Na Benzoate Reproducibility

[bgiles]

We are seeing reproducibity issues in Na Benzoate sample injections (same vial) when a 0.01M KPhosphate buffer is used vs. a pure water as the diluent. The Kphosphate buffer is pH 3.0 in both the diluent and mobile phase. THe mobile phase is 70/30 Buff/ACN.

Has anyone experienced these issues? What could be the reason for poor reproducibilty from the buffer?

[AdrianF]

By reducing the pH to 3 you are converting the NaBenzoate to benzoic acid which is only slightly soluble. It will slowly begin to come out of solution giving erratic results.

There should be no problem injecting a water solution. As an alternative you could dilute in mobile phase.