Chromatography Forum

Hello, I am a new member and am hoping to learn from some more experienced chemists here. I am running an Agilent 7890A GC with 5975C mass spec. I have always had very poor resolution from this machine. I changed my column, and cleaned my source, and did basic inlet maintenance. Apparently this has been an issue long before I started here. Compounds that I should be able to detect down to AT LEAST 100ppb, if not lower, I can't get good identifications below 1ppm, and even at 1ppm sometimes my peaks are indistinguishable from baseline. We usually don't use our GC/MS on a daily basis, but we are going to start using it more soon, so I need to get this figured out. I do always autotune before use and check water, air, EMVolts, etc. before I start up. Any assistance you can offer would be very helpful. Thank you in advance.

Column: HP-5MS 30m length 0.25mm I.D. 0.25 micron film thickness

We have an autosampler but I usually manually inject because I use the autosampler on the back side of our other 7890; however, this problem existed long before I started doing manual injections. I actually like manual injection, call me crazy but I am not quantifying at this stage so have no reason to worry about reproducibility (not that automated injections are any more reproducible than mine have been!)

One thing that is odd about our setup - we use hydrogen as carrier gas. Is this my problem? This was set up before I came here. If I want to change carrier gas, now is the time to do it because the end of the fiscal year is fast approaching and I can write it into the budget for next year.

Any other information you need, just ask, I am an open book Please forgive anything I say that sounds really stupid, I am only 2 years in this position and make "rookie mistakes" from time to time, this is why I ask for help. Many thanks!

Hello

First of all it'd be good to show us autotune report screen (make screen and paste it here).
Without this it is almost immposible to diagnose problem. There are so many factors that can be problematic.

Regards

Tomasz Kubowicz

OK, I am trying to post the image. Not entirely sure what I am doing here so patience please.

Hello

Autotune looks fine.
Problem can be with GC method (inlet settings) or with MS parameters (scan, SIM). You need to provide more information about method and compound so we can check it step by step.

Regards

Tomasz Kubowicz

Our method is for testing for pesticides in essential oils. We look for several different ones, like OPP, Bromopropylate, Phthalates, Prochloraz, Dicofol, Pirimiphos methyl, Tetradifon, you can see all in the SIM group table although they are abbreviated.

GC Inlet settings:

Inlet at 235C
Pressure 24.05mL/min total flow 1.4869 psi
Septum purge flow at 3mL/min
Gas saver off
Splitless injection
Purge flow to split vent 20mL/min at 1.5 minutes

MS we acquire both Scan and SIM data.

Scan parameters: Start at 5.00 minutes (long solvent delay on purpose) start at mass 35 end at mass 410, sampling rate 2^2

SIM parameters here is the SIM group table:


Is this enough information?

Hello

Could you also paste chromatogram please?
Thanks

Tomasz Kubowicz

I'm wondering about your oven temperature program... and the injection port temp may be a little too low, but I don't think that is the problem...

Tomasz, does that flow rate seem too high to you? I don't use hydrogen, so I'm not sure...

This is the standard I ran yesterday. It is a 50ppm standard made in our typical material, so it shows large peaks for our analytes.

Here is our oven temperature program:

Start at 70C
Ramp at 15C/min to 150C, hold for 12 minutes
ramp at 20C/min to 200C, hold for 2.3 minutes
ramp at 35C/min to 300C

Hello

You should try pulsed splitless injection as it is very useful for unstable compunds like you have - you minimise time that analytes spend in inlet. Check your GC manuall for correct settings.
I would strongly recommend ALS using.

Regards

Tomasz Kubowicz

Thank you for the idea. I will try it. I can put the ALS back now that I am using this machine more, I didn't use it at all for quite a long time and so was using the tower on another machine. I'll put it back today.

Can you suggest a starting point for the injection pulse pressure?

Hi

Peaks on SIM chromatogram for 50ppm are quite big...so what is your real problem? which compound you have problem with?

Tom

It's fine at large concentration. It's when I get down to below 1ppm I have issues. I don't have a low standard that has been run recently, I can dig out an old one and paste that chromatogram if it helps.

One compound in particular that I should see at 100ppb is ethion. I can only see to 1ppm for that compound. All compounds should show up at below 1ppm and none do, so the problem is across the board.