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Factors affecting the static headspace injection analysis of high performance gas chromatographs are sample properties, sample volume, equilibrium temperature, equilibration time, headspace vials and seal caps.
The biggest advantage of static headspace injection analysis is that it does not require complex processing of the sample, but directly takes the top air body for analysis, but the nature of the sample still has a direct impact on the analysis results. The sample refers to the original sample placed in the headspace vial, rather than the volatiles entering the gas chromatograph, so consider the nature of the sample in the entire headspace vial.
1. Gas sample:
For gas samples, there is only a gas phase in the headspace vial and no condensed phase (liquid phase or solid phase), which is not much different from ordinary gas chromatographic analysis. The sampling temperature and sample storage temperature of the gas sample may be different, generally the latter is lower than the former. When the sample is stored at a relatively low temperature, some components may condense, and the analysis is carried out at an equilibrium temperature for a certain period of time to achieve a uniform gas phase to eliminate the error caused by condensation of some components.
2. Liquid and solid samples:
It takes a certain time to convert the liquid sample into a gas. Incomplete gasification will make the composition of the headspace sample different from the original sample, thus affecting the accuracy of the analysis results, and should be balanced for a sufficient time at a certain temperature.
For liquid and solid samples, there are gas-liquid or gas-solid two phases in the headspace vial, even gas-solid three-phase coexistence. The content of each component in the headspace air is related to both its own volatility and the sample matrix, especially the matrix effect of the more soluble component in the sample matrix. The composition of the headspace gas is different from the composition of the original sample, and the effect on the quantitative analysis is particularly serious. The standard sample cannot be prepared only with the standard of the analyte, and must have the same or similar matrix as the original sample.
Methods for reducing the matrix effect include salting out, adding water to an organic solvent, adjusting the pH of the solution, pulverizing the solid sample, and diluting the sample.
(1) Use salting out:
Salting out means adding an inorganic salt to an aqueous solution to change the partition coefficient of the volatile component. Experiments have shown that there is little effect when the salt concentration is less than 5%. High concentration salts or even saturated salts are commonly used.
The effect of salting out on polar components is much greater than on non-polar components.
(2) Add water to the organic solvent:
The addition of water to the organic solvent (the water is compatible with the organic solvent used) reduces the solubility of the organics in the organic solvent and increases its content in the headspace gas.
(3) Adjust the pH of the solution:
For acids and bases, the degree of dissociation can be altered or the volatility of other analytes can be made greater by controlling the pH of the solution.
(4) pulverizing solid samples:
The diffusion coefficient of a substance in a solid is smaller than that in a liquid. The diffusion rate of volatile matter in a solid sample is very slow, and it takes a long time to reach equilibrium. It is advantageous to use a small particle solid sample to shorten the equilibrium time.
The general pulverization method causes sample loss, and the freeze pulverization technique is often used to prepare a solid sample.
Infiltrating the sample with water or an organic solvent reduces the adsorption of the solid surface to the analyte.
(5) Dilute the sample:
Dilution of samples is a common method of reducing matrix effects, but it reduces analytical sensitivity.
Second, the sample size:
The sample volume refers to the sample volume in the headspace vial. The amount of sample directly determines the impact on the analysis results.
The nature of the sample, the purpose and method of analysis are the main factors determining the amount of sample.
The injection volume is controlled by the injection time (pressure balance system) or the quantitative tube (pressure control loop system), which is affected by factors such as temperature and pressure. In fact, the absolute injection volume does not make much sense. What is important is the reproducibility of the injection volume. As long as the injection conditions are guaranteed to reproduce, the reproducible injection volume can be guaranteed. Even in quantitative analysis, it is generally not necessary to know the absolute injection volume.
Static headspace injection analysis typically only takes one sample from a headspace vial. When doing parallel experiments, several samples should be prepared and placed in different headspace vials. Whether the volume of each sample is reproduced will affect the analysis results. The smaller the partition coefficient of the component to be tested, the larger the error of the result caused by the fluctuation of the sample volume. Conversely, the larger the distribution coefficient, the smaller the impact. In practice, the partition coefficient of the sample is often unknown, and the volume of each sample should be as uniform as possible at all times.
The sample volume is also related to the volume of the headspace vial. The upper limit of the sample volume is 80% of the headspace vial volume so that there is sufficient headspace volume for sampling. Often the sample volume is 50% of the headspace vial volume.
Third, the equilibrium temperature:
The equilibrium temperature of the sample is related to the vapor pressure and affects the partition coefficient. In general, the higher the equilibrium temperature, the higher the vapor pressure, and the higher the concentration of the 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. However, in static headspace injection analysis, temperature changes only affect the partition coefficient and do not affect the comparison. For a given sample system, the comparison is constant. When the partition coefficient is compared, the temperature effect is very significant. When the distribution coefficient is compared, the temperature rise causes the partition coefficient to decrease, but the distribution coefficient and the change in comparison are small, and the change in the concentration of the headspace gas is small.
The equilibrium temperature should be selected according to the analysis object. In actual work, a lower equilibrium temperature is generally selected under the condition that the sensitivity of the analysis is satisfied. Excessive temperatures may cause some components to decompose and oxidize, causing the pressure of the headspace gas to be too high. Excessive pressure in the heading air will place higher demands on the next step of pressurization, which may cause the system to leak.
Temperature reproducibility must be guaranteed in static headspace injection analysis. In addition to balancing the temperature, the sampling tube, the loop, and the connection to the gas chromatograph are strictly temperature controlled. These temperatures are generally higher than the equilibrium temperature to avoid adsorption and condensation of the sample.
Fourth, balance time:
The equilibrium time is generally longer than the analysis time. The analysis period is mainly determined by the equilibrium time. Shortening the equilibrium time is the key to improving the speed of static headspace gas chromatography. The equilibrium time essentially depends on the rate of diffusion of the molecules of the component to be tested from the sample matrix to the gas phase. The faster the diffusion rate, the larger the diffusion coefficient and the shorter the required balancing time.
1. Determination of balance time: 20ml screw headspace vials
Since the nature of the samples varies widely, the equilibrium time is difficult to predict and is generally determined by experimentation. The same sample is placed in 5 to 10 headspace vials, each headspace vial is subjected to different equilibration time, and then analyzed by gas chromatography. The peak area of ??the analyte is plotted against the equilibration time to determine the required amount. Balance time.
The equilibration time is generally not less than 30 minutes to ensure that the gas-liquid or gas-solid phase of the sample solution has sufficient time to reach equilibrium. The balance time should not be too long, generally not more than 60min, otherwise the airtightness of the headspace vial may be deteriorated, resulting in a decrease in quantitative accuracy.
2. Ways to shorten the balance time:
(1) Increase the temperature:
The molecular diffusion coefficient is related to molecular size, medium viscosity, and temperature. The higher the temperature, the lower the viscosity and the greater the diffusion coefficient. Increasing the temperature shortens the equilibration time.
(2) Reduce sample volume:
A gas sample or a liquid sample that can be fully converted to a gas requires a shorter equilibration time. The equilibration time of a liquid sample is related to the sample volume, in addition to the nature and temperature of the sample. The larger the sample volume, the longer the required equilibration time. The sample volume is related to the analytical sensitivity requirement. For components with a small partition coefficient, increasing the sample volume greatly increases the sensitivity of the analysis, but the required equilibration time increases. For components with a large partition coefficient, increasing the sample volume is less sensitive to the analysis, and a small sample volume can be used to shorten the equilibration time.
(3) Adopt overlapping balance:
The Auto Headspace Sampler has this feature that allows you to preset time programs for automated analysis.
(4) Stirring:
Experiments have shown that for samples with small partition coefficients, agitation can reduce the equilibration time by more than half. However, for samples with a large partition coefficient, the effect is much smaller.
The solid sample requires a long equilibrium time. In addition to increasing the temperature, the equilibrium time can be shortened. The reduction of the solid particle size and the increase of the specific surface can effectively shorten the equilibrium time. The equilibrium time can also be shortened by dissolving the solid sample in a suitable solvent or by impregnating the solid sample with a solvent.
V. Headspace sample bottles:
Most of the headspace vials are made of borosilicate glass, and their inertness can satisfy the analysis of most samples.
1. Requirements for headspace vials:
(1) The volume is accurate.
(2) Can withstand certain pressures.
(3) Good sealing performance.
(4) No adsorption to the sample, etc.
2. Determination of the true volume of the headspace vial:
Quantitative analysis involves the exact value compared to the exact volume of the headspace vial. The simple method is to first weigh the empty bottle with a balance, then fill it with water and weigh it. According to the density of the water at the weighing temperature, the exact volume of the headspace vial can be calculated. For the same batch of headspace vials, the true volume of 5 of them can be accurately determined, and the average volume is used as the true volume of the batch of headspace vials.
3, the selection of the headspace sample bottle: 20ml screw headspace vials
The volume of the headspace vial is 5~22mL.
(1) According to the requirements of gas chromatography analysis.
(2) Depending on the sample. The liquid sample uses about 10 mL of the headspace vial because the sensitivity of the analysis depends on the concentration of the component to be tested in the headspace gas, not the amount of sample. Solid Samples Because of the large size of the sample itself, larger headspace vials are used.
(3) According to the column requirements. When a packed column or a capillary column is used for split injection, the injection volume is generally 0.5 to 2 mL, and a large volume headspace vial is used. When the capillary column is not split injection, the injection volume generally does not exceed 0.25 mL, and a small volume of the headspace vial is sufficient.
4, the cleaning of the headspace sample bottle:
The headspace vial is preferably used only once. If it is used repeatedly, be sure to clean it. The cleaning method is first washed with detergent, then washed with distilled water, rinsed with pure methanol, and finally dried in an oven.
Sixth, the sealing cover:
The sealing cap of the headspace vial consists of a metal cap and a gasket.
1. Metal cover:
The metal cover has a screw cap that can be used multiple times and a disposable gland. A single-use aluminum gland is currently used.
2, the gasket:
The gasket is mainly made of silicone rubber, fluororubber and butyl rubber.
(1) Silicone rubber mat: high temperature resistance.
(2) Fluororubber pad: good inertness. In order to prevent the gasket from adsorbing the sample components, a polytetrafluoroethylene gasket is currently used.
(3) Butyl rubber mat: low price. 20ml screw headspace vials
The seal may leak and lose its protection after being pierced once. When preparing the sample, all the samples are added to the headspace vial and then sealed. When multiple injections from a headspace vial are required, it is best to continue. Do not leave the headspace vial through the seal for a period of time.
This is the end of the introduction of Factors Affecting Static Headspace Injection Analysis of High Performance Gas Chromatograph .I hope it can help you.
