Chromatography Forum

Hi,
I am currently developing a method for simulated distillation of crude oils.
I am running a system blank and getting a very poor baseline (Please click on the link to reddit for an image of the baseline)
https://www.reddit.com/r/CHROMATOGRAPH ... baseline/
it is repeatable. I have tried baking the column and inlet, changing over consumables and adjusting FID temperatures, all to no avail.
does anyone have any recomendatios.
Thanks,

What are your temp program parameters?

It almost looks to me like you have something sticking around on the column that's coming off at higher temperatures.

I know you said that you tried baking, but my inclination would be to do it again at a temperature 10-25º higher than the highest temperature of your run(assuming your column will allow that) and leaving it there at a high flow rate for a couple of hours. I'd also try solvent flushing the column, but I've also been looking for excuses to play with my apparatus for doing that

Have you injected any crude samples into it? I'd have my concerns over the tars and other very heavy components sticking around. They SHOULD in theory stay on the liner(although it's also worth checking/changing the inlet seal if your GC has one and it hasn't been changed) but could also migrate into the column. If that's happening repeatedly, it might be worth dropping your inlet temperature a bit(if possible) to keep the heavy stuff from migrating to the column.

Also, you don't say what solvent you're using. When I've done SIMDIS in the past(on biodiesel in my case) we prepped everything in CS2. That can be a bit of a cantankerous solvent. As part of troubleshooting, I'd probably try injecting a different solvent with as close of a BP as realistically possible(maybe pentane, or possibly DCM) and seeing if it gives the same result. That can rule out possible solvent contamination.

Hi Ben thanks for the reply,
The method parameters are as follows.

Column Agilent J&W – DB-HT-SIMDIS 5 m x 0.53 mm x 0.15 µm
Autosampler 10 µL syringe
0.2 µL injection volume
Oven
Operated under constant flow
Initial holfd time 0 min
Temperature range -20°C – 425°C
Ramp rate 15°C/min
Final hold time 10 min


Injector 50⁰C to 425⁰C at 15⁰C/min
Hold at 425⁰C for 5 minutes
Carrier Helium, 20 mL/min
Detector FID at 430⁰C
Air: 450 mL/min
H2: 45 mL/min
Make-up:15 mL/min

yes I am using CS2 as my solvent

I will give your suggestions a go
Thanks,

Chemiger

I'd forgotten how extreme the temperatures in SIMDIS are...

From what I can find, the maximum isothermal temperature for that column is 400º, so a several hour long bakeout at a temperature higher than your analysis temperature is not really feasible(without wrecking the column at least).

You might try ramping to 430º(the temp programing limit for that column) and holding there for 5-10 minutes repeatedly, but I wouldn't want to spend an extended amount of time at that temperature.

One other thought-what does an air blank, or a run with no injection at all, look like?

yeah extreme at both ends of the temperature scale.

I don't know if you clicked on the reddit link but that chromatograph was the baseline blank.

Thanks,

Chemiger

Chemiger,

How many pAmps is the baseline rise at the end of the run? The scale to the left does not tell me. HT SimDis has a baseline rise at the end no matter what - even the roubust - 1's bleed some at that final temperature. The good news is that most of the crude is off by that point. The gunk in the middle is probably inlet "hairball" which is also not that surprising considering the application. So... if the baseline rise is on the order of 15-30 pA, then I would argue there is nothing wrong here.

Of course, you can help yourself by making sure you have an oxygen scrubber onyour carrier line!

Best regards,

AICMM