Josef Drozd

Quantitative and qualitative head-space gas analysis of parts per billion amounts of hydrocarbons in water: A study of model systems by capillary-column gas chromatography with splitless sample injection

Abstract Teh reliability of qualitative and quantitative head-space gas analysis of parts per billion * amounts of hydrocarbons in aqueous samples was studied on model systems by capillary-column gas chromatography. A simple all-glass splitless injection system is described that allows the introduction of head-space gas samples with a negligible decrease in efficiency. The applicability of different types of squalene capillary columns for head-space gas analysis was evaluated. The suitability of the standard addition method for quantitative head-space gas analysis is discussed for concentrations in the condensed phase varying from units to hundreds of parts per billion.

Versatile all-glass splitless sample-introduction system for trace analysis by capillary gas chromatography

Abstract A simple, versatile all-glass sample introduction system is proposed which can be easily coupled to the sample inlet part of commercially available gas chromatographic instruments without substantial modifications. The system allows the analysis of trace amounts of organic substances in large gaseous samples with narrow-bore open-tubular columns. The components are concentrated in a cooled capillary trap coated with OV-101 stationary phase and thermally vaporized in the presence of a negative temperature gradient across the trap in the direction of the carrier gas flow. A simple modification of the system is described for the introduction of concentrates released thermally from adsorbent-packed traps. The influence of various factors on trapping and separation efficiency, qualitative and quantitative reliability, possible applications and limitations are discussed.

Quantitative head-space gas analysis by the standard additions method : Determination of hydrophilic solutes in equilibrated gas-aqueous liquid systems

Abstract The use of the standard method for the quantitative trace headspace gas analysis of gas-aqueous liquid systems with acetone, methanol, ethanol and propanol as solutes was studied. The method makes it possible to determine the total content of the solute in the equilibrated system by analyzing defined volumes of the head-space gas. When performed under appropriately defined conditions, the procedure provides for the efficient elimination of the system matrix effects. At solute concentrations in the condensed phase of 1–100 ppm, the error in the determination was about 20–3%. With acetone and propanol, the sensitivity of analysis can be markedly increased by saturating the liquid phase with an inorganic salt.

Headspace determination of benzene in gas—aqueous liquid systems by the standard additions method

Abstract The standard additions method was studied as a means of quantitation in the determination of trace amounts of volatile hydrocarbons in gas—aqueous liquid systems by headspace gas analysis. Amounts of 1–1000 μg of benzene in 50/50-ml air— water systems, corresponding to about 0.02–20 ppm of benzene in the aqueous phase, were determined with errors from about 8 to 2% at these levels, respectively. Analogous systems with milk as the liquid substrate were also studied. and the results obtained by the standard additions method were compared with those obtained by external calibration using a model reference system with water is the liquid phase. While the standard additions method gave correct results, those obtained by the other method suffered from a negative error of about 70%. The sensitivity of the determination of benzene in water by gas chromatographic analysis of head space gas samples was about times higher than that attainable by direct analysis of the liquid phase.

Systematic errors with the use of internal standard calibration in gas chromatographic headspace analysis

Abstract The specificity of the use of internal standard calibration in headspace analysis was studied. An equation was derived that differed from the conventional equation by a factor (Vg + KgVl)/(Vg + KiVl), where Vg and Vl are the volumes of the gaseous and the liquid phases, respectively, and Ki and Ks are the distribution constants between the phases of the analyte i and the internal standard s, respectively. This factor is generally not equal to unity. Its magnitude, and hence that of the systematic error, is demonstrated by using literature values and experimental data obtained by analyses of model blood and water samples. It is shown that the use of the internal standard technique does not generally eliminated the matrix effect which is a serious problem in quantitatives headspace analysis.

Repetitive strippng and trapping applied to the determination of trace hydrocarbons in aqueous samples

Abstract The method of repetitive stripping and trapping of analytes was investigated to determine the reliability of the quantitative results. Different experiment variants of the method, viz., a closed circuit and an open arrangement, were tested by analysing water—air model system with low μg/l levels of benzene, toluene, n-decane, n- undecane, n-dodecane as analytes in the aqueous phase. Whereas in a closed circuit the stripping/trapping process can be conducted either in a conservation or in an equilibrium regie, in an open arrangement the conservation or pseudoequilibration (non-stationary conditions) mode of trapping is possible. All these variants yielded good quantitative results. The results of the deterination of n-decane, n-undecane and n-dodecane consistently suffered from systematic negative errors of 20–40%, which was attributed to non-constant (concentration-dependent) matrix effects associated with the adsorption of the analytes at the water–air interface. The ability of the repetitive stripping/trapping method ot eliminate such effects was tested by comparing the results for the determination of low μg/l levels of benzene, toluene and n-decane in a water—air and in a water—Carbowax 400 (9:1)—air system. Whereas the method of external calibration by means of a referecne water—air system gave erroneous results for the system with Carbowax 400, the results obtained by the repetitive stripping√apping method were correct.

Determination of hydrophilic volatiles in gas—aqueous liquid systems by Grob's closed-loop strip/trap method and standard-addition calibration

Abstract The application of Grobs closed-circuit strip/trap technique to the determination of hydrophilic volatiles in aqueous matrices by the standard-addition method was investigated. Tenax GC was used as the trapping material. Both the conservation and equilibrium alternatives of trapping were assessed, employing butyl acetate and ethanol as model compounds, respectively. With both alternatives the experimentally found dependences of the amount of concentrate recovered from the trap on the total amount of the respective component in the system analyzed were fairly linear. The technique gives an extremely high selectivity of analysis. An application of the procedure to the headspace gas analysis of apple juice is demonstrated.

Spurious adsorption effects in headspace-gas determination of hydrocarbons in water

Abstract The adsorption of benzene, hexane, heptane and octane at the phase interfaces in a gas—water system representing those used in static headspace-gas trace analysis was investigated. The apparent bulk water—gas distribution constants, calculated from the known total amounts of the model solutes adn their gas-phase contents determined analytically, true builk water—gas distribution constants, determined directly by analysis of both the aqueous and gaseous phases, and water surface—gas adsorption distribution constants, calculated from the solute mass balance, are presented and discussed. Adsorption phenomena were shown to play a signficant role with alkanes in gas—aqueous liquid systems.

Determination of Trace Hydrophobic Volatiles in Aqueous Media by a Technique of Multiple Stripping and Trapping in a Closed Circuit

Abstract The double-sampling method of quantitative headspace-gas analysis was assayed by analysing model water-gas systems with benzene, toluene, ethylbenzene, n-decane, and n-dodecane as analytes. With this method, two headspace samples are withdrawn successively from the system and analysed by gas chromatography. The total initial contents of the analytes in the system are calculated by virtue of the decrease in their concentrations in the gaseous phase, brought about by the withdrawal of the first headspace sample. Combined with Grobs closed-loop strip/trap technique, the method provides for the determination of ppb (109) concentrations of the above hydrocarbons in water with an average relative error of several percent.

Retention volume in high-pressure gas chromatography : III. Squalane-tetrachloromethane, isooctane, toluene-nitrogen, hydrogen, carbon dioxide systems

Abstract The specific retention volumes of tetrachloromethane, isooctane and toluene were measured on squalane at 50 and 75 °C with nitrogen and hydrogen as carrier gases at pressures of 10–100 atm and with carbon dioxide at pressures of 10–50 atm. Exact corrections of the data for the gas-phase non-ideality and liquid-phase compressibility effects were carried out, thus making it possible to distinguish and characterize unequivocally the role of the solubility of the carrier gas in the sorbent phase. The results show that the pseudo-binary model fails in a more refined interpretation of gas chromatographic retention data.