Sodium clusters driving me crazy!!

[hfredricks]

Hi everyone,

I recently started seeing a high background of sodium acetate clusters in my LCMS method - so much sodium that some of my analytes now show the sodium adduct instead of the preferred ammonium adducts.

I've vetted all my eluent ingredients and I don' think its there, I tried new (MilliQ washed) glass bottles for eluents, I've cleaned the MS source with copious amounts of MilliQ, I'm just about to flush MilliQ through the whole system and try to clean it up... I've no experience of sodium clusters before - anyone got any hot tips as to where they might be coming from?
The MilliQ is from a well-kept system, reading the usual 18.2 Mohms, so its not there!
It seems that there's no or very little sodium acetate clusters when I'm running high on the aqueous, but they seem to build up and become a constant high background when the organic eluent kicks in... right where my analytes are...

HPLC: gradient of 55% A [water with 1% 1M ammonium acetate + 0.1% acetic acid] 45% B [70:30 ACN:IPA + 1% 1M ammonium acetate + 0.1% acetic acid] to 99% B
Waters Xbridge C8 column w/guard and prefilter, QExactive... organic solvents are LCMS grade; either fisher or JTBaker, additives are LCMS grade; Fluka or Covachem.

gargh!

Helen

[sav]

Hi Helen.

1. Check ammonium acetate and acetic acid. We usually use ULC-MS grade (Biosolve).

2. Check isCID (fragmentation voltage, declustering potential etc.) - may be the voltage is too high for this situation (analytes, eluent composition etc.).

3. May be the main source of Na+ is sample.

4. Axiom: The Na+ is always in your LC-MS system

[hfredricks]

thanks sav.

Yup, sodium is always there, I'm used to seeing a small proportion of sodium adducts of my lipids, but this was way off the scale!

All the 'ingredients' checked out - I learned a long time ago to use at least LCMS grade for everything! Its as though the eluent bottle were contaminated with sodium from somewhere... perhaps not too surprising in an Oceanographic institution
now that I'm washing eluent bottles really well in MilliQ I do not see the sodium acetate clusters and the ammonium / sodium adduct proportions are back to normal...

there's always something new eh?

Helen