Gas Chromatography (GC) PDF

Gas Chromatography  Gas Chromatography Gas chromatography is a type of column chromatographic technique that can be used to separate volatile organic compounds. Mobile phase: Inert gas: nitrogen, helium, hydrogen Stationary phase: liquid/solid Key Components: Carri...

Gas Chromatography  Gas Chromatography Gas chromatography is a type of column chromatographic technique that can be used to separate volatile organic compounds. Mobile phase: Inert gas: nitrogen, helium, hydrogen Stationary phase: liquid/solid Key Components: Carrier Gas: Moves the sample through the column. Stationary Phase: Coated on the column's inner surface. Column: The site where separation of compounds occurs. Detector: Identifies and quantifies separated compounds. Factors Influencing Separation: Boiling Point: More volatile compounds elute faster. Polarity: Non-polar compounds typically elute before polar ones.  Principle of Gas Chromatography Separation Mechanism: Adsorption: Compounds interact with the solid stationary phase by sticking to its surface. Occurs when column consists of particles of adsorbent only Partitioning: When particles of adsorbent are coated with a liquid (Stationary phase). More volatile compounds spend more time in the gas phase and move faster through the column. Two major types: Gas-solid chromatography: Here, the mobile phase is a gas while the stationary phase is a solid. Used for separation of low molecular gases, e.g., air components, H₂S, CS₂, CO₂, CO, and oxides of nitrogen. Gas-liquid chromatography: The mobile phase is a gas while the stationary phase is a liquid retained on the surface as an inert solid by adsorption or chemical bonding. Properties of Samples that Can Be Analyzed by GC Volatility: Samples must be vaporized without decomposition. Thermal Stability: Compounds must withstand high temperatures (up to 400 °C). Low Molecular Weight: Ideal for analyzing small organic compounds, fatty acids, essential oils, and alcohols. Non-Polar/Moderately Polar: Samples with lower polarity perform well in GC.  Instrumentation - Carrier Gas The carrier gas transports the sample through the column. Types: Hydrogen (H₂), Helium (He), Nitrogen (N₂), Argon (Ar). Key Properties: Should be inert (non-reactive) with the sample. Must have good thermal conductivity for heat transfer.  Stationary Phase The stationary phase is where the separation occurs inside the column. Types: Uncoated solid material: Charcoal, silica gel. Inert solid with liquid coating: Diatomaceous earth coated with silicone. Direct coating of liquids: Silicone polymers bonded directly onto the column.  Flow control Regulates the carrier gas flow in GC Constant flow of carrier gas column efficiency & reproducible elution time. Magnitude of carrier gas flow rate depends on type of column Packed column — 10-60ml/min Capillary column — 1-25 ml/min  Sample Injector The sample injector introduces the sample into the column. Methods: Liquid samples: Injected using syringes. Gas samples: Injected via gas-tight syringes or gas- sampling valves. Key Considerations: The sample should not decompose during injection. The vaporization chamber is typically heated 50 °C above the lowest boiling point of the sample and subsequently mixed with the carrier gas to transport the sample into the column. Columns Packed Coulmn Capillary Column Packed column (i.d. 2-4 mm): stationary phase is a liquid film adsorbed or bonded on the surface of a finely divided, inert solid support tightly packed in coiled columns (length 2-50 m). The columns are made of glass, metal or Teflon. The most widely used solid support is diatomaceous earth – skeleton of single-celled plants, diatoms. The particle size needs to be small (dia. 0.15-0.26 mm) Wall Coated Open Tubular (WCOT) Columns Capillary column with stationary phase coated on the inner wall. Key Features: Thin layer on inner surface. Material: Fused silica/stainless steel. Diameter: 0.10-0.53 mm; Length: Up to 100 m. Advantages: High efficiency, low risk of overloading. Ideal for volatile/semi-volatile compounds.  Support Coated Open Tubular (SCOT) Columns Stationary phase supported by a thin layer of solid material. Key Features: Solid support (e.g., diatomaceous earth). Diameter: 0.25-0.53 mm. Advantages: Higher sample load than WCOT. Suitable for samples needing larger stationary phase amounts. Applications: Hydrocarbon compound analysis.  Porous Layer Open Tubular (PLOT) Columns Column with porous solid material as the stationary phase. Key Features: No liquid coating. Diameter: 0.32-0.53 mm; Length: 10-50 m. Advantages: High surface area for effective separation. Excellent for gas analysis. Applications: Natural gas, petrochemicals, permanent gases. Detectors in GC Different detectors based on concentration or mass flow. Common detectors: FID, TCD, ECD, NPD, and PID. Flame Ionization Detector (FID) FID detects organic compounds by ionizing them in a hydrogen flame. High sensitivity and linearity; temp limit: 400°C. Detection range: 10⁻¹¹g.  Thermal Conductivity Detector (TCD) TCD measures the change in thermal conductivity of carrier gas. Universal detector with excellent stability. Detection range: 10⁻⁶g, temp: 10-400°C. Electron Capture Detector (ECD) ECD detects halides, nitriles, peroxides, etc. Selective with a detection range of 10⁻¹²g. Excellent sensitivity for detecting organometallic compounds.  Nitrogen-Phosphorous Detector (NPD) Highly sensitive for nitrogen- and phosphorus-containing compounds. Uses hydrogen and air as mobile phases. Detection range: 10⁻¹³g for nitrogen. Working: Ionization in Hydrogen Flame: Compounds elute and are ionized in a hydrogen flame. Rubidium Bead: Enhances ionization of nitrogen and phosphorus atoms. Signal Generation: Ions are collected by electrodes, generating a proportional electrical signal. Flame Photometric Detector (FPD) Measures light emitted from burning sulfur or phosphorus. Detection range of 10⁻⁷g. Excellent specificity for sulfur compounds. Photoionization Detector (PID) Detects ethers, esters, and organosulfur compounds. Detection range of 20⁻⁹g. High linearity and stability. Temperature Programming in GC Elevated temperature improves separation efficiency and decreases retention times. Temperature programming optimizes analysis for complex samples. Derivatization in GC Converts non-volatile compounds (e.g., steroids, fatty acids) into volatile derivatives. Uses silylating agents like TMS to enable GC analysis. Derivitization i. The reagent should produce more than 95 % complete derivatives. ii. It should not cause any rearrangements or structural alterations of compounds during formation of the derivative. iii. It should not contribute to loss of the sample during the reaction. iv. It should produce a derivative that will not interact with the GC column. v. It should produce a derivative that is stable with respect to time. Silylating Agents in GC Silylating agents react with labile functional atoms present in functional groups like alcohols, amines, carboxylic acids, and esters. Common Silylating Agents: BSA: N,O-bis(trimethylsilyl)acetamide. BSTFA: N,O-bis(trimethylsilyl)trifluoroacetamide. TMSIM: N-trimethylsilylimidazole. Significance: Enhances Volatility: Silylation improves the volatility of non-volatile or semi-volatile compounds, making them suitable for GC analysis. Thermal Stability: Increases the thermal stability of sensitive compounds, allowing them to withstand the high temperatures used in GC. Reduces Polarity: Converts polar compounds into less polar derivatives, which improves peak shape and resolution in GC.  Analysis by Gas Chromatography: What Can Be Determined Compound Identification: Separates and identifies individual components in mixtures. Quantitative Analysis: Measures concentration of compounds. Purity: Assesses the purity of a substance by detecting impurities. Volatility: Determines how easily compounds vaporize. Molecular Weight: Analyzes low molecular weight compounds. Thermal Stability: Detects compounds that withstand high temperatures. Polarity: Differentiates between polar and non-polar compounds. Advantages of Gas Chromatography High Sensitivity: Detects trace amounts of compounds. Rapid Analysis: Quick separation with short run times. Minimal Sample Required: Works with very small sample sizes. Handles Complex Mixtures: Efficient for multi-component separations.  Limitations of Gas Chromatography (GC) Limited to Volatile Compounds: Only suitable for volatile or derivatized compounds. Requires Temperature Control: Precise temperature management needed. Not Suitable for High Molecular Weight Compounds: Larger molecules may require alternative techniques.  Applications of Gas Chromatography Purity Testing: Identifies impurities in drugs. Active Ingredient Quantification: Ensures correct dosage. Stability Testing: Detects degradation products. Residual Solvent Analysis: Measures trace solvents. Quality Control: Ensures consistent drug formulations. Bioanalysis: Monitors drug metabolism in biological samples.