Reflectiometric determination of trace urea using urease.

Abstract

Sample solution (10 μl) was mixed with the urease solution (5 μl), which contained 1800 units urease and 25 ml glycerin in 100 ml, in the tip of a micro pipette. After standing for 20 min, the mixture (15 μl) was dropped on the pH test paper, and the reflection absorbance (λ=570 nm) of the test paper was measured by spectrophotometry. The pH test paper was prepared by placing 40 μl of 0.1 % (w/v) Bromophenol Red {pH 5.2(yellow)pH 7.0(red)} aq. solution (pH 5.2) onto the filter paper (5B) and drying it in a desicator for 12 h. A linear relation was obtained between the concentration of urea (5120 ngN/10 μl) and the reflection absorbance of the test paper. The detection limit (S/N=3) was 2 ngN/10μl. Reproducibility tests (n=7) showed that the relative standard deviation of the response was 5.5% for 30 ngN/10μl.