Chromatography Forum

Does anyone have any thoughts as to the feasibility of injecting 0.9% saline solutions into an LC-MS? The injection volume will be somewhere in the range of 10-25uL, analytical column is a C18 with 1% acetic acid methanol/water mobile phase. I realize salts are generally frowned upon for LC-MS, but the concentration of the chemical I am needing to detect in the saline is already very low and I'd rather not dilute if this isn't a problem. Thanks for any advice!

10-25ul should not be too much of a problem when injected into the mobile phase, it should dilute it enough. Just be sure not to make too many injections without checking your source for salt buildup. I remember one of the old Agilent adds showing a source after multiple injections of undiluted sea water samples with the salt crusted on the source and the system still working, though I would not want to let mine get in such a shape.

It may not be too harmful but if possible I would still avoid it. I can think of 2 options:

-use a diverting valve that will direct the mobile phase to the waste until the solvent peak has eluted.

-use a C18 SPE step prior to LC-MS which would allow you to get rid of the salt and possibly concentrate your sample by evaporation of the eluting solvent.

I routinely analyze environmental samples of salt water and do everything to minimize the salt concentration, from diluting all samples when you have the sensitivity and always using a diverter valve. However, there are times when the sample can't be diluted further and I have analyzed straight salt water (usually with 25-50% organic added). I would not usually employ SPE solely to get rid of the salt. I would matrix match your standards if you are shooting samples with high salt content and keep a close eye on your source, especially API 3000 types.

jmmunson wrote:
Does anyone have any thoughts as to the feasibility of injecting 0.9% saline solutions into an LC-MS? The injection volume will be somewhere in the range of 10-25uL, analytical column is a C18 with 1% acetic acid methanol/water mobile phase. I realize salts are generally frowned upon for LC-MS, but the concentration of the chemical I am needing to detect in the saline is already very low and I'd rather not dilute if this isn't a problem. Thanks for any advice!

Ditto on the use of valve switching. As I said in another thread, pick a switch time later than the void but prior to the beginning of your first peak's elution. Salts will usually come out with the void.

If you have access to a syringe pump you can use post-column addition to visualize whether there is matrix suppression in the region where your peaks elute.

I also have to say - 1% acetic in your mobile phase is quite strong and I think it's likely to cause ion suppression. Normally I'd not go higher than 0.1% formic or acetic, although in one case with HILIC-MS I found 0.5% formic to be necessary for peak shape.