headspace gas chromatographyIt is an ideal chromatographic analysis instrument that connects a headspace sampler with various models of gas chromatographs, collects volatile components in liquid or solid samples, and directly introduces them into the gas chromatograph for separation and detection. Using headspace gas injection, the principle is to dry and collect volatile components in the sample through a heating device. Compared with liquid-liquid extraction and solid-phase extraction, it can avoid the loss of volatile components caused by solvent removal and reduce the noise caused by co extraction, with higher sensitivity and analysis speed. At the same time, it has less harm to analysts and the environment, high automation and good repeatability, easy operation, and is an analytical method that meets the requirements of "green analytical chemistry".
influence
headspace gas chromatographyThere are several parameters, which will be introduced one by one below:
1. Properties of the sample
The content of each component in the headspace gas is not only related to its own volatility, but also to the matrix, especially for those components with high solubility (distribution coefficient) in the sample matrix, the matrix effect is more significant. This is a major characteristic of headspace injection in gas chromatography, where the composition of the headspace gas is different from that of the original sample, which has a serious impact on quantitative analysis. Therefore, the standard sample must have the same or similar matrix as the sample, otherwise the quantitative analysis error will be significant. Reduce matrix effects: salting out, adding water, adjusting pH, solid crushing, dilution, etc.
2. Sample size
In headspace gas chromatography, the injection volume is controlled by injection time or quantitative tube, and is also affected by temperature and pressure. The injection volume of headspace analysis is not of great significance. What is important is the reproducibility of the injection volume, that is, the reproducibility of the injection conditions. The volume of the sample in the headspace bottle has a significant impact on the analysis results, and the sample volume needs to be determined based on the properties of the sample system.
Reproducibility of sample size: The smaller the distribution coefficient of the tested component (the greater its solubility in the condensed phase), the greater the impact of sample size fluctuations on the results. In practical analysis, the distribution coefficient is generally unknown, so consistency of sample size is required.
The volume of the sample bottle: The upper limit of the sample volume is 80% of the sample bottle volume, in order to have sufficient headspace volume, and less than 50% of the sample bottle volume is often used.
In summary, the nature of the sample, analytical purpose, and method are the main factors determining the sample volume.
3. Equilibrium temperature
The equilibrium temperature of the sample is directly related to the vapor pressure, which affects the distribution coefficient. Generally speaking, the higher the temperature, vapor pressure, and concentration of headspace gas, the higher the analytical sensitivity. The lower the boiling point of the component to be tested, the more sensitive it is to temperature. From this perspective, a higher equilibrium temperature is beneficial for shortening the equilibrium time.
In practical analysis, it is often necessary to choose a lower equilibrium temperature while satisfying sensitivity conditions. Excessive temperature can cause the decomposition and oxidation of certain components (with air in the sample bottle), and may also increase the pressure of the headspace gas, especially organic solvents (so high boiling point organic solvents should be selected). Excessive headspace gas pressure can cause system leakage.
4. Balance time
The equilibrium time essentially depends on the diffusion rate of the measured component molecules from the sample matrix to the gas chromatograph. The faster the diffusion speed, the shorter the required equilibrium time. In addition, the diffusion coefficient is also related to molecular size, medium viscosity, and temperature. The higher the temperature and the lower the viscosity, the greater the diffusion coefficient. So increasing the temperature can shorten the equilibrium time. Shortening the equilibrium time can be achieved through stirring methods such as mechanical vibration stirring or electromagnetic stirring. For samples with high analysis coefficients, stirring has a smaller impact.
5. Sample bottle
The headspace sample bottle requires volume, ability to withstand certain pressure, good sealing, and no adsorption effect on the sample. Nowadays, borosilicate glass is commonly used for manufacturing. The volume of the sample bottle is generally 10-20mL, and the volume is determined based on the instrument requirements and the sample. Generally, 20mL of liquid is used. The instrument factor should consider the size of the chromatographic column aperture, injection volume, etc.
The sealing cover is composed of a plastic or metal encrypted gasket. Usually disposable metal caps are used. There are three types of gaskets: silicone rubber (with good high temperature resistance), butyl rubber (with low price), and fluororubber (with good inertness). To prevent the gasket from adsorbing sample components, gaskets lined with polytetrafluoroethylene or aluminum are now commonly used. If necessary, the interference of non-volatile substances in the sealing gasket on sample analysis should be confirmed through analysis.