PH measurement problems affecting assay of carcinoembryonic antigen

Measurments with combination pH electrodes of the pH of the dilute buffers used in a commercial kit (CEA-Roche) for assay of carcinoembryonic antigen resulted in pH values 0.1 to 0.3 unit lower than pH values measured on an electrode system with a capillary junction. If the pH values of these buffers were adjusted, based on such measurements, an error in the assay of 0.2 to 0.6 ng/ml in the 1.5-3.0 ng/ml range would result. We recommend that the pH of dialyzed samples and of the working ethylenediaminetetraacetate and ammonium acetate-acetic acid buffers be monitored with pH electrodes that have a capillary junction between sample and saturated KCI, as is true of most blood-pH instruments. We also recommend use of a 1 mol/liter rather than 2.5 mol/liter stock ammonium acetate-acetic acid buffer, because of the closer similarity of the pH of buffers at this molarity to those at 0.01 mol/liter.