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Everything You Need to Know About EXP Guard Columns

13 May 2024

In this blog, we will cover everything you need to know about EXP guard columns.

What is a guard column?

A guard column is a short column that is installed before the analytical column in an LC system. Also called a pre-column, they are used to protect the analytical column from particulates and/or impurities that were not removed by sample preparation. Using a guard column can significantly improve the lifetime of the analytical column and improve chromatographic performance. 

Guard columns are typically made up of two components: a guard column cartridge and a guard cartridge holder. The guard column cartridge is typically packed with the same material as the stationary phase of the analytical column. The cartridge is placed in the guard cartridge holder, which is attached to the front end of the analytical column.

Figure 1: Guard column cartridges (left) and guard cartridge holder (right).

picture of guard column cartridges and guard cartridge holder

Are there different types of guard columns?

 There are many types of guard columns available. There are multiple configurations of guard columns just within Restek’s own catalog, so how do you distinguish them? Guard columns are designed to match with different LC column lines and for different techniques. Someone performing UHPLC would need a guard column configuration that can handle the higher back pressures associated with that technique, whereas someone performing traditional HPLC might require something different. For the remainder of this blog we are going to focus on EXP guard columns, but if you want to know more about the other guard column lines we offer, check out the article Restek LC Guard Column Selector

What are EXP Guard Columns?

EXP guard columns pair with our Force and Raptor LC column lines. EXP guard columns are compatible with both HPLC and UHPLC systems. 

When should you use an EXP Guard Column?

The less sample preparation you do, the more important it is to have a guard column. Even when dealing with relatively clean samples, a guard column is always a good idea to ensure your analytical column is protected. You should use an EXP guard column anytime your method utilizes a Force or Raptor LC column. 

EXP Guard Column Cartridges

The first component in EXP guard columns is the EXP guard column cartridge. EXP guard column cartridges come in several different particle sizes, diameters, and stationary phases. Guard cartridges should match the characteristics of the analytical column. For example, if a method utilizes a Raptor FluoroPhenyl 100 x 3.0 mm, 2.7 µm analytical column, the appropriate guard cartridge would be a Raptor FluoroPhenyl 5 x 3.0 mm, 2.7 µm EXP guard column cartridge. The stationary phase and dimensions of the guard cartridge should be compatible with those of the analytical column to ensure impurities are removed effectively. 

Just like HPLC columns, cartridges have a maximum pressure they can withstand. The maximum pressure capabilities for different particle sizes are listed below.

Table 1: Maximum guard cartridge pressures for UHPLC, 2.7 µm, and 5 µm particles.

Particle Size Maximum Cartridge Pressure
UHPLC         1034 bar/15,000 psi*
2.7 µm 600 bar/8700 psi
5 µm 400 bar/5800 psi

*For maximum lifetime, the recommended pressure for UHPLC particles is 830 bar/12,000 psi.

EXP guard cartridges will add a small amount of extra column volume to your separation system. When performing UHPLC, reducing extra column volume is extremely important, and EXP guard cartridges may not be the most ideal option. In this situation, an UltraShield UHPLC precolumn filter with a 0.2 or 0.5 µm frit could be used alternatively. These add less dead volume to the separation system, while still providing protection from particulates for the analytical column.   

EXP Direct Connect Holder for EXP Guard Cartridges

EXP direct connect holders are used specifically to hold EXP guard cartridges. EXP direct connect holders are made up of a holder cap and a holder body. The unidirectional guard cartridge sits in between these two parts (Figure 2). These holders can handle up to 1400 bar/20,000 psi of pressure.

Figure 2: EXP Direct Connect Holder

picture of a disassembled EXP direct connect holder

Installation

Installing an EXP guard column is quick, easy, and requires no tools. Below, you will find step-by-step instructions for installation.
To install an EXP guard column, you will need a Raptor or Force LC column, an EXP direct connect holder, and an EXP guard cartridge that matches the phase and dimension of your LC column, as shown below. While this demonstration uses EXP hand-tight fittings, EXP nuts can be used alternatively to provide a higher-pressure connection (1400 bar/20,000 psi). If using an EXP nut, a ¼” wrench is required for tightening. 

picture of guard cartridges, cartridge holder, and analytical column before they are assembled

 

Step 1: Slide the EXP hand-tight fitting and an EXP ferrule onto the tubing, with the PEEK end of the ferrule facing away from the EXP hand-tight fitting. Push the tubing into the EXP holder cap. While keeping the tubing fully bottomed in the holder cap, tighten the fitting by hand. Do not use tools when installing EXP hand-tight fittings, as overtightening can irreversibly damage the ferrule.  

two pictures showing disconnected and then connected EXP holder cap on EXP fitting that is attached to tubing

 

Step 2: Slide an EXP ferrule into the front end of the EXP holder body, with the PEEK end of the ferrule facing away from the holder body. Thread the holder body into the LC column to the point of initial resistance. Push the holder body into the column and fully tighten by hand. This connection is rated to 600+ bar/8700+ psi. For a 1400 bar/20,000 psi connection, continue to push the holder body into the column and use a 5/16” wrench on the provided wrench flats to tighten the holder body an additional ¼ - ½ turn past hand tight (not shown). Take care not to overtighten, as this can irreversibly damage the ferrule.

pictures of EXP holder being installed in an LC column

 

Step 3: Insert the unidirectional guard column cartridge into the holder body.

picture of guard column cartridge being installed in the holder

 

Step 4: Screw the holder cap onto the holder body and fully tighten by hand (about 1/8-turn past point of initial resistance).

picture of fully assembled guard system and analytical column

When should I replace my guard cartridge?

Guard cartridges are consumable pieces that periodically need to be replaced. The amount of use a single guard cartridge gets will differ based on how dirty the samples being injected are. If you notice significantly increased back pressures or chromatographic performance issues like inconsistent retention times or peak shapes, it might be time to change the guard cartridge. To change your EXP guard cartridge, simply unscrew the holder cap (where it says “Restek”) by hand, insert a new cartridge, and replace the holder cap. Fully tighten the holder cap by hand, and you’re good to go.

More Information

Still have questions or want to know more? Explore more great resources on this topic:

  1. EXP Direct Connect Holder instruction sheet
  2. Using Your EXP LC Guard Column Protection System
  3. Do I Need an LC Guard Column?
  4. Choosing a guard cartridge holder for LC
  5. Just don’t do it!! Don’t overtighten your LC direct connect guard.