Plasma aldosterone: comparison of a new automated assay with a standard extraction method.

hours to several days. We changed the concentration of ammonium hydroxide from 1 mmol/L to 2 mmol/L. We again prepared a standard solution of uric acid at the same molar ratio of 1.7:1 with this concentration of 2-mmol/L ammonium hydroxide. With this ammonium hydroxide concentration, uric acid dissolved completely in a few minutes. The standard solution was stable at least 3 months stored in a well-stoppered brown, allglass container at 20 °C. Ammonium hydroxide was selected because it was suitable for mass spectrometric analyses and also because uric acid was stable in the ammonium hydroxide solution. We therefore recommend the use of 2 mmol/L ammonium hydroxide, not 1 mmol/L ammonium hydroxide or other basic solutions, to prepare uric acid standard solutions.