Separation Science

Does a Guard Column Make Sense?

My short answer: Yes!... and No!... and Maybe. A guard column is one of those HPLC accessories that is a bit of a mixed grill – it can be a good thing or a not-so-good thing. Let’s look at some of the different aspects of guard column use to see if they make sense for you.

The guard column is a miniature column, ideally packed with the same packing material as the analytical column. This is mounted just upstream from the analytical column, as shown in Figure 1, to protect the analytical column. Guard columns come in a variety of dimensions. For example, I just checked the website for one popular column supplier and for a C18 guard column to go with a 150 x 4.6 mm i.d. column packed with 5-µm particles, they list three different options: 10 x 3.0, 10 x 2.1, and 10 x 1.0 mm i.d, all packed with 5-µm particles.

HPLC Solutions #23: Column Protection #3: Does a Guard Column Make Sense? 
Figure 1

Yes, Guard Columns are Great!
Guard columns inarguably will extend the life of the analytical column. This protection comes in two forms. First, the frits on the guard column are the same as those on the analytical column, so anything that might block the analytical column would block the guard column first, thus protecting the analytical column. Even if you use a guard column, I still recommend using an in-line filter in the system (see last week’s discussion), as shown in Figure 1. Typically the guard and analytical column will use 2.0-µm porosity frits (5-µm column packing assumed), so the 0.5-µm frit in the in-line filter will help to extend the life of the guard column, too.
In some cases, the guard column can be used to lower the costs of other parts of the HPLC analysis. For example, it may make sense to sacrifice guard columns as a trade-off for reduced sample preparation costs. If throwing away a $55 guard column every 50 samples means that you don’t have to spend $2-3/sample on solid-phase extraction cartridges, the economic advantage is obvious.

   The packing of the guard column should be as close to the same as the analytical column as possible (same silica and bonded phase), which means that it is best to get the guard column from the same brand and product line from the same manufacturer. If the packing is the same, anything that would stick irreversibly on the analytical column will stick on the guard column, instead, thus protecting the analytical column.

No, Guard Columns are Not Worth the Hassle!
On the other hand, guard columns are just one more thing to go wrong in the system. First, you need to figure out how long the guard column will last. If you wait until there is breakthrough of strongly retained material from the guard column to the analytical column, you have waited too long and may damage the analytical column. If you replace the guard column too early, you haven’t received your full value from this expensive device. Did I say expensive? Yes. I just took a glance at one manufacturer’s offering. A 150 x 4.6 mm, 5-µm C18 analytical column lists for $560. A pack of five 100 x 3.0 mm, 5-µm C18 guard columns costs $275, or $55 each. At 10% of the cost of the analytical column, you want to make sure you get the most out of the guard column. This means that you have to figure out how long they will last. Most workers replace the guard column either on a time basis (e.g., once a week) or on a per-sample basis (e.g., every 200 samples). Either way, you’ll probably build in some safety margin to discard the guard column before it no longer protects the analytical column. So you have to ask yourself, will the extra $55 really increase the life of my analytical column by 10%, and is it worth the hassle?

   And another less-recognized trait of the guard column is that it can cause deterioration of chromatographic performance. As a rule, guard columns are not as well packed as analytical columns and they are not tested, so their chromatographic performance suffers. When coupled to a high-performance analytical column, almost universally the result is a lower plate number (column efficiency) than with the analytical column alone. This may or may not be important in your method.

Maybe a Guard Column Will Work for You
So you can see that the use of a guard column has a lot to do with personal preference. My preference is not to use guard columns – I think the hassle-factor isn’t worth the potential financial savings. In any event, you should make a careful evaluation of the financial and chromatographic benefits and risks before you use a guard column. Does it improve the quality of your data? Does it improve the reliability of the HPLC system? Does it extend the lifetime of the analytical column? These and other questions will help you to decide if a guard column makes sense for your method.

This blog article series is produced in collaboration with John Dolan, best known as one of the world’s foremost HPLC troubleshooting authorities. He is also known for his ongoing research with Lloyd Snyder, resulting in more than 100 technical publications and three books. If you have any questions about this article send them to TechTips@sepscience.com

 

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