Chromatography Forum

Dear all
which TCD do you suggest me to measure from 99,5% to 100% N2 if I want to have an accuracy and repeatability on 12 hours of 0,01%?
thanks

almost anyone's TCD should work.

The issue is injecting a reproducible amount of N2

which does not overload the column, not the TCD per se.

The integration capabilities is quite important as well.

Usually folks measure the impurities in a balance like N2 and subtract their values from 100% to get a measurement of N2 purity at ±0.01% reproducibility.

Don't try to measure the balance peak with this precision as it is not possible due to the changes in atmospheric pressure which cause a variation larger than 0.01%.

best wishes,

Rod

Rod
We presently use a differential TCD which compare pure Helium at 30cc/min with the N2 sampling volume at 30cc/min. Sampling loop size is 2ml.
We use a molecular sieve 5A 8' heated at 80 Celcius and we get a integration time from 50seconds(peak starting time) to 90 seconds(peak ending time).
We calibrate the span with pure N2 at 100% and the zero with pure Helium. When we run cycles on pure N2 we have a variation around +/- 0,2% on 2 hours period. Is there a way to have better repeatability??

It would appear that you are getting normal performance from a sampling valve system from a laboratory GC.

2 out of 1000 repeatability is not bad.

I don't think there is any way of improving that unless you go with a very highly designed system, a special process analyzer.

Measuring a balance peak directly is not the way to get good repeatability on that peak.

You must measure by difference between 100 % and the sum of the impurities.

I repeat again:

You must measure nitrogen by difference between 100 % and the sum of its impurities.

If I have not made myself clear to you please give me a call or write me privately and I will attempt to clarify my answer.

best wishes,

Rodney George
Senior Research and Development Scientist
Gas Separations Research
Supelco
595 North Harrison Road
Bellefonte, PA 16823

814-359-5737 voice
814-359-5459 fax
rodney.george@sial.com