Horrible Toluene peaks

[travisdh]

Hi All,

I was hoping someone might be able to point me in the right direction in terms of troubleshooting. I have a strange peak (bottom of picture) which I am getting after the stringe bent and it was replaced. Normally the Toluene solvent peak is sort of okay as per the top part of the picture but for some reason it is being a bit temperemental at the moment.

I have trimmed the Column, baked the oven, changed the septa and liner, checked ferrules and the likes, the only thing left i can think of is to replace the column but i would LOVE to avoid that if possible.

Is there anything I may be missing that may cause such a crappy and intense peak?

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[Peter Apps]

First thing when comparing chromatograms is to look at them on the same scale - full scale on the bottom one is five orders of magnitude higher than the top one.

It looks to me as if the toluene peak is eluting later, this may be due to a leak.

Troubleshooting is next to impossible unless you tell us what hardware this is run on, and what the conditions were.

Peter

[travisdh]

Sorry about that, I don't have all of the conditions on me but will have a look for you. It is run on a Varian Saturn GCMS 4000 System. The column pressure is running at 7.6 PSI and is stable with no fluctuations, I checked the tightness of each end of the column in case there was a leak and didn't really detect anything there.

The conditions i do know are Injector Port 280 Deg C, Column Oven 50 Deg C, Column 7.6 PSI, I did have one blown filiment and switched over across to the second one, did autotunes and the likes and all came out alright.

The column is one of the Factor Four Varian colums,
Varian VF-5ms 30 m × 0.25 mm × 0.25 µm

I think it is also a split injection 5 or something similar. We run toluene as an extraction solvent for extraction of Formaldehyde off absorbtion tubes so normally the work is pretty clean, and when not formaldehyde it is Carbon Disulfide as the extraction solvent for carbon powder.

Thanks for any help.

[bhuvfe]

travisdh,

The bottom peak is perfectly normal for a solvent peak (broad because you are overloading the column).
The upper peak is not. Besides the intensity and rt (as Peter pointed out) there's notable tailing (since the column used is apolar I would consider this not normal).

Maybe there was something wrong with the previous settings ? The syringe wasn't injecting? stuff in a dirty liner?

Is the peak shape and intensity of your analytes OK for the bottom chromatogram?

If having a nice peak of toluene is necessary for your analysis you will have to use a split of 1:20. 1:5 for the column you use and for liquid injection is a little bit too much.

I don't know your GC-MS system but generally everybody switch off the detector when the solvent peak is eluting (this will increase a lot the lifetime of the filaments).

Good luck.

bhuvfe

[Peter Apps]

You need to check for leaks with a leak seeker, not just be tightening fittings - how about a leak at the septum ? If the autotune includes an air and water check, and it is passing then there is probably no leak. If the autotune does not include air and water then you need to run that separately.

Next step is to measure the flow velcoity through the column - inject a little bit of air and time how long it takes the peak to elute. With helium as carrier you will be looking for an average linear velocity of 30 - 50 cm/s, so 60 - 90s retention will be about right.

Check that the peak actually is toluene - maybe bottles got mixed up or mis-labelled.

Are the retention times of any other peaks affected ?

What is the retention time of the peak that you are interested in ?

Peter

[AICMM]

travisdh,

Your top peak suggests a splitter that never turns on. Usually you see a dramatic fall in signal shortly after you return to split. Your bottom peak suggests a split injection although you would have to have a tremendous split for that to account for all of the sensitivity difference.

Bhuvfe is certainly correct in the comment about having your filaments off until after solvent elution. I am assuming you are doing derivatized formaldehyde so there should be plenty of time to turn the filaments back on before your peak of interest.

Best regards,

AICMM

[travisdh]

I have done air water checks and each suggest that the Air / Water levels are correct within the system. The Vacuum in the MS is being held reasonabally well, that is it doesen't seem like there is quite a large leak of any sort.

In terms of the actual system, it runs at 7.6psi, with splitless injection. The column is a varian factorfour VF-5ms. I have also tried a new column and new septa as well as liner.

The analysis that is giving me grief is Formaldehyde, it is a derivitisation method that runs with extraction in Toluene solvent, the method was previously set up so I am hesitatant to change it (although I am strongly thinking about it right now!)

It runs for Ionisation off up to 0 - 3 mins, then full ionisation up to 10 mins finishing at up to 30 mins.

Injection Mode: Std Split / Splitless
Split Ratio: 50

Temp Ramp:
70 Deg C for 1 min,
90 Deg C for 1 min,
260 Deg C @ 30DegC/min up to 30 Mins.

Injector Temp = 280 DegC

Thanks for all the help!.

The gCounts have now come down to mCounts on the new column, but it looks like the column is still being overloaded with the sharkfin peak.

[travisdh]

This is the latest peak, and I also ran an air blank to see if there was anything strange showing up.

[chromatographer1]

Since the toluene peak is almost two minutes wide, can I guess how long you are keeping the splitter closed initially? ............... how about almost two minutes.

Even at 50°C toluene will migrate down a thin film column, so why are you surprised at the peak width? Perhaps you are trying to lower the detection limit of your analyte and are increasing the size of your injection and/or keeping the splitter closed longer in hopes of lowering the detection limit?

If you want to put more sample onto the column you have to get the analyte to refocus, and 0.25µm is too thin to do much.

You are also probably contaminating your injector pneumatics with deposits of your analyte and giving a background of it all through your chromatogram. Just a guess, but if your analyte is volatile enough that won't happen.

Just be aware that like the toluene peak, your analyte peak is also broadened. Having a wider peak will not decrease the limit of detection for it.

Good luck with your method development.

Rod