Peak 28 ! Help me !

[Albertospd]

Hello everyone! today I changed the helium cylinder (he) in my GC 2010ultra (shimadzu). after the vacuum, I tune and have the base peak 28 (no 69). I checked for leaks but can't find anything. What could it be ? the ferule of the transfer line? help please

[James_Ball]

If you have a can of the computer duster spray you can spray it at each point where a leak could be and if you see a mass in the 50-59 range show up you know there is a leak at that point. (I can't remember the exact mass to look for but it is between 50 and 59m/z)

The other thing to check, and I know because I have done it before, is to make sure the tank is Helium and not Nitrogen since they both have the same regulator fitting you can install a nitrogen tank on the helium line.

[benhutcherson]

To tag onto what James said-contaminated He cylinders are not unheard of, especially not with increasing scarce helium supplies.

Also, just to make sure we're on the same page-28 is almost certainly nitrogen.

One other big potentially obvious one is if you changed the carrier gas filter at the same time as the cylinder, or sometime recently. At least with the style that have a big upright plastic/glass color changing cylinder mounted to a baseplate(I currently have bases branded Varian, Agilent, and Restek in my lab while my current batch of filters is branded Trajan)-they ship filled with nitrogen, and you need to purge all of that out when installing a new one.

There are a couple of ways to check whether it's coming from the cylinder or from a vacuum leak. I try to avoid Shimadzus as much as I can, but all other GC-MSs I've worked with will generate an "air report" by-among other things-comparing the area of the 28 peak to the 32 peak. If it's the "correct" ratio for air, that's probably the result.

One thing I've done as a quick and dirty check is to vary the column flow rate and look at the absolute height of the 28 peak. In my experience, a low flow rate will usually increase the 28 height from a leak, while a higher flow rate should decrease it. Contamination is the opposite, with low flow rates showing lower intensities and high flow rates showing higher intensities.

[Albertospd]

Hello everyone!

today I try to change the flow rate in the column. I have already installed an Agilent filter and the coloring is correct.
could it be a ferule problem? or do I exclude it a priori?

Thank you !

[MJD]

I think you need to read again what ben wrote above, as it is very solid advice, and looking at the fact you have recently changed the gas filter, you need to compare the 28 peak to the 32 peak....as the the filters are normally shipped with nitrogen so if you did not purge it you will get a huge 28 peak for quite some time.

I am pretty sure it is due to the gas filter nitrogen packing. if not let us know and we will get to the bottom of it.

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[James_Ball]

Hello everyone!

today I try to change the flow rate in the column. I have already installed an Agilent filter and the coloring is correct.
could it be a ferule problem? or do I exclude it a priori?

Thank you !

Another possibility for air leak is if the ferrule at the detector/column interface has shrank. Once the oven cycles a few times they can shrink and begin to leak, usually requires about 1/8 to 1/4 turn more of the nut and then they seal until you need to remove it.

[aldehyde]

The gas filters are shipped filled with nitrogen, so it is normal to see a large amount of nitrogen if you ALSO changed the filter.

Run a 300 mL/min split for an hour or two then bring the flows back to normal and check--I'll be if the N2 isn't gone it is at least much smaller.

If the filter has say 1 L of N2 in it and you're running 1 mL/min column flow, 3 mL/min septum purge, and 10 mL/min split you'd need to run for many hours to get all the N2 purged out.

If the amount of N2 doesnt go down it is possible the leak is before the gas filter and the filter is stripping out O2 but not N2 from atmosphere. This is something to consider if you purge the filter for a bit and it isn't dropping at all.