Isotachophoretic determination of urea-ammonium in plasma: a candidate reference method.

Abstract

Separation and determination of sample constituents by capillary isotachophoresis are entirely based on physical phenomena. The method has therefore been proposed as a universal reference method for ionic constituents. The present paper shows that even neutral species can be adequately determined after suitable preceding reactions. Urea was completely hydrolysed by urease (EC 3.5.1.5) to ammonia and bicarbonate, followed by direct measurement of the ammonium ion concentration by capillary isotachophoresis. Standard Reference Material No. 912a urea (National Bureau of Standards) was used as a primary standard. The analytical linear range of the method extends to 64 mmol urea per litre. The precision of the method was in the range of 1.05-2.64% (CV) and the analytical recovery of added urea was excellent (99.4%, SD 1.13%). Further proof of accuracy was obtained by analysing the NBS human reference serum (standard reference material 909). The mean result by the capillary isotachophoretic method, 9.52 +/- 0.085 mmol/l, agrees well with the reference value, 9.64 mmol/l. The results obtained by capillary isotachophoresis showed good agreement with those obtained by the coupled-enzyme method (r = 0.995).