My First Experience With GC (Gas Chromatography) Part I

Typically, I strive to maintain the informative nature of my blog posts. Consequently, I tend to discuss topics I am familiar with. However, this time, I am eager to share my inaugural experience with a Gas Chromatography (GC) machine. My intent is to provide a unique perspective on the subject, whether you are a newcomer curious about its operation or an expert seeking a fresh outlook. It's worth noting that this marks my first encounter with any form of chromatography.

The genesis of this journey lies in an external request for the analysis of fatty acids (FA) in fish oil. Our mission was to identify the presence and proportion of FA within the sample.

To begin, let's demystify chromatography. Essentially, it entails the separation of individual components from a mixture. This is typically achieved by establishing a known order for each molecule type through various methods, often involving a column. Subsequently, a detector is employed to measure or detect these components. Depending on the time of detection, one can ascertain the types of molecules present in the sample.

In my particular case, I opted for gas chromatography, a method commonly utilized for analyzing volatile and semi-volatile compounds. What sets it apart from other chromatography techniques (such as HPLC, LC, and SFC) is that the sample is converted into a gas and transported through the column by another gas. At the column's terminus, there are numerous options; in my scenario, we employed one of the most prevalent detectors, known as the Flame Ionization Detector (FID). As the name implies, this detector identifies ions emitted from a flame when the sample is introduced.

Nevertheless, this device isn't a straightforward plug-and-play system; it entails a series of procedures. You cannot simply introduce your sample and expect immediate results. Initially, you must prepare your sample, a process that varies depending on the nature of the sample and the parameters you are measuring.

Before delving into the specifics of our approach for detecting the target molecules, let's familiarize ourselves with the structure of a GC system. Terminology and nomenclature may vary based on several factors, but the underlying principles remain consistent.

These are the main components of GC. Now lets expend the diagrams and visualize our variables. The ones I mentioned are not the only variables but they are the mains ones in my opinion. Variables are written in italic

 

So lets start evaluating our variables and think about their affects on our result.  

Variables

Carrier Gas

From left to right our first variable is type of the carrier gas. This can be;
  • Helium (He):
    • Advantages: Helium is the most commonly used carrier gas. It is inert, which means it does not react with the sample or the column, leading to good peak shapes and baseline stability. Helium is also nonflammable.
    • Disadvantages: Helium is becoming less available and more expensive due to supply limitations. It is less dense than other carrier gases, which can affect efficiency in some cases.
  • Hydrogen (H2):
    • Advantages: Hydrogen is often used as a carrier gas because it provides shorter analysis times compared to helium. It is also less expensive than helium.
    • Disadvantages: Hydrogen is flammable, so it requires extra precautions to avoid safety risks. It may not be suitable for certain detectors, such as the Flame Ionization Detector (FID), which relies on a hydrogen flame.
  • Nitrogen (N2):
    • Advantages: Nitrogen is readily available, relatively inexpensive, and nonflammable. It can be a good alternative when helium is not an option.
    • Disadvantages: Nitrogen is less efficient than helium, and analysis times may be longer. It can also exhibit broader peaks.
  • Argon (Ar):
    • Advantages: Argon is an inert gas, similar to helium, and is often used in specific applications where helium is not available or is too expensive.
    • Disadvantages: Argon is less commonly used as a carrier gas in GC, and its use may depend on the specific requirements of the analysis.
  • Air:
    • Advantages: Air can be used as a carrier gas in certain situations. It is readily available and cost-effective.
    • Disadvantages: Air contains oxygen, which can lead to oxidation of sensitive samples. It may not be suitable for all types of analyses.

Keep in mind your options can also be limited by type of detector you plan to use also as can be seen from this list there are some options and they all have their advantage.

Also way the gases used is important. As name suggests the sample is carried by this gas and it's pressure and flow affects result. Here most commonly the pressure of the carrier gas is determined but if your device supports you can also define the amount by setting certain flow or velocity. These variables may need to be adjusted during the run of the measurement.

This variable affects mainly; retention time, column efficiency, column loading, mass transfer, selectivity and boiling point.

Sample

The preparation of sample highly important and I believe it is a vastly different topic so I will not talk about it and presume the sample is prepared as ready to be given to the machine. After those steps only variable here is the amount of sample will be given to the system. This variable affects basically peak intensity and sensitivity. If the molecule you are looking for is appearing very weakly you might consider changing this variable. Be aware over doing this variable can result overload down the line to components like, injector, column and detector. 

Split

In this part you have two variables. One is temperature and carrier gas-sample ratio. First tackle the more complex one which is the ration. This variable exists because the sample is not given to the system all at once but gradually released. As carrier gas flows in constant manner split part of the device injects sample gradually to the stream. This ratio basically determines concentration of sample in the carrier gas. Ratio's affect is also more like sample amount.

The other variable is temperature. Heat in this stage is used for evaporating the sample so for this variable we used

Oven

This part of machine only has one variable, temperature. It indirectly affects columns temperature. Which has wildly different consequences. Also change of conditions in this part is more common in different methodologies. The temperature is usually changed during the run. As this variable affects column temperature we will approach this variable as temperature of the column. 

The change in column temperature affect;

  • Thermal Stability of Analytes:
    • Elevated temperatures may impact the thermal stability of certain analytes. Some compounds may degrade or undergo chemical reactions at high temperatures, leading to changes in the chromatographic profile.
  • Column Bleed:
    • The temperature of the oven can affect column bleed, which refers to the release of non-volatile compounds from the column material. Elevated temperatures can increase column bleed, potentially affecting baseline stability and detector sensitivity.

These affects can be said to almost every other column but if your column has special features like polarity then affect of is not limited to above mentioned affects, compounds with different polarities interact differently with the stationary phase at varying temperatures. In general, higher temperatures reduce the retention of polar compounds, making them elute faster, while less polar compounds may exhibit increased retention.

Column

Here we have type of column. More like sample preparation this topic is very deep and requires extensive knowledge on chromatography. There for I will provide basic variables within this variable and skip the consequences.

  • Column Length:
    • Longer columns generally provide better resolution but may result in longer analysis times. The choice of column length depends on the separation requirements and analysis goals.
  • Column Diameter (Internal Diameter):
    • The column diameter affects sample capacity and resolution. Smaller diameter columns provide higher resolution but may have lower sample capacity. Larger diameter columns allow for higher sample loads but may sacrifice resolution.
  • Film Thickness:
    • The thickness of the stationary phase film coated on the column influences the column's capacity and selectivity. Thicker films often provide greater sample capacity but may lead to broader peaks.
  • Stationary Phase:
    • The choice of stationary phase is crucial and depends on the type of analytes being separated. Common stationary phases include polar, non-polar, and intermediate polarity phases. Specialty phases may be chosen for specific applications, such as separating certain compound classes or providing unique selectivity.
  • Stationary Phase Polarity:
    • The polarity of the stationary phase should match the polarity of the analytes of interest. The interaction between the analytes and the stationary phase is critical for achieving good separation.
  • Column Temperature Range:
    • The temperature range over which the column can be operated is an important consideration. It should cover the temperature range required for the analysis, including the boiling points of the analytes.
  • Column Inertness:
    • The inertness of the column is crucial, especially for trace-level analyses. Inert columns minimize unwanted interactions between the analytes and the column, ensuring accurate and reproducible results.
  • Column Material:
    • Columns are often made of materials like stainless steel or fused silica. The choice of material can affect the chemical inertness and thermal stability of the column.
  • Phase Ratio:
    • The ratio of the film thickness to the column diameter is known as the phase ratio. It affects the column's efficiency and is often optimized for specific applications.
  • Manufacturer and Brand:
    • Different manufacturers offer columns with varying specifications and performance characteristics. Experience with specific brands and types of columns may influence the selection process.
  • Sample Matrix:
    • Consideration of the sample matrix is important. If the sample contains complex matrices or matrix components that may interfere with the analysis, the column should be chosen to minimize these effects.
  • Analysis Throughput:
    • The desired throughput and speed of analysis may influence column selection. Faster columns may be chosen for high-throughput applications, while slower columns may be preferred for more detailed separations.
  • Budget Considerations:
    • The cost of the column is an important practical consideration. Balancing performance requirements with budget constraints is essential.
  • Application-Specific Columns:
    • Some applications may require specialized columns, such as columns designed for specific compound classes (e.g., pesticides, drugs) or specific separation modes (e.g., chiral columns).

Detector

There are several options in terms of detector types. As they all have unique tradeoffs selection must be made based on target sample and molecules. These detectors can have variables needs to be defined during the run but each varies so I'm just listing the type as variable. There are several detector options;

  • Flame Ionization Detector (FID):
    • Principle: FID is a universal detector for organic compounds. It involves combustion of analytes in a hydrogen flame, producing ions that are collected and measured to generate a signal.
    • Advantages: High sensitivity, wide linear dynamic range, and suitable for a broad range of compounds.
    • Limitations: Non-destructive, not selective, and does not provide structural information.
  • Thermal Conductivity Detector (TCD):
    • Principle: TCD measures changes in thermal conductivity of the carrier gas caused by variations in sample concentration.
    • Advantages: Non-destructive, universal detector, and suitable for detecting non-volatile compounds.
    • Limitations: Lower sensitivity compared to some other detectors.
  • Electron Capture Detector (ECD):
    • Principle: ECD detects electron-capturing analytes by measuring changes in current resulting from electron capture.
    • Advantages: Extremely sensitive for compounds with electronegative functional groups (e.g., halogens).
    • Limitations: Limited applicability, not suitable for all compounds.
  • Mass Spectrometry (MS):
    • Principle: MS identifies and quantifies compounds based on their mass-to-charge ratio. GC-MS combines gas chromatography with mass spectrometry.
    • Advantages: High sensitivity, specificity, and ability to provide structural information.
    • Limitations: Costly instrumentation, complexity, and may require more expertise.
  • Flame Photometric Detector (FPD):
    • Principle: FPD detects compounds containing sulfur or phosphorus by measuring the intensity of light emitted during combustion.
    • Advantages: Selective for sulfur and phosphorus, suitable for environmental and petrochemical analyses.
    • Limitations: Limited to specific elements.
  • Nitrogen-Phosphorus Detector (NPD):
    • Principle: NPD detects nitrogen and phosphorus compounds by measuring changes in thermal conductivity.
    • Advantages: Sensitive for nitrogen- and phosphorus-containing compounds, selective for certain functional groups.
    • Limitations: Limited to specific elements.
  • Photoionization Detector (PID):
    • Principle: PID measures ionization of compounds by ultraviolet light, providing sensitivity to a wide range of volatile organic compounds.
    • Advantages: High sensitivity, broad applicability, and suitable for detecting low-concentration compounds.
    • Limitations: May lack selectivity for specific compounds.
  • Thermal Energy Analyzer (TEA):
    • Principle: TEA detects nitrogen and phosphorus compounds by measuring the heat generated during their combustion.
    • Advantages: Selective for nitrogen and phosphorus, applicable in environmental analyses.
    • Limitations: Limited to specific elements.

 

Last words,

I expected this text to be shorter but it turned out longer than I anticipated. so I will finish here for now and continue with describing steps for measurements later.