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Introduction to Quantitative Analysis of Headspace Gas Chromatography
Headspace gas chromatography can quantitatively determine volatile gaseous components in liquids or solids, but it is not a trivial matter to accurately determine the amount of components in a corresponding liquid (or solid) sample. This is because it is convenient to find out only when the component content in the sample is linear with the vapor pressure of the corresponding component in the equilibrium gas phase. Only the ideal solution has a linear relationship, and the actual samples are not necessarily all in an ideal state.
There are two different types of conditions for the headspace due to the nature and concentration of the sample.
(1) ideal solution
A solution in which the attractive forces between molecules are identical, the distances are completely equal, and the Raul's law is fully compliant is called an ideal solution. For example: oxygen and nitrogen, chlorobenzene and ethylbenzene, n-hexane and n-heptane, methanol and ethanol, a mixture of p-xylene and m-xylene, and the like.
If the mixture of n volatile components is the ideal solution, then according to Dalton's law:
Where P is the total pressure of the top air body (atm);
Pig - the partial pressure (atm) of the i component in the headspace air;
Ni - the number of moles of the i component in the headspace air (mol);
Rg - general gas constant (0.08206L·atm/K·mol);
T - absolute temperature (K);
V - head air volume (L).
According to Raoul's law:
Wherein - the mole fraction of the i component in the mixture sample;
- the vapor pressure of the i component in the pure state.
For an ideal solution, the content of the components in the sample can be determined based on chromatographic analysis and the above relationship.
(2) Non-ideal solution
The attraction between the molecules is not exactly the same, and the distance is not completely equal. The solution that causes the deviation between the actual vapor pressure of the solution and the calculated value of Raoul's law is called a non-ideal solution. The non-ideal solution includes the following three different kinds. Happening.
1. Positive deviation
In the sample of the mixture, if the attraction between different kinds of molecules is less than the attraction between the molecules of the pure substance, then they attempt to leave the mixture, which causes the pressure of the volatile gaseous components in the liquid or solid (partial pressure and total pressure). ) is greater than the calculated value of Raoul's law. This is generally the case when polar and non-polar molecules are present together, for example a mixture of ethanol and heptane.
2. Negative deviation
In the sample of the mixture, if the attraction between different kinds of molecules is greater than the attraction between the molecules of the pure substance, then the gas pressure (partial pressure and total pressure) of the mixture of volatile gaseous components in the liquid or solid is less than that of Rawu. The law calculates the value. If the molecule has a permanent or induced dipole moment, these electrostatic forces will cause hydrogen bonding to occur or create an unstable chemical bond between the molecules. This is the case for a mixture of acetone and chloroform; another example is an aqueous solution of hydrogen chloride. Due to the interaction between H2O and HCl to form H3O++ Cl-, the vapor pressure is so marked that it cannot be detected. The very dilute aqueous solution only has water vapor pressure. When the solution is very concentrated, only the HCl vapor pressure is seen.
3. Mixed deviation
There is also a type of mixture (for example, water and pyridine) whose vapor pressure curve shows a concave and convex curvature, that is, a negative deviation occurs under certain conditions, and a positive deviation occurs under another condition.
For non-ideal solutions, the following relationship is required:
Where - the activity coefficient of the i component;
The rest of the symbols have the same meaning as before.
Quantitative calculations are only possible if the activity coefficient values ​​of the components in the mixture are known.
(3) Related matters
1. Careful operation
In the quantitative analysis of the headspace gas, attention must be paid to the reliability of the equipment, the stability and reproducibility of the operating conditions. For example: the sample should be carefully prepared and the container must be well thermostated to establish a vapor pressure balance; the adsorption loss should be avoided when collecting the sample, and the composition of the sample should be avoided due to condensation; the cleaning of the sampling equipment is also very important, so as not to The residue is transferred to the next sample; careful operation reduces errors.
2. Fully mixed
Under constant temperature conditions, the rotation method can be used to make the partial pressure of steam balance faster. Binder pays special attention to the errors caused by the sample preparation process. The diffusion rates of various substances are very different, which will lead to quantitative errors in the headspace gas chromatography analysis, when the equilibrium period is short and the liquid space is available. This is especially true in large cases; this problem can be avoided if the impeller is stirred in it and mixed thoroughly.
3. Appropriate dilution
The change in partial pressure in the high concentration region is not linear with the concentration change. If you do not pay attention to this, you may get completely wrong results, so the quantitative correction work is especially important. In many cases, if the sample solution is properly diluted to bring it close to the desired mixture, it is possible to obtain a linear relationship.
Hachenberg suggested that the higher concentration of vinyl acetate solution should be diluted with methanol before analysis. Quantitative analysis of low concentrations of vinyl acetate in methanol solution demonstrated the use of headspace gas chromatography to analyze the reproducibility and accuracy of the dispersion; Table 1 is the analysis of headspace gas chromatography and bromide-bromine titration The method compares the quantitative analysis of vinyl acetate.
Table 1 Comparison of top air phase chromatography and titration
sample
Acetone acetate content % (w/w)
titration
Headspace gas phase chromatography
1
2
3
4
5
A
7.0
7.6
7.7
7.5
7.6
7.6
B
0.5
0.7
0.7
0.5
--
0.7
C
19.0
19.1
18.7
18.0
--
18.6
4. Cork effect
Special attention should be paid to the sealing of the headspace container with a rubber stopper, which may cause errors and affect the accuracy and reproducibility of the analysis results. When Maier analyzed the solid matter that produced the fragrance, it was found that the rubber stopper adsorbed a large amount of components to be analyzed. He believes that this adsorption can not be avoided even if the plug is heated or changed to aluminum foil or Teflon film. Changing to a silver hat may not give satisfactory results. Therefore, when analyzing a solid sample, it is best to first measure the blank test value. When the headspace analysis of the liquid sample is performed, the decrease in the concentration of the components in the gas phase is not observed, which may be because they are rapidly volatilized from the liquid phase. Figures 2-45 show the amount of steam of various components adsorbed in the rubber stopper as a function of time.
Davis believes that the size of the adsorption under the same conditions is related to the molecular weight and molecular structure of the adsorbed compound. His experiments show that the concentration of ethylene in a gas sample vial on a rubber stopper is 30 minutes. The reduction was 2.0%, the hexane decreased by 7.6%, the heptane decreased by 21.9%, the propionaldehyde decreased by 4.6%, the valeraldehyde decreased by 26.3%, and the heptaldehyde decreased by 64.5%. Therefore, he recommends using glass cock to reduce this effect.
5. Temperature effect
Experiments by Jentzsch et al. suggest that the temperature of the headspace vessel must be chosen to be as high as possible at the vapor pressure of the sample. For example, a sample containing 1% benzene (a sample of benzene-toluene mixture) is chromatographed as shown in the usual liquid sample to give a chromatogram as shown in Figure 2-46A, and the top air analysis at 40 °C gives The chromatogram shown in Figure 2-46B. That is, when the analysis was carried out at 40 ° C, the benzene content in the headspace gas chromatography analysis was 4.3 times that of the conventional liquid sample. Therefore, the detection limit for the component (herein referred to as benzene) can be lower, and a lower content can be detected.
6. Water sample analysis
One way to increase the sensitivity of the headspace gas chromatography analysis of aqueous samples is to add inorganic salts. For example, the addition of anhydrous sodium sulfate, the detection limit of carbonyl compounds can reach 0.01ppm; Kepner et al. added ammonium sulfate or sodium chloride to the diluted sample solution to saturation, the sensitivity can be increased by 7 times; Jets et al. added calcium carbonate to an aqueous solution of tert-butanol, which significantly improved the sensitivity, and the detection limit was reported to be 0.05 ppm.
7. Solid sample
Correction of the quantitative analysis of the top air phase chromatography of solid samples is more difficult. A solid sample with a suitable solvent can be prepared by first adding it to a solution and adding the internal standard.
Rohrschneider's experiments indicate that for the headspace analysis of solid samples, the equilibrium between the solid particles and their gas phase is too slow and takes a long time. Therefore, he suggested that a solid sample (eg, a high polymer) be dissolved in dimethylformamide to reach equilibrium within two hours; a comparison between the polystyrene sample and the corresponding standard sample for headspace gas chromatography. As shown in Figure 2-47.
8. Quantitative correction
If the solubility of a substance in an aqueous solution is lowered, the equilibrium vapor pressure is also changed, which must be considered in the analysis; when the concentration of each component of the sample is greatly different, it will cause great difficulty in quantitative analysis. . For example, high concentrations of ethanol have an effect on the solubility of other compounds at low concentrations and vapor pressure. Similarly, high concentrations of carbon dioxide have similar effects when doing headspace analysis of beer and lemonade. Experiments have shown that the peak heights of isoamyl acetate, isobutanol and isoamyl alcohol in aqueous solution are smaller than those in beer. However, some of the above effects have a relatively small effect on some compounds, such as ethyl hexanoate having the same peak height in water and in liqueurs. This indicates that each sample must be subjected to a specific quantitative calibration work during the headspace analysis.
After Hauck and Terfloth studied the causes of errors in the automated analysis of blood alcohols, it was strongly recommended to use internal standard methods for liquid gas analysis; they also found that for every 1 °C increase in vials The peak height of ethanol and internal standard (butanol) increased equally.