Gas chromatography, a powerful analytical technique, employs gas as a mobile phase within a chromatographic column. This core component of gas chromatography significantly influences the overall performance of the technique. Chromatographic columns come in two main types: packed columns and capillary columns.
Packed columns are filled with solid adsorbents or carriers containing fixed liquids. They are versatile, capable of separating compounds with high boiling points like nitrogen, oxygen, hydrogen, and argon. On the other hand, capillary columns offer high separation efficiency by coating the inner wall with a stationary phase liquid. This phase can be solid or liquid, each with distinct adsorption capacities for various components. Examples of weak adsorbents include sucrose, starch, inulin, talc, and potassium carbonate.
Gas chromatography boasts several outstanding advantages:
High Separation Efficiency: It can effectively separate complex mixtures of compounds with closely matched physical and chemical properties. This enables both qualitative and quantitative analyses, sometimes resolving tens or even hundreds of components simultaneously in a single analysis.
High Sensitivity: Gas chromatography can detect impurities at the ppm or even ppb level. Only minute gas or liquid samples, often less than 1 mL, are required for analysis.
Rapid Analysis: With the aid of computers, precise results can be obtained in seconds, making it a swift analytical method.
Wide Range of Applications: Gas chromatography is versatile, capable of analyzing gaseous, volatile liquid, and solid samples. It finds extensive use in organic analysis, covering approximately 20% of organic compounds, and can even analyze some inorganic substances following appropriate transformations.
The General Procedure for Gas Chromatography:
The sequence of peak times serves as the basis for qualitative analysis, while peak area or peak height is used for quantitative analysis.
Three Components of Industrial Gas Chromatography:
Industrial gas chromatography, compared to its laboratory counterpart, incorporates a sampling system, utilizes column-cutting technology, and employs complete automation for program control and information processing.
1. Sampling System:
The sampling system handles sample collection and pretreatment, serving as the interface between the production process and industrial gas chromatography. Initial sample pretreatment steps, such as decompression, water removal, and dust removal, occur external to the chromatography system.
Internally, the gas chromatography system features functionalities such as pressure regulation, flow path switching, flow monitoring, atmospheric balance, and calibration with standard gas or liquid. Design considerations encompass corrosion resistance of system components, leak prevention, explosion protection, reduction of transfer lag time, and control of contamination from evacuation.
2. Carrier Gas Flow System:
The carrier gas flow system comprises a source of carrier gas, purifiers, pressure and flow stabilization, and adjustment devices. Typically, high-pressure gas stored in cylinders serves as the carrier gas source, with a pressure-reducing valve reducing it to 0.1-0.5 MPa. High purity and stability are prerequisites for the carrier gas, necessitating the use of materials like silica gel, molecular sieve, and activated carbon to adsorb moisture and hydrocarbon compounds.
Carrier gases such as hydrogen, nitrogen, and argon may be employed. Maintaining constant gas flow with minimal variation (<1%) is crucial, necessitating the inclusion of flow meters, regulating valves, and pressure-stabilizing valves in the gas path.
3. Sampling Device:
Sampling involves the quantitative introduction of a gas, liquid, or transformed solid sample into the chromatographic column for separation. Consistency in injection quantity, injection duration, and sample vaporization rate significantly impacts the repeatability and accuracy of quantitative results.
(1). Gasification Chamber:
The gasification chamber features a metal block exterior with a controllable operating temperature ranging from 50-500°C. Temperatures exceeding 250°C utilize inner glass tube structures to prevent unwanted catalytic effects. The gasification chamber's primary function is to ensure instantaneous vaporization of liquid samples. It requires preheating of the carrier gas before entry, while the silicone rubber pad should remain cool to prevent unintended chemical reactions.
(2). Injection Valve:
Sampling valves must exhibit excellent airtightness, minimal dead volume, reliability, durability, and rapid switching times. In specific applications, resistance to corrosion and operation within defined temperature ranges may also be necessary.
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