Peak tailing

Discussions about GC and other "gas phase" separation techniques.

18 posts Page 1 of 2
Hi all,

I would appreciate some advice on peak tailing I am noticing in some of my runs.

I ran a reference standard through our GC and the resolution on higher BP alkanes is not too good. Resolution is great up until around C12, and tailing starts to become an issue after that. See the image below. It shows C12, C14, C16, C18, C20.

http://imgur.com/a/Tdt9s

What could be the cause of this, and is this a significant issue? I have noticed no tailing in standard sample analyses, and these include compounds across the BP range higher and lower than these alkanes. I have not noticed tailing in the alkanes present in my samples, though there is usually a high background profile that may be disguising it.
Hello

There are many possible sources of peak tailing.
I'd recommend:

1.Replace inlet liner (perhaps it has many active sites). You can also clean inlet
2.Cut first 10-20 cm of your column from inlet side
3.Make sure column is properly cut and has correct distance from ferrule tip (for Agilent S/SL is about 4-6 mm)

Good luck

Regards

Tomasz Kubowicz
I have noticed no tailing in standard sample analyses, and these include compounds across the BP range higher and lower than these alkanes. I have not noticed tailing in the alkanes present in my samples, though there is usually a high background profile that may be disguising it.


Could you elaborate?

So a solvent standard = tailing (like in your image)
Sample with same alkanes spiked or present = no tailing

?

If this is the situation, it is not uncommon in GC. Much more common however for stuff like pesticides compared to rather inert alkanes. It points towards activity in your system (can be anywhere like the poster above me said). Sample matrix (what kind of samples are you running?) protects your analytes from active sites in the column/inlet and give sharper peaks compared to solvent standards.

I would perform maintenance (see above). Visually check the old liner, inlet and column piece for septum particles, these are typical sources of tailing for high boilers. Also put a new septum above the inlet, and don't overtighten it.

and is this a significant issue


You're better off without it, but it doesn't make your data worthless. It makes the integration of peaks annoying, you'll have to pay attention to integrate the same peaks to the same extent troughout your calibration. The evaluation comes from your QCs of course.
Thank you both for the advice; next time I do some maintenance I will change out the liner and trim the column.
An update on this.

I changed the liner out and tailing has improved, but I still see small tails on C16, C18 and C20. Inlet and liner are both clean. The sample is a CRM diluted in DCM, so there shouldn't be any activity associated with the sample matrix.

What should I focus on next? Trim the head of the column?

http://imgur.com/a/bcT3W
cene wrote:
An update on this.

I changed the liner out and tailing has improved, but I still see small tails on C16, C18 and C20. Inlet and liner are both clean. The sample is a CRM diluted in DCM, so there shouldn't be any activity associated with the sample matrix.

What should I focus on next? Trim the head of the column?

http://imgur.com/a/bcT3W


Yes. Did you change the septum? Did you see septum particles in your liner?

If you trim the column, it's a good idea to check the old piece of column under a light microscope to learn something. Do you see dirt or particles in there? Was the cut well made?
Rndirk wrote:
cene wrote:
An update on this.

I changed the liner out and tailing has improved, but I still see small tails on C16, C18 and C20. Inlet and liner are both clean. The sample is a CRM diluted in DCM, so there shouldn't be any activity associated with the sample matrix.

What should I focus on next? Trim the head of the column?

http://imgur.com/a/bcT3W


Yes. Did you change the septum? Did you see septum particles in your liner?

If you trim the column, it's a good idea to check the old piece of column under a light microscope to learn something. Do you see dirt or particles in there? Was the cut well made?


Thanks for the advice.

The septum was already new; I changed it before I changed the liner.

The liner looked a little dirty, as was typical for a used liner, but no septum particles or other large particulates were present.

I will trim the column next time the instrument is down and see if things improve.
What are your analytical conditions ?, flows, temperatures, column diameter and length ? Without that information everyone is just guessing what the problem light be.

Peter
Peter Apps
While in complete agreement with Peter, I will hazard a guess as well and ask you what your injector temperature is. You have to be hot to keep from tailing on higher BP alkanes, plus it helps to work in split mode rather than splitless.
Mark Krause
Laboratory Director
Krause Analytical
Austin, TX USA
Peter and mckrause are right, it could be a problem with ramp temperature in oven too. But without your method parameters, we are only supposing what could be the problem.
Hi all,

I understand - should have provided these details in my first post. Column is an Rxi-5ms 0.25 x 0.25 x 30.

Column flow is set to 1 ml/min (with a 25:1 split). Injector at 250 C and temperature program going from 50 C to 280 C at 10 C/min.

Any advice on problems with the temperature program?
I think a thinner stationary phase wouldn't hurt for stuff like alkanes -correct me if i'm wrong- but I assume you want to keep working with this column.

1) Increase the injector temperature to 300°C.

2) You can speed up this analysis easily, which will also decrease tailing. Increase the flow to 1,5-2ml/min. A good starting point for flow somewhere between optimal speed and separation, if you work with helium in GC is 8 x column diameter (mm). Furthermore, I would increase the heating rate to 20 °C/min.


Apart from that, it's all about having good column cuts, no leaks, clean inlet/liner, no septum leaking into your system, undamaged columns,...
Now that we finally know that there is nothing serious wrong with your analytical conditions I suspect that you have a tiny bit of septum or ferrule, or an uneven patch of stationary phase somewhere in about the first meter of your column.

Rather than running a temperature ramp that is much faster than is optimal for the length of column, you could speed everything up by cutting the column in half.

Peter
Peter Apps
Thank you both for the new advice.

I will try increasing the injector temperature and see what happens. Other than that I would like to keep the analytical conditions the same, as we look at a variety of analytes on this column (not just alkanes) and changing the method would be detrimental to other analyses.

Other than that, it sounds like the column is the only possible site of activity. Next time the instrument goes down for service I will trim the column head and replace all the ferrules. Hopefully that will help.
cene wrote:
Thank you both for the new advice.

I will try increasing the injector temperature and see what happens. Other than that I would like to keep the analytical conditions the same, as we look at a variety of analytes on this column (not just alkanes) and changing the method would be detrimental to other analyses.

Other than that, it sounds like the column is the only possible site of activity. Next time the instrument goes down for service I will trim the column head and replace all the ferrules. Hopefully that will help.


When you replace the ferrules be sure to cut 2-3 cm off the ends of the column after you have threaded then through the ferrule to get rid of any little shavings.

Peter
Peter Apps
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