Top Gas Chromatography Interview Questions and Answers

Gas chromatography (GC) is an analytical technique used to separate and analyze volatile and semi-volatile compounds. It is widely used across various industries including pharma, environmental, food, cosmetics and more. GC is based on the principle that different compounds will interact differently with the stationary phase in the GC column, allowing them to be separated when passed through the system by an inert carrier gas.

With the ubiquity of gas chromatography, job candidates applying for roles involving GC are very likely to face GC-related questions during interviews. Being well-prepared to answer common GC interview questions is key to impressing your interviewers and landing the job. This article provides an overview of some typical GC interview questions, along with sample answers.

Common Gas Chromatography Interview Questions

Here are some of the most frequently asked gas chromatography interview questions:

Q: What is the purpose of the mobile phase in gas chromatography?

The mobile phase, also called the carrier gas, serves to carry the sample components through the GC column. Helium is most commonly used because it is inert. The mobile phase should not react with or absorb the sample components.

Q: What are some common stationary phases used in GC columns?

Common stationary phases include:

• Polydimethylsiloxane – A non-polar phase ideal for separation of non-polar compounds.
• Polyethylene glycol – A polar stationary phase.
• Porous polymer beads – Provides high surface area.
• Ionic liquids – Provide unique selectivities.

The stationary phase is chosen based on the polarity and properties of analytes to be separated

Q: What are the key components of a gas chromatograph?

The main components of a GC are

• Injection port – Introduces the sample into the continuous flow of carrier gas.
• Column – Houses the stationary phase. Sample components interact and are separated.
• Oven – Controls column temperature.
• Detector – Detects compounds as they exit the column.
• Data system – Captures and displays chromatograms.

Q: What is retention time in gas chromatography?

Retention time refers to the time taken for a particular analyte to travel through the GC column from injection to detection. Compounds interact differently with the stationary phase and elute at different times. Retention time is used to identify compounds by comparison with standards.

Q: How does temperature affect gas chromatography separation?

Temperature affects separation in GC by influencing compound partitioning between the mobile and stationary phases. Higher temperatures reduce compound interactions with the stationary phase, decreasing retention times. Too high of a temperature can prevent separation. Lower temperatures increase retention.

Q: What are van Deemter curves?

Van Deemter curves show the relationship between mobile phase linear velocity and separation efficiency. These curves are used to determine optimal flow rate for separation of compounds on a particular GC column. Flow that is too fast or too slow will result in poor peak resolution.

Q: What are isothermal and temperature-programmed analysis in GC?

In isothermal analysis, the oven temperature is held constant throughout the analysis. In temperature-programmed analysis, the oven temperature is increased throughout the run. This allows sequential elution of compounds from least to most retained.

Q: What are split and splitless injection techniques?

In splitless injection, the entire sample enters the column, providing maximum sensitivity. In split injection, a portion of the sample is diverted to waste, decreasing sensitivity but preventing column overload. Splitless injection is used for trace analysis and split injection for abundant analytes.

Q: How can you optimize resolution in gas chromatography?

Resolution can be optimized by:

• Using a column with higher efficiency, providing more theoretical plates.
• Lowering the column temperature to increase retention times.
• Reducing the carrier gas flow rate.
• Using temperature programming to elute compounds sequentially.
• Using a thinner film thickness stationary phase.

Q: What is a retention index in GC?

Retention index provides an identifier for compounds independent of column properties and conditions. It allows identification by comparing retention times to those of reference standards. Retention indices are calculated relative to retention times of n-alkanes.

Q: Explain the purpose of a guard column in gas chromatography.

A guard column protects the analytical column by trapping non-volatiles and impurities before they reach the analytical column. This preserves column integrity and lifespan. The guard column is designed to be more retentive than the analytical column.

Q: What are some common detectors used in gas chromatography?

Popular GC detectors include:

• Flame ionization detector (FID) – Sensitive to organic compounds.
• Thermal conductivity detector (TCD) – Detects changes in carrier gas thermal conductivity.
• Electron capture detector (ECD) – Selective for electronegative compounds like halogens.
• Mass spectrometer (MS) – Provides structural information and identification.

The detector is chosen based on analytes of interest and need for selectivity or sensitivity.

Q: How does a flame ionization detector work in gas chromatography?

An FID applies a flame to the column effluent to produce ions/electrons that are measured as an electric current. As analytes elute from the column, they produce an increase in current proportional to their concentration. FID is sensitive to organics but provides limited structural information.

Q: What information can be obtained from a GC-MS analysis?

GC-MS combines separation by GC with detection and identification by mass spectrometry. The MS provides the molecular weight and fragmentation pattern of compounds, allowing identification by comparison to spectral libraries. GC-MS provides both quantitative and qualitative analysis.

Q: What factors affect resolution in gas chromatography?

Resolution depends on efficiency (column plates), retention factor, selectivity, and peak width. Maximizing column efficiency and retention while minimizing peak width will optimize resolution. Selectivity is related to stationary phase properties.

Q: Explain how to calibrate a gas chromatography system.

Calibration involves injecting standards of known concentration to determine linearity, range and create a calibration curve. External standards calibration can be used by preparing standard solutions manually, while internal standards calibration employs adding standards directly to samples.

Q: What are some advantages of gas chromatography?

Advantages include:

• High resolution separation of volatile and semi-volatile analytes.
• Short analysis times.
• Compatible with small sample volumes.
• Various detection options providing both quantitative and qualitative data.
• Wide range of column chemistries for flexibility.

Q: What are some limitations of GC analysis?

Limitations include:

• Unsuitable for large or thermally labile compounds.
• Requires volatile mobile phase.
• Derivatization may be needed for polar or active compounds.
• Not universal detectors – response varies by analyte.
• Mostly destroys sample during analysis.

Q: How can gas chromatography analysis be validated?

GC method validation evaluates various performance parameters like:

• Accuracy/trueness – How close results match the true value.
• Precision – Repeatability under same conditions.
• Linearity – Calibration curve fit across expected range.
• Limits of detection and quantification.
• Robustness – Ability to withstand variations.

Proper validation ensures a method is fit-for-purpose.

Tips for Answering GC Interview Questions

When answering gas chromatography interview questions, it’s important to demonstrate both theoretical knowledge and practical abilities. Here are some tips:

• Use industry terminology appropriately – this shows you understand core GC concepts.
• Give examples of stationary phases or detectors you have experience with.
• Highlight troubleshooting skills and optimizations you have performed.
• Provide details on any method development or validation work.
• Mention safety precautions relevant to using GC instrumentation.
• Ask clarifying questions if an interview question is unclear.
• Admit if you are unsure on a topic and discuss how you would seek more information.
• Relate your answers to actual experiences in the lab working with GC, if possible.
• Emphasize your interest in and motivation to keep learning about GC technology.

With preparation and practice, you can confidently answer any GC interview questions that come your way. Mastering common technical and operational GC questions will help demonstrate you have the abilities needed to excel in a role requiring gas chromatography expertise. Use the sample questions in this article to study up before your next big interview opportunity.