FID signal lower with Nitrogen vs Helium carrier gas

[avitan]

Area of peaks in the test mixture of alkanes is about 2.5 times lower when using Nitrogen as a carrier gas vs. Helium. In both cases Nitrogen used as Make-up gas. Linearity and precision checks are passed in both cases. What might be the problem?

[gpronger]

Without pulling out my reference on the difference effect between He & N2 on the FID, there is a tremendous difference with chromatography.

In nearly all applications He will be a superior carrier gas improving overall chromatography, from shorter retention times to better overall resolution. N2 theoretically has a lower HETP, but given we're usually temperature programing, and N2 HETP curve increases fairly rapidly in both directions, it is generally more of a problem than a help. I would suspect for more specific info, you'll need to post more details on the analysis you're performing.

Greg

[avitan]

Greg, thanks for your input. I am aware of the differences in van Deemter curves for Nitrogen and Helium. The effect of the make-up gas on the FID signal (higher response with Nitrogen vs Helium make-up) was discussed on this forum earlier as well. Please, correct me if I wrong, but peak area should not change so drastically with the changes in HETP.

[larkl]

Is this an EPC instrument? Did you change the carrier gas type in the config? Maybe the split is wrong if you didn't change the gas type?

[gpronger]

Here's what may be going on; assuming this is capillary, and also that you're optimizing flow for the two different carrier gases, you're probably 2X the flow into the column between He and N2. If you've maintained the same splitless time due to the increase in flow into the column you're going to be delivering more analyte into the column.

Greg

[Peter Apps]

larkl and Greg have probably got it - this is an inlet pneumatics rather than a detector problem. Check actual split and septum purge flows with a bubble flow meter, and measure the retention of someting unretained (the gas hold up time), they will tell you if the GC knows which gas it is working with.

Peter