Hadamard transform CE-UV detection for biological samples
A Hadamard transform-capillary electrophoresis-UV (HT-CE-UV) detection technique is described for the analysis of biological samples. Pseudorandom injections of sample and buffer according to a simplex matrix obtained from the corresponding Hadamard matrix is performed with conventional capillaries. Alternating injections are achieved with a novel capillary "T" connector created by drilling conventional capillary dimensions through a 1-cm diameter polycarbonate disc. This connector design coupled with a switching system allows for rapid, electrokinetic injections of solution into alternating sample and buffer capillary arms for UV detection. The standard mixtures of nitric oxide (NO) metabolites, nitrite and nitrate, dissolved in physiological saline solution are injected into the separation capillary according to an 83-element injection sequence to obtain a signal-to-noise ratio (S/N) enhancement of ca. 4.5 over a single injection. Nitrite, being the less concentrated metabolite in NO detection and thereby more difficult to detect, was calibrated with the HT-CE-UV method and a limit of detection (LOD) of 0.56 lM was obtained. Rat blood plasma was analyzed with this detection system and demonstrated to be comparable with NO metabolite concentrations of previously published results. This HT-CE-UV method is described where a unique reservoir tube design that contains 8-lL standard nitrite sample volumes is placed over the end of the capillary arm to explore low volume limits for biological samples.