Determination of diffusion coefficients by gas chromatography

Gas chromatography (GC), apart from the qualitative and quantitative analysis of gaseous mixtures, offers many possibilities for physicochemical measurements, among which the most important is the determination of diffusion coefficients of gases in gases and liquids and on solids. The gas chromatographic techniques used for the measurement of diffusion coefficients, namely the methods based on the broadening of the chromatographic elution peaks, and those based on the perturbation of the carrier gas flow-rate, are reviewed from the GC viewpoint, considering their running though the history, the experimental arrangement and procedure, the appropriate mathematical analysis and the main results with brief discussions. The experimental data on diffusion coefficients, determined by the various gas chromatographic techniques, are compared with those quoted in the literature or estimated by the known empirical equations predicting diffusion coefficients. This comparison permits the calculation of the precision and accuracy of the techniques applied to the measurement of diffusion coefficients.     

Effect of efficiency improvement by injecting a sample at a lower carrier gas velocity in isothermal gas-liquid chromatography.

The effect of efficiency improvement of chromatographic system by injecting a sample at a lower carrier gas velocity (in comparison with the carrier gas velocity at subsequent separation) was studied experimentally and theoretically in isothermal gas-liquid chromatography. The suggested technique is based on sample introduction in the programmed carrier gas velocity operation mode: the injection is realized at low carrier gas velocity, then the velocity is increased rapidly up to the operation value. The technique can be applied in chromatographic practice.     

Carrier gas as a new factor influencing the selectivity of the gas-stationary liquid phase chromatographic system

This paper generalizes studies on the influence of carrier gas on relative and absolute retention values. This line of research is also of importance due to the fact that, in the opinion of many chromatographers, the role of the carrier gas is limited only to transporting analyzed compounds along the column. However, even under conditions of the conventional capillary gas-liquid chromatography (i.e. at column pressures under 5 atm) carrier gas (its nature and pressure) significantly influences retention and separation of the analyzed compounds. First, carrier gas (N2 and CO2, for example) dramatically affects relative retention values. For this reason, one should use limit values of alpha(ij) (0) = lim alpha(ij)(P(av)) and I(i)(0) = lim Ii(Pav) I(0) = limI(i) (Pav) with Pav-->0 as chromatographic constants, rather than traditional relative retention values alpha(ij)(P(av)) and I(i)(P(av)). Second, the average pressure Pav of the carrier gas in a column and the nature of the carrier gas influence the selectivity of the gas-stationary liquid phase chromatographic system. Third, wishing to maximize the role of the carrier gas as a factor that improves separation of analyzed compounds, we should design a special gas chromatograph that would allow work with pressures in the column up to 30-50 atm.     

High-speed analysis of residual solvents by flow-modulation gas chromatography

High-speed gas chromatographic (GC) separation of residual solvents in pharmaceutical preparations, using a flow-modulation technique, is described. These volatile compounds are separated on a series-coupled (tandem) column ensemble consisting of a polyethylene glycol column and a trifluoropropylmethyl/dimethylpolysiloxane column. This column ensemble is operated in stop-flow mode to enhance, or "tune", the separation. A valve between the junction point of the tandem column ensemble and a source of carrier gas at a pressure above the GC inlet pressure is opened for intervals of 2-8 s. This stops or slightly reverses the flow of carrier gas in the first column. Stop-flow pulses are used to increase the separation of target analytes that overlap in the total ensemble chromatogram, compared to non-stop-flow, or conventional, operation. All 36 target compounds, based on ICH Classes I and II residual solvent lists, are resolved in 12 min using the stop-flow technique and a single chromatographic analysis.     

Diffusion coefficients from stopped-flow gas chromatography

Summary Mutual diffusion coefficients of two gases A and B can be determined in an empty gas chromatographic column by letting component B enter at an intermediate position of the column and continuously flow through a part only of it, as a carrier gas. The other component A is injected in a small amount instantaneously at the closed end of the column with the detector placed at the other end. By repeatedly stopping and then restoring after a short time the flow of B, narrow extra peaks are produced on the chromatographic elution curve, owing to diffusion of A into B. An equation is derived giving the area under the curve of each stop-peak as a function of time of the corresponding stop. Plotting the experimental data according to this equation permits the determination of the diffusion coefficient of A into B. Some results obtained by this method show negligible variations with changes in the experimental parameters.     

Effect of the nature of the carrier gas on the chromatographic characteristics of monolithic silica capillary columns

Chromatographing a model mixture of hydrocarbons with various carrier gases (helium, hydrogen, nitrogen, carbon dioxide, and nitrous oxide) was used to study the separation ability of monolithic silica capillary columns. It was revealed that the nature of the carrier gas strongly affects the retention time of the sorbates and the height equivalent to a theoretical plate (HETP) of the column, with the values of both these parameters decreasing in the series He > H₂ > N₂ > CO₂ ～ N₂O. This effect was found to be more pronounced for normal hydrocarbons than for their isomers. For chromatographing with CO₂ or N₂O under optimum conditions, the HETP was 25–30 μm, a value indicative of a higher specific efficiency of monolithic capillary columns. Theoretical correlations between the HETP and the properties of the mobile phase were considered. As a result, it was concluded that elevated pressures of the carrier gas, which are required to ensure the optimum operation of monolithic capillary columns, may affect the properties of the chromatographic system.     

Hold-up time in gas chromatography I. New approach to its estimation

A new procedure to reach a good estimate of the hold-up time value (t_M) in gas chromatography is presented. The value of t_M obtained lies close to the experimental retention time of neon, the gas which has shown the shortest retention time in the columns studied. The new method, based on the retention of n-alkanes, is easily applied with any personal computer and may be used with detectors which do not respond to permanent gases. The procedure is reliable, produces very reproducible hold-up times and the value of t_M obtained may be safely used to calculate both chromatographic and thermodynamic parameters.     

Physicochemical measurements by the reversed-flow version of inverse gas chromatography

Since the first publication on the method, reversed-flow gas chromatography has been used to "separate" physicochemical quantities by measuring the value of one in the presence of another. The experimental arrangement consists of a small modification of a commercial gas chromatograph, so that it includes a four- or six-port gas sampling valve, and a simple cell placed inside the chromatographic oven. This cell suppresses the effects of the carrier gas flow on the physicochemical phenomena taking place in the stationary phase. These phenomena pertain to chemical kinetics, diffusion in gases, liquids and surfaces, mass transfer across gas-liquid and gas-solid boundaries, local adsorption on heterogeneous solid surfaces, etc.     

Application of capillary gas chromatography to studies on solvation thermodynamics

The potentiality of capillary gas chromatography (GC) as a means for research on solubility phenomena is focused. Basic thermodynamic information can be obtained in a simple and direct way from this technique relying on few parameters with their associated errors tightly controlled. An unexplored field of solvation phenomenology inaccessible to other techniques is revealed by the accuracy of capillary GC, provided that relevant chromatographic variables are utilized and an adequate treatment of the experimental information performed. The present article reviews different approaches for the attainment of basic thermodynamic information through capillary GC. Some traditional concepts on the treatment of chromatographic data for physicochemical measurement are questioned. Applications of the technique to research on solubility phenomena are depicted.     

2,3-di-O-methoxymethyl-6-O-tert-butyldimethylsilyl-beta-cyclodextrin, a useful stationary phase for gas chromatographic separation of enantiomers

Heptakis(2,3-di-O-methoxymethyl-6-O-tert-butyldimethylsilyl)-beta-cyclodextrin (2,3-MOM-6-TBDMS-beta-CD), synthesized by using methoxymethylchloride (MOM-Cl) as derivatization reagent, was used for capillary gas chromatographic separation of enantiomers. The new chiral stationary phase proved to be suitable for the enantiodifferentiation of volatiles from various chemical classes. Compared to the corresponding gamma-CD derivative (2,3-MOM-6-TBDMS-gamma-CD), the spectrum of compounds for which enantiomers could be separated was more limited and the enantioseparation achieved was generally less pronounced. Unusually high separation factors were observed for 2-alkyl esters of short chain acids (C2-C6). Phenomena underlying the enantioseparation of 2-pentyl acetate (alpha: 4.31; 35 degrees C) were investigated by determining thermodynamic parameters. Data show that only one enantiomer is retained significantly on the chiral stationary phase whereas the other one behaves like the hydrocarbons used as references.     

Chromatographic efficiency of polar capillary columns applied for the analysis of fatty acid methyl esters by gas chromatography

The chromatographic efficiency that could be achieved in temperature‐programmed gas chromatography was compared for four capillary columns that are typically applied for analysis of fatty acid methyl esters (FAME). Three different carrier gases, hydrogen, helium and nitrogen, were applied. For each experiment, the carrier gas velocities and the temperature rates were varied with a full 9 × 3 design, with nine levels on the carrier gas velocity and temperature rates of 1, 2 or 3°C/min. Response surface methodology was used to create models of chromatographic efficiency as a function of temperature rate and carrier gas velocity. The chromatographic efficiency was defined as the inverse of peak widths measured in retention index units. The final results were standardized so that the efficiencies that could be achieved within a certain time frame, defined by the retention time of the last compound in the chromatogram, could be compared. The results show that there were clear differences in the efficiencies that could be achieved with the different columns and that the efficiency decreased with increasing polarity of the stationary phase. The differences can be explained by higher resistance to mass transfer in the stationary phase in the most polar columns.     

Optimizing the relationship between chromatographic efficiency and retention times in temperature‐programmed gas chromatography

A methodology that can maximise the chromatographic efficiency that can be achieved within a defined time frame in temperature‐programmed gas chromatography is described. The efficiency can be defined as the inverse of peak widths measured in retention index units. This parameter can be described by a model similar to the van Deemter equation, which is expanded to account for the effect of the temperature rate in addition to the effect of carrier gas velocity. The model of efficiency is found by response surface methodology, where the temperature rates and the carrier gas velocities are systematically varied in the experiments. A second model that accurately explains the retention time of the last eluting compound can be found from the same experiments, and optimal conditions are found by combining the two models. The methodology has been evaluated with four capillary columns and three carrier gases, using fatty acid methyl esters as analytes. All experiments showed that there is a fairly linear decrease in efficiency with increasing temperature rates. At any temperature rate, optimal velocity is only marginally higher than the velocity that maximises chromatographic efficiency, since the carrier gas velocity has a limited effect on the retention times.     

Retention time prediction of compounds in Grob standard mixture for apolar capillary columns in temperature-programmed gas chromatography

This paper presents an extension of a previous investigation in which the behavior of nonpolar compounds in temperature-programmed gas chromatographic runs was predicted using thermodynamic (entropy and enthalpy) parameters derived from isothermal runs. In a similar manner, entropy and enthalpy parameters were determined for a Grob standard mixture of compounds with widely varying chemical characteristics. These parameters were used to predict the retention times and chromatographic behaviors of the compounds on four gas chromatography capillary columns: three that had phenyl-based stationary phases (with degrees of substitution of 0%, 5% and 50%) and one with (50%) cyanopropyl substitution. The predictions matched data empirically obtained from temperature-programmed chromatographic runs for all of the compounds extremely well, despite the wide variations in polarity of both the compounds and stationary phases. Thus, the results indicate that such simulations could greatly reduce the time and material costs of chromatographic optimizations.     

Headspace gas chromatographic-mass spectrometric analysis of light hydrocarbons and volatile organosulphur compounds in reduced-pressure cultures of Clostridium

A static headspace gas chromatographic method for the simultaneous separation of trace light hydrocarbons and volatile organosulphur compounds in gases of nineteen Clostridium cultures at reduced pressure is described. The separation was achieved on n-octane-Porasil C after sampling of the gaseous compounds in a PTFE loop without any pretreatment. Most peaks were identified by gas chromatography-mass spectrometry. The presence of methane and ethylene sulphide among Clostridium volatiles is confirmed and 3-methyl-1-butene, 2-methyl-2-butene, dimethyl trisulphide and S-methyl thioacetate are reported for the first time in the Clostridium group.     

Separation of mixtures of permanent gases and light hydrocarbons on spherical carbon molecular sieves by gas-solid chromatography

Summary Experimental results are presented on the application of Carbosieve S (Supelco) and Spherocarb (Analabs) spherical carbon molecular sieves for the gas chromatographic separation of mixtures of permanent gases and C₁–C₃ hydrocarbons using a single column or two columns in series. At a programmed temperature of 35–300°C, good separation of the sample components was obtained when using helium as the carrier gas. When hydrogen was used as the carrier gas and the analysis was carried out under isothermal conditions the elution sequence of oxygen and nitrogen reversed as the temperature was increased. This behaviour was observed within a temperature range of 35–225°C for Carbosieve S, and within a temperature range of 35–300°C for Spherocarb.     

Rapid gas analyses for the investigation of spontaneous combustion using capillary gas chromatography

A simple and fast but sensitive and precise gas chromatographic method is described for the quantitative determination of O(2), N(2), CO, CO(2), C1 and C2-hydrocarbons for coal research. Gas analyses are necessary to obtain parameters for modelling spontaneous combustion and to predict long term coal behaviour. The method is based on a single PLOT-type capillary column in a single channel gas chromatograph. Using a micro-volume TCD coupled in series with a FID detector containing a capillary methanizer it is possible to determine high and trace level gases simultaneously. Trace quantities of CO and CO(2) can be determined with a single analysis and the detection limits are improved significantly using the capillary methanizer. The detection limit of the described method is approximately ten parts per million CO(2) and one part per million CO. Using the same instrument configuration the O(2)/N(2) ratios (major components), as parameter for coal reactivity, are also determined. The proposed approach is restricted to the determination of gases evolved during coal studies and the application to other gas mixtures is not considered.     

The homogeneous nucleation of 1-pentanol in a laminar flow diffusion chamber: the effect of pressure and kind of carrier gas

The influence of total pressure and kind of carrier gas on homogeneous nucleation rates of 1-pentanol was investigated using experimental method of laminar flow diffusion chamber in this study. Two different carrier gases (helium and argon) were used in the total pressure range from 50 to 400 kPa. Nucleation temperatures ranged from 265 to 290 K for 1-pentanol-helium and from 265 to 285 K for 1-pentanol-argon. Nucleation rates varied between 10(1) and 10(6) cm(-3) s(-1) for 1-pentanol-helium and between 10(2) and 10(5) cm(-3) s(-1) for 1-pentanol-argon. Both positive and slight negative pressure effects were observed depending on temperature and carrier gas. The trend of pressure effect was found similar for both carrier gases. Error analysis on thermodynamic properties was conducted, and the lowering of surface tension due to adsorption of argon on nucleated droplets was estimated. A quantitative overview of pressure effect is provided.     

Dependence of the relative retention of organic compounds on the nature and pressure of the carrier gas and on the thickness of the PEG-20M film in the capillary column

The effects of the carrier gas nature and pressure on the relative retention values of organic compounds were studied using a series of capillary columns differing in the film thickness of the polar stationary phase (PEG-20M). Relative retention depends linearly on the carrier gas pressure. This dependence becomes more pronounced in the following order of carrier gases: helium < nitrogen < carbon dioxide. The limiting relative retention at a carrier gas pressure approaching zero rather than relative retention values measured experimentally (relative retention time, Kovats retention index,etc.) is an invariant characteristic of a compound subjected to chromatography. For the carrier gases studied, the limiting retention values almost does not depend on the nature of the carrier gas used. The limiting indicating the complex absorption-adsorption nature of these parameters. Dissolution of a carrier gas in the stationary liquid phase has an effect on the relative retention. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2177–2186, December, 1997.     

Study on the thermal decomposition of solids in fluidized bed

Reliable thermodynamic parameters for the thermal decomposition of diammonium hydrogenphosphate [(NH₄)₂HPO₄] may be obtained using a fluidized bed. For the same size of particle, at the same temperature, but for different carrier gases, the rate constant and activation energy increase in the following order: air, methane, hydrogen. For the same carrier gas (air) the rate constant increases when the particle size decreases.