Overview
This lecture explains the basics of gas chromatography, its components, operation, and interpretation of results for the separation and identification of volatile compounds.
Gas Chromatography Basics
- Gas chromatography separates volatile compounds that easily vaporize at room temperature.
- It uses a long coiled column to achieve separation.
Columns in Gas Chromatography
- Two types of columns: packed columns (glass/stainless steel, 1–3 m length, 2–4 mm diameter) and capillary columns (fused quartz, 10–100 m length, 0.1–1 mm diameter).
- The column is placed in a temperature-controlled chamber.
Phases in Gas Chromatography
- The stationary phase (silicon grease or wax) is coated on the inner wall of the column.
- The mobile phase is an inert or unreactive gas like helium or nitrogen.
- A molecular sieve removes unwanted impurities (hydrocarbons, oxygen, water) from the mobile phase.
Sample Injection and Separation
- Samples are mixed with a volatile solvent and injected through a septum.
- Injection zone is maintained 20–50°C higher than column to ensure quick vaporization.
- Column temperature is kept between 150–300°C for separation.
- Separation is based on interaction with stationary (slower, less volatile) or mobile phase (faster, more volatile).
Detection and Data Interpretation
- The detector (commonly Flame Ionization Detector, FID) sits at the end of the column.
- FID uses hydrogen and oxygen to create a flame that ionizes sample molecules.
- Electrons released are measured as current, generating a peak on the computer.
- Peak retention time identifies compounds; peak area indicates concentration.
Identification Using Standards
- Known standards (e.g., methanol, acetone) are run to determine retention times.
- An unknown sample's peaks are compared to standards for identification.
Key Terms & Definitions
- Gas Chromatography — technique for separating volatile compounds.
- Volatile Compound — substance that vaporizes easily at room temperature.
- Packed Column — glass or steel tube filled with stationary phase.
- Capillary Column — thin, long tube of fused quartz.
- Stationary Phase — non-moving substance inside the column aiding separation.
- Mobile Phase — carrier gas moving analyte through the column.
- Flame Ionization Detector (FID) — device detecting compounds by ionization in a flame.
- Retention Time — time a compound takes to travel through the column.
Action Items / Next Steps
- Review the concepts of stationary and mobile phases.
- Practice interpreting chromatograms using examples with known standards.