Gas-chromatographic headspace analysis: Metrological aspects

Various methods are studied for preparing stable gas mixtures with trace amounts of volatile substances based on a buffer effect of heterogeneous systems and implemented into headspace analysis, that is, static and dynamic versions of gas extraction. Information is provided on certified procedures for preparing such mixtures, that is, vapor-phase sources of gas mixtures included into the State Register of Measuring Instruments.     

Preparation of stable gas mixtures with microconcentrations of volatile substances in vapor-phase sources at elevated pressures

Tools are proposed for the preparation of stable vapor-gas mixtures with microconcentrations of volatile components (vapor-phase sources of gas mixtures). These work at elevated pressures and ensure the generation of gas flows with component concentrations differing within an order of magnitude from one and the same source.     

Measurement of the Solubility of Volatile Substances by Vapor-Phase Gas-Chromatographic Analysis

A new procedure is proposed for vapor-phase gas-chromatographic determination of the solubilityof partly soluble volatile compounds in liquids. The method is based on successive pneumatic sampling totake small, equal portions of the equilibrium gas from the gas-liquid system containing an excess ofthe volatile component forming the second liquid phase. The gas-chromatographic determination of thiscomponent in the samples of the equilibrium gas makes it possible to find the slope of the isotherm describing the distribution of the substance between the liquid and gas phases near the saturation point, estimate the nonlinear portion of the isotherm, and calculate the substance content in the saturated solution.     

Gas-chromatographic determination of volatile sulfur-containing impurities in industrial emissions and aqueous solutions

Procedures are developed for determining volatile sulfur-containing impurities, hydrogen sulfide, simple mercaptans, sulfides, and disulfides in air and aqueous solutions. The procedures are based on the principle’s headspace gas chromatography and equilibrium preconcentration. The procedures differ from the officially approved ones in that the operations of sampling and sample preparation to gas-chromatographic analysis considerably reduce the loss of hydrogen sulfide and mercaptans unstable in humid air. The procedures provide the determination of sulfur-containing substances at the level of their maximum permissible concentrations (MPC) in industrial emissions and natural waters and the analytical range of four orders of magnitude; they can be implemented on gas chromatographs with flame ionization and flame photometric detectors, a quartz capillary column, and a thermostated gas-sampling valve.     

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.     

Equilibrium Model in the Description of Gas Extraction and Headspace Analysis

The main regularities of equilibrium gas extraction were considered for the static, dynamic, and flow versions of headspace gas-chromatographic determination of volatiles in solutions. The efficiency of these analytical techniques was assessed.