Negative peaks when measuring nitrogen

[Excedere]

We are using an Inficon Micro GC fusion. We are trying to confirm that trace nitrogen in ultra-pure Hydrogen is lower than 2ppm. We have no issues with peaks when we use 100ppm-1000ppm nitrogen in balance Hydrogen, however when we are measuring down in the 0-10ppm range we have peaks in the negatives. We have tried everything in terms of troubleshooting: We have done a bakeout, verified that there are no leaks, our carrier gas is 99.9995% pure, our fittings are tight, contamination is not an issue, our inlet psi is right at 60, and ensured that the machine is sufficiently warmed up.

The negative peaks we are seeing are consistant but we are not sure if it is safe to use the data we have to verify our purity.

we run the tests at:
inject ms:35
column temp profile: 50 C (10 sec)- 100 C (2 degrees/30 sec)
heat injector: 50 C
column pressure: 25psi
sample pump time: 30 seconds

Thank you,
Frustrated Technician

[Excedere]

I failed to mention the carrier gas, its Helium

Thanks,
Still Frustrated Technician

[antonk]

H2 with more than 1% can be properly measured only with Ar carrier. With He carrier H2 peak would be inverted (thermal conductivity of H2 is less than one of He)

[Excedere]

Hydrogen is the balance, we are not trying to measure it, we are trying to measure nitrogen. Argon is less accurate than helium so that is why we are using helium. At higher concentrations (100-1000ppm) of nitrogen we see no problems, peaks are well defined.

[HPLC chemist]

I won't worry about this difference. It is the difference in the TCD (the detector I presume you are using) response between the Helium and the gas you are measuring. No 'real' samples should be down at this concentration (it should be below your LOQ).

[HPLC chemist]

Even an MS detector can have different responses for different molecules (gases in your case).

[Excedere]

It is well within our LOQ this machine is supposed to measure down to the single digit Ppms, and has done so with nearly every other gas we have examined

[AICMM]

Out of curiosity, why are you programming a micro-GC that is doing fixed gases?

Negative peaks usually indicate (at least with HID's) that the sample is cleaner than the carrier. Entirely possible at 2 ppm of nitrogen.

Also, how well resolved are you from the hydrogen peak? If not well resolved, then you could have a blend of hydrogen/helium in the detector which will change it's dynamics (although both are starkly different from nitrogen in their TC factor.)

Best regards,