Chromatography Forum

First the settings:

HP5890 w/ TCD, helium carrier, high TCD sensitivity, [+] signal setting, 30C isothermal run, Valco Sampling valve, tried sample loops from 250ul to 5mL, tried both a 1.5m Molsieve 13x and a 30' Hayesep DB

I run a 100ppm Nitrogen standard and have only my single nitrogen peak. If I run 'UHP' Hydrogen, however, I get my hydrogen coming off as a negative peak, but then at the time nitrogen should elute I get another, small negative peak instead of a positive peak one would expect from nitrogen. As mentioned above, I have tried two different columns to rule out any kind of flow upset coeluting with nitrogen, but the negative peak tracks with nitrogen on both columns. I have tried injections at both atmospheric and at carrier pressure (20psi for the molsieve), as well as sample loops ranging from 250ul to 5mL in size, and the negative peak size tracks with the amount of sample being injected. What can be causing this negative peak? If I had nitrogen as an impurity in my helium carrier, could conditions exist that give me a positive peak for nitrogen at 100ppm, but negative at a lower concentration, for example 10-20ppm?

Hi,

I suspect that you may have identified the problem, trace nitrogen in your Helium carrier - what's the specification for your helium?, and can you obtain a more pure grade?. This sounds like the most probable cause, given that you see the same problem with the two columns.

When you tried the two columns, did the relative peak % area stay the same?. What happens when you change sample size, carrier flow and oven temperature, do the % area peak ratios stay constant, even after conditioning the column.

I assume that you've also previously performed the analysis without the peak. If you haven't, I'd also check out your GC gas purification systems, making sure regulators are SS diaphragm and that any older drying traps don't have polymer bodies, and that the gas lines to the instrument are always pressurised 24/7. I assume that any detector makeup/reference gases come from the carrier gas system, and are also helium. I'd expect slight changes in area % ratios during repeat run if contamination was occurring.

I assume you are seeing no trace of oxygen or argon ( I can't recall gas elution position on MS13X and your Hayesep - haven't used them for ages ), so air contamination of carrier lines has been excluded?.

A less likely source of contamination would be your sampling valve and system, which you have already tested by sampling at pressure, but if you have doubts, put a small backpressure regulator and shutoff valve on the sampling line vent and ensure all parts of the valve are at a few psig pressure continuously. Although I'd expect the negative peak to become a positive peak if the sampling valve was the problem, but if you are desparate....

Good luck,

Bruce Hamilton

If I had nitrogen as an impurity in my helium carrier, could conditions exist that give me a positive peak for nitrogen at 100ppm, but negative at a lower concentration, for example 10-20ppm?

Sounds like you have nitrogen in your carrier. Any time your sample has a concentration of a component lower than the carrier you'll get a negative peak. And, indeed, your positive peaks are smaller than they should be.

Check everything for leaks. The smallest leak will aspirate air.

If you have no leaks, you may need to use UHP helium. Most applications can use Zero grade helium for the carrier, but when you start looking for trace levels of nitrogen, UHP may be neccessary.

A good helium purifier would probably be useful, too. Valco makes a helium purifier that works well, IME.

There could also be another reason that you are getting negative peaks. This depends on how good the separation is between your major hydrogen peak and the small nitrogen peak that you are trying to measure. If the hydrogen peak is still tailing, you would actually have a hydrogen/helium mixture as the carrier gas, and anybody who has tried to measure hydrogen at low levels with helium gas has found that the results can be unpredictable. Is there any reason that you cannot use hydrogen as carrier gas.

I would recommend that you first try out what the first two replies have suggested to you, but if this fails, try hydrogen as carrier gas ( if this allowed in your lab).

Gasman