GC/MS air leak?

[X]

I have an Agilent GC/MS 6890-5975 and I saw about 20% 28 and 32 against 69 ion, the 28 and 32 ions are about same intense. When there's air leak, the 28 peak should be about 3 times higher than 32 peak. Anyone know the reason for this weird same intensity of the 28 and 32 ions? Thanks a lot!

BTW, I capped the beginning and the end of column, the 28 and 32 peak were much lower (about 5%).

X

[Krashan]

Do you have still the same relation between 28 and 32 after capping your column or 28 is higher?

[X]

Do you have still the same relation between 28 and 32 after capping your column or 28 is higher?

Hi Krashan, 28 is higher when I capped the inference nut or column. Thank you!

[Krashan]

It could be some contamination from injector (or some extra connection), during the leak you will probably get higher 28 and 32 together comparing to other peaks, often higher than 69. 20% of 69 is ok. If you do not need small ions for your determination it should not interfere you. However you can check difference during tuning with or without capping (all the system).
Regards

[X]

It could be some contamination from injector (or some extra connection), during the leak you will probably get higher 28 and 32 together comparing to other peaks, often higher than 69. 20% of 69 is ok. If you do not need small ions for your determination it should not interfere you. However you can check difference during tuning with or without capping (all the system).
Regards

Thank you! I'm afraid that 20% O2 may harm the GC column, I'll try change the gas trap.

X

[Don_Hilton]

To check the question of air entering through the inlet - set the inlet to split mode and set the split ratio to a high enough value. I use a split rate of 35:1 or more with a 20 m x 0.18 mm id column to avoid measuring air that is coming through the inlet.

[X]

Thank you Don, I tried with split ratio of 50:1 for a 30 m*0.25 mm column, the 28 and 32 peaks are still there at similar abundance, so the leak is not from the inlet.

I capped the column and there was no leak, so I think the leak may come from the carrier gas (He) or the vent trap?

X

To check the question of air entering through the inlet - set the inlet to split mode and set the split ratio to a high enough value. I use a split rate of 35:1 or more with a 20 m x 0.18 mm id column to avoid measuring air that is coming through the inlet.

[Don_Hilton]

Do you see m/z 18? What grade of helium are you using?

If you see m/z 18, the background is not a contaminant coming from the gas cylinder.

[jdezeeuw]

better do some leak checking. There are sensitive helium leak detectors available. another way is to spray some CFC around the target area. These fragmentation ions are also easily picked up by MS.. We have 2 types: C2H4F2 and C2H2F4

rgds

jaap de Zeeuw, Restek corporation

[carras]

[quote="Krashan" 20% of 69 is ok. If you do not need small ions for your determination it should not interfere you.
Regards[/quote]
Hello X,
20% 28 or 32 is definitely not OK for a MSD, your sensitivity would suffer and it would break havoc with your column. A quick test to acertain if the air is coming from the carrier gas is to increase column flow, say two fold. If the carrier gas is the culprit your 28/32 signal would increase.
I find very handy to check for leaks using the Ar/CH4 mixture used as make up gas in ECDs monitoring m/z 40.

Good luck

[X]

Do you see m/z 18? What grade of helium are you using?

If you see m/z 18, the background is not a contaminant coming from the gas cylinder.

No, the 18 peak is tiny, less than 1% of 69 peak, so gas cylinder could be a contamination source.
The helium is ultra high pure grade.

[X]

better do some leak checking. There are sensitive helium leak detectors available. another way is to spray some CFC around the target area. These fragmentation ions are also easily picked up by MS.. We have 2 types: C2H4F2 and C2H2F4

rgds

jaap de Zeeuw, Restek corporation

Thank you, Jaap, I'll buy some later.
X

[X]

[quote="Krashan" 20% of 69 is ok. If you do not need small ions for your determination it should not interfere you.
Regards

Hello X,
20% 28 or 32 is definitely not OK for a MSD, your sensitivity would suffer and it would break havoc with your column. A quick test to acertain if the air is coming from the carrier gas is to increase column flow, say two fold. If the carrier gas is the culprit your 28/32 signal would increase.
I find very handy to check for leaks using the Ar/CH4 mixture used as make up gas in ECDs monitoring m/z 40.

Good luck[/quote]

I doubled the carrier flow, the 28 and 32 peaks increased about 50%, so it's the carrier gas.
Thank you so much for your help!
X

[Don_Hilton]

Do you have an indicating trap at the back of the instrument? I use these even though I use UHP helium. On occasion, I 've seen the color begin to change in the trap - warning that the UHP wasn't so UHP...

[carras]

Hello again X,

Check the indicators in your gas trap, it probably is gone by now.