Comprehensive Guide to Guard Columns and Pre-Columns

Guard Columns and Pre-columns (Online Filters)

These are items commonly used in daily laboratory experiments.

However, many people have doubts,

They look so similar,

Aren't they the same thing?

What is the difference between them?

This article will explain in detail~

Differences Between Guard Columns and Pre-columns

Similarities:

Both have ferrules.
Both are connected in the same position, directly in front of the chromatographic column.
Both have frits to prevent solid particles from reaching and clogging the chromatographic column frit.

Differences:

Guard Column: The ferrule contains a guard column cartridge, which is filled with packing material. This packing material can adsorb some strongly retained, strongly acidic, and strongly basic substances in the sample, preventing them from contaminating and modifying the packing material inside the chromatographic column. Therefore, using a guard column provides comprehensive protection for the chromatographic column and is the best way to protect it.

The use of a guard column has a drawback: the packing material type matching requirement is relatively high. The packing material of the cartridge needs to be the same as that inside the chromatographic column. If different, it may interfere with the analysis or reduce the protective effect on the chromatographic column.

It is important to note that the guard column can only protect against a small amount of contaminants in the sample. It cannot provide effective protection against issues such as excessive pH levels in the mobile phase because the volume of the mobile phase is too large, exceeding its protection limit.

Pre-column (Online Filter): It has only one frit and its function is relatively simple, solely preventing solid particles from reaching the chromatographic column. However, it has some significant advantages: it can be used with columns from various manufacturers, and a one-time purchase can be used for a long time (the replaceable filter can be ultrasonically cleaned).

Functions of Guard Columns and Pre-columns

▸ Guard Column: Contains packing material and frits, performing both chemical adsorption and physical filtration to remove contaminants that may pollute or clog the analytical column.

▸ Pre-column (Online Filter): Removes particulate matter from the sample or mobile phase through physical filtration only, protecting the analytical column, pump, and tubing from clogging.

When to Replace Guard Column Cartridges and Filters

Replacing Guard Column Cartridges:

When the column pressure increases by more than 10%.
When column efficiency decreases by more than 10%.
When resolution decreases by more than 10%.
When peak shapes show significant tailing, fronting, splitting, broadening, or other abnormalities.

When to Replace Pre-columns (Online Filters):

When the column pressure sharply rises or exceeds the pressure limit, the online filter or the filter inside needs replacement.
For replaceable filters, the filter can be removed and cleaned with isopropanol using ultrasound.
For portable stainless steel or PEEK online filters, as they are integrated structures and cannot be disassembled, it is recommended to replace them directly when their performance is poor.

How to Choose Guard Columns and Pre-columns

The packing material of the guard column must match that of the analytical column.

Now, let's address a small question: My chromatographic column is a C18 column from Company A. Can I use a C18 guard column from Company B?

Answer: It's not recommended. Because the C18 packing materials from two different companies may have different adsorption capabilities for substances, the guard column may not effectively protect the subsequent column. Additionally, while differences in packing materials from different manufacturers might not matter for insensitive analyses, they could lead to changes in peak behavior for sensitive analyses.

The inner diameter of the guard column is determined by the inner diameter of the analytical column.

When the inner diameter of the HPLC analytical column is >3.0mm, choose a guard column with an inner diameter of 4.6mm.
When the inner diameter of the HPLC analytical column is ≤3.0mm, choose a guard column with an inner diameter of 2.1mm.
When the inner diameter of the UHPLC analytical column is >3.0mm, choose a guard column with an inner diameter of 4.0mm.
When the inner diameter of the UHPLC analytical column is ≤3.0mm, choose a guard column with an inner diameter of 2.1mm.

The pore size of the filter in the online filter is usually chosen based on personal usage habits and the type of HPLC/UHPLC analytical column. In normal situations, 2μm and 0.5μm filters are used, and there won't be significant differences in column pressure between the two.

Welch currently offers stand-alone and direct-connect guard column ferrules and pre-column products. For detailed product prices and information, please contact our sales team.