nitrogen leak

[Rocknroll99]

Hello,

I am getting a big 28 peak in my MS, but virtually no 32, 44 or 18.

I was wondering if the He carrier gas I have is contaminated or there is another source of the N2? I am thinking if it was an air leak I would see more O2, CO2 and H20 in the MS.

I am doing the usual leak tests right now but what I wanted to know is, is N2 contamination of He Gas common?

thanks for any help on this, I changed the cylinder about 2-3 weeks ago and did not pick this up in my tuning last week.

[Peter Apps]

If you have oxygen and water scrubbers on your carrier line this can be from a leak upstream of the scrubbers. Contaminated helium is not unknown, but it would appear very soon after a cylinder change.

m/z 28 can also be from column bleed.

Peter

[Rocknroll99]

Hello Peter,

Thanks for the advice,

Column bleed is interesting to me and could be the culprit, I have just changed columns.

We are using a a Restek GTx silica capillary column (30 m x 0.25 mm i.d., 0.25 um film thickness)

I have not seen this before. I usually run Fatty acids through it, but have been running some sylated fatty alcohols through of late.

Could this 28 be coming from the column?

[Peter Apps]

Evidence that m/z 28 is column related is mainly from eliminating every other possible cause, and associations with column type and changes between columns.

Your use of silylated samples is interesting, because other incidents of m/z 28 in "column bleed" have been with siloxane columns.

How anything to do with siloxanes could plausibly give rise to an m/z 28 fragment is an open question.

Peter

[dblux_]

Hello Peter,

...

Could this 28 be coming from the column?

Execute a tune with cold oven and inlet. Any changes regarding m/z 28 ?

[AICMM]

Yes, the 28 could easily be N2 in your helium (been there, done that.) The example(s) I have seen showed messed up tunes though (because the nitrogen will rob ionization.)

Have your vendor swap a new lot with you.

Best regards,

aicmm

[Rocknroll99]

Yep, Still looks like it is coming from the He.

Blocked off the MS and the 28 peak disappeared.

Changed columns to see if it was still there, yep, in 2 different phase column. Therefore not column bleed

Pretty sure there is no leak (as I have changed all the ferrules, o rings and septum)

Waiting for the gass supplier to show up with the new bottle to see if it from the He source

Thanks for your help on this one

BTW does anyone use the dust off spray to find leaks? I was going to go down this path but was wondering if anyone uses it, and what brand do they buy (I am in New Zealand so I dont have much choice)

[BMU_VMW]

BTW does anyone use the dust off spray to find leaks?

Yes I do
but only to check transferline-nut, etc. its no good to test e.g. an injector, because it takes to long to travel trough a column.

I'm in Belgium and use: Dust off 67 from kontakt chemie. I don't think it is available in NZ.

[Yama001]

The problem with DustOff is the darn can is always empty

One place I worked at would recharge thier cans with an air compressor. Not much use leak checking there (though those would almost always be empty as well). Great fun watching an unsuspecting service tech borrow one.

I tend to (sparingly) use methanol for this purpose since it is always available in the labs I have worked in. Only ever messed up some electronics once.

I have seen a spate of helium tanks in the last year or so that had percentage levels of Nitrogen or of Argon. Perhaps our supplier is getting unreliable.