Chromatography Forum

Hey all,

I've got a problem I can't quite get rid of. I recently inherited a GC and a method and I cannot for the life of me get rid of fronting on a peak. I've cleaned the injection port (replaced the septum, gold seal and washer and liner with new parts), replaced the column with a new one, increased the split ratio to crazy high amounts, and even replaced the FID jet. Here's the method that I'm using.

HP6890
250 C inlet temp
120-320 oven temp @ 25 C/min
HP-5 column (30 m x 0.25 um x 0.32 mm)
split ratio 300:1
H2 carrier gas
FID
goose neck liner

Here are the offending chroms:

[img]http://s616.photobucket.com/albums/tt242/GC_User/?action=view&current=Image001.jpg[/img]

[img]http://s616.photobucket.com/albums/tt242/GC_User/?action=view&current=Image002.jpg[/img]

What is the solvent and do you know what compunds these are?

The solvent is pyridine and the compound is a derivative of a lactam that has been silanized. By the way, are the pictures showing up in the post? I was having internet connection difficulties earlier. I can't speak too much about the compound itself, but I was assured that such peak fronting was not there several months ago. I've only been working on this project for about a week and a half now and the issue came up before I started.

Your chroms aren't showing up in your post. Are you having fronting on one peak or all your peaks? Have you tried adjusting your injection temp and how does that affect your peak shape?

Just thought of something else, you could try running a guard column and see if that works better.

We really need to see the chromatograms.

Peter

You can copy and paste the links into your browser and see the images, but the image quality is poor, it's hard to see the peaks.

The peak looks really big, I am thinking column overload. have you tried diluting?

Thaks Larkl, now I can just about see them.

This does not look like overloading to me - that would give a straight profile to the leading edge, whereas what we have is a curve (diificult to tell though becuase the peak is so far off scale).

Considering that this is done with a 300:1 split (which might cause its own problems) the amount injected must be huge, because this peak is way off scale. If fact, with minimum detectable quantity not an issue, so the peaks can be made as big as you need, then little bits of distortion at the base of the peak are irrelevant - they do not make any significant difference to the integrated peak area, and you do not have any peaks close to the target peak to make squeezing the last bit of resolution a necessity. So to make the tails go away just display the chromatogram with the peak on scale.

Peter

I've adjusted the injection port temp on levels down to 150 C and the fronting doesn't go away, it doesn't seem to affect it at all. I've also tried diluting by quite a bit. The method called for 0.5 mL of sample mixed with 0.5 mL of BSTFA in a 2 mL GC vial. I recently tried 25, 50, and 100 uL of sample into a 0.5 mL of BSTFA and 0.5 mL of pyridine, and while the peaks get smaller the fronting still exists. I'll try to post better pictures son.

Oh and it's 1 uL of a 5,000 ppm sample.

Hi GC-user

Do yourself a big favour and stop worrying about minor deviations in peak shape that will not make any difference to your final result. With peaks as big as these you do not need to be blowing the chromatogram up until you can see the baseline noise.

If you were scraping the bottom of the barrel of ultra-trace analysis in complex mixtures these peak shapes might be a problem, but you have nice concentrated samples with few components, so don't worry, be happy

Peter