Choosing Gas Chromatography Columns: Key Parameters

Hello, everyone!
The length, inner diameter, and film thickness of gas chromatography columns
How do they affect separation efficiency and time?
Today, the editor will share with you The specific effects of these three factors.

• Column length mainly affects three important parameters: column efficiency, retention value (analysis time), and carrier gas pressure.

1. Column efficiency (usually expressed as theoretical plate number N or plates per meter N/m) is directly proportional to column length. The resolution is a function of the square root of plates per meter. For example, doubling the column length theoretically increases the resolution by 1.41 times. Common column lengths include 15m, 30m, 50m, 60m, etc.

   • 15m column: Used for rapid screening of simple mixtures or compounds with extremely high molecular weights.

   • 30m column: The most common column length.

   • 50m, 60m column: Used for more complex samples. If a higher column efficiency is desired to achieve narrow and sharp peak shapes, it is recommended to use longer columns.

2. The retention time of analytes is inversely proportional to column length. This effect is particularly significant under isothermal conditions, but even under programmed temperature conditions, differences in runtime may be noticeable. While increasing column length is advantageous for increasing column efficiency, the analysis time will also correspondingly increase.
3. The column head pressure is almost directly proportional to the column length. Chromatographic column bleeding increases with increasing column length. Longer chromatographic columns have more stationary phases, thus producing relatively more degradation products and slightly larger bleed. However, this does not affect the selection and use of long columns. The cost of chromatographic columns is directly related to the column length, and doubling the column length will almost double the price of the column. When increasing column efficiency by increasing column length, the cost of the chromatographic column also significantly increases, and the analysis time is extended accordingly.

Selection of chromatographic column length:

1. If the optimal length is unknown: Start with a chromatographic column length of 25~30m.

2. For samples containing easily separable components or samples with fewer target and other components: Use chromatographic columns with lengths of 10~15m. Chromatographic columns with smaller inner diameters are usually shorter to reduce column head pressure.

3. If satisfactory separation cannot be achieved through other methods (such as reducing the inner diameter, changing the stationary phase, increasing the film thickness, changing the temperature program, etc.): Use chromatographic columns with lengths of 50~60m. These columns are suitable for separating complex samples containing multiple components, but the analysis time is longer, and the cost is higher.

For example, in GB 5009.168-2016 National Food Safety Standard Determination of Fatty Acids in Foods; GB 5009.257-2016 National Food Safety Standard Determination of Trans Fatty Acids in Foods. Due to the variety of fatty acids, including many cis-trans isomers, for these complex samples, the standards recommend using a 100-meter strongly polar chromatographic column.

02 Inner Diameter

• The inner diameter of the chromatographic column mainly affects five parameters: column efficiency, retention value, pressure, carrier gas flow rate, and column capacity.

1. Column efficiency (N/m) is inversely proportional to the inner diameter of the chromatographic column. The smaller the diameter of the chromatographic column, the higher the theoretical plate number per meter. The resolution is a function of the square root of theoretical plate number. Therefore, doubling the column efficiency theoretically increases the resolution by about 1.41 times, but in practice, the increase may be around 1.2~1.3 times. For obtaining narrow peaks and high column efficiency, it is recommended to use chromatographic columns with smaller inner diameters.
2. Retention value is inversely proportional to the inner diameter of the chromatographic column (under the same length and film thickness). Decreasing the inner diameter increases the retention value; however, column inner diameter is generally not chosen based on retention value. In reality, decreasing the inner diameter increases column efficiency, thereby improving separation efficiency. By selecting an appropriate length column, the same separation work can be completed in a shorter time.
3. The column head pressure is approximately a negative square function of the column inner radius. For example, a chromatographic column with an inner diameter of 0.25mm requires about 1.7 times the column head pressure of a column with an inner diameter of 0.32mm (under the same length, carrier gas, and temperature conditions), indicating that a column with a smaller inner diameter requires higher pressure under the same conditions.

4. Under atmospheric pressure, the carrier gas flow rate increases with the increase of the chromatographic column's inner diameter. For methods or hardware requiring high flow rates, chromatographic columns with larger inner diameters are typically used. To operate properly, headspace injection systems and purge-and-trap systems require higher carrier gas flow rates. Therefore, chromatographic columns with inner diameters of 0.45mm or 0.53mm are usually chosen to be used with these systems. For methods or hardware requiring low carrier gas flow rates, chromatographic columns with smaller inner diameters are typically used. GC-MS is a typical system that requires low carrier gas flow rates; therefore, chromatographic columns with inner diameters not exceeding 0.25mm are usually selected for GC-MS systems.

5. The column capacity increases with the increase of the chromatographic column's inner diameter. The actual column capacity also depends on the stationary phase, solute properties, and film thickness.

Selection of Chromatographic Column Inner Diameter:

1. If high column efficiency is needed: chromatographic columns with inner diameters of 0.18~0.25mm can be used. However, columns with smaller inner diameters have smaller sample capacities and require higher column head pressure.

2. If a larger sample capacity is needed: chromatographic columns with an inner diameter of 0.32mm can be used. Compared to columns with an inner diameter of 0.25mm, this type of column typically provides better separation for components that elute earlier during splitless or large-volume (>2μL) injections.

3. Only when the instrument is equipped with a large-diameter direct injector and high column efficiency is required: chromatographic columns with inner diameters of 0.45mm and above are used. Especially in cases requiring high carrier gas flow rates, such as purge-and-trap, headspace sampler, and valve injection, large inner diameter columns are particularly suitable.