"front tailing" peak problems

[matthieu]

Hi all,

I'm looking for help on a situation I had recently.

I'm experiencing front tailing of peaks in my chromatograms.
I'm performing C14-C16 in hexane analysis; conditions are as following:

GC Varian 3800.
Liquid Injection 3µl, split mode (ratio 30/1)
Injector temp 250°C
Oven temp 160 to 180°C
detector: FID temp 280°C
Carrier Gaz: helium, flow 2ml/min
Column: HP1 MS 15m*0.25mm*0.25µm

The GC has 2 lines, exactly identicals. I tested on the front and the middle lines and noticed the same front peaks problems (with the same column).
The injections are done with CTC Combipal sampler, syringe was changed but no improvements were seen.
I unfortunatly cannot post a chromatogram...

Any help will be welcome!

Cheers,

Matthieu

[zokitano]

Matthieu,

What is the concentration of your standards/samples?
Do you get a huge response (signal) from the detector?
Maybe you're dealing with column overload. You use narrow column (0.25mm) so the loadabilty of the column is less compared to megabore column (e.g 0.53mm i.d).
Please provide more information about sample's/standard's final concentration (before injection).

Regards

[Peter Apps]

Try injecting less and increasing the split ratio.

Peter

[matthieu]

Samples are used to test linearity of the fid,
We make 5 dilutions (5, 10, 25 and 50%) from a 100% solution; original concentrations in the 100% solutions are 0.2 g/L for each alcanes.

The FID gave correct signals (peak areas are around 150000 µv/s for 100% samples)

I also thought about column overloading but I didn't see any better peak shape by injecting lower concentrated samples or by injecting less (1 or 2µL instead of 3!!)

Best regards;

Matthieu

[Peter Apps]

At the concentrations that you are using I doubt that you are seeing concentration overloading. A solvent effect is possible given the larger than usual injection volume, but less likely at the high oven temperature.

What exactly do you mean by front tailing ? - if you cannot post a chromatogram we need a really good description.

Peter

[matthieu]

a description is very difficult as my english is very rusty

let's say that the signal start raising quite slowly and then rise "normally" (i have big, clear, well separated peaks). Thus I clearly see a change of slope in the first part of my peak. However the second half of peak (return to baseline) is going well and looks "gaussian".

Consequently I had a rsd of around 2% on areas instead of the classical 0.5%.
The error made is then not so big but peaks really look bad!!

what do you mean by solvant effect?

Best Regards,

Matthieu

[chromatographer1]

A solvent effect is a disturbance of the analyte plug caused by a large amount of solvent covering the initial length of the column's phase and not allowing the plug to be focused sharply. This 'smears' the plug and gives a broadened shape of the plug until the solvent moves ahead of the plug and allows proper contact with the column phase surface. The result is a distorted peak shape.

This effect can occur when too large an amount of solvent is injected onto a column with small diameter or thin layer of phase.

Peter's advice is well given. Reduce the amount of sample placed on the column. Increase your split ratio to 100 or 200:1.

This FID test mix is usually performed with a 0.53mm ID and 1µm film column.

best wishes,

Rod

[AICMM]

Matthieu,

I agree with increasing split ratio or, especially, decreasing injection volume. According to my interpretation of your concentration you are shooting 3 uL of a 200ng/uL standard (worst case) at split 30:1. That is only 20 ng on column which means that the 0.25 film/id column should not be overloaded. Another possibility is that you are exceeding the expansion volume under these conditions especially if you are using a narrow split liner.

The other thing I would try would be a starting temp of 50C and see what that does. Starting below the bp of hexane could refocus your analytes and give you very sharp peaks.

Best regards.

[chromatographer1]

Just for sake of you chromatography history buffs, when the situation is one where the peak can be described as having "frontal tailing", that peak distortion is called
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wait for it
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really, it is the term originally used......
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it is a BEARDING PEAK.

best wishes,

the bearded Rod

[shaolinseperation]

i call them shark fins

[matthieu]

Hi,

thank you for your input.
I'll give a try next time about increasing split ratio to 100:1..
we'll see!

Cheers,

Matthieu