pH adjustment of human blood plasma prior to bioanalytical sample preparation.

pH adjustment in bioanalytical sample preparation concerning ionisable compounds is one of the most common sample treatments. This is often done by mixing an aliquot of the sample with a proper buffer adjusted to the proposed pH. The pH of the resulting mixture however, does not necessarily have to be the same as the pH of the used buffer due to the significant buffer capacity of the sample. Calculation methods from titration technology were adapted and applied to this problem. The acid-base characteristics of human blood plasma and serum samples were determined and used to calculate the pH of buffer-plasma mixtures. Based on these parameters and the characteristics of the used buffers, two alternative methods were described to prepare buffers that lead to the proposed pH when mixed in the right volume ratio with human plasma samples. The resulting pH of several mixtures of different buffers with human blood plasma were in good accordance with the calculated pH. The proposed calculation methods and recommended buffer preparation methods may lead to more robust bioanalytical methods.