Gas Chromatography (GC) in Qualitative and Quantitative Analysis

Qualitative Analysis

• Identification of Species

  • Retention times or volumes are used for identification.
  • Standard samples are used to identify compounds in unknown samples by matching retention times.
  • Injection technique consistency is crucial to avoid shifts in retention times.
  • Purity and Contaminants
    • Additional peaks in a supposed pure sample indicate contaminants.
    • Confirm presence or absence of species by adding authentic samples and observing peak changes.

• Selectivity Factor (α)

  • Calculated as Kb/Ka, where K is the distribution constant.
  • Formula: ( \frac{T_R'B - T_0}{T_R'A - T_0} )
  • Used for characterizing solutes relative to common standards.

Quantitative Analysis

• Measurement

  • Accuracy within 1% relative attainability.
  • Based on peak height or peak area.
  • Peak heights inversely related to peak width.
  • Variability controlled by column temperature, effluent flow rate, and sample injection rate.
  • Errors from sample injection rate can be 5-10%.
  • Avoid column overload by injecting small amounts.

• Peak Analysis

  • Peak Height
    • Maximum height and height at half peak width are measured.
    • Dependent on consistent column conditions.
  • Peak Area
    • Independent of peak broadening effects.
    • Considered a more satisfactory analytical variable than peak height.

Calibration and Standards

• Preparation

  • Series of standard solutions prepared for calibration.
  • Plot peak height or area as a function of concentration.
  • Frequent re-standardization is crucial.
  • Standards and unknowns must be run simultaneously.

• Sources of Error

  • Uncertainty in the volume of sample.
  • Rate of injection.