How to Adjust LC Flow Rates after Changing a Column’s Inner Diameter (ID)

For research use only. Not for use in diagnostic procedures.

Answer

Columns are available in different lengths and inner diameters (IDs) for liquid chromatography (LC) methods. For example, the Phenomenex Luna Omega C18 UHPLC column is available in lengths of 50, 100 and 150 mm. This column is also available with different IDs of 2.1, 3.0 or 4.6 mm. If a user is following an existing LC method but is using a column with an ID that differs from the original method, how should the LC parameters change?

If a new method is developed for a Triple Quad™ 6500+ system using an old LC/MS method with a limited flow rate of 0.200 mL/min, the ID of the column must be taken into account. Due to unique design of the Turbo V™ ion source and its V-shaped heaters, the column could easily handle flow rates of up to 1.2 mL/min or higher. Taking advantage of this capability to save time and labor, users may directly inject a high sample volume without performing sample concentration or extraction and use a higher flow rate, requiring a different column size.

To reproduce chromatography obtained on columns with differing IDs, the linear velocity of the mobile phase should be kept the same. The linear velocity is proportional to the total LC flow rate, but it is inversely proportional to the column cross-section area. The area of a circle is proportional to the circle's radius squared, so if the column ID is doubled, then the flow rate must be quadrupled to obtain the same linear velocity. For flow rates of 0.200 mL/min on a 2.1 mm column, switching to a 4.6 mm column would require a flow of almost 1.000 mL/min to keep the same linear velocity and similar separation.


This formula provides a proper starting point, and users should optimize the flow rate based on other parameters such as LC pressure, peak shape, and separation. The following table provides common flow ranges for different C18 column IDs:



Also See:

How does Column Temperature Affect the Liquid Chromatography Method?
https://sciex.com/support/knowledge-base-articles/column-temperature-affect-on-liquid-chromatography-method_en_us

How to Reduce the Column Pressure in Liquid Chromatography (LC)?
https://sciex.com/support/knowledge-base-articles/reduce-the-column-pressure-in-liquid-chromatography-lc_en_us