Determination of unconjugated catecholamine in urine as dopamine by thermal lens spectrometry

Abstract

A thermal lens spectrometric (TLS) method to determine the sum of the concentrations of unconjugated catecholamines in urine, giving rise to a parameter with diagnostic value, the dopamine index, is proposed. After isolation with alumina, urinary catecholamines are oxidized with hexacyanoferrate(III), and the TLS signal is measured with a 100 mW argon ion laser-pumped spectrometer in ethanol–water (1 + 1). Limits of detection are about 1 ng cm^–3, the repeatability is 2% and the dynamic range extends up to 1 µg cm^–3. The analysis of urine samples is performed by the standard additions method. The concentration of dopamine in urine is always much higher than the concentrations of other catecholamines, and catecholamine TLS sensitivities are similar, dopamine having an intermediate value. This makes dopamine an adequate standard, the expected systematic errors being within the ±5% range.