Electronic linearization of a d.c. electron-capture detector via constant-current operation

An electronic control mode has been developed for operating a d.c. electron-capture detector at constant-current levels by means of a feed-back circuit. A linear range of 1·10⁴ and a detection limit of 100 fg have been observed with aldrin.     

变压器油中溶解气体分析专用气相色谱仪的检定与校准

为解决检定变压器油中溶解气体分析专用气相色谱仪时遇到样品进样量计算不准,检测器对甲烷气体标准物质无响应,专用软件分析方法不能更改等问题,建立变压器油中溶解气体分析专用气相色谱仪的检定与校准方法。当进样模式为一次进样双柱分流时,按分流比计算分流到检测器的样品体积,其它进样模式按进样体积计算进样量;因色谱软件禁止积分导致CH_4在热导检测器上无响应时,应采用有证标准物质中的H_2或O_2进行检定与校准。以7890B型专用气相色谱仪为例,详细说明了变压器油中溶解气体分析专用气相色谱仪的检定与校准流程。该法为检定人员检定该类专用仪器时提供了参考。     

Comparison of list mode data acquisition and shift register measurements using the large epithermal neutron multiplicity counter (LEMC)

Neutron multiplicity analysis has been a valuable technique for safeguards measurements of plutonium oxide and mixed oxides. Historically, most of these measurements have been performed using shift register based electronics. The shift register data acquisition lacks certain flexibility because the basic coincidence parameters (e.g., pre-delay, gate width, and long delay) must be fixed prior to the start of the measurement and the values may potentially, therefore, be sub optimal. List mode or time stamped data acquisition records the arrival time of each pulse thereby preserving the history of the pulse stream and allowing analysis and reanalysis using software analogs to the shift register circuit with adjustable parameters. Until recently, the data rates encountered in the assay of modest amounts of plutonium using efficient multiplicity counters were beyond the capacity of readily available personal computers. The calibration of the large epithermal neutron multiplicity counter (LEMC) for assay of plutonium scrap materials is used as a vehicle to compare the performance of the multiplicity shift register and a commercially available list mode acquisition module.     

A theoretical model of the electron capture detector

Summary This paper reports a theoretical model of the ECD detector. The model presented here can be used to examine the influence of pulse parameters on the current and signal characteristics of the detector. On the basis of this model it was found that a space charge is created in the detector when it is supplied with pulse voltage. Due to the electric potential generated by the space charge, in the time between the pulses the electrons and negative ions move towards the detector electrodes. The ionization current of the detector is the sum of the electron current flowing to the anode under the influence of the supplied pulse voltage and the current flowing under the space charge potential in the time between the pulses. It was also found that the detector signal is the sum of the differences between those two currents caused by introducing the sample molecules to the detector. The model was tested for a detector with different electrode configurations which worked at temperature of 300 K or 573 K and which was supplied with nitrogen or Ar+10% CH₄ as the carrier gas.     

Comparison of On-Column and Splitless Injections for the Capillary Gc Quantitation of Organics Extracted from Landfill Leachatesl

Abstract

An extensive study of the organic components of the leachate from the University of Connecticut landfill has been carried out. A modification of EPA Method 625 for base/neutral extractable organics was used to obtain both the gas chromatographic profiles and the mass spectrometric identification of the organics in groundwater samples from six test wells and several private wells in the vicinity of the landfill. A characteristic fingerprint representing a number of components was consistently found in the analysis of the landfill leachate but not in the drinking water of the private residences located on an adjoining road. Gas chromatographic parameters for optimum qualitative and quantitative analysis of field samples were determined using an acetone solution of the model leachate sample. It was confirmed that the on-column injection mode resulted in greater flame ionization detector (FID) response and more reproducible peak areas than the splitless mode. The major variable in obtaining reproducible data was the field sampling at the landfill, not the solvent reduction step or the injection mode used to introduce the sample into the gas chromatograph.     

Characteristics of Dielectric Barrier Discharge Ozone Synthesis for Different Pulse Modes

This paper features the pulse polarity effect on ozone generation efficiency by adjusting the applied voltage and the flow rate in a coaxial dielectric barrier discharge reactor. Results show that utilization of unipolar pulse has better performance when compared with the bipolar mode, but on the other hand, utilization of the positive pulse has slightly higher efficiency than that of negative mode. Meanwhile, changing the gas flow rate shows a minor effect on ozone generation. Utilization of bipolar pulse would decrease the breakdown voltage and ozone generation efficiency when compared with unipolar pulse while it would lead to higher ozone concentrations at fixed applied voltage. The maximum ozone yield reaches 186.9 g/kWh at 6 kV positive pulse with ozone concentration of 11.9 g/Nm³.     

Halogenated compound response in an oxygen doped ion mobility detector for capillary gas chromatography

An ion mobility detector that has been specifically developed for interfacing with capillary gas chromatography is investigated in the negative ion mode. Like the electron capture detector, to which this instrument is closely related, the ion mobility detector shows an enhanced response to low molecular weight halogenated compounds when a small quantity of oxygen is doped into the make-up gas flow. Under O₂ doping conditions, the device can operate in a reactant ion monitoring mode responding universally to compounds capable of capturing thermal electrons and in a tunable selective product ion mode providing increased selectivity over that achieved by the ECD. At an oxygen concentration of 0.5%, minimum detectable amounts as low as 600 femtograms have been realized for carbon tetrachloride. Selectivity of chloro- versus bromo- compounds is demonstrated using a mixture of p-dichlorobenzene and p-dibromobenzene.     

Gas chromatographic determination of ethyl 2-cyanoacrylate in the workplace environment

Low levels of airborne ethyl 2-cyanoacrylate (ECA) were monitored by drawing air through sampling tubes containing Tenax GC. After desorption with acetone, ECA was quantified by gas chromatography using a thermionic specific detector in the nitrogen mode. Polymerised ECA was shown to yield ECA monomer under the conditions of gas chromatographic analysis.     

Glass capillary gas chromatography with simultaneous flame ionization (FID) and Hall® element-specific (HECD) detection

Two glass capillary gas chromatographic systems were equipped with inert effluent splitters which allowed simultaneous data acquisition using nonspecific and element-specific detectors. Simultaneous detection was achieved using the nonspecific flame ionization detector (FID) and the Hall^® electrolytic conductivity detector (HECD) operated in either the sulfur-or the nitrogen-specific mode. Typical application of the simultaneous detection system as applied to analysis of petroleum residues is briefly described. The Hall electrolytic conductivity detector can be made element specific for halogen-, sulfur-, or nitrogen-containing compounds. Simultaneous detection enhances the information yield from a single sample injection and proves to be a powerful complementary technique when used with computerized gas chromatography/mass spectrometry.     

Optimizing the gas chromatographic separation and detection of polychlorinated biphenyls by use of electronic pressure programming and experimental design

The applicability of electronic pressure control and Taguchi L₂₇ experimental design to the optimization of the gas chromatographic separation and detection of polychlorinated biphenyls has been evaluated. The influence of several experimental variables, column temperature program, carrier gas pressure program, on-column injector temperature program, and make-up gas pressure program, was studied using analysis of variance. Simultaneous optimization of sample introduction, column efficiency, and detector performance could be achieved without compromising system performance. The relationships between system performance and experimental variables were established using regression analysis. Agreement between the simulated and experimental results obtained using suggested optimum conditions demonstrated the applicability of the technique developed in this study. The improvement achieved in the chromatographic separation of PCBs is presented.     

Simultaneous acquisition of X-ray spectra using a multi-wire, position-sensitive gas flow detector

A multi-wire, gas-filled position-sensitive detector has been developed for the simultaneous recording of wavelength-dispersed X-ray signals that enables X-ray fluorescence spectrometry with a limited multi-element capability in the low Z element range. Details of the modular construction of the detector are given. The detector performance was characterized using Al–K radiation and a variable slit system. The detector has been applied in a laboratory spectrometer equipped with an electron source and a double multilayer mirror device as the wavelength-dispersing element. Spectra from Al and Si obtained in the simultaneous acquisition mode show good agreement with calculations performed using a ray-tracing model.     

Injection by Programmed Temperature Vaporization Injection (PTV) of Gaseous Samples for Gas Chromatography – Atomic Emission Spectrometry (GC-AED)

Programmed temperature vaporization injection (PTV) coupled to gas chromatography and atomic emission detector (AED) has been studied for large volume injection of gaseous samples. As examples of the effectiveness of the technique, the results of the analysis of a series of headspace samples of foods such as garlic and onion, and of landfill gases are presented. The volumes of gaseous samples reconcentrated varied from a few milliliters up to liters depending on analyte dilution, through focusing onto a sorbent trap, then rapid liberation into the GC-AED system by programmed thermal desorption. Despite the high carrier gas flow rates associated with direct PTV-GC, AED performance and sensitivity were unaffected. The detailed elemental information obtained from the PTV-GC-AED analyses was confirmed using a PTV coupled to a gas chromatograph with ion trap detector mass spectrometer as detector (PTV-GC-ITD/MS).     

Large diameter open tubular columns in gas chromatographic analysis

Large diameter open tubular columns provide the packed column chromatographer with a simple route to higher resolution gas chromatography. They can be operated in a high-flow (lower resolution) mode that permits their direct substitution for a packed column, or they can be operated in a low-flow (higher resolution) mode that maximizes separation at the cost of longer analysis times. Inlet design and column installation can influence both the chromatographic results and quantiative reliability. Make-up gas is not required in the high-flow mode, and its benefits in the low-flow mode are restricted to enhanced detector sensitivity, provided the outlet end of the column resides in the detector jet (FID). The columns seem fully compatible with all common modes of detection.     

Direct quantitative analysis using the flame-ionization detector

It has been generally accepted that the use of the flame-ionization detector (FID) in the quantitative determination of differing types of solute in gas chromatographic eluents necessitates prior detector calibration for each species in question.Methods of operating the FID using hydrogen-rich flames with oxygen as the supporter of combustion have been evaluated, which permits the direct quantitative determination of the solutes investigated on both a molar and a weight mode basis and which therefore enables estimates of molecular weights to be calculated. It has also been shown that the detector response to carbon can be minimized while allowing an enhanced selective response for chlorine.     

Comparison of a gas chromatography-optical fibre (GC-OF) detector with a gas chromatography-flame ionization detector (GC-FID) for determination of alcoholic compounds in industrial atmospheres

An analytical methodology based on an optical fibre detector coupled to gas chromatograph has been developed for the speciation of some volatile alcoholic compounds. This methodology combines the separation capability of gas chromatography with an optical fibre detector made of an optical fibre sensitized with a thin polymeric film of poly[methyl(3,3,3-trifluoropropyl)siloxane] (PMTFPS). The response of the detector has been characterized at 650 nm for nine different alcohols (allyl alcohol, n-propyl alcohol, sec-butyl alcohol, isobutyl alcohol, n-butyl alcohol, isoamyl alcohol, methyl isobutyl carbinol, cyclohexanol and diacetone alcohol). An alternative method based on gas chromatography-flame ionization detector (GC-FID) was also used in order to evaluated the performance and compare the analytical results with the proposed method. The time of analysis, the analytical error and the analytical performance were similar for both methods. However, the analytical apparatus based on the GC-OF detector is much less expensive than the GC-FID and show high accuracy and suitability for actual monitoring on indoor atmospheres.     

Gas analysis using a gas density detector

Summary A gas-chromatographic setup equipped with a gas density detector has been built and tested with regard to its performance in gas analysis. The results show that this detection system can be successfully applied to many practical gas analytical problems withouth the necessity of calibrating for individual gas components. The accuracy of the results is not inferior to that obtained using a thermal conductivity or a flame ionization detector, the requirements on the choice and constancy of the operational parameters remaining in justifiable limits. There are nearly no limitations to the kind of gas components. Analyses of natural gases and calorimetric test gases are presented as examples of application.

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Gasanalyse mit Hilfe eines Gasdichtedetektors

Zusammenfassung Eine gas-chromatographische Meßapparatur mit einem Gasdichtedetektor wurde aufgebaut und im Hinblick auf ihre Leistungsfähigkeit in der Gasanalytik untersucht. Die Ergebnisse zeigen, daß dieses Meßsystem auf viele in der Praxis wichtige Analysenprobleme ohne Notwendigkeit einer stoffspezifischen Kalibrierung mit Erfolg angewendet werden kann. Die Genauigkeit der Ergebnisse steht der mit einem Wärmeleitfähigkeitsdetektor oder Flammenionisationsdetektor erreichbaren nicht nach, wobei die Anforderungen an die Auswahl und Konstanz der Betriebsparameter in vertretbaren Grenzen bleiben und nahezu keine Einschränkung bezüglich der Art der Analysenbestandteile besteht. Als Anwendungsbeispiele werden Analysen von Erdgasen und calorimetrischen Prüfgasen aufgeführt.

     

亲水作用色谱-电雾式检测用于5种抗病毒药物的定量分析

抗病毒药物具有广泛的应用,对人类和动物的健康有着重要的作用。但抗病毒药物紫外吸收差异大、极性分布广,要实现该类药物的同时分析较为困难。本实验以Click TE-Cys为固定相,结合电雾式检测(CAD),建立了多种抗病毒药物同时分离分析的方法。考察了不同的检测器、色谱模式、色谱柱和流动相对抗病毒药物峰形、分离选择性及检测信号响应的影响。在优化的条件下定量分析了5种抗病毒药物,结果证明该方法具有较好的日内重复性(RSD≤3.06%)和日间重复性(RSD≤5.38%)。同时,该方法还具有较宽的线性范围(0.07~2.28 mg/mL)和较高的灵敏度(LOQ≤0.04 mg/mL),可用于相关药物的定量、定性分析。     

Electroplating of metal nanotubes and nanowires in a high aspect-ratio nanotemplate

Ordered metal (Co, Pt, and CoPt alloy) nanotube and nanowire structures were fabricated by a simple electroplating method in high aspect-ratio anodic aluminum oxide (AAO) membrane. The growth rate in pulse mode is always larger than that in constant-current mode, which represents that diffusion limitation exists in this electroplating condition. It is also found that the sputtered Au layer structure could influence the electroplating. Traditional nanowires could be fabricated in the template with a uniform Au layer as conducting contact. In case of unblocked AAO membrane, metal electroplating begins from the Au particles which were attached inside the holes during the sputtering step and produces metal nanotubes. Pt and CoPt nanotubes could be easily prepared by this method and might be applied as catalyst and magnetic material.     

直流等离子体离子化检测器的研究：管道煤气中可燃气体的测定

本文提出一种改进的直流等离子体离子化检测器（ＤＣＰＩＤ），用于管道煤气中可燃气体Ｈ２、ＣＨ４和ＣＯ的测定。采用氢气作工作气体，对该检测器的工作参数和响应特性进行了优化。本法结果与微波诱导等离子体离子化检测器（ＭＩＰＩＤ）的结果作了比较，结果满意。