Gas Chromatography and Retention Index

Introduction

• Gas chromatography can be used for qualitative analysis to identify the species present in a sample.
• Retention index (I) is proposed as a parameter for identifying solutes.

Stationary Phases

Non-Polar Stationary Phase

• Non-polar compounds are retained longer than polar compounds (like dissolves like principle).
• Retention is determined by the volatility of solutes.
• Compounds elute in the order of their boiling points.

Polar Stationary Phase

• Polar compounds are retained longer than non-polar compounds.
• Retention is determined by the strength of intermolecular forces, such as hydrogen bonding.
• Alkanes (non-polar) elute first, then polar compounds based on intermolecular force strength.

Retention Index (I)

• The Kovats retention index is derived from a chromatogram with a solute and at least two normal alkanes.

• Formula:

  I = 100n + (N - n) * log(t'r unknown) - log(t'r small n) / log(t'r big N) - log(t'r small n)
  • n: Number of carbon atoms in the smaller alkane.
  • N: Number of carbon atoms in the larger alkane.
  • t'r: Adjusted retention time.

• The retention index for normal alkanes = number of carbons * 100.

• Independent of temperature and column packing.

• Retention index of other compounds can vary with column conditions.

Examples and Calculations

• Plot of logarithm of adjusted retention times versus number of carbons.
• Benzene falls between 6 and 7, dimethylbutane between 5 and 6.
• Example calculation:
  • Given: CH4 retention = 0.5 min, Octane = 14.3 min, Unknown = 15.7 min, Nonane = 18.5 min.
  • Adjusted retention times:
    • Unknown: 15.7 - 0.5 = 15.2 min
    • Octane: 14.3 - 0.5 = 13.8 min
    • Nonane: 18.5 - 0.5 = 18 min
  • Calculation of I for unknown:
    • I = 836. Falls between 800 (Octane) and 900 (Nonane).

Order of Elution

• Polar column (e.g., polyethylene glycol):
  • Non-polar compounds elute first in the order of boiling point.
  • Polar compounds elute based on intermolecular forces.
• Example with compounds:
  • Heptane first (boiling point 98°C), then Octane.
  • Ketone (dipole-dipole) before Alcohol (hydrogen bonding).

Conclusion

• Ensures identification of solutes through retention indices.
• Essential to understand column type and compound properties for accurate analysis.