of carbon and hydrogen in amino acids

A detailed evaluation of the analytical characteristics of a particle beam/hollow cathode glow discharge atomic emission spectroscopy (PB/HC-AES) system is described for applications in the area of organic sample analysis. The optimization of sample introduction, nebulization parameters, and glow discharge conditions was performed for the elemental analysis (focusing on C and H) of a group of amino acids. By use of a high-efficiency thermoconcentric nebulizer, analyte particles are introduced into a heated hollow cathode glow discharge source, in either flow injection or continuous-flow mode, for subsequent vaporization/atomization and excitation. Nebulization temperature, solvent composition, and liquid flow rate were studied to elucidate their roles in the ultimate analyte emission characteristics for organic compound analysis. The hollow cathode operating discharge current and gas pressure were optimized, with the general responses found to be similar to those for the case of metal analysis. Background interferences from solvent and additive media on carbon and hydrogen determinations were studied and substantially reduced. The analytical response curves for carbon and hydrogen present in amino acids were obtained using 200 μL injection volumes, showing less than 10% RSD for replicate injections over a concentration range of 10-250 ppm, with detection limits of 3 and 1 ppm, respectively, for C (I) and H (I) emission. Subsequent studies of the response of carbon and hydrogen emission signal intensities to differences in amino acid stoichiometries suggest a capability of the PB/HC-AES system for the determination of empirical formulas based on H (I)/C (I) intensity ratios.     

Design and evaluation of a new thermospray liquid/liquid extractor for the extraction of semivolatile and nonvolatile organic compounds from water

The recovery of several semivolatile organic compounds (SVOCs) using a new thermospray liquid/liquid extractor (TSLLE) was investigated. The base system includes a 300 mL multiport extraction vessel, jacketed in a 500 mL cooling flask, a dual-stage condenser for progressive cooling, several thermospray probes, and solvent/sample delivery systems. Aqueous mixtures of SVOCs were used to evaluate the TSLLE. For most compounds, recovery values of 80-100% were obtained during a single cycle in <1 h. The design, evaluation, and extraction capability of the TSLLE are discussed.     

Simple coupling of gas chromatography to electrospray ionization mass spectrometry

A simple method for direct coupling of gas chromatography (GC) with electrospray ionization mass spectrometry (ESI/MS) has been developed. The outlet of the GC capillary column was placed between the ESI needle and the atmospheric pressure ionization (API) source of a mass spectrometer. The ionization occurs via dissolution of neutral compounds into the charged ESI droplet followed by ion evaporation or via a gas-phase proton transfer reaction between a protonated solvent molecule and an analyte. The mass spectra of organic volatile compounds showed abundant protonated molecules with little fragmentation, being very similar to those produced by normal liquid ESI. The quantitative performance of the system was evaluated by determining the limit of detection (LOD), linearity ( r (2)), and repeatability (RSD). The GC-ESI/MS method was shown to be stable, providing high sensitivity and good quantitative performance.     

FT—IR监测溶剂蒸发对CAB透气膜的性能影响

报道了用干／湿相转换方法制得的不对称CAB膜的形态，氧氮透气性与铸膜液中混合溶剂蒸发时间的关系，以红外光谱监测混合溶剂的蒸发过程各个时间段的主要蒸发成分及剩余成分，说明膜的形态及其气体渗透性随铸膜液浸入到凝聚介质－水时不同的溶剂组成而变化。     

Synergistic Dual-Mechanism Localized Heat Channeling and Spectrum-Tailored Liquid Metal Hydrogels for Efficient Solar Water Evaporation and Desalination

Photothermal hydrogels featuring broadband light absorption abilities and highly hydrated networks provide an appealing mass-energy transfer platform for water evaporation by using solar energy. However, the targeted delivery of solar heat energy to power the water evaporation process remains challenging. Herein, enlightened by metal-phenolic coordination chemistry and camouflaged architecture, photothermal hydrogels with dual-mechanism vaporization structure are tactfully designed via a rational interfacial engineering and integration strategy to enable near-µm heat confinement and highly efficient light-to-heat conversion ability. The spectrum-tailored liquid metal droplet (LMGAs-Fe^III) and optimized carbon-wrapped silver nanowire sponge (Ag@C₇₅₀) are integrally built as photothermal promotors/channels and jointly embedded into a highly hydratable poly(vinyl alcohol) hydrogel, denoted as PALGH, to synergistically boost water molecule activation and interfacial vaporization behavior by triggering robust photothermal performance. As a result, under one sun irradiation, the all-embracing PALGH hydrogel evaporation system achieves a brine evaporation rate to a high level of 3.47 kg m⁻² h⁻¹, and >19 L m⁻² clean water of PALGH is ideally delivered daily when purifying natural seawater. This work offers not only a rational design principle to create sophisticated photothermal materials but also replenishes insight into solar heat generation and water transportation in a cross-media system.     

Observation of liquid hydrogen jet on flashing and evaporation characteristics

The evaporation speed of liquid hydrogen jet has been measured using high speed CCD camera. In the evaporation process the diameter of injected liquid hydrogen droplet plays important role. The experimental parameters for injection condition had been selected such as injection temperature and size of injection hole diameter which influences injected diameter of liquid droplet. Liquid hydrogen had been injected into the spray chamber that was filled with gaseous helium at room temperature. The liquid hydrogen jet moves in the horizontal direction and the images of the evaporation processes had been acquired from the observation window using high speed CCD camera. With this method it has been shown that evaporation speed of liquid hydrogen is influenced by injection temperature and size of injection hole diameter.     

Experimental Study of a Microchannel Bubble Injector for Microgravity Applications

We perform a quantitative characterization of a microbubble injector in conditions relevant to microgravity. The injector pregenerates a slug flow by using a capillary T-junction, whose operation is robust to changes in gravity level. We address questions regarding the performance under different injection conditions. In particular we focus on the variation of both gas and liquid flow rates. The injection performance is characterized by measuring bubble injection frequency and bubble sizes. We obtain two distinct working regimes of the injector and identify the optimal performance as the crossover region between them.     

The Effect of Gas Properties on Bubble Formation,Growth, and Detachment

Bubble formation and growth play an important role in various processes and industries, where the dispersion of gas bubbles in a liquid medium occurs frequently. In this paper, the formation, growth, and detachment of gas bubbles produced from a submerged needle in water are numerically and experimentally investigated. The effect of injected gas properties on bubble characteristics, including bubble diameter, contact angle, and the frequency of bubble formation, is evaluated. In particular, the changes in bubble characteristics during the injection process are investigated for three different gases to evaluate the effect of density and surface tension on the bubble detachment criteria. The present numerical results show an acceptable agreement with experiments under different operating conditions. The results show that the increase in surface tension, and the decrease in gas density result in larger bubble sizes before detachment occurs. Moreover, the bubble generation frequency is found to strongly depend on the contact angle and the surface tension.     

Feasibility of collection and analysis of airborne carbonyls by on-sorbent derivatization and thermal desorption

The most commonly used method for analysis of airborne carbonyls is to collect the analytes on solid sorbents coated with a suitable derivatization agent, followed by solvent desorption and liquid injection for analysis by high-pressure liquid chromatography. We have explored a new approach by combining on-sorbent derivatization and thermal desorption to measure airborne carbonyls. More specifically, carbonyls in the air are collected onto an O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA)-coated Tenax sorbent packed in a tube with dimensions identical to those of a gas chromatography (GC) injector liner. The derivatives are then released by in-injection port thermal desorption to a GC column for analysis. Gaseous carbonyls, including formaldehyde, acetaldehyde, benzaldehyde, glyoxal, and methylglyoxal, at ppbv levels are shown to be effectively collected (> or = 92% collection efficiency) onto the sampling tubes at a flow rate of 20 mL/min under both < 1% and 71% relative humidity. The collection efficiency drops as the sampling flow rate increases, and the degree of decrease is compound dependent. The derivatization agent, at a level of approximately 127 nmol/tube, is thermally desorbed and eluted from the GC column without compromising the determination of any carbonyl-PFBHA derivatives. Detection limits of low ppbv to sub-ppbv are achieved for a sample air volume of 4.8 L. Using this new method, we have measured formaldehyde, acetaldehyde, benzaldehyde, glyoxal, and methylglyoxal to be 4.9-16.3, 0.6-8.2, <5.9, 0.5-4.1, and <2.4 ppbv, respectively, in the ambient atmosphere at the university bus stop. This method is less labor intensive than the solvent desorption technique and avoids use of organic solvents. Other classes of airborne polar species can be measured through the same approach by selecting an appropriate derivatization agent.     

A continuous film-recirculable drop gas-liquid equilibration device. Measurement of trace gaseous ammonia

A miniature gas-liquid equilibrator or a gas collector, intended as a low-volume interface between a soluble gaseous sample and a liquid phase analyzer or between a liquid phase sample and a detector designed for use with gas samples, is described. This paper addresses the application of the device for the measurement of trace atmospheric ammonia. Gas collection occurs solely by diffusive sampling such that aerosol particles are not collected. The device essentially consists of a tube surrounded externally by a jacket. Gas flows through the jacket and contacts a liquid film flowing on the surface of the tube. The flowing film forms a drop at the tube terminus and is aspirated off through the inner bore of the tube. The collected analyte can be (a) directly sent to an analysis system or (b) preconcentrated on a suitable stationary phase; the preconcentrator effluent can be recycled, if desired. With a fluorometric flow injection analysis system harnessed to measure ammonia with such a collector, the limit of detection (LOD, S/N = 3) for a sample drawn for 18 min at 200 mL/min was 4.5 parts per trillion by volume, with the linear range extending up to 30 parts per billion.     

Evaporation heat and mass transfer in natural convection pipe flow

Abstract

A numerical analysis has been performed to examine film evaporation on natural convection heat and mass transfer in a vertical pipe. Coupled governing equations for liquid film and induced gas flow were simultaneously solved by the implicit finite difference method. Results for interfacial heat and mass transfer coefficients are specifically presented for ethanol film and water film vaporization. The predicted results indicate that the heat transfer from gas‐liquid interface to the gas flow is predominated by the transport of latent heat in association with film evaporation. The results are also contrasted with those of zero film thickness and show that the assumption of extremely thin film thickness made by Chang et al. [5] and Yan and Lin [19] is only valid for a system with a low liquid Reynolds number Re _l1. But as the liquid Reynolds number is high, the assumption becomes inappropriate.     

Enhanced analysis of sulfonated azo dyes using liquid chromatography/thermospray mass spectrometry

Modifications to a thermospray vaporizer probe and ion source have been made that enhance ion evaporation resulting in increased sample ion current. These modifications include restricting the thermospray vaporizer exit orifice and the addition of a needle-tip repeller electrode to the thermospray ion source. The interaction and effect of probe-tip size, repeller voltage, and flow rate on sulfonated azo dye detection are reported. An increase in signal response for sulfonated azo dyes was observed. An efficient chromatographic separation procedure for sulfonated azo dyes, which is compatible with thermospray mass spectrometric detection, is also presented. Five disulfonated azo dyes were separated and detected by using selected ion monitoring. Mass spectra of disulfonated dyes show a number of molecular ions and adduct ions that provide unequivocal molecular weight information.     

Protocol for liquid chromatography/mass spectrometry of glutathione conjugates using postcolumn solvent modification

A novel protocol for thermospray liquid chromatography/mass spectrometry (LC/MS) analysis of mixtures of glutathione conjugates is reported. Solvent conditions for optimal high-performance liquid chromatography are not always the same as for optimal thermospray ionization mass spectrometry. Labile glutathione conjugates that give poor spectra in aqueous ammonium acetate yield more intense molecular ion signals with increased percentages of acetonitrile. Direct injection thermospray ionization using 30-60% acetonitrile in aqueous ammonium acetate produced protonated molecular ions for glutathione conjugates of menadione, styrene oxide, pentachlorophenyl methyl sulfone, chlorodinitrobenzene, and chlorambucil. Since, the high percentages of organic modifier needed for good molecular ion intensity preclude chromatographic separation of these polar compounds, successful graphic separation of these polar compounds, successful LC/MS was facilitated by postcolumn addition of organic modifiers to the mobile phase. This new methodology allowed excellent chromatographic separations and thermospray ionization mass spectra to be obtained for a mixture of haloalkane glutathione conjugates. Moreover, cleavage of the gamma-glutamyl-cysteine amide bond of glutathione results in class-characteristic fragment ions. Changes in the fragmentation pathways in spectra acquired with and without organic modifiers shed light on the importance of the desolvation process in obtaining good molecular ion sensitivity in thermospray.     

Numerical Simulation of Fluid Flow and Mixing in Gas-Stirred Ladles

In this work air injection into a water physical model of an industrial steel ladle was simulated. Calculations were developed based on a multiphase Eulerian fluid flow model involving principles of conservation of mass, momentum, and chemical species for both phases to predict turbulent flow patterns and mixing times for centric and eccentric injections. Effects of gas flow rate, injector position, number of injectors, and geometry of ladle on mixing time were analyzed. Optimum injection conditions are: single injector at 2/3 of radius and high gas flow rates. Quantitative correlation of mixing time as a function of main process variables was obtained.     

Pesticide residue analysis by off-line SPE and on-line reversed-phase LC-GC using the through-oven-transfer adsorption/desorption interface

A new method to determine pesticide residue in water is presented. The described method includes using off-line solid-phase extraction (SPE) and on-line reversed-phase liquid chromatography-gas chromatography (RPLC-GC). An interface, based on a modified programmed temperature vaporizer (PTV) injector, packed with a suitable trapping material, is used for on-line RPLC-GC. The changes made in the PTV injector affect the pneumatic system, sample introduction, and solvent elimination. The new interface is easily capable of automation. Methanol/wate (70/30) is used as the eluent in the LC preseparation step. The LC column flow during elution is different from the flow during the transfer step. The transferred volumes range from 500 to 1400 microL (volume of the fractions of interest). Solvent elimination is almost 100% before the sample reaches the GC column. The described system does not show any variation of the peak retention times. The detection limit for real samples ranges from 0.04 to 1.5 ng/L, using NP detection.