What is headspace sampler?
Headspace injection gas chromatography
is a method in which a sample is placed in a small closed glass vial and the gas phase portion (called headspace) is introduced into the separation column of gas chromatography for detection and quantification. This allows the analyst to eliminate the sample pre-treatment step and concentrate on interpreting quantitative results and developing new analytical methods, allowing more time for research and development evaluation. Headspace sampler
is a convenient and fast sample pre-treatment method in gas chromatography. The head space sampler
eliminates complex, error-prone steps compared to other GC sample processing techniques. It allows you to obtain a large amount of useful information in a relatively short period of time.
Function of headspace sampler
In chromatography, it is always the goal to improve efficiency, which means improving the efficiency of the number of samples that can be analyzed per unit of time. Sample treatment is often one of the most time-consuming tasks. Statistics show that chromatography laboratories usually spend 60% of their time on sample processing and only 10%-15% on real GC analysis, while the rest of the time is spent on data processing and report editing. Therefore, how to speed up or simplify the sample processing becomes a key issue to improve efficiency. In this regard, many good methods have been developed, such as solid phase extraction (SPE), solid phase microextraction (SPME), supercritical fluid extraction (SFC), and so on. However, if we are only interested in volatile components in complex samples, such as organic volatiles in wastewater, blood alcohol levels in drunk drivers, and so on, it is obvious that the extraction methods described above are time-consuming. In this case, headspace chromatography is often a simple and effective method. Headspace analysis is the chromatographic analysis of the gas phase portion of the sample matrix (liquid and solid) above the sample.
Working principle of headspace sampler
is a method to determine the content of a component in the original sample by the composition of the gas above the sample matrix. This is an indirect analysis method. The basic theory is that there is a partition equilibrium between the gas phase and the condensed phase (liquid and solid phases) under certain conditions so that the composition of the gas phase reflects the composition of the condensed phase. Headspace analysis is a gas phase extraction method in which a gas is used as a "solvent" to extract the volatile components of a sample. Thus, headspace analysis is an ideal method for sample cleanup.
As an analytical method, headspace analysis is very simple. First of all, it takes only the gas phase portion for analysis, which greatly reduces the interference of the sample matrix with the analysis. As a sample processing method for GC analysis, headspace
is the simplest. Secondly, different modes of headspace analysis can be optimized for various samples by optimizing the operating parameters. Thirdly, the sensitivity of headspace analysis can meet the regulatory requirements. Finally, if combined with the quantitative analysis capability of GC, headspace GC is fully capable of performing accurate quantitative analysis. Therefore, having a headspace sampler
in your laboratory is worthwhile.
Feature of headspace sampler
There are five headspace analysis methods: manual injection, gas-tight syringe injection, balanced-pressure system, quantitative-loop system, and static-dynamic compensation system.
A. Manual injection
a. The sample is heated and reaches thermal equilibrium.
b. The sample is withdrawn by syringe.
c. Quickly get the sample into the gas phase for analysis.
B. Gas-tight syringe injection
a. The sample is heated to thermal equilibrium.
b. Sample is withdrawn through a heated gas-tight needle.
c. Move to gas phase injection analysis.
C. Balanced-pressure system
a. The sample is heated and reaches thermal equilibrium.
b. Pressurize the sample by passing the carrier gas through the tube.
c. The sample is injected with the carrier gas.
D. Quantitative-loop system
a. The sample is heated to reach thermal equilibrium.
b. Pressure is applied to introduce the sample into the dosing ring.
c. Valve injects sample into the transfer channel.
E. Static-dynamic compensation system
The patented technology is used to provide additional means for sample extraction, including purge mode, constant mode, multiple headspace mode Progressive temperature mode.
Use of headspace sampler
a. When the gas chromatograph is in the steady flow valve, the balanced injection method is used, and the carrier gas is received at the carrier gas interface.
b. Put the injection needle into the chromatographic inlet.
c. When using the pressurization method of injection, you should turn on the pressurization switch and adjust the pressure. The headspace injector pre-column pressure should be 0.02 MPa higher than the gas chromatograph pre-column pressure. Then start feeding the sample after the instrument is stabilized. (Note: when adjusting the pressure, the injection needle is not inserted in the inlet port, and then inserted after adjustment.)
d. The setting of headspace time and injection time of the headspace sampler can be set freely by pressing the setting key with up and down keys.
e. You should use the pipette or pipette gun to accurately move the standard sample into the headspace bottle to seal it, put it into the heating furnace to equilibrate, and wait for the equilibration time to arrive, then start to feed the sample.
f. Insert the sampling needle into the headspace bottle and press the Run button. When the display shows the end of sampling, pull out the sampling needle.
g. After the sample is out, press the cleaning key to clean it, then proceed to the next sample.
h. When doing the standard curve, six different concentrations of standard samples can be accurately pipetted or pipetted into the headspace vial and sealed. Then after equilibrating the same time headspace injection, the standard curve and regression equation can be calculated.
i. When doing reproducibility testing, you can use a pipette or pipette gun to accurately pipette 6 standard samples of the same concentration into a sealed headspace vial. Then equilibrate the headspace injection after the same time, you can exclude one chance error and can calculate the RSD value of the peak area of the 6 injections.
Headspace sampler precautions for operation
a. As the carrier gas will not only enter the headspace but also will also enter the GC, the gas used for headspace should also be purified.
b. The headspace vial heating temperature, quantification tube temperature, and transfer line temperature should be set from small to large. The transfer line should be less than or equal to the temperature of the sample inlet.
c. When applying headspace, the total GC gas flow should be the headspace gas stream plus the GC gas flow. When calculating you should pay attention to the split ratio. It can be calculated after measurement with a flow meter.
d. In the time setting, the sample filling time of the quantification tube should be sufficient, the equilibrium time of the quantification tube should not be too long, and the time of the injection should be long enough.
e. If the pressure of the headspace injector needs to be manually adjusted, it is recommended that the sample pressurization and carrier gas pressure values should be recorded after implementation to avoid pressure changes based on the changes in valve status.
Maintenance of headspace sampler
The spacer mainly plays the role of sealing the sample and cleaning the injection needle. Generally, the spacer can be used more than 100 times. If you find that the pressure at the inlet port drops, check if the spacer is badly worn and the seal is deteriorating, and replace it if necessary. The debris of the spacer will cause the background to rise and may contaminate the liner and block the column, so it should be replaced frequently to avoid the above situation. If the spacer is screwed too tightly during installation and replacement, it will cause the spacer to shrink and harden too much, and the spacer will easily produce debris when feeding the sample, which will significantly decrease the lifetime. It is recommended that the spacer should be tightened slightly without leaking when screwing it.
B. Lining tube
The liner tube acts mainly as a sample vaporization chamber in GC. The sample is vaporized in the liner tube and carried into the gas phase with many classifications. Failure to replace regularly or use properly can lead to poor peak shape, solute discrimination, poor reproducibility, sample decomposition, ghost peaks and other results in the liner tube. The maintenance of the liner tube includes cleaning, silanization, and reasonable use of glass wool.
a. When cleaning the headspace sampler, it usually uses pure water, methanol, or anhydrous ethanol rinse or ultrasonic cleaning.
b. If the contamination is severe, you should gently wipe it with a cotton swab. Please note that you should not use excessive force to avoid damaging the inner surface to produce active spots. Then place to oven 70 degrees drying and dry and cool sealed storage can be.
c. Silylation is one of the effective ways to eliminate the carrier surface activity. It can eliminate the silanol group on the surface of the carrier, weaken the generation of hydrogen bonding force and make the surface inert. The general method is to soak or reflux with toluene solution of 5~8% silylation reagent for more than 1 hour, then wash with anhydrous methanol to neutral, dry and spare. Commonly used silylation reagents are dimethyldichlorosilane (DMCS), trimethylchlorosilane (TMCS) and hexamethyldisiloxane (HMDS). The silanization effect of DMCS is the best, followed by HMDS and TMCS is poor.
d. In most practical applications, a certain amount of glass wool can usually be filled inside the liner tube to increase the vaporization efficiency of the sample and also to prevent spacer debris from clogging the column. However, if the glass wool is not de-activated or has a high number of breakpoints, it can make the active point increase and can be counter-productive. The following applications are not recommended to use glass wool: phenols, organic acids, pesticides, amines, drugs of abuse, reactive polar compounds, thermally unstable compounds, etc.
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