Guard columns have very low efficiency. For the sake of numbers, a 0.5 x0.46 cm i.d. cm guard column packed with 2.7 micron particles may generate only 1000-1500 plates at best if used alone. Narrow diameter guard would produce worse results. You are adding another complication of changing diameters as well as chemistry. No wonder it is producing peak tailing. It is better to play by changing mobile phases as well as the column temperature to obtain best possible separation. Both variables can even change peak elution order.

Hi,

have a look here: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4445503/

Hyphenated the guard column? I'm reading the question as placing a silica guard on a C18 (for example)? For the example I just used, the "weak" solvent for the C18 is the "strong" solvent for the silica. Compounds that don't retain on silica under reverse phase condition may retain well on the C18. Other crosses, such as ion exchange guards with C18 columns may give different results since the IEX column isn't acting in ion exchange mode, you are more likely to get mixed chromatography. which may or may not be helpful.

Guard columns have very low efficiency. For the sake of numbers, a 0.5 x0.46 cm i.d. cm guard column packed with 2.7 micron particles may generate only 1000-1500 plates at best if used alone. Narrow diameter guard would produce worse results. You are adding another complication of changing diameters as well as chemistry. No wonder it is producing peak tailing. It is better to play by changing mobile phases as well as the column temperature to obtain best possible separation. Both variables can even change peak elution order.

Hi Mel,

I agree with the others, a guard column in comparison to the main column has a really low effect on separation since the add up of plates is not that significant. However, you should always use identical column materials for guard column and main column not to screw up the analysis and you don't know where it comes from. Guard columns are always available in all the stationary phases as offered for the columns! Check phenomenex for instance.

Marcus

guard column is designed to protect the column from strongly retained compounds which otherwise will stay in the analytical column and cause the stationary phase changes in selectivity and also clog the column. If you need more efficiency from your analytical column, use a longer column, smaller particle size, or smaller column diameter. You will need only a few mm thickness of guard column to do this such as Phenomenex Security guard which is a small disc 3 mm in diameter with 2 mm thick. You will throw it away as often as you see fit (high back pressure or dirty look). It should be the same material as the analytical column so it does not change overall selectivity.

In general, the recommendation is to stay away from and not use guard columns. Note: guard columns should not be confused with true "Pre-Columns" which are usually much larger in volume and do contribute to the separation or method. Guard columns have a very specific role in liquid chromatography and are often overused (because "someone else used one") in methods. Guard columns are not and should not be a substitute for a "real" column. Guard columns can have practical uses (do a search as there are too many to list here), but are not needed for most well developed methods (they do provide extra income for column suppliers). Develop the method on an appropriate column of the required size and chemistry first. The addition of a second column, such as a guard column, is usually a step backward.

First off, you shouldn't mix phases with a guard column as Farooq points out. It is a bad idea and will only lead you to problems. Guard and analytical column should be the same phase.

I disagree with Bill about the guard columns; I use them routinely to keep garbage off of my analytical column. We deal with pesticides in food, and the matrices are nasty. I change my prefilter and guard column routinely, and get much more life out of my analytical column. I do agree with him about income for the column manufacturers; the price of guard columns is ridiculous!

Hi Mark, I did specifically state, "Guard columns have many practical uses" and the one you cite is an example.

I would like to express my opinion.   Guard column is not necessary at all, when high-recovery analysis is performed.   Thus, I have not used any guard columns yet in my articles (please see file; HPLC-Surf-SEC protein determination method).   This SEC column for protein determination has been safely used in good condition for c.a. 12 years without using guard column (from National Research Institute for Child Health and Development, Ohkura, Setagaya-Ku, Tokyo (2001) to my Arbeit at The University of Tokyo, Bunkyo-Ku, Tokyo (2013); my unpublished experience).

This is not my propaganda but my experience.

By the way, it is recommended that "A Model ultra-low-volume pre-column line-filter (GL Sciences, Tokyo, Japan) was inserted between the injector and the column" (personal communication kindly provided by Dr. Tomio Fujita, Shimadzu Co., Kyoto, Japan (at Tokyo Research Laboratory, Shibasaki, Chofu-city, Tokyo, Japan) in 1986; please see file, JCB Fucoidan active transport).

@ Mark wrote " about income for the column manufacturers; the price of guard columns is ridiculous!"

I totally agree with this observation. However, with an investment of less than $ 6-7k, you can pack your own guard columns. I think one guard column costs about $ 200, which is perhaps $ 100 (max) in reality. 

The guard column will be "dirty" only the first mm so you don't need a long one and I used the Securityguard cartridge format (from Phenomenex which comes in pack of 10 and you can check when you need to change it.

Are there any more journals on this testing their protectiveness and efficiency with proof of efficiency and how low it can be?. I tried keywords "Guard column". "Serial column. Liquid chromatography" in web of science and google scholar. 

I do not see how the paper you reference is related?

Mel: A guard column is a generic term to describe any "column" which is placed in front of the actual method (analysis) column to protect it. We characterize columns, not 'guard columns' so I am unaware of any articles designed to compare or describe them. Normally, a guard column only contains a filter or a tiny amount of stationary phase and the goal is to make them invisible (you may introduce more error by adding them). They add delay volume and can result in band spreading (undesirable). Regarding efficiency, you asked "how low it can be"? That question can not be answered as guard columns are not used for (and should not be used for) separation, so 'efficiency' is not used. This is a small filter we are talking about. 

To generalize, Guard Column dimensions should be very small as we do not want them to negatively contribute or impact the chromatography method being used at all. They principally serve as an expensive filter or solvent saturation device (esp in NP) only. Normally, they contain the same packing material as the working column (so as to NOT change or interfere with the actual method). They are NOT replacements for an actual HPLC column at all. We do not normally use them in method development. "Efficiency" is not relevant and measuring it also is not relevant. Those metrics are reserved for actual columns used in the analysis, not a pre-filter such as a guard column. Guard columns are normally described by their packing material or frit size details (support type, pore size, functionality and particle size for "column" type guard columns).

Why are you interested in these accessories? We do not use them for analysis alone.

I haven't seen anything but they are basically supposed to guard (because they are cheaper) your main analytical column from all the crap that is injected into the HPLC.  You do realize that B-Lactams have to be manufactured in a dedicated facility (no other drugs).

Just my dollar 0.02: perhaps some confusion may sometimes be generated by the term "precolumn", which, in preparative HPLC, is sometimes used as a means to enhance purification/separation, this being either a short column of the same packing and added in front of the main column (say 50x50 + 250x50 mm combo), or a precolumn containing suitably selected different stationary phase.

However, as Bill very correctly described it, for analytical HPLC purposes, the role of a guard column is... to guard.

Wow, I was away and missed this juicy conversation and wanted to join. Here's is my two cents. Using a guard column is like wearing shoes or walking barefoot into a wood. You have a choice and both will work. However, you will get hurt more easily without the shoes. As I said above, the contamination will occur only the first 1 mm (no experiment shown), to compromise between protecting the column and reduce void volume, I use a guard disc from phenomenex (SecurityGuard) whcih is relatively inexpensive, easy to change, and you can see how dirty it is by just looking at it and yes, observing the increasing pressure and poor peak shape. I am not trying to change your opinion, but just showing mine.