Performance characteristics of a new prototype for a portable GC using ambient air as carrier gas for on-site analysis

The performance characteristics of a portable GC instrument requiring no compressed gas supplies and using relatively lightweight transportable components for the analysis of volatile organic components in large-volume air samples are described. To avoid the need for compressed gas tanks, ambient air is used as the carrier gas, and a vacuum pump is used to pull the carrier gas and injected samples through the wall-coated capillary column and a photoionization detector (PID). At-column heating is used eliminating the need for a conventional oven. The fused silica column is wrapped with heater wire and sensor wire so that heating is provided directly at the column. A PID is used since it requires no external gas supplies and has high sensitivity for many compounds of interest in environmental air monitoring. In order to achieve detection limits in the ppb range, an online multibed preconcentrator containing beds of graphitized carbons and carbon molecular sieves is used. After sample collection, the flow direction through the preconcentrator is reversed, and the sample is thermally desorbed directly into the column. Decomposition of sensitive compounds during desorption is greater with air as the carrier gas than with hydrogen.     

Programmed cold sample introduction and multidimensional preparative capillary gas chrornatography. Part I: Introduction,design and operation of a new mass flow controlled multidimensional GC system

A new design of a mass-flow controlled multidimensional switching system for capillary gas chromatography is introduced and discussed. Large sample amounts can be introduced with a required input band width onto the first column. A combination of cold sample introduction and successive high speed programmed heating of the injector liner is used for this purpose. The potential of such an approach for selective and fast high resolution separations with series coupled wide bore, thick film columns is emphasized, and illustrated for various concentrations.     

Increasing the rate of sample vaporization in an open air desorption ionization source by using a heated metal screen as a sample holder

Rapid vaporization of sample into the ionizing gas exiting a direct analysis in real time (DART®) source has been enabled by directing a high electrical current through a metal wire screen to which sample has been applied. This direct heating of the screen enables rapid vaporization of sample as the wire temperature rises from room temperature to greater than 400°C in less than 20 s. Positioning the screen between the DART source and atmospheric pressure inlet of the mass spectrometer ensures that the ionizing gas is in close proximity to the sample molecules, resulting in efficient ionization while significantly reducing the time required for mass spectrometric analysis. The capability to modulate the electrical current flow through the wires facilitates either rapid desorption for the determination of single component samples or slower desorption where analysis of mixtures might be desired. The technology also enables deployment of strategies for the determination of chemicals present as powders that might otherwise require dissolution prior to analysis. Results from the use of this thermally assisted DART (‘TA‐DART’) system for the analysis of pure compounds, simple mixtures, solids and low vapor pressure samples are presented. Copyright © 2011 John Wiley & Sons, Ltd.     

Temperature calibration of a mass spectrographic evolved gas analysis system

Because of the vacuum used in mass spectrographic evolved gas analysis, the usual effects of temperature lag between actual and apparent sample temperatures are exaggerated. Factors contributing to this temperature difference are discussed. The melting point of various metals in the range 110–1100°C are used to obtain insights and estimates regarding these temperature discrepancies at different heating rates, utilizing a variety of sample holders. In general, if the sample is in good contact with the heated supporting surface, the agreement between the observed and reported equilibrium melting temperatures is good at heating rates of ? ～ 20°C min. At higher heating rates the differences become larger (?10°C) and the effect increases with increasing temperature of melting. For sample holders which are not in good contact with the sample, hot spots can develop at high temperatures due to unequal thermal radiation. Under these circumstances the apparent melting point can be considerably lower than the actual equilibrium temperature and less dependent upon heating rate.     

Fully automated in-tube solid-phase microextraction for liquid samples coupled to gas chromatography

An online device is described in which analytes are extracted from a liquid sample by means of in-tube solid-phase microextraction (in-tube SPME), pulse released by rapid heating, and transferred to a gas chromatograph in a fully automated way. Switching of the sample and gas flows as well as the heating of the extraction tube and the valves is controlled by a remote computer system. Results obtained for river water and for aqueous standard solutions of phenanthrene are presented and are compared to the performance of standard SPME.     

Oxidative coulometric trace determination of sulphur in hydrocarbons

A new combustion system for the oxidative coulometric determination of sulphur in liquid hydrocarbons is described. The conditions were selected so that the recovery as SO(2) was close to 100%. The relative standard deviation was < 1% for sulphur in the range 2-1000 mg/l., with thiophene in cyclohexane as a test substance. Thermodynamic data on the equilibrium between SO(2), O(2) and SO(3) were used to select the operating conditions. To increase the recovery of SO(2) the combustion gas mixture was diluted with an inert gas to lower the partial pressure of oxygen. A temperature of 1000 degrees in the equilibrium zone resulted in a recovery of 99%. The SO(2) was titrated with coulometrically generated iodine, the concentration of which was controlled by a Pt-redox electrode. The response of this electrode has been examined. A rather high concentration of I(-) was used to suppress iodine losses during the analysis. The time of analysis was 2-5 min, and sample sizes were 3-7 mul. An LKB 16300 Coulometric Analyzer governed the titration procedure.     

吸附丝富集化探样品中烃类的色谱—质谱检测

通过吸附丝富集样品中的游离烃，热解析后直接进行色谱－质谱分析，该法适用于土、岩石、水及油气样品，操作简便，灵敏度高，可检测至Ｃ１６的烃类，是油敢化探中行之有效的一种新方法。     

Determination of lead in preserved egg by flame atomic absorption spectrometry after chemically modified preconcentration

A sensitive method for the determination of lead in preserved egg by flame absorption spectrometry using ammonium pyrrolidine dithiocarbamate-polystyrene chemically modified platinum wire matrix is presented. The modified platinum wire matrix, after preconcentrating the lead, is placed in a flame burner for direct atomization and measurement. The concentration range is linear between 5 and 500 ng/ml lead in solution and the detection limit is 0.65 ng/ml. This new technique is sensitive and convenient.     

铂丝富集-火焰原子吸收光谱检测皮蛋中痕量铅

一种运用火焰原子吸收测定皮蛋中铅含量的灵敏方法，它是通过用APDC－聚苯乙烯的化学方法修饰铂丝基质来实现的，预浓缩铅溶液后，将修饰的铂丝基质，旋转在火焰燃烧器上，以便直接雾化并测定，在溶液中铅的浓度为5－500ng/mL范围内一船呈线性，检测的极限是0.65ng/mL，这种方法既灵敏又方便。     

Development and applications of sample controlled thermomicroscopy

Sample controlled thermal analysis techniques such as constant rate transformation analysis or stepwise isothermal analysis, where the transformation rate of the sample itself is used to control the experiment, are becoming increasingly important [1]. The measurements are normally carried out using changes in the sample mass, sample dimensions or in the evolved gas, as the property used to control the experiment, and enable reactions to be studied in greater detail than is possible using linear heating techniques. A new approach is described here where a thermomicroscopy system has been developed to enable the intensity of the light reflected or transmitted by the sample to be used as the controlling signal [2]. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sampling small volumes of ambient ammonia using a miniaturized gas sampler

The development of a gas sampler for a miniaturized ambient ammonia detector is described. A micromachined channel system is realized in glass and silicon using powder blasting and anodic bonding. The analyte gas is directly mixed with purified water, dissolving the ammonia that will dissociate into ammonium ions. Carrier gas bubbles are subsequently removed from the liquid stream through a venting hole sealed with a microporous water repellent PTFE membrane. A flow restrictor is placed at the outlet of the sampler to create a small overpressure underneath the membrane, enabling the gas to leave through the membrane. Experiments with a gas flow of 1 ml min(-1), containing ammonia concentrations ranging from 9.4 ppm to 0.6 ppm in a nitrogen carrier flow have been carried out, at a water flow of 20 microl min(-1). The ammonium concentration in the sample solution is measured with an electrolyte conductivity detector. The measured values correspond with the concentration calculated from the initial ammonia concentration in the analyte gas, the fifty times concentration enhancement due to the gas-liquid volume difference and the theoretical dissociation equilibrium as a function of the resulting pH.     

直热式毛细管柱对直馏汽油组成的分析

采用柱内电阻丝加热升温的直热式１３Ｘ分子筛薄层毛细管填充柱分析汽油的族组成。分析时间比炉温加热短５０ｍｉｎ,定量分析结果与炉温加热和多孔层开管柱基本相同。保留时间的相对偏差小于３％,直馏汽油中绝大部分组分定量结果的相对偏差小于５％。     

The gas temperature in and the gas expulsion from a graphite furnace used for atomic absorption spectrometry

A simplified model for heat transfer based on thermal conduction is used to calculate the radial gas temperature distribution inside a semi-enclosed, commercial graphite tube furnace used for atomic absorption spectrometry. In the absence of a forced convective flow of a purge gas, the gas temperature inside the graphite furnace during its heating is lower than the wall temperature. After the wall temperature has attained a steady-state value, the gas temperature approaches the wall temperature and the radial temperature gradient in the gas decreases. The difference between the wall temperature and the gas temperature depends on the temperature program used, the thermal properties of the purge gas, and the atomizer geometry. The residence time of relatively volatile analyte elements is largely controlled by expulsion when wall atomization at high heating rates and high atomization temperatures are used. Analytical sensitivities are often enhanced by vaporizing the analyte into a gas having an approximately constant temperature.     

Thermoanalytical characterization of polymers: A comparative study between thermogravimetry and evolved gas analysis using a temperature-programmable pyrolyzer

Evolved gas analysis (EGA) was carried out on 15 synthetic polymer samples using a temperature programmable pyrolyzer as a heating unit which was on-line coupled with a MS detector. A deactivated stainless steel tube and a vent free adapter were used to couple the pyrolyzer with the MS detector, and they were placed in a GC oven at 300°C to avoid condensation of evolved gases with high boiling point. Thermograms of polystyrene measured by this system (Py-EGA-MS) showed shifts of the peak temperature to the higher temperature region as the sample mass increased. It was found that the S/N ratios of EGA thermograms were 420-fold superior to those of differential thermogravimetry (DTG) thermograms for the same sample mass of 0.20 mg. Since a good linear relationship was obtained between peak temperatures obtained by Py-EGA-MS and DTG, it can be concluded that Py-EGA-MS can be used to obtain reliable data on thermal properties of samples with high sensitivity and using less sample.     

A new method for solvent-free application of polymers and inorganic materials to ferromagnetic wires used for pyrolysis-capillary gas chromatographic studies

A method is described that enables soluble and insoluble samples to be applied to ferromagnetic wires. The principle of the method consists in pressing the sample on to a flattened Curie-point wire at a pressure of 15 tonnes. It is shown that this method is suitable for a wide variety of polymers and for inorganic materials. As the pyrolysis—gas chromatographic results obtained are not only influenced by the pyrolysis itself but also to a large extent by the gas chromatographic process, a number of considerations are given with respect to the use of capillary gas chromatography in combination with a Curie-point pyrolyser. The sample application technique described has also been used to check the inertness of the chromatographic system by the preparation of Curie wires loaded with Tenax. These wires have been used for the “injection” of volatile compounds, such as a Grob mixture, with the pyrolyser system.     

Hot embossing of electrophoresis microchannels in PMMA substrates using electric heating wires

A simple method based on electric heating wires has been developed for the rapid fabrication of poly(methyl methacrylate) (PMMA) electrophoresis microchips in ordinary laboratories without the need for microfabrication facilities. A piece of stretched electric heating wire placed across the length of a PMMA plate along its midline was sandwiched between two microscope slides under pressure. Subsequently, alternating current was allowed to pass through the wire to generate heat to emboss a separation microchannel on the PMMA separation channel plate at room temperature. The injection channel was fabricated using the same procedure on a PMMA sheet that was perpendicular to the separation channel. The complete microchip was obtained by bonding the separation channel plate to the injection channel sheet, sealing the channels inside. The electric heating wires used in this work not only generated heat; they also served as templates for embossing the microchannels. The prepared microfluidic microchips have been successfully employed in the electrophoresis separation and detection of ions in connection with contactless conductivity detection.     

A modified combustion method for the determination of the surface and bulk carbon contents of high-purity metals

A newly developed apparatus for the determination of carbon in high-purity metals is described. By controlled heating of samples in an oxygen or oxygen/helium flow, it is possible to distinguish between the carbon contents of a thin surface layer and of the bulk metal. The oxidation is done within a device of fused silica by means of micropyrometer-controlled h.f. induction heating. The carbon dioxide formed is quantified after absorption in 10^-2 M sodium hydroxide by measurement of electrolytic conductivity. Calibration is done by injection of various volumes of a certified calibration gas from a gas syringe. Investigations on Si, Fe, Ni, Cu, Zr, Nb, Mo and W samples of high purity as well as of technical grade showed that the surface carbon can be determined by oxidation at 630–650°C. Its quantity depends on the kind of sample preparation chosen. Increasing the temperature to a maximum of 1500°C yields the carbon concentration of the bulk by complete combustion of the sample. Combustion additives are useful for a variety of matrices. At sample weights of ? 0.5 g, bulk carbon concentrations of ? 0.1 μg g^-1 can be determined.     

Flame-atomic absorption analysis for trace metals after electrochemical preconcentration on a wire filament

To avoid unspecific light losses in the atomic absorption analysis of trace metals in the presence of a high concentration of salts, electrochemical preconcentration on a wire filament is employed, prior to flame atomization. The elements in question are electrolyzed on to a platinum spiral filament which is then heated in an air-acetylene flame. A quartz tube is placed above the filament, to increase the sensitivity of the method. The separation technique is simple and non-destructive; a sample volume of 5–20 ml and an electrolysis time of 2–5 min are recommended. Because of the relatively low filament temperature, the method is limited to elements which are readily volatilized such as silver, bismuth, cadmium, mercury, lead, selenium, tellurium, thallium and zinc. For most elements, the detection limits are of the same order of magnitude as those of the “Delves cup” technique.     

Digitally controlled high-precision thermostat for X-ray investigations on lyotropic and thermotropic liquid crystals

The construction of a special hot air generation system for X-ray diffraction investigations of lyotropic and thermotropic mesophases at different temperatures is described. The sample is positioned at the center of the X-ray diffractometer contained in a long cylindrical capillary, and resides at a small goniometer head used to adjust the precise position and inclination of the sample. It was decided to use air as the medium and initially the medium was intended to flow axially. A stream of air is blown from a small and high rectangular opening sideways toward the sample. The gas streaming out of the vessel has to pass through a vortex cell. The air leaving the vortex cell forms a core of whirling gas with constant temperature. A temperature sensor is mounted longitudinally above the sample and is positioned inside the same airflow core. The sensor provides the input signal for a microprocessor-based controller which regulates the power of the heating or cooling system in the inlet tube providing constant temperature in the gas core. Received: 15 January 1999 Accepted: 1 February 1999