Noobie's corner

[Peter Apps]

Dust-off has mass 69 and 83 (at least the one we use here in Belgium)

The Thermo 1300-series GC's can be equiped with a He-saver module.
It is a very recent feature (about a month) that uses N2 for all split/purge flows and He for the injection and column-flow.
Since you are likely to have your system on standby for a long time it might be worth considering this option.

Unless they have a Maxwell's demon in the inlet liner sorting helium from nitrogen and sending only the helium down the column I'm having a bit of difficulty how they mange to do this.

Peter

[MSCHemist]

Dust-off has mass 69 and 83 (at least the one we use here in Belgium)

The Thermo 1300-series GC's can be equiped with a He-saver module.
It is a very recent feature (about a month) that uses N2 for all split/purge flows and He for the injection and column-flow.
Since you are likely to have your system on standby for a long time it might be worth considering this option.

I can confirm that there is a pronounced 52 m/z (higher than 83) in the dustoff here in the states which is weird. According to the article it is chlorodifluoromethane which is a CFC which should be illegal and of course the dusters all say CFC free.

Another tip is to be sure to put some Apiezon L vacuum grease on the side plate O-ring and vent valve O-ring every now and then so they do not dryrot and leak. It is in the Agilent manual. The O-rings on the MS particularly the side plate O-ring are often troublespots. I was getting pump down failures until I lightly greased my 5975.

[GRovian]

Here, in Canada, the CAS number on DustOff can is 75-37-6 which will produce strong peaks at 51 and 65, tested. Also confirmed by NIST http://webbook.nist.gov/cgi/cbook.cgi?I ... 6&Mask=200

The confusion might be because aside from the 1,1- version there is also a 1,2- version that produce peaks at 33 and 45,46 and who knows where else.



Something weird happened today. The tech said he finished working on the instrument and a small peak of air at 28 is normal. To test that statement, I switched to splitless, I waited for 3...5 minutes for the helium to make it through the column, I measured the peak 28 and peak 69 in MANUAL TUNE. With the cal gass off, the 28 peak read like 4E7. With the cal gas on, the 69 peak measured 4.5E7. Which mean that, in splitless mode, the N2 and cal gas read almost the same. I was expecting something like 5..10% or less for N2, as many people suggested. The tech said that's normal and according to the training he received from the factory.

Am I doing something wrong here or I'm given king size portions of BS from my tech?!

GRovian

[Don_Hilton]

When you have a low inlet pressure relative to atmosphere, you will get a significant quantity of air into the GC in splitless mode relative to m/z 69 in the cal gas. What you see will depend on head pressure, column flow rate, septum purge rate - at least.

[GRovian]

When you have a low inlet pressure relative to atmosphere, you will get a significant quantity of air into the GC in splitless mode relative to m/z 69 in the cal gas. What you see will depend on head pressure, column flow rate, septum purge rate - at least.

Ah, thanks. Column flow rate is 1 mL/min, septum purge is 5 mL/min.

So, when people were talking about 5...10% reference to 69 cal gas, what split ratio should be used? I mean to compare apples with apples.

[James_Ball]

When you have a low inlet pressure relative to atmosphere, you will get a significant quantity of air into the GC in splitless mode relative to m/z 69 in the cal gas. What you see will depend on head pressure, column flow rate, septum purge rate - at least.

Ah, thanks. Column flow rate is 1 mL/min, septum purge is 5 mL/min.

So, when people were talking about 5...10% reference to 69 cal gas, what split ratio should be used? I mean to compare apples with apples.

All of my injections are done with splits ranging anywhere from 20:1 through 100:1 and I always have low nitrogen of 5-10% or less sometimes. Unless I turn the flow off and let it pull vacuum on the column long enough to evacuate all the helium and begin to pull in air it will be this way.

[GRovian]

All of my injections are done with splits ranging anywhere from 20:1 through 100:1 and I always have low nitrogen of 5-10% or less sometimes. Unless I turn the flow off and let it pull vacuum on the column long enough to evacuate all the helium and begin to pull in air it will be this way.

Thanks James. Again. As I said already, that's invaluable practical information that you can't find in user manuals.

Reading a bit around the forum, I apologize to the admin for not using the beginner section, "Student Projects". An honest mistake, I'll use that section for further trivial questions in the future.