Purity level of nitrogen for drying gas in LC/MS

[thohry]

Dear all,

We are quite new to LC/MS (compared to GC/MS). We often use N2 generator for the drying gas. The nitrogen purity as in catalogue is about 99.5% and it's all right for us. But the flow rate is only about 9L/min. If we want to run at higher flowrate, the purity level is reduced.
I really do not know how low the purity of nitrogen for drying gas I can still run the LC/MS. Anyone have any idea about this ?

Thanks for any inputs.

[lmh]

The purity really doesn't have to be all that high. It's really a question of making sure that the mixture hasn't got enough oxygen to support anything explosive in the spray chamber! Generators sold for use with LC-MS equipment always have a quoted maximum flow, and you shouldn't exceed this (many generators in any case are fitted with some sort of flow-limitation device). The purity will drop with increased flow, but provided you don't exceed the maximum, it will be good enough. Most generators seem to go in steps of 15L/min, with 15L/min being sold as a "1 instrument" generator, and 30L/min designed to cope with two typical MS instruments. The exact details, though, will depend on your instruments, and whether you run them flat out at high flow-rates.
Depending on pipe-work, if you try to get too much flow out of 1 generator you may also find that the pressure drops, and your MS complains its nebuliser pressure or flow is too low.

My personal viewpoint is that if you've got a 15L/min generator and you're trying to run two 9L/min instruments, you will almost certainly find that the instruments will complain about restricted gas flow and if your generator has built-in compressors, they'll be failing frequently. You'd do better to scale up.

[thohry]

Thank you very much lmh. I was a bit concerned about the contamination of trace analysis if the drying gas is not pure enough. So I can increase the gas flow rate if it requires so.