Chromatography Forum

Hi everybody.

I recieve a new Agilent 6850 last month, and when I want to analyse my sample it appears a problem. I think it's a leak on the split but not sure.

The baseline after the solvent peak goes under 0, and this isn't normal for this method

see below the chromatogram



The area are 4 times lower than another Agilent GC (one 6850 and one 7890).

I changed the septum / liner / O-ring / etc. Base maintenance was effectued but the problem persists. I can call the Agilent technician, but I want fix it by myself and understand the problem.

Method works perfectly on two other GC, so I know the problem went from the new GC and not from the method.
Carrier gaz: H2 / Split: 1/20

Can anybody help me?

Thanks
Jeremy

When you inject, watch the inlet pressure. Does it change? I assume that the phenomenon does not occur if you merely push start (no injection).

If you change the split ratio, does the magnitude of the problem change or does it stay the same?

I have had a similar issue. In my case the problem was a Flame tip on the FID not tightened.

@rb6banjo

Inlet pressure: no variation (except due to heating)
Blank run (condition only, no injection): perfect baseline
The problem stay the same with different ratio (10/20/30/40/50)

This morning I'd retry to tight different valves I can access and change one more time the O-ring (another O-ring batch), and my area go back to a normal value (same as my other GC). But baseline problem persists.

@WillyOne
Jet is new, good tightened. Detector corps too. Do you see another thing to tight on the Agilent 6850? (I never go under the jet in any maintenance)


Thanks for your two response, have a nice day.
Jeremy

That's really odd. I don't think I've ever seen that happen in my 25+ years of GC analysis. FID is a mass sensitive detector so somehow, there's less "mass" getting to the base of the flame at about 0.5 minutes.

Does it matter if you do a split or splitless injection? Does anything change if you change to one or the other?

Same problem with splitless injection.

Check that there is no programme entry that zeros the detector during the run.

Peter

Autozero is at 0 min. No other zero entry in the method.

Hi Jeremy

Please post all your operating conditions; temperatures, flows, pressures, injection volume etc and the column dimensions and stationary phase. What is the solvent and what are the analytes ?.

Peter

Instrument: Agilent 6850

Detector: FID

Column: DB-5, 15m ; 0.32mm id, 1um

Oven temp:
Isothermal 180°C, 1 min
Program 180-230°C, 10°/min
Isothermal 230°C, 7 min
Program 230-280°C, 20°C/min
Isothermal 280°C, 5 min

Injector Temp: 250°C

Detector Temp: 300°C

Injection mode: split 1/20

Carrier gas: Hydrogen constant flow 2ml/min
Makeup: Helium, 30ml/min

Injection volume: 1ul

Solvent: Isobutylmethylketone
Analytes are Docosan (ISTD) and a fungicide.

But as I say, method works perfectly on two other instrument in my lab.

Nothing in the method is an obvious cause of the baseline step. It is the signal before the solvent that is odd and the solvent peak is suspiciously narrow - the rest of the chromatogram looks normal. Check for a change in detector attenuation or range.

Does the time between the end of one run and the start of the next make any difference ? If you are cooling quickly after that very high final temp you might still have column bleed coming off when you inject.

Is the step visible is you get your analyte peaks on scale ?, and does it make any difference to the results ? I suspect not.

In general with a new instrument you need to get your money's worth out of the guarantee.

Why are you using helium as make-up ? Nitrogen would work better and be much cheaper.

Peter

Thanks for your time Peter

Does the time between the end of one run and the start of the next make any difference ? If you are cooling quickly after that very high final temp you might still have column bleed coming off when you inject.

--> I have the problem with the first injection the morning.

Is the step visible is you get your analyte peaks on scale ?, and does it make any difference to the results ?

--> No, this is not obviously visible. The signal is just at -1pA after the solvent peak. I don't have any difference on my results, my RSD is under 0.2% and I have the same results with my other GC.


Yes, I can use the guarantee. I don't use it before beacause of good RSD since I made a big maintenance on this GC. If the technician went and change all the pieces it didn't explain me what is the problem. This is the reason beacause I want fix it by myself, using help I can find on this forum.

Helium beacause no Nitrogen installed in my lab to my dismay.

I just check my solvent area, and on this GC it is 1074 pA*min. On my two other GC, it is around 6500 pA*min (same GC model). But the areas of my ISTD and analyte are the same.

I'm sorry, I can't explain , but it seems that it happens some kind of flame collapse due to changes in amount or composition of mixture of gases arriving the flame tip.
Perhaps you should look at the detector connections: column, make-up, air or even the FID tower fixation...

Hi Jeremy

One reason to get this fixed now under guarantee is that it might be a symptom of a problem that will become more serious later, when you will have to pay to get it sorted out. I always watch what service techs do and get them to explain what and why.

Peter

Another thought

What happens if you run a blank with no injection (or a zero volume injection, or an injection from an empty vial) ?

Peter