Gas chromatograph

Date：2025/7/7 17:40:18

I. Core Principle

Separation is achieved by utilizing the difference in distribution coefficients of different substances between the stationary phase and the mobile phase (carrier gas), combined with quantitative analysis by a detector. After vaporization, the sample passes through the chromatographic column with the carrier gas. Each component flows out successively due to the difference in the strength of interaction with the stationary phase. The detector converts the concentration signal into an electrical signal to form a chromatogram. Qualitative analysis is based on retention time, and quantitative analysis is based on peak area/peak height.

II. Key Structures and Functions

Components	Functions and Characteristics
Injection System	- Vaporization chamber: Instantly heats and vaporizes liquid samples (temperature is usually 20-50℃ higher than the sample boiling point).- Injection method: Manual injection (micro syringe) or automatic sampler (precisely controls injection volume, such as 1μL).
Chromatographic Column	- Types: Packed column: Made of stainless steel/glass, with an inner diameter of 2-4mm, filled with solid adsorbents or coated with stationary liquid. Capillary column: Made of fused silica, with an inner diameter of 0.1-0.5mm, and the inner wall is coated with stationary liquid (such as polydimethylsiloxane), with higher separation efficiency.- Stationary phase selection: Based on sample polarity (such as non-polar OV-17 column for separating hydrocarbons, polar PEG-20M column for separating alcohols).
Carrier Gas System	- Carrier gas: High-purity nitrogen (N?), helium (He) or hydrogen (H?), which needs to be purified and dried.- Flow control: Precisely adjusted by pressure sensors or electronic flow controllers (EFC) (such as 1-50mL/min).
Detector	- Flame Ionization Detector (FID): Sensitive to carbon-containing organic compounds, with a detection limit as low as 10?12g/s, widely used in pesticide residue and VOCs analysis.- Electron Capture Detector (ECD): Sensitive to electronegative substances (such as halogen-containing and nitro compounds), used for halogenated hydrocarbon detection.- Thermal Conductivity Detector (TCD): A universal detector based on the difference in thermal conductivity between components and carrier gas, suitable for permanent gas analysis.
Data Processing System	Workstation software (such as Agilent ChemStation): Collects signals, draws chromatograms, automatically integrates and calculates content, and supports method development and result report generation.

III. Typical Analysis Process

Sample Pretreatment:

Liquid samples: Direct injection or dilution (such as alcohol detection in blood);

Solid/semisolid samples: Extraction (such as Soxhlet extraction), derivatization (such as fatty acid methylation) followed by filtration and injection.

Instrument Parameter Setting:

Column temperature: Programmed temperature rise (such as initial 50℃ for 2min, then rise to 250℃ at 10℃/min) to optimize separation efficiency;

Injection port temperature: Higher than the sample boiling point to avoid condensation (such as set to 280℃ when analyzing fatty acids).

Separation and Detection: The sample passes through the chromatographic column with the carrier gas, and the detector records the signal in real-time.

Result Analysis: Qualitative analysis based on the retention time of standard samples, and quantitative analysis by external standard method/internal standard method (such as preparing a series of concentrations of benzene standard solutions with n-hexane to establish a standard curve).

IV. Core Application Fields

Industry	Examples of Detection Objects
Environmental Monitoring	Volatile organic compounds (VOCs) in air, benzene series in water, organochlorine pesticides in soil (such as DDT, hexachlorocyclohexane).
Food and Pharmaceuticals	Pesticide residues in food (such as dichlorvos, chlorpyrifos), methanol content in alcohol, solvent residues in pharmaceuticals (such as dichloromethane).
Petrochemical Industry	Analysis of hydrocarbon composition in gasoline, detection of natural gas components (such as methane, ethane), monomer residues in polymers (such as ethylene).
Forensic Science and Security Inspection	Trace analysis of alcohol/drugs (such as morphine, cocaine) in blood, explosives (such as TNT).
Materials Science	Additives in polymer materials (such as phthalate plasticizers), volatile components in coatings (such as formaldehyde).

V. Comparison with High-Performance Liquid Chromatography (HPLC)

Comparison Item	Gas Chromatography (GC)	High-Performance Liquid Chromatography (HPLC)
Mobile Phase	Gas (carrier gas, not involved in separation)	Liquid (mobile phase, polarity can be adjusted to affect separation)
Sample Requirements	Volatile, thermally stable (boiling point < 300℃)	Soluble, no volatility required (suitable for macromolecules and thermally unstable substances)
Separation Mechanism	Mainly based on differences in distribution coefficients	Based on multiple mechanisms such as adsorption, distribution, ion exchange, etc.
Detectors	FID, ECD, TCD, etc.	Ultraviolet (UV), Fluorescence (FLD), Evaporative Light Scattering (ELSD), etc.
Typical Analysis Time	5-30 minutes (rapid separation)	10-60 minutes (longer separation time for complex samples)

Ion chromatograph

Needle Flame Testing Machine