Thermal Lens Spectrometric Detection of Catecholamines after Oxidation to Aminochromes

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Abstract
Experimental conditions for the spectrophotometric and thermal lens spectrometry (TLS) detection of catecholamines after oxidation to aminochromes with hexacyanoferrate (III) are optimized. At the low concentrations used in TLS, and in a 0.07 M citrate buffer, catecholamine oxidation can be performed at pH 7 and is immediate, whereas a lower pH value is required in spectrophotometry to avoid aminochrome polymerisation, the oxidation reactions being much slower. Similar TLS sensitivities are obtained for all catecholamines which facilitates HPLC evaluation. Sensitivity can be enhanced using a 50% ethanol-water medium. The linear dynamic range extends over two orders of magnitude, the limit of detection being about 1 ng ml−1 with a reproducibility of 2%, indicating that TLS is adequate to detect catecholamines as aminochromes in physiological samples after HPLC separation. A chromatogram of a urine sample extract is given.

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