Chromatography Forum

Analysis parameters:
Column: Agilent Zorbax 300-SCX (50mm)
Eluent: 5mM nitric acid (in 18MOhm water)
Flow: 1.5mL/min
Column temp: 40C
Analyte of interest: Ammonium
Sample: Ammonium acetate in eluent

We have recently been seeing an unknown (cation) peak eluting ~15 seconds before ammonium, in both samples and to a lesser extent blanks. Because it appeared in the blanks, and its peak area is linear with injection volume, I assumed there was a contamination issue and prepared new mobile phase with separate water/nitric acid, but we're seeing the same issue.

This unknown peak, in addition to being linear with injection volume, is also fairly Gaussian (tailing ~1.1) and reproducible in terms of retention time and peak area. It doesn't appear when doing an injection-less acquisition, which to me indicates the problem is somewhere in the sample introduction. I was considering the possibility of column damage leading to split peaks, but a 15s difference seems pretty extreme.

Any suggestions would be appreciated.

Without knowing the chromatogrphic details, a peak just ahead of ammonium reminds me to sodium. Spike your sample with sodium to check whether the area increases or whether an additional peak occurs.

Generally Sodium is coming just before the Ammonium ion and many time this will confuse.
Sodium will comes in the water also.

Please confirm the sodium i am sure you will get sodium as unknown peak.
After getting that kindly work on the concentration of mobile phase and flow so that you will get very clear separation in both the peaks.
I have experienced this one