Liquid gas techniques for GC trace analysis

Liquid gases (LG), i.e. low boiling compounds with vapor pressures below 5 bar at room temperature, are introduced as solvents for trace analyses. A system for preparin, diluting and handling LG solutions safely and conveniently in 5 to 500 μl amounts was developed as well as a syringe for direct injection of μl-LG samples into capillary GC. Even technical grade LG are of high purity. GC/FID of LG solutions (starting at ?60°C) allows the separation of volatile traces from the solvent peak: e.g., dichloromethane can be measured in the picogram range.     

Ultra sensitive trace gas detection for biomedical applications

We present an overview of our recent progress on spectroscopic trace gas detection for biomedical applications. The latest developments of cavity-enhanced spectroscopy as well as magnetic rotation spectroscopy lead to unprecedented sensitivity and specificity. The current detection limits of our laser spectroscopic approaches are in the picomolar to nanomolar range, depending on the molecular compound. The time resolution of the measurements is down to the sub-second range. This very high sensitivity and time resolution open up exciting perspectives for novel analytical tasks in biomedical research and clinical diagnosis.     

Calibration method for the infrared-spectrometric trace gas analysis

Summary A simple and versatile calibration method for the infrared spectrometric trace analysis of gases and vapours is described. A dynamically operating set-up consisting of a capillary dosage system and a dilution unit with a set of micro-orifices was used for producing exactly known small volume fractions of trace constituents in gas mixtures. The infrared absorption measurements were performed using a Fourier transform spectrometer equipped with a long-path gas cell. Absorption coefficients of selected vibrational-rotational bands obtained by a linear least-squares fit around the threshold limit values are reported for NH₃, SO₂, CCl₄ and 1,4-dioxane as examples of trace constituents in air.

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Methode zur Kalibrierung in der infrarotspektrometrischen Spurenanalyse von Gasen

Zusammenfassung Ein vielseitig einsetzbares einfaches Kalibrierverfahren für die infrarotspektrometrische Spurenanalytik von Gasen und Dämpfen wird beschrieben. Zur Realisierung genau bekannter kleiner Volumenanteile von Spurenkomponenten in Gasmischungen diente eine nach dem dynamischen Prinzip arbeitende Apparatur, bestehend aus einem Capillardosierer und einer Verdünnungseinheit mit Mikroblenden. Zur Messung der Infrarotabsorption wurde ein Fourier-Transform-Spektrometer mit Langweggasküvette verwendet. Für NH₃, SO₂, CCl₄ und 1,4-Dioxan als Beispiele für Spurenkomponenten in Luft werden die mit Hilfe einer linearen Ausgleichsrechnung im Bereich der MAK-Werte erhaltenen Absorptionskoeffizienten ausgewählter Schwingungs-Rotationsbanden angegeben.

     

Anodic stripping voltammetry with medium exchange in trace element speciation

The application of medium exchange in anodic stripping voltammetry (ASV) was investigated by determing the fractions of ASV-labile copper, lead, cadmium and zinc in the presence of a variety of natural and sysnthetic ligands in a soft water. Pristin and polluted samples of river water were also tested. Medium exchange, where the test solution is replaced after electrodeposition by a simple electrolyte such as acetate buffer, before the oxidation (stripping) of the deposited metals, can ensure that the ASV-labile metal fraction depends only on the parameters of the electrodeposition step. Significant differences were found for ASV-labile fractons when medium exchange was used for the two river water samples, and for several mertal/ligand combinations such as copper/chloride, copper/humic acid, and zinc/tannic aid. It is recommended that medium exchange be used routinely for ASV-labile determinations.     

Trends in atmospheric trace gas measurement instruments with membrane-based gas diffusion scrubbers.

A diffusion scrubber (DS) is an excellent tool for gas analysis, and there are many types of DS devices, varying both in structure and construction. In this paper, recent work on atmospheric trace gas measurements by means of DS devices are reviewed. Theoretical considerations on representative DSs are summarized first. Then, the characteristics of the key material, a gas-diffusion membrane, are discussed, and recent improvements and novel scrubbers for highly effective collection are outlined. A chromatograph is a suitable tool for the multi-gas analysis of collected species. On the other hand, solid-state fluorescence/absorbance detectors have been developed for the continuous measurement of target gases. The methods based on DS collection and subsequent detection have high sensitivities, and the detection limits can be in the low-pptv levels. Accordingly, they are capable of measuring background levels, and detecting very low levels of contaminants in a cleanroom. Miniaturized advanced DS units, perhaps the next generation of DS devices, are introduced at the end of this review. DS systems have contributed significantly to our knowledge of the atmospheric dynamics and atmospheric chemistry.     

Wavelength-modulated fabry-perot interferometry for quantitation of trace gas components

The change in refractive index of a gas sample caused by molecular absorption is monitored by Fabry-Perot interferometry in a continuous flow system. To reduce the background noise level from other absorption processes, wavelength modulation is incorporated into the excitation laser. An enhancement in the signal-to-noise ratio of up to 146 s is found for the present method relative to intensity-modulated schemes or Stark-modulated schemes.     

Diffusion of a trace gas into a flowing carrier

Numerical Methods were used to solve the differential equation for diffusion of a trace gas into a flowing carrier gas having a parabolic velocity profile in a cylindrical tube. Steady state solutions are given in the form of contour diagrams of constant trace gas concentration.     

Fast gas chromatography and its use in trace analysis

There is revived interest in the development and implementation of methods of faster GC. The paper summarises the advantages of faster GC analysis, general approaches to faster GC method development and practical aspects of fast gas chromatography with the utilisation of open tubular capillary columns with the stress on trace analysis. There are a number of ways to take the advantage of the improved speed of analysis by faster GC. Numerous options exist for pushing the speed of capillary gas chromatography (CGC) analysis. The scope of this paper is also to give an overview of the present state of faster GC instrumentation which is already available for trace analysis. The practicality of fast CGC is a function of sample preparation and the matrix interferences and how they affect the resultant resolution that may be achieved. Researchers have demonstrated the applicability of fast GC to trace and ultratrace analysis of volatile and semivolatile compounds also with narrow bore columns and difficult sample matrices (such as food, and soil extract). The main development of faster GC methods has been observed in the field of environmental analysis. Practical applications are presented. Both optimised sample preparation and experimental conditions for faster GC are the future perspective of trace analysis.     

The atomic emission detector in gas chromatographic trace analysis

Summary The atomic emission detector for element-selective detection in gas chromatography shows high selectivities for many elements vs. carbon, coupled with a large dynamic range and relatively low limits of detection. In this work we report on our experience with the detector with respect to the response in dependence on the chemical form of the element monitored, the limit of detection, the selectivity vs. carbon and efforts to improve the limit of detection by varying the flow rate of the make-up gas. Applications discussed include the quantification of a dibenzodioxin/dibenzofuran standard solution, dual-isotope monitoring and the determination of elemental ratios in organic compounds. The work centers on the elements carbon, sulphur, chlorine, bromine and oxygen.Dedicated to Prof. Dr. V. Krivan on the occasion of his 60th birthdayIn part presented at the 20th International Symposium on Environmental Analytical Chemistry, Strasbourg, France, April 1990.     

Analysis of trace components in gases by gas chromatography

Summary Relationships are derived describing how the detection limit of a chromatographic system depends on the minimum detectable limit of the detector and the chromatographic parameters such as column length, efficiency, carrier gas flow rate and the capacity factor. Performance data of detectors developed in the last 25 years at the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences are given and a few selected application examples are listed. These include trace analysis by preconcentration and by direct analysis and the utilization of multidimensional gas chromatography with two columns, two detectors, a 12-port valve, and a catalytic conversion reactor.     

A new test gas generator for atmospheric trace compounds

A new device for generating atmospheric trace gases for calibration purposes is described, which allows to adjust test gas concentrations with fast response times and constant carrier gas flow. It consists of a permeation tubing surrounded by a solution of the substance of interest. Inside of the tubing a movable piston is placed. The position of this piston determines the available fraction of the permeation surface and hence the output of the test gas source. The device has been tested for the trace gas production of SO₂ and formaldehyde in the ppbv range. It shows linear dependence of the produced concentrations on the position of the piston, fast response times and good reproducibilities.Dedicated to Professor Dr. Dieter Klockow on the occasion of his 60th birthday     

Gas chromatographic analysis of trace gas impurities in tungsten hexafluoride

Highly reactive fluorinated gaseous matrices require special equipment and techniques for the gas chromatographic analysis of trace impurities in these gases. The impurities that were analysed at the low-microg/l levels included oxygen, nitrogen, carbon dioxide, carbon monoxide, sulfur hexafluoride and hydrogen. This paper describes the use of a system utilising backflush column switching to protect the columns and detectors in the analysis of trace gas impurities in tungsten hexafluoride. Two separate channels were used for the analysis of H2, O2, N2, CO, CO2 and SF6 impurities with pulsed discharge helium ionisation detection.     

Slow proton exchange in water in the gas phase

To study proton exchange in water the technique of quantitative measurements of water content at very low concentrations of water in solutions or at low vapor pressure in the gas phase (for water content of 1–10 μg/ml) has been developed. Under these conditions the rate of proton exchange slows down significantly. In particular, in mixtures of H₂O and D₂O vapors under the pressure of a 10–17 mm Hg proton life-time during the exchange process reaches the values of 10–30 min. The lifetime was measured using the gradual slow growth of the signal intensity of the mixed isotopomer HDO.     

Generating gas flows with a constant trace concentration of hydrogen sulfide

The dynamic gas extraction of hydrogen sulfide from aqueous solutions of sodium sulfide was studied theoretically and experimentally. The effects of the number of bubblers, the concentration of sodium sulfide, and the pH of solution on the concentration of hydrogen sulfide in the generated gas flow were found. The proposed method for generating gas flows with a constant trace concentration of hydrogen sulfide can be used for calibrating the verifying instruments for gas analysis.     

Use of backflushing and peak-cutting technique in gas chromatographic trace analysis

Summary A backflusing, peak-cutting method was developed for the analysis of trace components present in the tail of a major component peak. Using a helium-ionization detector, the minimum detectable concentration (MDC) was: oxygen in hydrogen, 0.5ppm; nitrogen in oxygen or argon, 1ppm; and methane and carbon monoxide in nitrogen, 1ppm. The corresponding MDC values for the conventional single-column method were 7.2ppm, 8.3ppm, and 14 and 5ppm, respectively. Equations used to predict the retention time are given. The accuracy of the predicted retention time is about two percent compared to experimental results.     

A simple gas chromatographic technique to determine trace impurities in gases

A new method is described which allows the determination of trace impurities in flowing gases without removing a sample. The technique is based upon gas fractionation by a sudden change of flow through a small orifice or capillary.