“Education costs money, but then so does ignorance” — Claus Moser
Your understanding of a topic and technical skills are gauged in interviews. We now provide you an opportunity to familiarize yourself with some typical questions that you may face in interviews when seeking career opportunities in analytical laboratories involving extensive use of gas chromatography technique.
Q1. What are the main differences between High Performance Liquid Chromatography and Gas Chromatography?
- In HPLC the mobile phase is a liquid whereas in Gas Chromatography the mobile phase or carrier is a gas.
- HPLC is useful for analysis of samples which are liable to decompose at higher temperatures. GC involves high temperatures so compounds are stable at such temperatures.
- Gas Chromatography is applied for analysis of volatile compounds whereas non volatile compounds can be easily analyzed on HPLC
- Gas Chromatography cannot be used for analysis of high molecular weight molecules whereas HPLC has applications for separation and identification of very high molecular weight compounds
- HPLC requires higher operating pressures than GC because liquids require higher pressures than gases for transport through the system
- HPLC columns are short and wide in comparison to GC columns
Q2. Which type of GC detector is most commonly used? Explain its working principle and what are its limitations?
Ans. The most commonly used detector is the flame ionize detector. The sample is combusted with the help of fuel gas and oxidant in the detector body. Combustible sample components burn and produce ions and electrons which can conduct electricity through the flame. A large potential difference is applied at the burner tip and the collector electrode located above the flame and the current between the electrodes is measured. The detector is mass sensitive and response is not affected by carrier gas flow rate changes. However, the detector is not responsive to inorganic gases such as CO, O2, NH3, N2, CS2, CO2, etc.
Q3. What are the commonly used carrier gases in GC analysis when using FID detector?
Ans. Inert gases commonly used in analysis when using FID detector are Nitrogen and Helium. Nitrogen is more commonly used as it is less expensive than Helium. Purity of carrier gas should be more than 99.995% and on-line traps should be used to prevent residual moisture or other impurities from entering the system.
Q4. What are the desirable characteristics of a GC detector ?
Ans. The detector chosen for particular analysis should :
- Give reproducible response to changes in concentration of eluting compounds in the carrier gas stream.
- Should provide a large linear dynamic range
- Should have high sensitivity
- Should have small internal volume to give narrow peaks and also facilitate flushing of previous sample traces
- Should preferably be non-destructive
Q5. What do you understand by specificity of a detector?
Ans. Detectors falls into three categories depending upon response to the eluting compounds:
- Non-selective – Respond to all component in the gas stream except for the carrier gas
- Selective – Respond to a particular class of compounds with common physical or chemical properties
- Specific – Respond to a single specific compound only in the carrier gas stream
Q6. What are the commonly used types of capillary columns?
Ans. Capillary columns are generally 10 – 100m long tubes having an internal diameter ranging from 0.1 – 0.5mm made of flexible material such as fused silica. Common types of capillary columns are
- Wall coated open tubular (WCOT) – Internal wall is coated with a very fine film of adsorbing liquid
- Surface coated open tubular (SCOT) – Inner wall is lined with a layer of solid support on to which the liquid phase is absorbed.
The columns are flexible and wound into several turn coils supported on a SS cage inside the column oven
Q7. What do you understand by column efficiency and how it is expressed?
Ans. On continuous use a column gradually loses its original resolution power. Column efficiency is expressed on the basis of plate theory concept. Each component under separation spends a finite time in each theoretical plate. Smaller the plate height the larger the number of plates (N) and better is the column efficiency.
where N = Number of theoretical plate
tr = Retention time of a peak
w = Peak width at base
N can also be expressed as
where w1/2 is peak width at half peak height
Column efficiency is expressed in terms of Height Equivalent to a Theoretical Plate (HETP)
HETP = L/N
where, L = Length of column in terms of mm
Small value of HETP signifies greater column efficiency
Q8. What do you understand by temperature programming in GC analysis?
Ans. Temperature programming means change of temperature of the column at a rate predetermined rate during the analytical run. This has the same influence on elution time of separated components as gradient programming in HPLC analysis. Temperature programming helps reduce analysis time by permitting early elution of less volatile components.
Q9. When is isothermal operation useful?
Ans. Isothermal operation is useful when high resolution is required for separating compounds having narrow boiling range. Temperature is set to around mid range of boiling points of constituents. This results in good resolution of low boiling components but band broadening of higher boiling components can result due to their longer retention in the column.
Q10. What measures you would adopt to extend useful life of a column?
- Condition a column before first use or after long time storage
- Take care not to exceed upper temperature limit specified by the manufacturer
- Avoid injection of solutions which are strongly acidic or basic in nature
- Rinse columns by injection with blank solvents such as methanol, methylene chloride or hexane to remove contamination of column after excessive usage
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