Minimum analysis time in capillary gas chromatography. Vacuum- versus atmospheric-outlet column operation

Previous studies on open tubular column operation at vacuum outlet vs. atmospheric outlet pressures focused on comparisons of given columns, or comparisons of columns with the same inner diameters. It was demonstrated that, for a given separation problem, vacuum outlet operation of columns with a constant i. d. always yields the shortest analysis times (under minimum plate height conditions). In this paper, the comparison of vacuum vs. atmospheric outlet operation is broadened to columns with different dimensions. A general equation for the gain in speed of analysis by vacuum outlet operation of any column, as compared to atmospheric outlet operation of all possible open tubular columns with the same maximum plate number is presented. The resulting equation is further evaluated for thin film columns of different dimensions. It appears that vacuum outlet operation is beneficial only, in terms of speed of analysis, if low maximum plate numbers are required. The gain in speed of analysis is more pronounced for wide-bore than for narrow-bore columns

Development of an on-line analyzer for organic anaesthetics in inspiratory and end-tidal gases

An analyzer for measuring the concentrations of volatile organic anaesthetic agents in inspiratory and end-tidal gases has been constructed. Respiratory gas from an anaesthetized patient is led continuously through a heated capillary transport tube (length 5.7 m, I.D. 0.25 mm) to a hydrogen flame-ionization detector. The pressure drop across the capillary tube necessary to transport the gas is applied by operating the detector at reduced pressure. The ionization current, caused by the organic anaesthetic agent in the detector, is measured with an electrometer amplifier. The transport time, at an optimal pressure drop of 600 mm Hg, is 4.3 sec, and the flow-rate of respiratory gas in the tube is then 3.9 ml/min. The time constant of the system is 0.2 sec. It is shown that mixing between successive inspiratory and expiratory samples can be neglected. The use of the system is demonstrated by two examples. Firstly, the end-tidal concentration of diethyl ether during the wash-out after a combined intravenous infusion-inhalation anaesthesia was measured. Secondly, the analyzer was used during experiments to measure the ventilation:perfusion ratio by administration of small concentration of halothane

Development of an on-line analyzer for organic anaesthetics in inspiratory and end-tidal gases

An analyzer for measuring the concentrations of volatile organic anaesthetic agents in inspiratory and end-tidal gases has been constructed. Respiratory gas from an anaesthetized patient is led continuously through a heated capillary transport tube (length 5.7 m, I.D. 0.25 mm) to a hydrogen flame-ionization detector. The pressure drop across the capillary tube necessary to transport the gas is applied by operating the detector at reduced pressure. The ionization current, caused by the organic anaesthetic agent in the detector, is measured with an electrometer amplifier. The transport time, at an optimal pressure drop of 600 mm Hg, is 4.3 sec, and the flow-rate of respiratory gas in the tube is then 3.9 ml/min. The time constant of the system is 0.2 sec. It is shown that mixing between successive inspiratory and expiratory samples can be neglected. The use of the system is demonstrated by two examples. Firstly, the end-tidal concentration of diethyl ether during the wash-out after a combined intravenous infusion-inhalation anaesthesia was measured. Secondly, the analyzer was used during experiments to measure the ventilation:perfusion ratio by administration of small concentration of halothane

Minimum analysis time in capillary gas chromatography. Vacuum- versus atmospheric-outlet column operation

Previous studies on open tubular column operation at vacuum outlet vs. atmospheric outlet pressures focused on comparisons of given columns, or comparisons of columns with the same inner diameters. It was demonstrated that, for a given separation problem, vacuum outlet operation of columns with a constant i. d. always yields the shortest analysis times (under minimum plate height conditions). In this paper, the comparison of vacuum vs. atmospheric outlet operation is broadened to columns with different dimensions. A general equation for the gain in speed of analysis by vacuum outlet operation of any column, as compared to atmospheric outlet operation of all possible open tubular columns with the same maximum plate number is presented. The resulting equation is further evaluated for thin film columns of different dimensions. It appears that vacuum outlet operation is beneficial only, in terms of speed of analysis, if low maximum plate numbers are required. The gain in speed of analysis is more pronounced for wide-bore than for narrow-bore columns

Investigation of parameters affecting the supercritical fluid extraction of polymer additives from polyethylene

Polymer additives were extracted from polyethylene with supercritical carbon dioxide. The two-film theory, which considers mass transfer across a phase boundary, is applied to qualitatively describe the kinetics of mass transfer from the core of the polymer particles into the supercritical fluid extractant. The effects of pressure, temperature, addition of benzene as a modifier, properties and concentrations of the solutes, static time, and supercritical fluid extractant flow rate on the extraction process are investigated systematically. At constant temperature the extraction rates first increase with increasing pressure. When pressure reaches a certain level, a further increase of the pressure does not further increase the extraction rates. At constant pressure, the extraction rates were found to increase first and then decrease with increasing temperature. In addition to pressure and temperature, the SFE extraction kinetics is also influenced by the solute concentration, and the rate-limiting parameter in the extraction can be changed from solubility to diffusion during the course of the extraction. The magnitude of the effects of the experimental parameters depends on the properties and molecular weights of the solutes. The role of benzene as a modifier in the extraction of polymer additives from polyethylene is swelling the polymer particles and improvement of the solvent strength of the supercritical fluid extractant. Modifier effects were found to be more pronounced at lower temperatures

Ionization mechanisms in capillary supercritical fluid chromatography-chemical ionization mass spectrometry

Ionization mechanisms have been studied for supercritical fluid chromatography (SFC) with mass spectrometric (MS) detection. One of the problems associated with SFC-MS is the interference of mobile phase constituents in the ionization process, which complicates the interpretation of the resulting mass spectra. This interference can be reduced by adding a reagent gas to the ion source. It was found that the properties and the pressure of this reagent gas control the ionization process. In this study ammonia was used as a chemical ionization (CI) reagent gas. An increase in the reagent gas pressure generally resulted in higher abundances of the protonated molecular ion. The presence of an excess of reagent gas suppresses charge exchange processes between the mobile phase constituents and the solutes. Charge exchange causes a more pronounced fragmentation than proton transfer in CI processes. The spectra obtained by charge exchange ionization, with helium as the reagent gas at moderately high pressures, are comparable to electron ionization spectra from standard MS libraries

Preparation of thermally stable phenylpolysiloxane fused silica capillary columns : optimization and evaluation of the deactivation by capillary GC and solid state 29Si NMR

The deactivation of fused silica capillary columns with a laboratory-made poly-diphenylvinylmethylhydrosiloxane copolymer has been investigated. The deactivation obtained at different temperatures and reaction times is characterized with a dual column capillary GC system [1]. In parallel, the effect of the silylation temperatures and reaction times on the nature, the structure, and the chemical properties of the deactivation layer has also been studied by solid-state 29Si NMR spoctroscopy. A fumed silica, Cab-O-Sil M5, was used as a model substrate for these spectroscopic studies. The deactivated fused silica capillaries show an excellent thermal stability (up to 400°C), a high resistance to solvolysis, and a minimal interaction to various critical test components. A good wettability of the fused silica capillary columns deactivated with this reagent was confirmed by successful subsequent coating with polysiloxanes with different phenyl content