Instrumental Analysis: Gas Chromatography

Gas Chromatography (GC) Overview

• Sample is injected and vaporized in a chromatographic column.
• Components are separated based on partition or adsorption between mobile (carrier gas) and stationary phases.
• Mobile phase is inert and does not interact with the sample.

Types of Gas Chromatography

• Gas-Solid Chromatography (GSC): Uses a solid stationary phase; separation occurs through adsorption.
• Gas-Liquid Chromatography (GLC): Involves a liquid stationary phase supported on solid; relies on solubility of components in the stationary phase.

Suitable Samples for GC

• Compounds must be volatile and thermally stable (e.g., hydrocarbons, aldehydes).
• Non-volatile samples can be converted to volatile derivatives (e.g., fatty acids to methyl esters).
• Suitable samples have molecular weights between 2 and 1000.

Advantages and Limitations of GC

• Advantages:
  • High sensitivity (detection limit: 10^-9 to 10^-12 g).
  • High accuracy and rapid analysis times.
• Limitations:
  • Samples must be volatile and thermally stable below 400°C.
  • Dirty samples require cleanup processes.
  • Identification requires coupling with techniques like mass spectrometry.

GC Instrumentation

• Components include:
  • Carrier gas system (with regulator, purifier, flow meter).
  • Sample injection system.
  • GC oven for temperature control.
  • GC column for separation.
  • GC detector for analysis.
  • Data integration/computer station.

Carrier Gas Requirements

• Must be pure (99.999% purity) and inert, to prevent interaction with samples.
• Common carrier gases: nitrogen, helium, hydrogen, argon.
• Gases must be dry to avoid hydrolysis and adjusted for pressure based on viscosity and column length.

Sample Injection System

• Introduces sample into GC column as liquid or gas.
• Modes include:
  • Split Injection: Diverts a portion of the sample for concentrated samples.
  • Splitless Injection: Introduces the entire sample into the column.

GC Oven Functions

• Houses the chromatographic column and heats samples for separation.
• Temperature determines the degree of separation and affects sample boiling points.

Types of GC Columns

• Packed Columns:

  • Made of glass, metal, or Teflon; lengths of about 2 meters.
  • Filled with solid inert support coated in liquid stationary phase.
  • Carrier gas flow rate: 30-50 ml/min.

• Capillary (Open Tubular) Columns:

  • Made from glass or fused silica; lengths between 15-100 meters.
  • Coated on the inner wall with liquid stationary phase.
  • Lower flow rates of about 1 ml/min due to minimal resistance.

Comparison of Packed vs. Capillary Columns

• Packed Columns: Larger diameter (2-5 mm), max temp 280°C, larger sample sizes.
• Capillary Columns: Smaller diameter (0.2-0.5 mm), higher max temp (370°C), suitable for smaller sample sizes (100 times smaller than packed).

Key Factors in GC Separation

• Retention in GC influenced by:
  • Solubility in stationary phase.
  • Boiling points of compounds.
  • GC oven temperature.

Resolution in GC

• Influencing parameters include:
  • Column temperature.
  • Length of the column (√L).
  • Film thickness and internal diameter of the column.

This quiz covers the basics of gas chromatography, including sample injection, vaporization, and fractionation. Learn about the partition and adsorption processes between the mobile and stationary phases.