Gas Liquid Chromatography Column
2.Chromatographic Column (Rotation Type)
3.Chromatographic Column (Manual)
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Description
Technical Parameters
Gas chromatography and Liquid chromatography are two different chromatographic techniques, the construction design of their instruments have their own characteristics.
Columns in Gas Chromatographs
Gas chromatography is a chromatographic technique that uses gas as the mobile phase (carrier gas), and its core component is the chromatographic column. A column is used to separate the components of a mixture and usually consists of a column tube, a stationary phase and a mobile phase (carrier gas). Column materials include metal, glass, quartz, etc., while the stationary phase is selected according to the analytical needs. Chromatographic columns are divided into two types: packed columns and capillary columns, of which capillary columns have higher separation efficiency and faster analytical speed, so they are more common in practical applications.
Columns in liquid chromatographs
Liquid chromatography is a chromatographic technique that uses liquid as the mobile phase, and the design of its column is equally critical. In a liquid chromatography column, sample molecules undergo partitioning and adsorption between the mobile phase (liquid) and the stationary phase. Partitioning refers to the distribution of the sample between the mobile phase and the stationary phase. Different components have different partition coefficients between the mobile phase and the stationary phase, and therefore different degrees of separation between the two phases will occur. Adsorption refers to the presence of an adsorbent on the surface of the stationary phase, and the sample molecules are adsorbed by the adsorbent in the mobile phase, thus separation occurs.
Parameters
The limitations of capillary columns in high-sensitivity analysis
Small column capacity:
Due to the small inner diameter of capillary columns (usually 0.1-0.7mm), their column capacity is relatively small. This means that the limited sample size that can be accommodated during analysis may impose certain limitations on high-sensitivity analysis, especially when analyzing large amounts of samples or trace substances.
High requirements for injection technology:
The small inner diameter of capillary columns requires more precise injection techniques. Excessive injection volume may cause column overload, affecting separation efficiency and detection sensitivity. Therefore, when conducting high-sensitivity analysis, more sophisticated injection techniques such as split flow injection are needed to ensure the accuracy and reliability of the analysis.
Accurate control of carrier gas flow rate:
The capillary column requires more precise control of the carrier gas flow rate. The variation of carrier gas flow rate will directly affect the separation efficiency and peak shape, thereby affecting the sensitivity of detection. Therefore, when conducting high-sensitivity analysis, it is necessary to strictly control the flow rate of the carrier gas to ensure the stability and accuracy of the analysis.
High sensitivity requirements for detectors:
Due to the small column capacity of capillary columns, the amount of sample entering the detector is correspondingly reduced, which puts higher demands on the sensitivity of the detector. In order to obtain accurate analysis results, it is necessary to choose a high-sensitivity detector and optimize the detection conditions, such as increasing the detector temperature to reduce background noise.
Peak broadening problem:
The flow rate of the mobile phase inside the capillary column is low and the flow rate is small. The sample will undergo severe longitudinal diffusion due to a sudden increase in dead volume behind the column, resulting in peak broadening. The broadening of peak shape may affect the clarity and sensitivity of separation, especially in high-sensitivity analysis, where even small changes in peak shape can have a significant impact on the analysis results.
some suggestions for optimizing injection techniques
Sample concentration:
When the sample concentration is below the detection limit of the instrument, the concentration method can significantly improve the analytical sensitivity. Common concentration methods include liquid-liquid extraction followed by solvent evaporation, solid-phase extraction (SPE), etc.
In recent years, the development of new technologies such as supercritical fluid extraction (SFE) and solid-phase microextraction (SPME) has provided more options for chromatographic analysis. Especially SPME technology, as a solvent-free extraction method, can be directly combined with gas chromatography (GC) to achieve automatic analysis, greatly improving analysis efficiency.
Choose the appropriate injection method:
Non split injection, cold column head injection, and programmed temperature injection techniques can all improve analytical sensitivity and simplify sample processing steps to a certain extent. These injection methods can reduce the loss of samples during the injection process and improve the efficiency of sample entry into the chromatographic column.
For samples with extremely low concentrations, Large Volume Injection (LVI) technique can be used. The core of this technology lies in effectively eliminating solvents and controlling the amount of sample entering the chromatographic column, thereby achieving large volume injection and improving sensitivity. Some instruments are equipped with specially designed LVI injection ports, while others achieve LVI functionality by attaching accessories to existing injection ports.
Using an endotracheal tube or microinjection device:
For small volume samples or samples with low liquid levels, an inner tube or microinjection device can be used to ensure accurate and complete entry of the sample into the chromatographic column. These devices can reduce the volatilization and loss of samples during the injection process, and improve the injection accuracy.
Optimize instrument parameters:
The injection volume is an important instrument parameter that needs to be set reasonably based on the concentration of the sample and the detection limit of the instrument. Generally speaking, increasing the injection volume appropriately can improve sensitivity while ensuring no overload.
The heating program is also one of the key factors affecting sensitivity. A reasonable heating program can ensure effective separation of samples in the chromatographic column, thereby improving the sensitivity and accuracy of detection.
Using an automatic sampler:
Automatic sampler can accurately control the injection volume and injection time, reducing errors caused by human operation. In high-sensitivity analysis, using an automatic sampler can greatly improve the accuracy and repeatability of the analysis.
Pay attention to sample purification and matrix effects:
Impurities in the sample may interfere with the analysis and reduce sensitivity. Therefore, before conducting high-sensitivity analysis, it is necessary to purify the sample to remove impurities and interferences.
The sample matrix may also have an impact on the analysis. To eliminate matrix effects, techniques such as headspace injection and internal standard method can be used to correct and eliminate the influence of matrix on the analysis results.
Supercritical Fluid Extraction
1. Basic principles
The principle of supercritical fluid extraction technology is to utilize the relationship between the solubility of supercritical fluid and its density, by adjusting the pressure and temperature to change the density of supercritical fluid, thereby adjusting its solubility. In a supercritical state, the supercritical fluid is brought into contact with the substance to be separated, selectively extracting components with different polarities, boiling points, and relative molecular weights in sequence.
2. Supercritical fluid
Supercritical fluid refers to a fluid that is above the critical temperature (Tc) and critical pressure (Pc), where the fluid has both gas diffusion and liquid solubility. Commonly used supercritical fluids include carbon dioxide, nitrous oxide, sulfur hexafluoride, ethane, heptane, ammonia, etc. Among them, carbon dioxide is widely used due to its critical temperature close to room temperature, colorless, non-toxic, odorless, non flammable, chemically inert, inexpensive, and easy to produce high-purity gas.
3. Main advantages
High extraction efficiency: Supercritical fluids have lower viscosity and higher diffusion coefficient, making them easier to pass through porous matrices than liquid solvents, thereby increasing the extraction rate.
High selectivity: By adjusting temperature and pressure, effective ingredients can be selectively extracted or harmful substances can be removed.
Environmentally friendly and pollution-free: Carbon dioxide is commonly used as an extractant, reducing pollution to the environment.
Mild operating conditions: Extraction can be carried out near room temperature and under the cover of carbon dioxide gas, effectively preventing the oxidation and escape of thermosensitive substances.
Extraction and separation combined: When carbon dioxide containing dissolved substances flows through the separator, the pressure drop causes the carbon dioxide and extract to quickly become two phases (gas-liquid separation) and immediately separate, resulting in high extraction efficiency and low energy consumption, saving costs.
4. The use of entrainers
For hydrophilic molecules with high polarity, metal ions, and substances with high relative molecular weight, the extraction effect using supercritical carbon dioxide alone may not be ideal. At this point, suitable entrainers (such as methanol, ethanol, acetone, etc.) can be added to improve and maintain extraction selectivity, and increase the solubility of non-volatile and polar solutes.
5. Process flow
Preparation stage: Pre treat the material to be extracted, such as drying, crushing, etc.
Extraction stage: Place the pre treated material in an extraction kettle and introduce supercritical fluid for extraction. By adjusting the pressure and temperature inside the extraction kettle, the solubility and selectivity of the supercritical fluid can be controlled.
Separation stage: After extraction is completed, supercritical fluid containing dissolved substances is introduced into the separator for separation. By reducing pressure or increasing temperature, supercritical fluid is transformed into ordinary gas, and the extracted substance is completely or almost precipitated.
Collection stage: Collect and process the separated extracts to obtain the final product.
6. Application Fields
Supercritical fluid extraction technology has a wide range of applications in multiple fields, including:
Food industry: used to extract edible oil, natural pigment, essence, spice, etc.
Pharmaceutical industry: used for extracting effective ingredients from traditional Chinese medicine, preparing drug particles, etc.
Chemical industry: used for separating and purifying chemicals, preparing catalysts, etc.
Environmental protection: used for treating harmful substances in wastewater, exhaust gas, etc.
In summary, supercritical fluid extraction technology has shown broad application prospects in multiple fields due to its high efficiency, environmental friendliness, and mild operating conditions.
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