Equilibrium Headspace Gas Chromatography Research Articles

Development of an equilibrium headspace gas chromatographic method for the measurement of noncovalent association and partitioning of n-alkylbenzenes at infinite dilution in fulvic acid pseudophase

Fulvic acid (FA), the most important water soluble fraction of humic substances in nature, is known to form aggregate pseudophase and complexes with organic and inorganic species. Here, we report a novel equilibrium headspace gas chromatography (eHSGC) and a two-step reaction model to measure n-alkylbenzene–FA association constant (K11) and n-alkylbenzene–pseudophase FAn association constant (Kn1) without solute concentration and response factor. The K11 and Kn1 values were 2–3 orders of magnitude higher than those for sodium dodecylsulfate. Changes in peak area were used to calculate the critical FA-aggregation concentration (cfc), mole fraction based partition coefficients (Kx), activity coefficients of solute inside the aggregate pseudophase (γm∞), and transfer free energies of alkyl CH2 at infinite dilution. The cfc was found to be 10±0.5μM. The Kx values are of the order of 107 in the FA-aggregate pseudophase. The data shows that benzene has the lowest (0.0002) and n-butylbenzene has the highest (0.01) γm∞ values, which are seven orders of magnitude smaller than γw∞ in water. The transfer free energy of association of a CH2 group, −155cal/mol, compared to that of benzene, −9722cal/mol, indicates that the FA-aggregate pseudophase is more polarizable benzene-like and less n-alkane aliphatic-like.

Effect of starch and amylopectin concentrations on volatile flavour release from aqueous model food systems

AbstractThe influence of starch and amylopectin (0, 1, 2, 3, 4 and 5% w/v) in aqueous model food systems was evaluated for 20 volatile flavour compounds. To understand the influence of the gelling agents on the thermodynamic and kinetic component of volatile flavour release, the release of six alcohols, five ketones, four esters, three aldehydes, one sulphur compound and one terpene was determined under equilibrium conditions and mouth conditions. For equilibrium analysis, the air/product partition coefficients of the compounds were determined by equilibrium headspace gas chromatography. Under mouth conditions, the flavour released in a model mouth system was analysed by gas chromatography–mass spectrometry. Both gelling agents and their concentration affected the release of the volatile compounds significantly, resulting in a distortion of the flavour composition. Under equilibrium conditions, starch showed greater flavour retention than amylopectin. Under mouth conditions the opposite was observed; amylopectin affected flavour release more than starch. Both functional group and chain length of the flavour compounds influenced the effect of the gelling agent. Since the effects of the gelling agents differed under mouth and equilibrium conditions, it is conceivable that the kinetic component of flavour release is of greater importance for the effect of these gelling agents on volatile flavour release than the thermodynamic component. Copyright © 2003 John Wiley & Sons, Ltd.

Quantitation of the Residual Solvent Naphtha in a Pharmaceutical Soft Gelatin Capsule Product by Equilibrium Headspace Gas Chromatography

An equilibrium headspace gas chromatographic (EHS-GC) method for the quantitation of the residual solvent naphtha in a pharmaceutical soft gelatin capsule product is described. The drug product, which is an encapsulated oil suspension, is exposed to the solvent naphtha during the manufacturing process. Headspace gas chromatography is the preferred method for the analysis of residual solvents in bulk pharmaceutical and drug products because the sample matrix is not introduced into the gas chromatographic system, resulting in a simpler and cleaner assay method. In EHS-GC sampling, the gas phase above the sample matrix is analyzed after it has reached thermal equilibrium. The EHS-GC method parameters are optimized by monitoring the n-octane gas-liquid partition coefficient of naphtha in the sample matrix via the EHS-GC phase ratio variation method of

Simultaneous determination of volatile aromatic and halogenated hydrocarbons in water and soil by dual‐channel ECD/PID equilibrium headspace analysis

AbstractVolatile aromatic and halogenated hydrocarbons are determined in water and soil samples by equilibrium headspace gas chromatography. Simultaneous analysis is carried out in a dual column/dual detector arrangement with ECD and PID. Their determination in contaminated soil is performed with the same instrumental configuration. However, in contrast to water analysis, an increased sample temperature was found necessary for highly adsorptive soil specimens. At sample temperatures of 95 °C, recoveries near 100% have been found. Both adsorption and desorption processes were found to be strongly time‐dependent.

Determination of gas-liquid partition coefficients by automatic equilibrium headspace-gas chromatography utilizing the phase ratio variation method

The new phase ratio variation method is described which represents a convenient way for the determination of gas-liquid partition coefficients for practical purposes, utilizing equilibrium headspace-gas chromatography (EHS-GC). This method is based on the relationship between reciprocal peak area and the phase ratio in the vial containing the sample solution; it involves regression analysis of the EHS-GC measurements of a number of sample vials containing the same sample solution but with a wide variation of phase ratios. Examples are given for both aqueous systems and systems consisting of a stationary (liquid) phase used as the solvent; comparison of the measured values with results obtained by other methods shows satisfactory agreement. A critical discussion of the conditions influencing the accuracy of the analytical results is given.

Determination of partition coefficients by automatic equilibrium headspace gas chromatography by vapor phase calibration

Equilibrium headspace gas chromatography has been applied to the determination of the partition coefficients of volatile compounds in water-air systems. Only techniques that are suited to a fully automatic headspace procedure using the pneumatic headspace sampling-technique have been considered. Particularly simple is the technique of “vapor phase calibration —VPC” where an external vapor standard is used to calibrate the concentration of the volatile analyte in the headspace, while the concentration in the sample is found from the difference in the total amount in the vial. This technique is described in detail for 2-butanone in water. Finally, the water-air partition coefficients of several selected volatile compounds at different temperatures are listed together with their temperature functions.

Headspace-gas chromatography: The influence of sample volume on analytical results

The role of the volume of the sample and the sample vial in equilibrium headspace-gas chromatography is discussed. A new term, thesample phase fraction (ΦS) is introduced. It is shown that if the value of ΦS is kept constant, the vial's volume has no influence on the “sensitivity” of the headspace analysis (which is proportional to the concentration of the analyte in the headspace). In a given headspace sampling system, concentration of the compound of interest in the headspace (cG*) at equilibrium is related to the value of ΦS: a higher ΦS will increase cG*. However, the influence is important only in the case of low distribution coefficients: in the case of higher distribution coefficients this influence is negligible. This conclusion is also true for small changes in the sample volume in duplicate analyses: exact reproducibility of the sample volume is important only in the case of low distribution coefficient values.

Analysis of water in liquid and solid samples by headspace gas chromatography

Equilibrium headspace gas chromatography is applied for the analysis of water in liquid and solid samples which are first dissolved in a water miscible solvent. The preferred quantitation technique is the method of standard addition. A particular problem is the water blank, caused by the humidity of the enclosed air in the headspace vial and by the residual water content in the solvent. It has been found more convenient to include this water blank in the calculation rather than to avoid it by working under dry conditions. Both, precision and detection limit, depend on the reproducibility of this water blank. The combination of a hot wire detector with a fused silica capillary column was found superior to that with a packed column. Good agreement of the analytical result with the theoretical value was achieved with oxalic acid dihydrate as a test sample to prove the accuracy of the headspace approach. Practical applications are given for water analysis in a liquid solvent and in new and used engine oil.

Flavor characterization of dentifrices using equilibrium headspace gas chromatography

Equilibrium headspace gas chromatography was used to measure quantity and composition of flavor released from dentifrices into the headspace of sealed vials. Dentilfrice formulations varying in composition were analyzed for volatile flavor components after thermodynamic equilibrium between the gas (headspace air) and liquid phase (dentifrice) was established. Comparison of dentifrice formulations allowed for determination of the effect of individual components of flavor binding. This information is useful to formulators, since knowing the extent of the effect of dentifrince components on flavor release allows the formulator to intelligently modify the dentifrice components/flavor. This technique is advantageous over the previously reported kinetic procedure in that the results of equilibrium headspace gas chromatography are not complicated by the effect of varied viscosity.

Headspace analysis with cryogenic focusing: A procedure for increasing the sensitivity of automated capillary headspace analysis

A convenient procedure has been described for enhancing the sensitivity of equilibrium headspace gas chromatography. The technique involves the cryogenic focusing of headspace vapors at the head of a 0.32 mm fusedsilica capillary column which has been coupled to a packed column injection port. Cryofocusing is accomplished automatically by cooling the GC oven and results in enhanced sensitivity with improved resolution. With this simple focusing technique no sample splitting is required which gives at least a 10-fold increase in sensitivity. A Multiple Headspace Injection (MHI) technique has been described which allows several rapid headspace injections to be made at the start of a single chromatographic run. Cryofocusing causes the injections to be superimposed, thus increasing the sensitivity even further. These procedures have been demonstrated for flavor and aroma analyses of toothpaste and soft drink samples.

Quantitative headspace analysis of solid samples; a classification of various sample types

Volatile compounds in various solid samples have been analyzed quantitatively by equilibrium headspace gas chromatography with the technique of multiple headspace extraction (MHE). Solid samples can represent either a partition system, such as certain polymers, or an adsorption system, particularly when present as porous material. It has been found that the humidity of the sample has a strong influence on the equilibration in an adsorption system with a hydrophilic matrix and that the addition of water increases the volatility and speeds up the equilibration time. The influence of various important parameters, such as temperature, time, size and structure of the solid samples on the equilibration is demonstrated on the examples of the quantitative determination of vinyl chloride monomer in PVC, styrene in polystyrene, ethylene in polyethylene, ethylene oxide in a contact lens, halogenated hydrocarbons in coffee and residual solvents in drugs.

Methods for the quantitative analysis of volatile halocarbons from aqueous samples by equilibrium headspace gas chromatography with capillary columns

Volatile halogenated hydrocarbons (halocarbons) from aqueous solutions are analysed by headspace gas chromatography with fused-silica capillary columns and an electron-capture detector with detection limits below the parts per billion level. Special emphasis is placed on quantitative determination. Calibration by the internal and external standard techniques and by the method of standard additions is discussed and compared with the multiple headspace extraction procedure, which is based on a repeated headspace extraction of the sample.