Introduction of injection methods for gas chromatography

In gas chromatography analysis, it is required that the injection volume of liquid sample is less, and the injection needs to be accurate, fast and with high reproducibility. But in the daily gas chromatography analysis, especially for capillary gas chromatography, there are some problems in liquid sample injection. Only a reliable injection system can solve these problems. There are four common liquid sample injection techniques: split injection, non split injection, column injection and temperature programmed injection. The following will mainly introduce the application of these injection methods in the analysis of liquid samples.
1. Split injection
For split injection, the liquid sample is injected into the heating chamber of the injector, and the heating chamber rapidly heats up to make the sample evaporate instantaneously; under the purging of carrier gas with high flow rate, the sample and carrier gas are rapidly mixed, and most of the mixed gas is discharged when the mixture passes through the shunt port, and a small amount of mixed gas enters the chromatography for analysis. The split flow has two purposes: one is to reduce the content of sample in carrier gas to meet the requirements of capillary chromatography; the other is to make the sample enter into the chromatographic column with narrow bandwidth. However, only 1-5% of the samples can enter the chromatographic column, which is not suitable for the analysis of trace components in samples. When flame ionization detector (FID) is used, the detection limit of analysis is about 50pm (w /). During the injection process, some volatile components will be lost when the sample is injected into the heating chamber by the injection probe, so the analysis repeatability of this injection method is not high. The split mode injection is suitable for the analysis of volatile compounds, and the boiling point of the compounds to be measured should be lower than that of n-c20 in quantitative analysis. Split mode injection is not suitable for the analysis of thermally unstable substances. Because the decomposition reaction of the substance to be measured often occurs in the heating chamber, especially when using glass fiber filler liner. Although the split injection method has many disadvantages, it is still one of the more commonly used injection methods in analytical work because of its simple operation and strong adaptability.
2. Undivided injection
The equipment required for non split injection and split injection is similar. The sample evaporates rapidly after being introduced into the heated liner. At this time, the shunt tube is closed and the sample is introduced into the chromatographic column. After 20-60 seconds, open the diverter valve to drain the trace steam from the heated liner. At a lower column temperature, the components to be determined are enriched again at the top of the column due to the solvent effect, so that the samples are separated with a narrow bandwidth. The ideal enrichment is to form a liquid membrane at the entrance of the column. This effect can be achieved by using a weak polar solution as a solvent. For solvents with strong polarity, such as methanol, only a small volume (< 2 μ L) can be injected. If the injection volume is large, the peak shape of the sample will be distorted. A similar situation can occur in split injection mode. Because the sample needs to be placed in the heating chamber for a longer time, the thermal decomposition effect of the non split injection mode is more obvious than that of the split injection mode. Compared with split injection mode, non split injection is more suitable for the analysis of trace components.
3. Column injection
Column injection is to inject the liquid sample into the capillary column without heating without evaporation. During the temperature programmed process, the vapor pressure of solute increases continuously, and the analysis begins. Because the initial temperature is lower than the boiling point of the solvent, the thermal discrimination effect is avoided. For volatile components, solvent effect is used in both column injection mode and non split injection mode. By connecting a short intercepting precolumn to the column head, the broadening of liquid sample spectral band caused by column overflow is avoided. This technique is suitable for the determination of trace components and thermally unstable substances in samples. The seed characteristics of column injection were better than that of split injection and non split injection. Although column injection has so many advantages, it can not be widely used in daily analysis due to the particularity of technology and operation.
4. Comparison of three injection methods: split injection, non split injection and column injection
In order to select a suitable injection method, the content of the components to be tested, the boiling point and thermal stability of the components, and the properties of the components to be tested should be considered. Finally, we need to consider the practicability of the injection mode. Figure 1 shows several applications of the three injection methods. However, in the analysis of special samples, only one injection method can not meet the needs of analysis. The content of the substance to be determined in the sample is the main factor affecting the choice of injection mode. When the content is higher (> 50pm, FID), hot split injection, no split injection or cold column head injection can be used. When the content of the components in the sample is between 0.5-0 PM, it is necessary to use hot split injection or cold column head injection. For this kind of sample, split injection is only suitable for application in the pretreatment stage. Another factor to consider is the polarity of the solvent. When a large volume of polar solvent is introduced into a non-polar or medium polarity column, it will cause overflow of the column and produce abnormal peaks. It can be seen from Figure 1 that the above three injection methods can not be used for the detection of samples with low content, and the above three injection methods are not suitable for large volume injection of strong polar solvents. However, temperature programmed evaporation (PTV) injection can successfully solve the above problems。
5. Temperature programmed vaporization (PTV)
PTV injection mode combines the traditional split / no split injection technology, and adds a temperature control system. It can realize hot split / no split injection, cold split / no split injection and cold column head injection. The combination of cold injection and temperature controlled evaporation overcomes the shortcomings of traditional hot injection technology. In the cold injection mode, the liquid sample is introduced into the column without thermal discrimination. In addition, the application of PTV also reduces the probability of thermal decomposition reaction. In addition to the above five injection methods, PTV system can also achieve large volume injection. This injection method is also called solvent exclusion injection. The mass injection mode is to inject large volume of sample into the liner at a certain sample introduction speed, and the solvent is discharged through the splitter tube. At this time, the solute is enriched and trapped, and then the shunt valve is closed to heat the sample liner. In this way, 250 ml sample can be introduced into 320 m capillary column. Large volume injection mode is also suitable for polar solvent injection. The solvent is discharged before the sample enters the capillary column, so the injected polar solvent will not affect the analysis results.
6. Conclusion
PTV injection device is a multifunctional sample introduction device. It has six different injection modes to choose from. The PTV system can be used to analyze the samples that are not suitable for analysis, such as the samples with less components to be tested and dissolved in polar solvents.
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