Headspace sampling theory

Tipler, A. An introduction to headspace sampling in gas chromatography. Fundamentals and theory, www.perkinelmer.com... 

See also Air Analysis Sampling. Chromatography Overview Principles. Clinical Analysis Sample Handling. Drug Metabolism Metabolite Isolation and Identification. Extraction Solid-Phase Extraction. Food and Nutritional Analysis Sample Preparation. Forensic Sciences Volatile Substances. Headspace Analysis Purge and Trap. Perfumes. Sample Handling Sample Preservation Automated Sample Preparation. Sampling Theory. 

See also Extraction Solvent Extraction Principles Solid-Phase Extraction. Fluorescence Environmental Applications. Gas Chromatography Mass Spectrometry Environmental Applications. Gravimetry. Headspace Analysis Static Purge and Trap. Immunoassays Overview. Infrared Spectroscopy Overview. Liquid Chromatography Overview. Sampling Theory. 

Headspace sampling is probably the simplest and easiest technique. A brief introduction to the topic has been published by Hinshaw [16], and a complete coverage of the theory and practice has just appeared [17]. The sample (liquid or solid) is placed in a sealed vial and heated to a predetermined temperature for a fixed period of time. Volatile components of the sample partition between the gas and sample phases, usually reaching equilibrium. Residual monomers diffuse only slowly from some highly cross-linked polymers, so sufficient time must be allowed for the vaporization from these samples. 

One theory advanced about the canine detection of explosive is that the dog actually identifies the specific explosive molecule. This line of reasoning concludes that training on a pure sample of a single explosive compound should enable a dog to detect any target containing that explosive, regardless of the presence of other materials in the vapor headspace. This does not appear to be the case. 

Solid sample treatments involving the removal of volatile species Static or equilibrium headspace theory... 

SPME is a multiphase equilibrium technique and, therefore, the analytes are not completely extracted from the matrix. Nevertheless, the method is useful for quantitative work and excellent precision and Unearity have been demonstrated. An extraction is complete when the concentration of analytes has reached distribution equilibrium between the sample and coating. This means that once the equihbrium is achieved, the amount extracted is independent of further increase in extraction time. If extraction is terminated before the equihbrium is reached, good precision and reproducibihty is still obtained if incubation temperature, sample agitation, sample pH and ionic strength, sample and headspace volume, extraction and desorption time are kept constant. The theory of the thermodynamic, kinetic and mass transfer processes underlying direct immersion and HS-SPME has been extensively discussed by Pawhszyn [2]. The sensitivity and time required to reach adsorption equilibriiun depends on the partition coefficients between the fiber and the analytes, and the thickness of the phase. Limits of detection and quantitation are often below 1 ppb. 

Quantitative relationships exist between the concentrations of volatiles in equilibrium with liquid samples and these form the basis of quantitative theory for headspace techniques. The partition coefficient, K, is the parameter of fundamental importance and it is defined as the ratio of concentration of analyte a in the sample phase s, and that in the vapor phase v. Cl, at equilibrium (eqn [1]) ... 

A series of headspace analyses is carried out on the same sample, which is conditioned to equilibrium between each analysis step. From the progressive, measured decrease in concentrations determined, the analyte concentration in the original sample can be calculated. The theory is outlined below. 

The balanced-pressure injection entails the headspace vial being pressurized and allowed to reach an equilibrium then a valve is switched to direct part of the sample into the transfer line and the GC for a specific time interval (Fig. 2B). The absolute volume of the sample injected into the GC is unknown because this technique uses a theoretical amount of time to inject the sample. A number of contact parts are minimized in this design which should in theory lessen the chance of analyte adsorption or condensation within the system. An example of an instrument utilizing the balanced-pressure technique is the Perkin-Elmer TurboMatrix model HS-40. 

It is not easy to determine which factors play the greatest role in obtaining good accuracy and precision. One must consider the assumptions inherent in the theory as well as the chemical, mechanical, and instrumental parameters. In general, gas chromatographic methods agree within 1-5% with other physicochemical methods. For example, Hussam and Carr (22) showed that in the measurement of vapor/liquid equilibria via headspace GC, complex thermodynamic and analytical correction factors were needed. These often came from other experimental measurements that were not necessarily accurately known. Another source of significant error can be in determination of the mass of stationary phase contained within the column (59). Other sources of error include measurement of holdup time (60), flowrate, sample mass, response factors, peak area, or baseline fidelity. 

SPME has proven to be a simple, rapid, and sensitive method for collecting the volatile compounds from the headspace of a sample (4,5). In theory, under ideal conditions, the volatile compounds in a liquid sample partition between the liquid and gas phase and between the gas phase and the SPME fiber. After sufficient time, equilibria are established between the sample and the headspace and the headspace and the SPME fiber. In liquid samples, the equilibria can be shifted by adjusting the temperature, pH, using mechanical mixing, by the addition of salts such as NaCl, or a combination of these treatments. 

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