Here’s a straightforward explanation of how Gas Chromatography–Mass Spectrometry (GC-MS) works:
1. Sample Introduction and Vaporization
- The sample must be volatile (able to vaporize).
- It is injected into the GC injector, where it is heated and turned into a vapor.
- A carrier gas (like helium or nitrogen) pushes the vaporized sample through the GC column.
2. Separation by Gas Chromatography (GC)
- The GC column is coated with a stationary phase.
- Different compounds interact differently with the stationary phase:
- Compounds that interact less move faster.
- Compounds that interact more move slower.
- This separates the compounds, producing retention times for each.
3. Ionization in the Mass Spectrometer (MS)
- After exiting the GC, compounds enter the mass spectrometer.
- They are ionized (turned into charged particles) for detection.
- The common method is Electron Ionization (EI):
- High-energy electrons knock electrons off molecules, forming positively charged ions.
- Molecules may also fragment, providing structural information.
4. Mass Analysis
- Ions are separated based on their mass-to-charge ratio (m/z).
- Types of mass analyzers include:
- Quadrupole: Filters ions by m/z.
- Time-of-Flight (TOF): Measures ion flight time.
- Ion Trap: Captures and analyzes ions sequentially.
5. Detection
- The detector measures the abundance of each ion.
- A mass spectrum is produced, showing:
- Molecular ion peaks (molecular weight).
- Fragment ions (helpful for identifying structure).
6. Data Interpretation
- Combining the GC retention time and MS spectrum allows you to:
- Identify unknown compounds.
- Quantify compounds.
- Analyze complex mixtures.
In short:
GC separates volatile compounds, MS identifies and measures them, and together GC-MS is a highly sensitive and precise method for analyzing complex samples like drugs, pollutants, or forensic evidence.