Help on mobile phase replacement for ELSD

[Tristan]

Hi all,
I am trying to transfer a method from the literature to my needs. The column used is a NH2 column and the detector from the literature is a refractive index. I want to use an ELSD detector rather than an RI. The mobile phase is 50 mM sodium sulfate:0.5 TFA. What is the sodium sulfate used for, and is there a replacement that is suitable for ELSD detection?

Thank you all in advance!!!

[Kostas Petritis]

Just use ammonium formate and use the TFA to buffer your mobile phase at the same pH...

[dkollmorgen]

Tristan,

Did the ammonium formate/TFA work out for you? I am in a similar situation, only with ammonium sulfate.

thanks,
Diane

[JGK]

Can i suggest you check out Page 37 of the document in the link

http://www.knauer.net/dwnld_fls/elsd2000_manual_e.pdf

it's a useful table of Volatile buffers and mobile phase modifiers

[Vlad Orlovsky]

I am not sure that ammonium formate/TFA is a good combination. I expect background to be high due to non-volatile nature of ammonium TFA, we use ammonium formate for TFA analysis with ELSD. Also it might be a good idea to stay with the same acid when adjusting pH (formic acid for ammonium formate, acetic acid from ammonium acetate, etc).
I would suggest either using ammonium formate or TFA.

What is your analyte?

[tlm]

Try Ammonium acetate with acetic acid to adjust pH.

[CSV]

I'm not sure ammonium acetate works so good on ELSD! I've tried it a few times but it seems that it's not volatile enough (not so surprizing since it's crystaline in the container ). Maybe it's enough with an acetate buffer (pH3.7-5.7) or formate buffer (pH 2.7-4.7).

[Vlad Orlovsky]

we are using ammonium formate and ammonium acetate with ELSD daily with concentrations of up to 500 mmol. Never had a problem with these two buffer. You just need to set up ELSD temperature correctly. We are using Sedex 75/85 models with 30-100*C.

[HW Mueller]

My guess would be that NH4+X- doesn´t evaporate as a crystal, but rather as NH3 and HX.
Also, if you want to use acetic or formic at pH´s where they buffer you will have to have their conjugate bases present with a counterion other than H+, so what counterions are you suggesting, CSV?

[CSV]

You are right Mueller, the counter ions I can think of would also produce crystalline matters. Actually, I was thinking about just to acidify the mobile phase with the acids. The obtained pH will maybe not be within the buffer range, but in this case it may be suffient to keep the pH low.

Regards,

CSV

[XL]

Good quality ammonium acetate or format is the crucial to be compatible with ELSD. When I tried both salts with 99% purity in the past, I saw random spikes during the run. Try higher purity ammonium acetate to improve your situation.
Also as Vlad pointed out, ELSD parameter setup is also important. My experience of Sedex-85 is I could use 200 mM MH4OAc buffer with little problem. The evaporating temperature was set at 50 C.