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Front Tailing HPLC & GC Peaks

Mar 8, 2022

What are fronted tailing peaks in HPLC and GC?  Why do they occur in your chromatogram?  How can you reduce them and do better chromatography?

Before I talk about front tailing HPLC peaks, aka peak fronting, let’s differentiate peak fronting with peak tailing.

What is a peak tail?  That is when the back half, or the later half of a peak tails.  Check out my video HPLC peak tailing for a deeper dive. What is a peak front? That is when the front of the peak – the early part of the peak angles forwards towards the beginning of the chromatogram.

Fronted peaks are very unusual, we aren’t used to seeing them, so when you see one it should jump out at you.  A lot of people describe them as ‘shark fins’ or ‘sailboats’.

What causes front tailing HPLC peaks? & GC peak fronting?

If you see fronting, it is almost always caused by column overload.  This is true in both HPLC and in GC.

I like to think of a column as having ‘X’ number of sites.  Let’s say it has 1000 sites.  It’s got way more than that, but let’s call it 1000 sites. 

That means that the column has room for 1000 molecules.  If you inject 1001 molecules, that last molecule has no place to stick.  It cannot interact with the stationary phase because the first 1000 molecules occupy all the sites on the column.  So that last molecule moves forward to the front of the peak.  It does not slow down as the other 1000 molecules do, so it comes off the column first. 

That is why we tend to get a front on the peak if you overload the column.

Overloading the column does not hurt the column, does not damage the column, do not worry about it.  Why you should worry about it though is that it results in a lousy peak shape. 

If you are interested in quantifying that peak – measuring that fronting peak – you would prefer that it doesn’t front. 

How to reduce peak fronting in HPLC and GC

So the easy solution is to dilute your sample. Do a 1 to 10 dilution and the fronting goes away.

Fronting in HPLC is fairly unusual.  It is almost always caused by column overload.

There may be another answer to the question ‘why are my HPLC peaks fronting?’ 

Some give the answer, ‘Well you have forward tailing peaks because you actually have two peaks.’ I think that is a dumb answer becauyse you should know the difference between two peaks that are overlapping and one peak that is fronting.

But if it is one peak that is fronting, dilute the sample and that will solve the problem.  Or inject less; do a one microliter injection instead of 10 microliter.  

The same is true in GC.  Use a higher split ratio to inject less.  Or inject less volume.

GC Troubleshooting – GC Peak Fronting

One other odd thing that produces peak fronting in GC:

It’s fairly unusual but it could be that your oven temperature is too low.

If you are running GC isothermally (constant temperature) and you notice that the first couple of peaks look beautiful, but the further out you go the more of a front you are seeing on each peak, that is a temperature issue.  So if you see that in GC that means that you need to raise your temperature.  It is a fairly obscure thing so don’t focus on that too much.  But most of the time when you see fronting peaks the sample is too concentrated – you put too many molecules on the column so dilute the sample. 

I hope you have learned to identify a fronting peak in HPLC and GC, and how to solve it.

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