A new gas chromatographic methodology for the estimation of the composition of binary gas mixtures

The relatively new technique of reversed-flow gas chromatography (RFGC) is used to determine the diffusion coefficients of pure gases into gas mixtures (D(mix)(exp)). The pure gases are CO and CO(2), and the mixtures consist of H(2) and He in various volume percentage compositions. A linear regression analysis of D(mix)(exp) of CO and CO(2) in various mixtures of H(2) and He against the percentage composition (X(H2) or X(He)) of the mixtures at different temperatures results in an empirical equation relating D(mix)(exp) to the corresponding theoretical values of the diffusion coefficients of CO and CO(2) in the pure gases H(2) and He, as they are calculated from the Fuller-Schettler-Giddings equation. The empirical equation shows that the diffusion coefficient of an analyte gas in a gas mixture is the partial sum of its diffusion coefficients in the component gases, therefore making possible the determination of the mole fractions of the components of the mixture. The found percentage volume compositions are very close to those determined independently by routine gas chromatography, indicating that the proposed RFGC methodology could be successfully applied to the accurate determination of the volume composition of binary gas mixtures.     

Quantitative accuracy in the gas chromatographic analysis of solvent mixtures

Quantitative accuracy is of great importance in the analysis of bulk mixtures of solvents, particularly when the analysis is related to quality control of very large product volumes like in solvent recovery plants. Serious errors can be made if the effects of density differences between the pure solvents and volume contractions are not properly addressed. In earlier work, the use of an iterative process for correcting such errors has been suggested. However, in the case of volume contractions and mixtures of several solvents, this procedure is difficult to apply. In the present paper, we describe a simple procedure where calibration curves based on mass concentration are utilized. The densities of calibration mixtures of known compositions are determined with a density meter, in order to provide for correction factors caused by volume contractions. Model experiments with mixtures of water, ethanol, acetone and methanol showed a significant improvement in quantitative accuracy. when the suggested calibration strategy was applied.     

Chromatographic methods for petroleum fractions analysis

The state-of-the-art problem of the analysis of various petroleum fractions by the methods of high performance liquid and capillary gas chromatography, IR-spectroscopy, and mass-spectrometry has been considered. The core principle of high performance liquid chromatography as the principal method for petroleum fractions’ separation has been described. Some methods for the chemical modification of domestic silica have been presented. Methods for aromatic hydrocarbons in benzine, kerosene, and diesel petroleum fractions by the chromatography method have been described.     

Computerized capillary gas chromatographic analysis of hydrocarbon mixtures in workplace air

The quantitative composition of white spirit vapours in workplace air may differ considerably from the solvent being used, although all components are the same. By calculating the hygienic effect from the threshold limit value (NGV) of each component, a more reliable estimate is obtained of the occupational hazard than by using the NGV for white spirit. In this method the analyses were performed by on-column injection onto a temperature-programmed capillary column. Retention indices based on n-paraffins and isooctane were calculated using spline functions. Index tables were established for different hydrocarbon mixtures. The validity of the retention indices was found to be satisfactory, depending on the stability of the column and the possibility of optimizing the indices when replacing one column by another of the same type and dimensions. Comparisons were made with alternative methods for determining the concentration of white spirit vapours in air samples. A polar column was used to check the total content of aromatics.     

Procedure for preparing infrared spectra on gas chromatographic fractions

A convenient technique is described for the preparation of microinfrared spectra on materials which elute from a gas chromatograph. The technique involves trapping the sample on KBr and then pressing this material in a die between two portions of pure KBr. Samples as small as 10 μg grams and as low boiling as 150 °C produce acceptable spectra. Since no solvents are employed, solvent band compensation is unnecessary.     

Aqueous solubility calculation for petroleum mixtures in soil using comprehensive two-dimensional gas chromatography analysis data

An assessment of aqueous solubility (leaching potential) of soil contaminations with petroleum hydrocarbons (TPH) is important in the context of the evaluation of (migration) risks and soil/groundwater remediation. Field measurements using monitoring wells often overestimate real TPH concentrations in case of presence of pure oil in the screened interval of the well. This paper presents a method to calculate TPH equilibrium concentrations in groundwater using soil analysis by high-performance liquid chromatography followed by comprehensive two-dimensional gas chromatography (HPLC-GCXGC). The oil in the soil sample is divided into 79 defined hydrocarbon fractions on two GCXGC color plots. To each of these fractions a representative water solubility is assigned. Overall equilibrium water solubility of the non-aqueous phase liquid (NAPL) present in the sample and the water phase's chemical composition (in terms of the 79 fractions defined) are then calculated using Raoult's law. The calculation method was validated using soil spiked with 13 different TPH mixtures and 1 field-contaminated soil. Measured water solubilities using a column recirculation equilibration experiment agreed well to calculated equilibrium concentrations and water phase TPH composition.     

The routine use of selective gas chromatographic detectors

Summary The availability of gas chromatographic detectors selective for phosphorus, sulfur, nitrogen and halogens and the possibility of their simultaneous operation with each other or with conventional universal detectors has tremendously increased the amount of qualitative information that can be gathered from an unknown sample. Selective detectors when used simultaneously permit the calculation of percentage ratios of heteroatoms which represent additional information for qualitative identification. Combination of parallel columns with selective detectors reduces the total time needed for analysis and introduces additional information due to the different polarity of the columns.     

Quantitative thermogravimetric analysis of binary mixtures

Thermogravimetric analysis is used to determine the amounts of Mg(OH)₂ and Mg(CH₃COO)₂in a mixture thereof. The application and suitability of different analysis methods are discussed. In the first method the mass losses in the temperature ranges as indicated by the decomposition of the pure compounds were used. Results obtained using these temperature ranges were unusable. The percentage mass losses due to the decomposition of Mg(OH)₂ and Mg(CH₃COO)₂ were then determined in a second method using the minimum in the derivative mass vs. temperature curves. The results obtained by this method compared well with the actual values for mixtures containing more than 15% magnesium acetate. The third method employed the total experimental mass loss of both decomposition reactions. The results obtained using this method compared well to the actual values, giving a R ² value of more than 0.99. This method of using the total mass losses can however only be used for binary mixtures that consist only of magnesium hydroxide and magnesium acetate. This revised version was published online in August 2006 with corrections to the Cover Date.     

Quantitative analysis of binary antioxidant mixtures

Quantitative analysis of individual inhibitors and their binary mixtures has been carried out. It has been established that the effect of binary mixtures (additive, synergistic or antagonistic) depends on the structure, concentration ratios and activity of their components. The conditions of the synergistic effect have been determined.

---

, , . , , . , .

     

Application of a flow-tunable, serially coupled gas chromatographic capillary column system for the analysis of complex mixtures

Summary A capillary column gas chromatographic system employing two serially coupled fused-silica columns and a simple coupling element is described. The system is operated in flow-tunable mode (flow-tunable tandem system). The very fact of continuous tuning over a large polarity (selectivity) range, ultimately determined by the two constituent columns, offers several possibilities in the analysis of complex mixtures. In this paper two applications are discussed in detail: optimization of peak separation and peak identification. For these applications it is feasible to use, retention data collected from experiments on the tandem system, and empirical formulas. A relatively simple theoretical mathematical model valid for the flow-tunable tandem system, however, furnishes an easy way of calculating retention data on the system from data collected from the individual single columns, thus, creating a new possibility for optimization and peak identification. Optimization and peak identification processes using the empirical and theoretical models are both demonstrated by analysis of solvent mixtures. Dedicated to the memory of Dr. Tibor Tóth Presented at Balaton Symposium on High-Performance Separation Methods, Siófok, Hungary, September 1–3, 1999     

The permeation of binary gas mixtures through support structures of composite membranes

The dusty gas model (DGM) is used to describe transport of binary gas mixtures through porous membrane supports to quantify the resistance towards permeation. The model equations account for three different transport mechanisms for the permeating components: conventional viscous pore flow, Knudsen diffusion, and binary diffusion. Experimental data obtained with the uncoated membrane supports are used to determine the morphological parameters needed in the DGM equations. Flat sheet and hollow fiber membrane supports are characterized by the permeation of a TCE/nitrogen vapor. The DGM shows an excellent fit to experimental data when the asymmetric structure of the membrane supports is taken into account, but the morphological parameters cannot necessarily be related to precise physical structure parameters such as pore size, porosity, and tortuosity. The DGM works well even when the membrane supports are modeled as a single homogenous structure. The membrane supports exhibit different resistances towards the various transport mechanisms that occur within the porous support and the resistances vary with process conditions so that support optimization is not straightforward. With the analysis presented in this paper and transport equations specific to the dense coating and module geometries, the influence of the support layer on gas or vapor separation can be quantified.     

Parallel factor analysis of HPLC-DAD data for binary mixtures of lidocaine and prilocaine with different levels of chromatographic separation

A set of 17 samples containing a constant amount of lidocaine (667 μM) and a decreasing amount of prilocaine (667-0.3 μM) was analysed by LC-DAD at three different levels of separation, followed by parallel factor analysis (PARAFAC) of the data obtained. In Case 1 no column was connected, the chromatographic resolution (R_s) therefore being zero, while Cases 2 and 3 had partly separated peaks (R_s=0.7 and 1.0). The results showed that in Case 1, analysed without any separation, the PARAFAC decomposition with a model consisting of two components gave a good estimate of the spectral and concentration profiles of the two compounds. In Cases 2 and 3, the use of PARAFAC models with two components resolved the underlying chromatographic, spectral and concentration profiles. The loadings related to the concentration profile of prilocaine were used for regression and prediction of the prilocaine content. The results showed that prediction of prilocaine content was possible with satisfactory prediction (RMSEP<0.01). This study shows that PARAFAC is a powerful technique for resolving partly separated peaks into their pure chromatographic, spectral and concentration profiles, even with completely overlapping spectra and the absence or very low levels of separation.     

Highly accurate gas chromatographic analysis of reference gases for use in vehicle emission measurements

 A highly accurate gas chromatographic analytical method has been developed for the determination of the composition of gas mixtures. It was tested using a reference gas as an example consisting of 3.5% of CO, 14% of CO₂, 0.2% of propane and residual N₂ intended for the use in vehicle emission measurements. The method is based on comparison measurements with samples of a calibration gas, whose composition is iteratively adapted to that of the sample investigated using a gravimetric gas mixing method. For the gas chromatographic measurement, a molecular sieve column and a polymer column are used in parallel and in isothermal operation. All gas components can be determined by a single gas chromatographic measurement, and the relative uncertainty of measurement achievable is ?0.4%. Received: 6 March 1996/Revised: 30 May 1996/Accepted: 6 June 1996     

Highly accurate gas chromatographic analysis of reference gases for use in vehicle emission measurements

A highly accurate gas chromatographic analytical method has been developed for the determination of the composition of gas mixtures. It was tested using a reference gas as an example consisting of 3.5% of CO, 14% of CO₂, 0.2% of propane and residual N₂ intended for the use in vehicle emission measurements. The method is based on comparison measurements with samples of a calibration gas, whose composition is iteratively adapted to that of the sample investigated using a gravimetric gas mixing method. For the gas chromatographic measurement, a molecular sieve column and a polymer column are used in parallel and in isothermal operation. All gas components can be determined by a single gas chromatographic measurement, and the relative uncertainty of measurement achievable is ?0.4%.     

Simple gas chromatographic method for furfural analysis

A new, simple, gas chromatographic method was developed for the direct analysis of 5-hydroxymethylfurfural (5-HMF), 2-furfural (2-F) and 5-methylfurfural (5-MF) in liquid and water soluble foods, using direct immersion SPME coupled to GC-FID and/or GC-TOF-MS. The fiber (DVB/CAR/PDMS) conditions were optimized: pH effect, temperature, adsorption and desorption times. The method is simple and accurate (RSD<8%), showed good recoveries (77-107%) and good limits of detection (GC-FID: 1.37 microgL(-1) for 2-F, 8.96 microgL(-1) for 5-MF, 6.52 microgL(-1) for 5-HMF; GC-TOF-MS: 0.3, 1.2 and 0.9 ngmL(-1) for 2-F, 5-MF and 5-HMF, respectively). It was applied to different commercial food matrices: honey, white, demerara, brown and yellow table sugars, and white and red balsamic vinegars. This one-step, sensitive and direct method for the analysis of furfurals will contribute to characterise and quantify their presence in the human diet.