Chromatography Forum

I am running an Agilent 7890 with autosampler and FID @ about 250C Air 300, H2 30 Make-up Gas is set to N2, but the true Make-up gas is He, oven program 40C hold 11 min then ramp @15C to 115C no hold then ramp 25C to 215C hold 4 min. Inlet temp180C Split 3:1. New base deact liner w/ deact wool. New septa. New Syringe. Column: Restek Rtx624 60m, 320um,1.8 df. Carrier He constant flow 5 ml/min. Internal Std= 0.05% n-Propanol in DMSO. Use Int Std to prep an Alcohol Std 95uL/100mL Int Std. Very clean chromatography 10 replicate inj of W.Std yeild a %RSD of about 0.8% Super set-up, super system very reproducible. So my question is what effect does the Make-Up gas set to N2 instead of He have.

The EPC won't give the proper flow for your makeup.

You are running a little rich on the FID. Only 300 air? As long as you are happy with the sensitivity, then who cares? And why are you concerned about the makeup?

Read your manual and follow it if you want the best sensitivity.

best wishes,

Rod

Thank You for the reply Rod. Do you know how the EPC determines the flow to be applied? Also I do not understand your comment about the H2. Sensitivity is good, and typically I run H2/Air 30/300, 30/325, or 40/400 with good sensitivity. One reason for my running 30/300 vs. 40/400 is the conservation of gas. I just cleaned my detector and it was full of soot. An embarasment. I was experiencing flame outs. Do you expect this is due to the richness of the H2, or just poor maintenance? Can you suggest a different ratio, and your reason for this?

Thanks Gus

5.3ctwptr,

I would suspect your solvent of choice for the soot and too low a detector temperature. That is where I would start looking.

Regarding N2 as make-up, there is anecdotal evidence and, as far as I can tell, 1 article that says that N2 is more sensitive than He. If you system is running fine, I would not bother plumbing in yet another gas. If you were really hurting for sensitivity, it might be different.

I believe flow is a function of pressure applied to a fixed (known) restrictor. Thus, the differences in viscosity of the gas will affect the flow rates (and the corresponding pressure applied.)

Best regards,

AICMM

Thank You Very Much
Gus

I am accustomed to running a FID with a 35/450 ratio of He/air

Running the flame lean minimizes the soot buildup on the flame tip.

Usually wrong air ratios indicates the detector is dirty and is requiring a different ratio in order to keep the flame lit.

best wishes,

Rod

Ok 35/450 next time i will try this ratio
Thanks gus

First rule of troubleshooting: If its not broken, don't fix it.

Peter

First rule of troubleshooting: If its not broken, don't fix it.

Peter

absolutely