Ammonium nitrate for LC/MS/MS corrosion problem

[Chelsea]

Does anyone know about using ammonium nitrate as a mobile phase to prevent corrosion in a LC/MS/MS? Also do you know if there would be damage to the instrument if the heat was 350-600 deg and at 4500 V?
Thanks.

P.S. Any other comments on how to deal with corrosion issues in LC/MS/MS would be greatly appreciated.

[lmh]

I'd worry about nitrate as it's quite a strong oxidiser. Can you use ammonium acetate or ammonium formate, these being volatile non-oxidising buffer salts, with safe acids...

[HW Mueller]

HNO3 is used to passivate stainless steel, lots of info on this in this forum. The NH4NO3 might be an adaptation of this? (Isn´t the solid form of NH4NO3 quite explosive under some conditions?)

[Chelsea]

I know that ammonium nitrate is explosive but I was wondering if a dilute form of it could be used to help with the corrosion problem.

[lmh]

Passivating is, I believe, an oxidation process. I can't swear to what nitric acid does in spray chambers, but I can promise faithfully that 5mM sulphuric acid will oxidise a stainless steel spray chamber to dripping green gunk (apart from the bits that are pitted and black) within 30min. Don't ask how I know (lovely sulphate adduct ions though).

Obviously a neutral salt isn't as bad as an acid, but strange things go on when you're working at 350 degrees in the presence of high voltages. There's a difference between using something to passivate tubing at low temperature, and running it into a spray chamber over long periods. I'd keep a very close eye on my spray chamber to be aware of difficulties before they get to the green-and-runny stage.

[HW Mueller]

lmh, I really wonder whether something else was in your H2SO4, or whether your stainless was a special variety, because I don´t remember seeing any corrosion with many different aqu. or neat H2SO4 concentrations. One does not generally consider H2SO4 an oxidation agent. I will be happy to take this back if I am confusing something.

[lmh]

It was an Agilent 1100 MSD, 1946A (since upgraded); Agilent now use a teflon-coated (black) spray chamber, but in those days everything was whatever grade of stainless steel they specified. The spray shield was pitted, and the cage that surrounds the electrospray needle went an odd green/black colour (mostly washing off...). Agilent were very courteous about it, but spoke loudly, and slowly, using simple words!

Fount-of-all-wisdom Wikipedia declares sulphuric acid to be a typical acid at low temperatures, and an oxidising agent when hot and concentrated. It also reminds me that sulphuric acid attacks iron in any case, to FeSO4. In a spray chamber it's hard to know exactly what conditions will happen; there's lots of heat about, and plenty of drying, so it's possible, depending on source conditions and chamber design that it might get a bit hot and concentrated in places. I don't know, and I'm afraid I'm not a good chemist. I had assumed it oxidised because of what it does to paper, but that's actually extreme dehydration.

Wikipedia also reminds us that nitric acid only passivates Fe at high concentration; at low concentration it simply dissolves Fe.

[HW Mueller]

I bet that you will have real fun trying to write an equation for
Fe° >>> FeSO4

OK, it didn´t register that you introduced H2SO4 onto 350° metal, but obviusly you had other things there as well.

[lmh]

Wikipedia gives Fe (s) + H2SO4 (aq) → H2 (g) + FeSO4 (aq)
Yes, you're right, there would have been solvents, all sorts of stuff going on. A spray chamber can be a chemically and physically exciting place. It was silly of me, but we all learn from mistakes...

[HW Mueller]

Of course! I was hooked on trying to think how SO4-- might cause Fe° >>> Fe+3 to the point of momentarily forgetting the common reaction of M + H+ >>> H2. I played with that reaction in high school, it was quite fun to light the H2 formed this way in a test tube. Not really recommended unless you don´t care for your ears.

[lmh]

.... ooh! Pity a spray chamber has such a high gas flow. That could have been quite exciting.