Chromatography Forum

Hi Everyone,

Our forensic chemist has his lab doing blood alcohol content with our headspace autosampler. Their data looked great and I have to say that I actually resurrected the headspace by following some advice on the forum!

We're using a method with parameters:
Agilent 7890 GC
Inlet temp: 250 C
Split ratio 20:1
Column Flow: 12 mL/min (He)
Column Dimensions: 30m x 0.53mm x 3.0 micron
Oven temo: 40 C isothermal for 5 mins

FID:
Makeup (He): 35 mL/min
Air: 400 mL/min
H2: 30 mL/min
Temp: 300 C

To further summarize the issue we had a 65 sample sequence. I observed that the FID would extinguish during the GC post-run and then have to reignite during the pre-run. I asked Agilent about it and the engineer said that since there's no erroring out there isn't an issue. When I said that the FID extinguishing and reigniting 65 times seems bizarre he was silent. A video is below:

https://drive.google.com/file/d/1AeSTfk ... sp=sharing

It seems like there should be a way to keep the flame on from the start of the sequence to the very end.The output isn't going below the lit offset, so I don't think that's the issue. It seems like it would be a method parameter that in the post run? But I can't find that. I used this version of ChemStation when I was in industry, but that's been too many moons ago, sadly.

We're using the older MSD ChemStation E.02.00.493. Any ideas would be appreciated!

...
Column Flow: 12 mL/min (He)
...

Hi. What column ?
Isn't it huge helium flow blows out FID flame ?

Agilent likes 40 H2 and 400 Air (due to big FID size)

35 ml/min makeup - is blowing away flame

...
Column Flow: 12 mL/min (He)
...

Hi. What column ?
Isn't it huge helium flow blows out FID flame ?

I can't believe I forgot the column dimesnions in my original post... sigh.
It's a megabore - 30m x 0.53mm x 3.0 micron.

Agilent likes 40 H2 and 400 Air (due to big FID size)

35 ml/min makeup - is blowing away flame

I'll try adjusting those as you recommended and update the post. If I remember correctly, we always used 30 mL/min in industry - I think. I can't remember to save my life.

It's a megabore - 30m x 0.53mm x 3.0 micron.

Recommended carrier flow for Megabore is 2.9 to 4.2 mL/min.
At 12 mL/min linear velocity in the column is 76 cm/min which exceeds optimal velocity for helium.
BTW - Agilent recommends for FID:
H2 - 30-35 mL/min
carrier+makeup - 30-35 mL/min
air - 400 mL/min

General rule is to avoid fuel lean and fuel rich ratios and to avoid high velocities at the jet orifice, otherwise flame won't be stable.

mburleson,

A megabore can handle 12 ml/min no problem, we use to run them much higher back in the old days. They were meant to replace packed. That being said, you have a lot of flow through the jet in this configuration so do you have a capillary jet or packed jet? With 12 on the column you probably do not need that much make-up.

The fact that it does it consistently (35 runs/35 FID off) makes me suspicious of a post-run command or macro designed to put you in standby. Then when a new sequence run is initiated the flame is asked to re-light. (I would look at headspace method as well for this type of command.) The other question would be if there is a valve switch at the headspace at the end of the run which could, potentially, puff out the detector?

Finally, I would recommend a FIDIGbolt modification to your FID castle to ensure it will light every single time. At 300 C the odds are good but a FIDIG will ensure ignition every time on an Agilent. Besides, you could save a lot of money turning the detector off while idle -especially since you are using He make-up.

Best regards,

AICMM

I seem to recall this happening to me. What is your Lit Offset value? If it's too large and your background is too small, the instrument will think that your flame has gone out and it will try to light again.

Go to "Config" button on the front of the GC and find the detector. Scroll down and select the problem detector. Make that value smaller and see if your problem goes away.

I had to change it to 1.0 on one systems because it had an extraordinarily low background level at my starting temperature.

I'm thinking turn down the makeup to around 20 to get your carrier + makeup into the preferred window...