Gas chromatography (GC) is a powerful analytical technique with outstanding resolution and sensitivity, it is used in a wide range of applications including environmental, petroleum, chemical, food and beverage, pharmaceutical and many others. By separating and quantifying individual components in complex mixtures, GC allows scientists to delve into the complex world of molecular interactions.
Types of GC Column
Central to the success of gas chromatography is the GC columns, which play a crucial role in the separation process. GC columns come in various types, each designed to cater to specific analytical requirements. The two primary classifications are packed columns and capillary columns, each offering distinct characteristics and applications.
| GC Columns |
Characteristics |
Applications |
| Packed columns
|
Inner diameter of 2 to 4 mm.
Column is completely filled with particles Adsorbent – GSC.
Support coated with liquid stationary phase – GLC.
Column length is limited due to high flow resistivity (< 10 m).
Column material
: copper, stainless steal, glass, quartz. |
Packed columns are ideal for the analysis of complex mixtures, especially when a broad range of compounds needs to be resolved. |
| Capillary columns
|
Inner diameter < 1 mm.
Column length: 5 to 100 m.
Column material: glass (fragile), fused silica (quartz) made from ultra pure SiO2
, fused silica coated stainless steal (high temperature resistant). |
Suitable for rapid analysis of individual trace compounds.
For highly sensitive analytical assays, e.g., pesticide residues, isomer analysis, fatty acid, and medicine analysis. |
Fig. 1 Column types and common dimensions in GC (Dettmer-Wilde, K.; Engewald, W.).
How to Choose GC Columns?
Selecting the right GC column is a delicate process influenced by several key factors, including the nature of the analytes, their volatility, the desired resolution, and the analytical conditions.
Analyte Characteristics
Knowledge of analyte properties such as molecular weight, polarity, volatility, and thermal stability is critical to the selection of stationary phase and column dimensions.
Stationary Phase Selection
The stationary phase plays a vital role in the separation process. It interacts with the analytes and facilitates their separation based on specific interactions such as polarity, size, or volatility. Therefore, it is necessary to select the stationary phase that best matches the analyte characteristics and separation objectives.
Column Dimensions
The column length, internal diameter, and film thickness significantly impact separation efficiency, resolution, and analysis time. Longer columns provide higher resolution but take longer for the analytes to elute. A balance must be struck between resolution and analysis time.
Column Diameter
The column diameter will be affected by five parameters that are efficiency, retention value, pressure, carrier gas flow rate and capacity. If high column efficiency is required, use a column with an internal diameter of 0.18-0.25 mm. 0.18 mm internal diameter columns are well suited for GC/MS systems with low pump capacities. Smaller diameter columns have the lowest capacity and require the highest column head pressure. If larger sample volumes are required, use a 0.32 mm ID column. These columns generally provide better separation of solutes that flow out earlier without splitting the sample or with large volume (>2 µl) injection than 0.25 mm ID columns. Columns with 0.45 mm or 0.53 mm bore are used only when the instrument is equipped with a large-bore direct feeder and high column efficiency is required.
Operating Temperature
The choice of column temperature is vital to achieve effective separation. The temperature affects the volatility and interaction of analytes with the stationary phase. Adjusting the temperature gradient can improve resolution and optimize separation for specific analyte classes.
Compounds Analyzed by GC
GC is suitable for the analysis of 10% - 20% of all known compounds. To be analyzed using GC, compounds must be sufficiently volatile and thermally stable. If all or part of the compound is in the gas or vapor phase at or below 400-450 °C and does not decompose at these temperatures, then it is possible that the compound can be analyzed by GC. However, not all compounds can be analyzed by GC, such as the following compounds.
- Non-evaporable compounds, such as inorganic metals, ions and salts.
- Highly reactive and chemically unstable compounds, such as ozone, hydrofluoric acid, strong acids, NOx and other highly reactive substances.
- Highly adsorbent compounds containing amino, hydroxyl, carboxyl or sulfur groups.
Reference
- Dettmer-Wilde, K.; Engewald, W. Practical gas chromatography. A Comprehensive Reference. Springer, 2014: 902.
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