Ghost peaks

[WillyOne]

I'm very sorry. I do not understand the procedure to include images in a message.
I'm using a 6890N Agilent GC column is 30 m 0.32mm, 0.5µ thin film of a 5% Phenyl-95% methyl silicone bonded phase.
Detector is a FID and injection is 1µl of air (empty vial). Condiitons are Carrier He 1.5 ml/min constant flow, H2 40 ml/min, air 400 ml/min, make up 48.5 ml/min of He. Split 1:100.
Temperatures Inlet 100°C, Detector 280°C and column 50°C during 30 min, then 20°C/min to 250°C, wait 30 min.

I'm finding the strange peaks you can see in the image. They are not column contamination because their shape is independent of the temperature.
Perhaps someone of you can help me to fix the issue.
Thank you so much :





http://farm9.staticflickr.com/8236/8502 ... 284a_b.jpg

[WillyOne]

I have found it. Please see here the chromatogram

img001 por WillyOne, en Flickr

[chromatographer1]

You have an electrical problem, possibly outside your GC. I would guess another instrument or an outside lab is causing your power supply to vary. Or a power supply within your GC is causing the voltage output of your detector to vary. It is not a chromatography problem.

best wishes,

Rod

[dblux_]

What is the source of air to FID ?
Cylinder or compressor ?
If compressor, how often does it switch on ? Can you find a corelation with baseline disturbancies ?

[cleh]

Try baking out your column at 300 degrees for about an hour. Please let us know if it works.

[dblux_]

I'm very sorry. I do not understand the procedure to include images in a message.
...
Temperatures Inlet 100°C, Detector 280°C and column 50°C during 30 min, then 20°C/min to 250°C, wait 30 min.
...

Don't you think your inlet temperature is to low compared to your temperature program.
Can you increase it to 200 degC ?

And please, only one change of parameters at time. When it doesn't work - try another change. Be systematic.

[WillyOne]

Thanks a lot you all.
Rod, Dblux, electrical supply comes from an uninterrupted supply System that services all Chromatographs at the Lab. Means that supply is a filtered and stabilized one. Any way it could be an internal problem of the 6890N.
Otherwise it seems clear that this is a detector problem, so I will check the supply of air at Lab because I think it comes from a generator system that perhaps needs maitenance.
If it is not the problem, I will call for Agilent assistance.

Cleh, I have purged the column several times before arriving to that chromatogram collection. Anyway, If it was a contaminated column problem, peak shape should change with temperature. But that I observe exactly the same shape at 50°C than 250°C.

I will keep you all informed.
I'm sorry, my english skill is not really high

[chromatographer1]

If your supply of air is not consistent, that is a possible cause for your baseline rise and drop.

It is something causing the signal output to change, either electronic within the GC or a hydrogen or air flow variation from some cause which is affecting the flame and the signal output. To be so regular in size and timing indicates it is a non-chromatography problem. No need to check or replace column at this point.

best wishes,

Rod

[dblux_]

To be sure that your problem is not column or inlet related you may remove column from FID, cap FID with "no-hole" ferrule and run your method several times.

[GasMan]

You need to approach this systematically.

1. Is is a flow/ detector problem or electrical. Turn the hydrogen and air flow off and then check your baseline. If the problem is still there it will be an electrical problem.

2. If step 1 shows that it is not an electrical problem, do as dblux_ suggests. This will tell you if it is a detector/flow problem.

3. If step 2 shows no problem, then you have a column, inlet problem.

You have something changing every 20 minutes. The chromatograms suggest to me a flow problem, electrical problems do not normally show gaussian peaks. I would look at the hydrogen supply and make up supply. On Agilent FID detectors, the air flow is not that critical at the flows you are using. If you plot the detector output signal against air flow, you will see it increase and then levels out. However, hydrogen flow is more critical. For a given carrier gas flow, the detector output will increase with increase in hydrogen flow, reach a maximum and then decrease. The Agilent FID will give best results with a total carrier gas flow ( column flow plus makeup flow) of about 30ml/min and the hydrogen flow should be the same as the total carrier gas flow. I would suggest that you decrease your make up flow to 28.5 ml/min and also drop your hydrogen flow accordingly. However, these changes are not the casue of your probem, it just means that your FID will more sensitive to flow changes.

I would also check the pressure regulators that you are using to supply your hydrogen, air and makeup gases. I have seen regulators which are sticking, and the pressure needs to drop to overcome internal friction in the regulator which causes the regulator to 'jump' at periodical intervals.

Good luck with your trouble shooting.

Gasman

[dblux_]

You need to approach this systematically.

...

You have something changing every 20 minutes. The chromatograms suggest to me a flow problem, electrical problems do not normally show gaussian peaks.
...

100% agree. That's why I omitted your step nomber 1.

BTW - good job Gasman. I hope it will help WilliOne to find the culprit.

[WillyOne]

Thanks a lot you all.
This morning we have checked other GC that are connected to the same Gas piping lines. They are a 3400 and 3800 Varian GC.
All of them showed the same symptoms.
After that we have unplugged the sytentic air generator and installed a couple of cylinders for air supply.
I have just had time to see one and a half runs, but the issue seems have been dissapeared.
I will wait the complete sequence before affirm that everything is OK.
If so happens, next step is to call the air generator technical service.

Again thanks a lot

[dblux_]

bingo

[WillyOne]

Aire-2001 por WillyOne, en Flickr

Happy end. The syntetic air generator has been declared guilty.
The first chromatogram corresponds to changement to air cylinders.

Thanks people

[dblux_]

Thanks for information.