A.G. Vitenberg

Application of phase equilibria to gas chromatographic trace analysis

SummaryThe different ways of GLC determination of trace impurities using phase equilibria of the objects under investigation (head-space analysis and determination of impurities in a gas using the data of their content in the liquid saturated with this gas) are discussed and compared. The limits of application and the possible source of errors of each of these methods are discussed. Examples for application of the method of equilibrium concentration for GLC analysis of impurities in food, waste water, air and biological objects are given.

Application of the headspace analysis to systems with unknown partition coefficients

SummaryPossibilities for the quantitative determination of volatile constituents in solutions by headspace analysis are investigated in cases where the partition coefficients (K) depend on the composition of the object under study. The timeconsuming work of revealing and taking into account the effect of the analytes and the other present constituents on the magnitude of K can be eliminated if simultaneously with determining the analyte content in the equilibrium gas one also measures the partition coefficients characteristic for the solution under study. These quantities can be obtained by repeated analysis of the equilibrium gas after partial removal of the volatile solution constituents. At small K (0.1 to 10) the required change of analyte concentration in the solution can be attained by single replacement of the equilibrium gas by pure gas, and at large K, by passing a known volume of pure gas through the solution under study under equilibrium conditions. The paper establishes the range of applicability of each version, provides the necessary equations and presents the results of an experimental check in the case of determining alcohol admixtures in water solutions.

Efficiency of gas extraction in headspace analysis

A study of the variation of volatile concentrations in static and dynamic versions of headspace analysis was carried out to compare the efficiencies of single- and multistep gas extraction procedures with a stationary liquid or gas phase, and with two moving phases.

Methods of equilibrium concentration for the gas chromatographic determination of trace volatiles

Abstract This review deals with a gas chromatographic technique based on the use of equilibria in the condensed phase-gas system, which is currently in the stage of intense development. Problems bearing on the usefulness of different variants of headspace analysis and of related methods in reducing the threshold of the gas chromatographic determination of volatile impurities in objects taken in any aggregate state are considered. When analysing impurities in solid and liquid objects, major attention is focused on the specific features of headspace analysis and methods of improving its sensitivity. For gas objects the same problems are treated in the context of the reverse technique, i.e. , equilibrium saturation of a condensed phase with the gas to he analysed. The theory and practice of equilibrium concentration, including headspace analysis techniques, are developing so fast that even fairly recent reviews and monographs no longer reflect the state of the art of this area in organic analysis. The methods of headspace analysis and its new modifications provide fairly accurate and selective determinations of a variety of impurities present in complex mixtures at the μg/1 level or lower, and find broad application in the analysis of environmental samples and in biology, medicine, geochemistry, oceanology, power engineering, etc. The review covers studies made in the field of headspace concentration and related methods in the last 10–12 years. Attention is focused primarily on the description of the fundamentals of these methods and the areas of their applicability.

Gas chromatographic headspace analysis with pneumatic sampling

Abstract A derivation is given of the major equations describing the multiple headspace extraction process involving partial replacement of the gas phase by pneumatic sampling from pressurized heterogeneous systems. New modifications of the quantitative headspace analysis and concentration of compounds present in liquid and solid states, as well as new methods of chromatographic detector calibration based on the specific features of pneumatic gas sampling from vials pressurized with inert gas, are proposed.

Injection of a gas equilibrated with a liquid into the gas chromatograph

SummaryApplicability limits of the existing methods for the introduction of a gas equilibrated with a liquid into a gas chromatograph are discussed. A new device as well as the method of its application are described. This new device permits gas sampling from an equilibrium system without changing the concentration of the substance distributed between the two phases.

Basic equations in continuous gas extraction and their application to headspace analysis

Abstract The evolution of ideas on the rules of continuous gas extraction based on dynamic headspace analysis is outlined. The application of a simpler exponential equation that describes well the course of gas extraction under the conditions of a sufficiently small volume of the vapour phase is recommended for practical purposes. The possibility of performing under non-equilibrium conditions not only stripping but also variants of the analysis that do not require full extraction of the sample components is considered. The expediency of the application of continuous gas extraction is demonstrated by the determination of volatile substances with low liquid—gas partition coefficients, where high sensitivity can be achieved by direct headspace analysis under static conditions.

Application of gas chromatographic headspace analysis to the characterization of chemical equilibria in solutions

Abstract The possibility is considered of using headspace analysis in studies of chemical equilibria in solutions of volatile organic compounds. The main theoretical relationships for the determination of the constants of X + Y ⇄ XY type equilibria by the gas chromatographic analysis of equilibrated vapour above a solution are derived. New methods are proposed and have been checked experimentally for the measurement of the ionization constants of volatile organic compounds, and of the stability constants of metal complexes with unsaturated and aromatic hydrocarbons.

High-Temperature Gas-Chromatographic Headspace-Analysis of Volatile Polar Impurities in Aqueous Solutions

SummaryHigh-temperature headspace analysis performed under static conditions above 100°C increases the sensitivity of gas chromatographic determination of volatile phenols in aqueous solutions 20–100 fold and reduces the detection threshold, to 10 μg×1−1. To improve the sensitivity and reduce the threshold still further, preliminary concentration by high-temperature microdistillation at elevated pressure is proposed. A theoretical model of this process has been developed. The possibility of concentrating volatiles by high-temperature microdistillation has been experimentally confirmed.

Determination of trace impurities in gases by their equilibrium accumulation in volatile liquids

Abstract A brief discussion is given of work devoted to the theoretical substantiation and experimental testing of a new method for the determination of trace amounts of organic compounds in gases by their concentration in a suitable volatile solvent and subsequent injection into a chromatograph after equilibrium has been reached. The method has substantial advantages over the known techniques of concentration of micro-amounts of impurities by adsorbents and non-volatile liquids, but its application is limited by the availability of a suitable solvent. Results are presented that illustrate the determination of simple aromatic hydrocarbons, carbonyl compounds and diethylamine in air at the level of 0.1–65 mg/m3.