Interview questions on Gas Chromatography
Interview questions on Gas Chromatography (GC)
Gas Chromatography (GC) is one of the widely used tools to control the quality, safety and efficacy of pharmaceuticals at various stages. This is the reason why most of the questions are asked in pharmaceutical analytical or QC interviews. In this post I will discuss all the questions asked during the interview and their answers. If you want to clear the pharma interview then this post is for you.
What is GC?
GC is a chromatographic system to separate organic volatile molecules. In GC mobile phase is gas and the stationary phase is immobilized liquid. Separation is achieved by the interaction of molecules between the stationary phase (immobilized liquid) and mobile phase (carrier gas) and it depends upon both polarity (of both stationary phase and molecule) and the vaporization point of the molecules. Inert gas like Helium or Nitrogen is used as carrier gas.
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What is the difference between GC and HPLC?
- The basic difference between GC and HPLC is that GC is used for testing volatile components whereas HPLC is used for testing non-volatile components. Some of the volatile components can be analysed in HPLC also.
- In GC capillary columns are used whereas in HPLC packed columns are used.
- In GC sharp peak is obtained compared to HPLC.
- In GC higher theoretical plate (up to 100000 or more than that) compared to HPLC
- In GC paek having less tailing factor compared to HPLC
What is the difference between GC and TLC?
- The basic difference between GC and TLC is that GC is used for testing volatile components whereas TLC is used for testing non-volatile components.
- GC comes under continuous chromatography whereas TLC comes under complete chromatography.
- GC is used for both qualitative and quantitative analysis of volatile components whereas TLC is used for mainly qualitative analysis of non-volatile components.
What are the different components of GC?
The different components of GC are:
- Carrier gas
- Flow control valve
- Sample injection port
- GC column
- Column oven
- Detector
- Outlet
- Computer controller
Why GC is required for analysis?
Most of the volatile components have no UV absorbance and hence they can not be analysed on HPLC using a UV detector. These can also not be analysed on other detectors due to volatility. However, these volatile components can easily be analysed on GC.
What is the role of carrier gas in GC?
Answer: Carrier gas carries the volatile components.
Which gases are used as a carrier gas and why?
Generally, Helium and Nitrogen are used as carrier gas. These gases are inert which is the reason to use as carrier gas.
What should be the purity of carrier gas?
Answer: More than 99.5%
What is the difference between Gas solid chromatography (GSC) and Gas-liquid chromatography?
In GSC (gas-solid chromatography) packed columns are used whereas in GLC (gas-liquid chromatography) capillary columns are used.
Why Gas solid chromatography (GSC) is rarely used in industries?
GSC is not used due to the following reasons:
- Broad peak
- Low column efficiency
- High tailing &
- Poor separation
Why GLC is mostly used in industries?
Due to fast analysis and high accuracy.
What is the difference between a packed column and a capillary column?
- In the packed column low theoretical plate (N) whereas in the capillary column high theoretical plate (N)
- In the packed column broad peak is obtained whereas in the capillary column-sharp peak is obtained.
- In the packed column high tailing factor (T) whereas in the capillary column less tailing factor (T) &
- In the packed column, poor resolution (R) is obtained between the peaks whereas in the capillary column higher resolution (R) is obtained between the peak
Why packed column is rarely used in industries?
In the packed column, the reproducible result is not obtained and that is the reason the same is rarely used in the industries
Why capillary columns are mostly used in industries?
Capillary columns are widely used in the industries compared to packed column due to the following reasons:
- In the packed column low theoretical plate (N) whereas in the capillary column high theoretical plate (N)
- In the packed column broad peak is obtained whereas in the capillary column-sharp peak is obtained.
- In the packed column high tailing factor (T) whereas in the capillary column less tailing factor (T) &
- In the packed column, poor resolution (R) is obtained between the peaks whereas in the capillary column higher resolution (R) is obtained between the peak
What is the role of the injection port in GC?
The sample is injected in the injection port. It is at a high temperature and it converts the sample into a vapour phase.
What is the role of the column oven in GC?
It helps in managing the column temperature as per method requirement.
What is the function of the column in GC?
Answer: Column is the heart of the GC system. In the column, different components are getting separated.
What is the role of the detector in GC?
Answer: The detector converts eluted components into a signal and then the signal is converted into a peak by the data processor.
What are the different types of GC detectors?
Generally following detectors are used in GC:
- Flame ionisation detector or FID
- Thermal conductivity detector of TCD
- Electron capture detector or ECD &
- Mass detector or MsD
Can HCOOH (formic acid) be analysed on GC using an FID detector?
No
Can CH3COOH be analysed on GC using an FID detector?
Yes
Why FID detector is mostly used in industries?
Because most of the organic compounds have oxidisable carbon atoms.
For which type of samples Electron capture detector (ECD) is used?
For halogenated carbon compounds like CCl4, CH3CH2Cl
How peaks are formed on the GC chromatogram?
The detector converts eluted components into a signal and then the signal is converted into a peak by the data processor.
What are the different stationary phases of the GC column?
Dimethylpolysiloxane, diphenyl dimethyl polysiloxane etc
What is the principle of selecting a stationary phase?
The column is selected based on the polarity of the components.
What is the internal standard?
Internal standard is used to nullify injection error.
Why the internal standard is used in GC quantitative taste?
Internal standard is used to nullify injection error.
For which type of compound derivatization technique is used for GC analysis?
For non-volatile molecule.
What are different applications of GC?
GC is used for both qualitative and quantitative analysis in:
- Food industries
- Pharmaceutical industries
- Pesticide industries &
- Petroleum industries
What are limitations of Gas chromatography?
Non-volatile components can not be analysed by GC
What will be the elution pattern of Ethanol and Isopropyl alcohol on the DB5 column?
First Ethanol will elute and then Isopropyl alcohol will elute.
Which detector will be used for the analysis of the following molecules:
- Ethanol
- Carbon tetrachloride
- Dichloromethane]
- Benzene
Answer: Ethanol and Benzene will be analysed on an FID detector. Carbon tetrachloride and Dichloromethane will be analysed on an ECD detector.
Which one is the polar column between DB5 and DB624 column?
Answer: DB 624
What is GC column Bleeding?
Column bleed is the normal background signal generated by the column stationary phase.
What is GC column bleed?
Column bleed is the normal background signal generated by the column stationary phase
Why does column bleed occur?
Column bleeding results from the thermal breakdown of the stationary phase at temperatures close to the upper-temperature limit of the column.
Factors affecting column bleed:
- The level of bleeding depends on the film thickness of the stationary phase layer on the adsorbent. Thicker film columns are used to handle higher sample loads but also have a higher bleed rate. Bleeding at high temperatures is natural for all columns, regardless of the thickness of the stationary phase film, but excessive bleeding results in a shortened useful lifetime of the columns.
- The presence of oxygen at higher column temperatures contributes to increased column bleeding and consequently increases the baseline. It is essential to ensure freedom from leaks in injectors and the use of oxygen traps in carrier gas lines.
How column bleed can be minimised?
Steps to minimize column bleed:
- Always operate columns at least 20 – 30 degrees C below the upper specified temperature limit
- Install oxygen and moisture traps in carrier gas lines and perform leak checks after column installation
- Condition columns as per the manufacturer’s recommendations before use
- Cap the open ends during storage
- Replace septa frequently to prevent damage due to coring
What are the different effects of column bleeding on analysis?
- Problems in both qualitative and quantitative analysis
- The problem with column conditioning
- Column life getting short
What is the importance of low bleed column GC-MS?
Column manufacturer manual must be followed while developing and finalising the GC method to avoid column bleeding. Use a low bleed column where analysis is required at higher temperatures. Low bleeding columns offer advantages such as the extension of the useful life of the column and improved
detection sensitivity at elevated temperatures. The advantages are of greater significance in high-sensitivity GC – MS analysis.
What is the principle of GC?
Gas Chromatography (GC) is a chromatographic system to separate organic volatile molecules. In GC mobile phase is gas (such as nitrogen and helium) and the stationary phase is immobilized liquid. Separation is achieved by the interaction of molecules between the stationary phase (immobilized liquid) and mobile phase (carrier gas) and it depends upon both polarity (of both stationary phase and molecule) and the vaporization point of the molecules.
What are the two types of GC?
Gas solid chromatography (GSC) and Gas liquid chromatography (GLC) are the the two types of GC
Which carrier gas used in GC?
Nitrogen and Helium are used as carrier gas in GC
What are the mobile phases used in Gas Chromatography (GC)?
Carrier gas Nitrogen and Helium are used as a mobile phase in GC
What do you mean by inert Gas?
Inert gases like helium and nitrogen that do not undergo chemical reactions under given conditions are called inert gases
Which types of components are used to separate in GC?
Volatile compounds
What is the difference between FID and TCD?
Operating Principle of Flame Ionization Detector (FID)
- The detector is highly sensitive to hydrocarbons and organic molecules.
- FID detects ions produced during the combustion of organic compounds in a hydrogen flame.
- When a sample elutes from the GC column and passes through the flame, carbon-based compounds (like hydrocarbons) are ionized, and the resulting ions produce a current that can be measured.
Operating Principle Thermal Conductivity Detector (TCD)
TCD measures changes in the thermal conductivity of the eluting gas relative to a reference gas.
As the sample components pass through a heated filament (the detector’s sensing element), any change in thermal conductivity (due to the different properties of the sample compared to the reference gas, usually helium or hydrogen) causes a change in the temperature of the filament, which is measured and used to generate a signal.
Difference
| Feature | FID (Flame Ionization Detector) | TCD (Thermal Conductivity Detector) |
| Detection principle | Ionization of organic compounds in a flame | Change in thermal conductivity of sample compared to reference gas |
| Sensitivity | Very sensitive to hydrocarbons and organic compounds | Lower sensitivity, detects a wider range of compounds |
| Specificity | Highly specific to organic compounds (especially hydrocarbons) | Non-specific, responds to any compound with different thermal conductivity |
| Response | Linear over a wide range, especially for hydrocarbons | Less sensitive, linear but for a broader range of compounds |
| Applications | Organic compound analysis (e.g., hydrocarbons) | General gas analysis (e.g., inorganic gases, non-organic compounds) |
What are the 3 types of autosamplers in GC?
- Liquid: It works with liquid samples
- Headspace: It works with volatile organic samples, and
- SPME: which uses fiber to extract the components of your sample
What is the length of the packed & capillary column in GC?
The length of the packed is 6 feet & length of capillary column is 10 to 100 meter in GC
What is the function of Makeup Gas in GC?
In gas chromatography (GC), makeup gas is used to ensure proper detector function and to maintain a stable baseline. Its primary functions include:
- Maintain Detector Sensitivity: In certain detectors, like the flame ionization detector (FID) or thermal conductivity detector (TCD), the makeup gas serves to stabilize the signal by ensuring consistent flow and reducing baseline fluctuations. It helps maintain an optimal environment for the detector to function properly.
- Flow Rate Adjustment: Makeup gas can be used to adjust the flow rate to the detector, particularly when the flow from the column is not sufficient. This is common when using narrow-bore columns or when the detector requires a higher flow than what the column alone provides.
- Increase Signal-to-Noise Ratio: By improving the performance of the detector and stabilizing the overall system, makeup gas can enhance the signal-to-noise ratio, leading to more accurate and reliable measurements.
Makeup gas is typically an inert gas like nitrogen or helium, chosen based on the type of detector and the experimental requirements.
What does zero air contain which includes gas chromatography?
Zero air, as used in gas chromatography, refers to air that has been purified to remove specific contaminants, primarily any hydrocarbons or other reactive gases, that could interfere with the analysis of the sample. The composition of zero air typically includes:
- Nitrogen (N₂) – The primary component of air, nitrogen serves as an inert carrier gas in gas chromatography.
- Oxygen (O₂) – Present in the atmosphere and generally included in zero air, though in very low concentrations to avoid any reactive interactions.
- Argon (Ar) – A noble gas that may also be present in trace amounts.
- Carbon Dioxide (CO₂) – Often removed or maintained at a very low concentration, since it can interfere with the chromatographic process.
The air is typically treated through filtration and sometimes additional chemical scrubbers or catalytic converters to remove hydrocarbons, carbon monoxide, sulfur compounds, and other potential contaminants. The goal is to create an atmosphere that does not introduce interfering substances that could skew the results of the gas chromatography analysis.
What is the split ratio in GC?
In gas chromatography (GC), the “split ratio” refers to the proportion of the sample that is split between the column and the waste during the injection process. It is typically used when injecting a sample into the GC system with a split injector, which divides the sample flow between the column and the vent (waste).
The split ratio is defined as:
Split Ratio = Flow of sample to waste÷Flow of sample to column
For example, if the split ratio is 10:1, it means that for every 1 part of the sample that goes into the column, 10 parts of the sample are directed to the waste. A higher split ratio reduces the amount of sample entering the column, which can help prevent overloading the column with too much analyte.
Split ratios are chosen based on the concentration of the sample, the sensitivity of the detector, and the capacity of the GC column
What are the parameters of the GC calibration?
The following parameters are performed in GC calibration:
- Column oven temperature
- Flow rate of the gases
- Detector performance
- Detector precision and
- Detector linearity
What is the advantage of Gas chromatography?
The main advantages of GC are fas analysis, cost effective and reliable result
What types of molecules elute first in GC?
More volatile molecule elutes first
What is the role of GC oven?
The role of the GC (gas chromatography) oven is to heat the sample and the GC column to facilitate the separation of the sample components.
What is USP general chapter for Gas chromatography?
<621> Chromatography
How to identify peak in gas chromatography?
Using reference standard you can identify peak in gas chromatography.
What is the difference between spitless injecting and split injection in GC?
In Gas Chromatography (GC), spitless injection and split injection are two different modes of introducing a sample into the column, and they have distinct differences in how the sample is handled. Here’s an overview of each:
1. Split Injection:
- Mode: In split injection, a small portion of the sample is injected into the GC, and the majority of the sample is diverted to the waste. This is done by using a split valve.
- Split ratio: The amount of sample that enters the column compared to the amount of sample that is discarded is controlled by the split ratio. For example, a 1:10 split ratio means that 1 part of the sample goes into the column, and 10 parts are diverted to the waste.
- Application: Split injection is typically used when the sample is concentrated or the analytes are present in high concentrations, and you don’t want to overload the column. It helps in minimizing the risk of overloading the detector.
- Sample size: A relatively small amount of sample is injected into the system because most of it is vented out.
2. Splitless Injection:
- Mode: In splitless injection, the entire sample is directed into the column without any diversion. This is typically done by closing the split valve during the injection so that all of the injected sample goes into the column.
- Application: Splitless injection is useful when the sample is very dilute or when you’re analyzing trace levels of analytes that are difficult to detect. Since all the sample enters the column, it allows for better sensitivity and is ideal for low-concentration samples.
- Sample size: In splitless injection, a larger volume of sample is injected into the system, and the sample is retained within the column for detection.
Key Differences:
- Split ratio: Split injection has a split ratio (e.g., 1:10), while splitless injection uses a ratio of 1:1 (no splitting).
- Sample handling: In split injection, most of the sample is discarded, whereas in splitless injection, all of the sample is transferred into the column.
- Application: Split injection is preferred for concentrated samples, while splitless injection is ideal for trace analysis and low-concentration samples.
In summary, split injection is best suited for samples with higher concentrations, while splitless injection is designed for trace-level analysis where all of the sample is introduced into the column for better sensitivity.
What is role of liner in GC?
In Gas Chromatography (GC), the liner plays an important role in the injector system. It serves as a critical component within the injection port, which is where the sample is introduced into the chromatograph. Here’s the specific role of the liner:
- Sample Introduction and Vaporization: The liner provides a chamber where the liquid or solid sample is injected. Once introduced, the sample is rapidly vaporized due to the high temperature of the injector. The liner ensures that the sample is evenly vaporized and then directed onto the column.
- Improved Sample Mixing: The liner helps mix the sample with the carrier gas, aiding in efficient vaporization and helping to prevent sample degradation or degradation of thermally sensitive compounds.
- Preventing Contamination: Liners help prevent the sample from directly contacting the injector’s metal parts, which could otherwise cause contamination or adsorption. It acts as a barrier, ensuring that the sample does not react with components of the injector.
- Reducing Peak Broadening: The liner is designed to promote a smooth flow of vaporized sample into the column, preventing the introduction of impurities that could cause peak distortion or broadening in the chromatogram.
- Maintaining Reproducibility: Since the liner is a part that comes into contact with each sample, its material and cleanliness can influence the precision and reproducibility of results. Regular cleaning or replacement of the liner is necessary for consistent results.
In short, the liner ensures efficient sample vaporization, minimizes contamination, and helps maintain the integrity of the chromatographic analysis.
If boiling point is the same which one elute first?
Then separation will be decided by polarity of the sample and the column.
What is the retention time in Gas Chromatography?
The time at which analte peak elutes in GC is called retention time
How does temperature affect Gas Chromatography separation?
Temperature plays a very important role in peak separation and retention time optimization.
What is the function of the GC injection port?
In injection port sample is injected and it is at higher temperature.
What is the purpose of a guard column in Gas Chromatography?
The purpose of using guard columns is to protect the GC column from contamination since the sample that is introduced may be impure like reaction mixture.
How can you optimize resolution in Gas Chromatography?
Resolution in Gas Chromatography can be optimised by oven temperature programming, carrier gas flow rate by split ratio.
How does the choice of carrier gas impact Gas Chromatography?
Carrier gas must be inert to avoid any reaction with sample components.
Explain the significance of the FID (Flame Ionization Detector) in Gas Chromatography.
FID (Flame Ionization Detector) is widely used in Gas Chromatography since most of the organic compounds contain oxidisable carbon atom.
How does Mass Spectrometry enhance Gas Chromatography analysis?
How GC is calibrated?
Using mass detector in GC molecular mass of unknown analyte can be determined and structure can be characterised.
How often should you calibrate a Gas Chromatograph?
Once in six months.
Conclusion
I hope this article has cleared all douts related to GC interview questions and now you answer all GC related question in the interview.
You may also want to check out other articles on my blog, such as:
- Difference between HPLC and GC
- How to develop a method using GC-MS?
- Need of Chromatographic Method in Drug Development
- How to decide system suitability test (SST)?
- Allowable GC Method Adjustment
- How to develop the GC method?
- What should be the Analytical method development approach?
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Thank you sir to teach deep knowledge about GC and GC column.
Thank you sir for nice explanation.