Top 10 Interview questions about GCMS apparatus.
1. What is GC-MS and how does it work?
Answer: GC-MS combines gas chromatography (GC) for separation of compounds and mass spectrometry (MS) for identification and quantification. The sample is vaporized, separated by GC based on volatility, then ionized in the MS and detected based on mass-to-charge ratio (m/z).
2. What are the main components of a GC-MS system?
- GC Components: Injector, column, oven, and carrier gas.
- MS Components: Ion source, mass analyzer (e.g., quadrupole), and detector.
These work together to separate and identify complex mixtures.
3. What type of samples can be analyzed using GC-MS?
Volatile and semi-volatile organic compounds are suitable, including environmental pollutants, drugs, flavors/fragrances, and biological samples after derivatization if needed.
4. What is the role of the carrier gas in GC?
The carrier gas (e.g., helium, hydrogen) transports the vaporized sample through the column. It should be inert and have consistent flow to ensure proper separation.
5. How does the ionization process occur in the mass spectrometer?
Electron ionization (EI) is common, where high-energy electrons collide with sample molecules to form ions. These ions are then analyzed based on their m/z ratios.
6. What are retention time and mass spectra, and how are they used for identification?
- Retention Time: The time a compound takes to travel through the GC column.
- Mass Spectrum: A plot of ion abundance vs. m/z.
Together, they provide a “fingerprint” for compound identification.
7. What is a quadrupole mass analyzer and how does it function?
It uses oscillating electric fields to filter ions by m/z. Only ions with a specific m/z reach the detector at a given time, allowing selective analysis.
8. How do you ensure accurate quantification in GC-MS?
By using internal standards, calibration curves, and consistent sample preparation. Quantification depends on comparing peak areas to known standards.
9. What are common maintenance steps for a GC-MS system?
Regular tasks include:
- Replacing septa and liners,
- Trimming/replacing the column,
- Cleaning the ion source,
- Checking for leaks and ensuring proper vacuum levels.
10. What are some common troubleshooting issues in GC-MS and how do you address them?
Issues include:
Poor peak resolution: Check column condition, temperature program.
Signal drop: Clean ion source or replace filament.
Baseline noise: Check carrier gas purity or leaks.
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