Gas Chromatography

What is Gas Chromatography?

 

Gas chromatography (GC) is a highly significant physical-chemical separation and analytical technique primarily used to analyze volatile compounds that are thermally stable and capable of vaporization. Its core principle relies on the differences in partition coefficients between various substances in the mobile phase (carrier gas, an inert gas) and the stationary phase (the coating or packing material within the chromatographic column) to sequentially separate the components of a mixture for qualitative and quantitative analysis.

1. Basic Procedure

1. Carrier Gas (Mobile Phase)

High-purity nitrogen, helium, or hydrogen are commonly used as the carrier gas to propel the sample forward.

2. Injection

Liquid or gaseous samples are rapidly vaporized and enter the chromatographic column along with the carrier gas.

3. Chromatographic Column (Separation Core)

The column is coated with a stationary liquid or packed with a solid adsorbent (stationary phase).

4. Separation Process

Different components interact with the stationary phase with varying strengths:

Weak interaction with the stationary phase → Moves faster, peaks earlier

Strong interaction with the stationary phase → Moves slower, peaks later

Ultimately, the mixture is separated sequentially based on differences in migration rates.

5. Detection and Recording

The separated components enter the detector in sequence; the signals are recorded as chromatographic peaks. Peak positions are used for qualitative analysis, while peak area and peak height are used for quantitative analysis.

Principle of Core Separation:

The chemical properties of different substances (such as polarity, boiling point, and molecular weight) determine their degree of “preference” between the two phases, i.e., the partition coefficient (K):

K = concentration of the component in the stationary phase / concentration of the component in the mobile phase

• Substances with a low distribution coefficient: tend to remain in the mobile phase (gas), move rapidly with the carrier gas, and elute from the chromatographic column first.
• Substances with a high distribution coefficient: tend to be adsorbed or dissolved in the stationary phase, move slowly, and elute from the chromatographic column last.
• This difference in migration rates allows different substances, which were originally mixed together, to be separated one by one over time.

Stage 1: Preparations Before Power-Up

Before turning on the instrument, complete the following checks:

Gas Line Inspection:

Confirm that the main valves of the carrier gas (typically helium or nitrogen), hydrogen, and air (if an FID detector is present) cylinders are open, and that the output pressure of the pressure regulators is within the instrument’s specified range (typically 0.5–0.7 MPa for carrier gas).

Confirm that the gas purifiers (dewatering and deoxygenation) are operational.

Environmental and Consumables Check:

Check the injection pad (seal) for leaks or wear; replace if necessary.

Check that the liner is clean and that the quartz wool is properly positioned.

Confirm that the chromatographic column is installed correctly and that the nuts at both ends of the column are tight.

Stage 2: Instrument Startup and Condition Setup

The core of this stage is to establish a stable analytical environment, typically performed using the accompanying chromatography workstation (software).

Turn on the instrument power: Turn on the carrier gas (start gas flow first), the GC main unit, the computer, and the workstation software in sequence.

Edit the method (set parameters):

Injection port parameters: Set the temperature (to instantly vaporize the sample, typically 20–50°C higher than the sample’s boiling point), split/splitless mode, and purge flow rate.

Column oven parameters: Set the initial temperature, hold time, temperature ramp rate, and final temperature. Programmed temperature ramping is key to separating complex mixtures.

Detector parameters: Set the detector temperature (typically at least 20°C higher than the maximum column oven temperature to prevent condensation), gas flow rates (e.g., hydrogen and air flow rates for FID), and ignition.

Flow rate settings: Set the carrier gas linear velocity (typically in constant flow mode).

Wait for Equilibrium: Proceed to the next step only after the injection port, column oven, and detector have reached their set temperatures, and the detector baseline (signal value) has stabilized and become flat. This process typically takes anywhere from 30 minutes to several hours.

Stage 3: Sample Analysis

Once the instrument is ready, proceed to the analysis stage. Currently, most laboratories use autosamplers, though manual injection is still common.

Create Sequence: Create an analysis sequence in the workstation, entering the sample vial position, sample name, analysis method used, data save path, etc. Typically, the sequence begins with one blank injection (solvent) and one standard injection to verify system cleanliness and calibrate retention times.

Run Samples: Start the sequence.

Auto sampling: The instrument automatically draws the sample, injects it into the injection port, and simultaneously triggers data acquisition.

Manual Injection: Using a micropipette (e.g., 10 μL), draw up the sample and expel any air bubbles. At the injection port, quickly and vertically push the needle all the way in and pull it out rapidly, while simultaneously pressing the “Start” button (or foot switch) to initiate data acquisition.

Chromatogram Acquisition: Based on the preset analysis time, each component will elute from the column in sequence and be recorded by the detector. During this process, observe whether the chromatographic peak shapes are normal (check for overloading, tailing, splitting, etc.).

Stage 4: Data Processing

After analysis is complete, perform qualitative and quantitative analysis on the obtained chromatograms.

Integration Parameter Settings: In the workstation, set the minimum peak area (to filter out noise) and slope (to identify the start and end points of the peak).

Qualitative Analysis: Determine the position of the target peak by comparing it to the retention times of standard samples. If necessary, confirmation can be performed via spiking or gas chromatography-mass spectrometry (GC-MS).

Quantitative Analysis:

External Standard Method: Establish a standard curve and calculate the concentration based on the peak area (or peak height) of the target peak in the sample. This is the most commonly used method.

Internal Standard Method: A known quantity of an internal standard is added, and the concentration is calculated based on the response ratio between the target compound and the internal standard. This method is suitable when the injection volume is difficult to control precisely.

Area Normalization Method: Calculate the percentage of each peak area relative to the total area. This is primarily used for a rough assessment of purity or component ratios.

Stage 5: Shutdown and Maintenance

To ensure instrument longevity and column safety, there are strict requirements for the shutdown sequence:

Turn off the detector flame (for combustion-type detectors such as FID): First close the hydrogen and air valves, or use the software to “extinguish the flame.”

Cooling:

Set the temperatures of the injection port, column oven, and detector to lower values (typically, the column oven below 50°C, and the injection port and detector below 100°C).

Wait for the temperatures to drop (approximately 20–30 minutes). If the carrier gas is shut off while temperatures are still high, the influx of air can damage the stationary phase of the column.

Shut off the carrier gas (final step): Once the temperatures of all heated zones have dropped to a safe range, close the workstation software, turn off the power to the GC main unit, and finally close the main valve on the gas cylinder (or the main gas line shut-off valve).

I. The Power Industry

This is the primary application scenario for gas chromatographs specifically designed for the power industry, primarily used for condition monitoring and fault diagnosis of electrical equipment:

• Dissolved Gas Analysis in Insulating Oil: Detects components such as H₂, CO, CO₂, CH₄, C₂H₄, C₂H₆, and C₂H₂ in the insulating oil of equipment such as transformers, reactors, and current/voltage transformers to determine whether the equipment is experiencing faults such as overheating or electrical discharge. This is a core method for preventive testing in the power industry.
• SF₆ Gas Analysis: Detects the purity, humidity, and decomposition products (such as SO₂, H₂S, and CF₄) of SF₆ gas in equipment such as GIS and SF₆ circuit breakers to identify potential hazards such as gas leaks, electrical discharge, and overheating.

Power Industry Material Testing: Analyzes volatile components in cables and insulating materials to evaluate material insulation performance and the degree of aging.

1. Food Safety and Agriculture

This is one of the most widely applied fields for gas chromatography, primarily used to detect trace amounts of harmful substances and nutritional components.

• Pesticide Residue Analysis: Detects pesticide residues such as organochlorines, organophosphates, and pyrethroids in vegetables, fruits, and tea. Typically, an Electron Capture Detector (ECD, sensitive to halogen-containing pesticides) or a Flame Photometric Detector (FPD, sensitive to phosphorus- and sulfur-containing pesticides) is used.
• Food Additives and Packaging Materials: Analysis of antioxidants and preservatives in food (such as benzoic acid and sorbic acid, which require derivatization), as well as residues of printing inks, plasticizers, and adhesives (such as phthalates) that may migrate from packaging materials.
• Fatty Acids and Nutritional Analysis: Through derivatization, the fatty acid methyl ester composition in edible oils and dairy products is determined to assess trans-fatty acid content or detect adulteration of vegetable oils.
• Flavor Compounds and Aromatic Components: Combining techniques such as solid-phase microextraction (SPME), volatile aromatic components in alcoholic beverages, fruit juices, and flavorings are analyzed for product flavor control.

2. Environmental Monitoring

Gas chromatography is the standard method for detecting organic pollutants in the environmental field.

• Air Pollutants: Monitoring volatile organic compounds (VOCs) in ambient air or industrial exhaust gases, such as benzene compounds (benzene, toluene, xylene), halogenated hydrocarbons, aldehydes, and ketones. Portable gas chromatographs are commonly used on-site for rapid emergency monitoring.
• Water Quality Analysis: Detection of organochlorine pesticides, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and volatile halogenated hydrocarbons (such as chloroform and carbon tetrachloride) in surface water, drinking water, and wastewater.
• Soil and Solid Waste: Analysis of semi-volatile organic compounds in soil, including total petroleum hydrocarbons (TPH), pesticide residues, and polychlorinated biphenyls (PCBs), commonly used for contaminated site assessments and remediation effectiveness verification.

3. Petrochemicals and Energy

Analysis of complex hydrocarbon mixtures in this field relies heavily on gas chromatography.

• Refinery Gas and Natural Gas Analysis: Qualitative and quantitative analysis of permanent gases (such as hydrogen, oxygen, and nitrogen) and C1–C6 hydrocarbon composition, used for calculating calorific value and quality control.
• Simulated Distillation: Utilizing high-temperature gas chromatography to simulate traditional distillation processes, rapidly determining the boiling range distribution of petroleum products (such as gasoline, diesel, and lubricating oil).
• Purity Analysis of Solvents and Chemical Products: Detection of impurity levels in industrial solvents (e.g., ethanol, acetone, ethyl acetate) and trace impurities in polymer monomers (e.g., ethylene, propylene).
• Biodiesel and Renewable Fuels: Analysis of methyl ester purity, glycerol content, and free glycerol residues in biodiesel.

4. Healthcare and Clinical Applications

In the pharmaceutical field, gas chromatography is primarily used for drug quality control, in vivo drug analysis, and toxicological research.

• Drug Solvent Residues: Detection of Class I, II, and III organic solvent residues (e.g., methanol, dichloromethane, benzene) in active pharmaceutical ingredients (APIs) and finished dosage forms, in accordance with the limits specified by the Chinese Pharmacopoeia or the ICH (International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use).
• Clinical Diagnostics: Detection of ethanol in blood (for DUI testing), ethylene glycol (for antifreeze poisoning), and volatile anesthetic gases (for intraoperative monitoring).
• Biomarker Analysis: Analysis of organic acids and fatty acids in urine or blood using gas chromatography-mass spectrometry (GC-MS) to aid in the diagnosis of inherited metabolic disorders (such as phenylketonuria).
• Forensic Toxicology: Detection of sedatives, stimulants, illicit drugs (such as marijuana and cocaine), and their metabolites in biological samples.

5. Forensic Examination and Criminal Investigation

Gas chromatography (particularly GC-MS) provides legally admissible evidence in forensic science.

• Arson Analysis: Residuals collected from fire scenes are analyzed using gas chromatography to identify characteristic hydrocarbon fingerprints of accelerants (such as gasoline, kerosene, and diesel).
• Explosive Residues: Detection of explosive components such as nitroaromatic compounds and nitrates.
• Ink and Ink Analysis: Analysis of ink components on documents to authenticate documents or determine the time of writing.

6. Life Sciences and Metabolomics

With the advancement of gas chromatography-mass spectrometry (GC-MS) technology, gas chromatography is now used in the study of complex biological systems.

• Metabolomics: High-throughput detection of small-molecule metabolites (such as amino acids, sugars, and fatty acids) in biological samples (plasma, urine, tissue) to identify disease-related biomarkers.
• Microbial Identification: Enables rapid taxonomic identification of bacteria and fungi by analyzing the specific fatty acid composition (fatty acid fingerprint) of microbial cultures.

7. Quality Control and General Industry

• Fragrances and Flavors: Monitors the consistency of fragrance ingredient ratios in consumer chemicals (e.g., perfumes, detergents).
• Tobacco Industry: Analyzing alkaloids and humectants in tobacco leaves, as well as tar and nicotine content in tobacco smoke.
• Electronics and Electrical Appliances: Testing electronic products for restricted substances (such as polybrominated biphenyls and polybrominated diphenyl ethers) to ensure compliance with the RoHS (Restriction of Hazardous Substances) Directive.

Domestic:

Wuhan Goldhome Hipot Electrical Co., Ltd.

• Flagship Model: HM9890 Fully Automatic Oil Chromatograph (Gas Chromatograph for Power Applications)
• Technical Highlights: Triple-detector system (TCD + dual FID); completes a full analysis of 7 dissolved gases (H₂, CH₄, C₂H₂, etc.) in insulating oil with a single injection; complies with GB/T 17623, GB/T 7252 power industry standards; industrial-grade chips + Ethernet remote monitoring, high temperature control accuracy and strong stability; suitable for fault diagnosis of oil-filled equipment such as transformers and current transformers.
• Applications: Online/offline oil sample testing for power grid companies, power plants, and transformer manufacturers; analysis of cooling media for hydrogen-cooled generators.
• Services: Rapid local response, customizable solutions for the power industry, and O&M training.

Fuli Instruments

• Core Models: GC9790Plus, FL9720; contract win volume on par with international brands.
• Advantages: Strong versatility; FID detection limit as low as 1.5×10⁻¹² g/s; supports multiple detector combinations; high cost-effectiveness; suitable for general research and industrial analysis.
• Applications: Chemical, environmental protection, food, and third-party testing laboratories.

Panuo Instruments

• Core Models: A91Pro Series; frequently outperforms international brands in Q4 bid wins.
• Advantages: Precise electronic flow control (EPC), fast programmed temperature ramp rates, suitable for high-throughput analysis of complex samples; user-friendly software with remote diagnostics support.
• Applications: Petrochemicals, environmental monitoring, and pharmaceuticals.

Overseas:

Agilent (Global Leader)

• Core Models: 7890B/7820A; Long-term leader in contract awards.
• Advantages: Most comprehensive range of detectors (FID, TCD, ECD, NPD, FPD, etc.); detection limits down to the pg level; long instrument lifespan; comprehensive global service network; suitable for ultra-trace analysis and international standard comparisons.
• Applications: High-end research, pharmaceuticals, customs, and international testing laboratories.

Shimadzu (Japanese )

• Core Models: GC-2030, GC-2014; High market share in the Asia-Pacific region.
• Advantages: High stability, user-friendly operation, and low maintenance costs; suitable for routine industrial testing and quality control.
• Applications: Food, environment, chemicals, electronics.

Thermo Fisher

• Core Models: Trace 1600/1300; mature coupling technologies (GC-MS, GC-IR).
• Advantages: Excellent compatibility with mass spectrometry and other coupling methods; suitable for qualitative and quantitative analysis of complex unknowns; powerful software and efficient data processing.
• Applications: Forensics, environmental protection, biopharmaceuticals, metabolomics.

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