Chromatography Forum

Is anyone else running PAHs using hydrogen as a carrier gas having linearity problems especially with the higher MW compounds?

We had switched to hydrogen carrier several years ago - we did not see any issues. The high end PAHs can often have linearity problems, but they did not get worse after the switch. (We did see an elution order swap with naphthalene and dodecane - that was kind of entertaining from a physical chemistry point of view).


Jaap from Restek has spoken of some things to pay attention to on these forums regarding hydrogen carrier - search here or on Resteks blog to see what he has to say. Many of the other experts here have had some good points as well.

I thnk he mentioned something about hydrogen activiating the inlet liner? Can't really remember. One question though, is this a method you've already been using with helium or another carrier gas, or is this the first time you've tried it?

What GC/MS and how much flow are you using?

Best regards,

AICMM

On top of the switch to Hydrogen, we made several other changes: Liner, column (switch from DB17MS to DB-EUPAH 30mx0.25x0.25).

Instrument: Agilent6890/5973. Constant Flow - 2.0ml/min.

Actually have 2 instruments...one equipped with a turbo and the other a diffusion pump. I've altered many things over the last 8 months including changing liners arounds, switching to a smaller bore column to get the flows down to 0.8-1.0mL/min. The thinking was that we may be getting a soft chemical ionization with the increased flow. No matter what we did, the problem remained with the same problem on each instrument.

The problem is this. We have seven standards from 0.002-10ppm (inject vol. = 1µl). Right up to and including Pyrene, everything is great (R2-1.000). For all compounds after this, the 5.0 & 10.0ppm standards bias high above the curve. Not horribly but enough to cause problems (R2 typically b/w 0.990-0.995). I've tried to attached an example but can't seem to get by out firewalls here. I'll keep trying.

Now, these are 2 instruments that have ben running PAHs for 11-12 years. Another thought was maybe that the hydrogen is just acting as a scrubber and will be cleaning systems that have been collecting crap for that long. Thought that maybe 3-4 months and it would clean itself out. At first, we did source cleanings at least every 2 weeks, changed all the lines, traps, etc. The systems appears to be clean now but the problem remains. I've tried switching back to Helium and the problem dissappeared but returned on going back to Hydrogen.

Hopefully I've explained this well enough. It all sounds fine in my head but I don't always translaate to print clearly. If anything is unclear, please let me know.

If anyone has any thoughts or insight here please let me know!!

Thanks

First, make sure you change the gas type from helium to hydrogen.

Second, I don't think carrier gas has anything to do with the linearity of the ICAL. It sounds more like a discrimination issue in the injector port.

Hydrogen is selected as the gas type.

We've also switched back to Helium at one point with all other conditions being the same. The problem disappeared with linear calibration curves. Upon switching back to hydrogen the problem came back.

This problem has been consistent over 6-8 months on 2 different instruments (turbo & diffusion pumps) with many maintenances performed over this period.

Simple experiment you might try, pretty quick too. Try cutting the hydrogen flow down to something around (below) a mL/minute and run the curve. I've seen this work in the past with some MS's so it is something to think about.

Best regards,

AICMM