Gas Chromatography (GLC) Concepts

Gas Chromatography (GLC)

• Isocratic elution: A constant mobile phase composition used in gas chromatography.
• Volatile compounds: Substances readily transformed into a vapor at elevated temperatures.
• Chromatographic peaks: Represent different components in a sample.
• Peak broadening: Leads to decreased resolution and difficulty in identifying components
• Tailing peaks: An asymmetric peak shape in a chromatogram, indicates sample interactions with the stationary phase.

Section A.1 & A.2 (GLC):

• Two problems with chromatogram: Peak broadening and tailing peaks.
• Reducing temperature: Lower temperature can improve resolution and reduce peak broadening.
• Correcting problems: Using a more polar stationary phase or reducing the injection volume.
• Flame ionization detector (FID): It responds to organic compounds.
• FID Principle: Organic compounds are burned in a hydrogen flame, generating ions that produce an electrical current. The size of this current is proportional to the amount of organic compound present.

Section A.2 (van Deemter Equation):

• Van Deemter equation: Describes the relationship between plate height (H) and linear velocity (u) in a chromatography column.
• A: Eddy diffusion coefficient, relates to the non-ideal flow paths within the column.
• B: Longitudinal diffusion coefficient, relates to the diffusion of the analyte in the mobile phase.
• Cs: Stationary phase mass transfer coefficient, relates to the time it takes for an analyte to equilibrate between the mobile and stationary phases.
• CM: Mobile phase mass transfer coefficient, relates to the time it takes for an analyte to equilibrate between the mobile phase and the bulk of the stationary phase.
• Csu: The term Csu represents the contribution of mass transfer in the stationary phase to the overall band broadening. Higher values of Csu result in lower efficiency.

Gas Chromatography (GLC)

• Increasing Csu: The efficiency decreases. This is because it means that the analyte is spending more time transferring between the mobile and stationary phases, leading to broader peaks and less resolution.
• Decreasing Csu: The efficiency of the separation increases. This is because the analyte is spending less time transferring between the mobile and stationary phases, resulting in sharper peaks and better resolution.

UV-Vis Spectrometry & Beer-Lambert Law:

• Absorbance: The amount of light absorbed by the sample is related to the concentration of the substance and the path length of the light beam.
• UV-Vis spectrophotometer: Measures the absorbance of a solution at different wavelengths of ultraviolet and visible light.
• Transmittance: The fraction of light that passes through a sample.
• Beer-Lambert Law: Relates the absorbance of a solution to the concentration of the analyte, the path length of the light beam, and the molar absorptivity. (A = εbc)
• ε: Molar absorptivity, a constant that is specific to the substance and the wavelength of light used.
• b: Path length, the distance the light travels through the solution.
• c: Concentration, the amount of substance dissolved in a given volume.

HPLC (High Performance Liquid Chromatography):

• HPLC: A technique that separates components in a mixture based on their differences in polarity.
• Stationary Phase: HPLC column, composed of silica particles with different functionalities.
• Mobile Phase: Solvent pumped through the column, usually a mixture of water and an organic solvent.
• Reverse Phase: A type of HPLC where the mobile phase is more polar than the stationary phase.
• C-18: A type of stationary phase composed of silica particles with a hydrophobic C-18 group, commonly used in reversed-phase HPLC.
• UV-Vis Detector (VWD): Detects compounds by measuring the amount of UV-Vis light they absorb.

Section A.4 (HPLC):

• Type of column: Reverse-phase C-18 column.
• HPLC & TLC discrepancy: The two peaks in the HPLC chromatogram might be due to the presence of isomers, enantiomers, or other forms of the betamethasone valerate.
• TLC (Thin-Layer Chromatography): A technique that separates components in a mixture based on their differences in polarity.
• TLC: Less sensitive than HPLC.

Section A.4 (HPLC):

• Mobile Phase: A mixture of methanol:water (75:25) is commonly used to elute betamethasone valerate.
• Increasing the percentage of organic solvent (methanol): Decreases elution time by allowing the compound to elute faster.
• Increasing the percentage of water: Increases elution time by slowing down the elution of the compound.

Section B.5 (Elution Strength):

• Elution strength: A measure of how strongly a solvent can displace an analyte from the stationary phase.

Section C.1 (Counterfeit Drug):

• TLC for drug authentication: TLC can be used to identify counterfeit drugs.
• Comparing TLC profiles: The TLC profile of the suspect drug is compared with the profile of an authentic drug.
• TLC Procedure: The sample is spotted on a TLC plate. The plate is placed in a chamber containing a mobile phase. The mobile phase travels up the plate, separating the components of the sample.
• Visualizing TLC: The components are visualized using a UV lamp, iodine vapor, or a chemical stain.

Section C.2 (Aspirin Analysis):

• Calibration Curve: A plot of absorbance vs. concentration.
• Standard curve: Used to determine the concentration of an unknown sample by comparing its absorbance to the absorbance of known standards.
• Standard solution: Solution of known concentration, used to create a calibration curve.
• Aspirin analysis procedure: A sample of aspirin is dissolved in a solvent. The absorbance of the solution is measured using a UV-Vis spectrophotometer.
• Concentration determination: To obtain the concentration of the unknown sample, the absorbance is compared to the standard curve.
• % stated content: Calculated by dividing the calculated amount of aspirin in the sample by the amount of aspirin claimed on the label.

This quiz explores key concepts in Gas Chromatography (GLC), including isocratic elution, chromatographic peaks, and issues like peak broadening and tailing peaks. It also discusses methods to improve resolution and the function of flame ionization detectors (FID). Test your understanding of these important GLC principles.