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The routine maintenance of analytical laboratory instruments should always form part of the research budget. Unexpected instrument breakdowns can cause budget constraints – a situation recently experienced at our laboratory. We investigated the use of a more affordable alternative sample injection loop which was installed and validated for the Thermo Scientific™ DionexTM UltiMateTM 3000 series HPLC system’s WPS-3000TSL analytical autosampler. Linearity and precision validation parameters were used to test the alternative sample injection loop viability. A total of eight different injection volumes were tested. Both parameters tested gave acceptable validation values, with a linearity regression value of r2 = 0.999 over the eight different injections and %RSD reproducibility of less than 1.0% per injection volume over 10 repeated injections. These results support the use of the alternative sample injection loop on this HPLC analytical autosampler instead of the more expensive original. [ABSTRACT FROM AUTHOR]

The determination of catechol-O-methyltransferase (COMT) activity is considered valuable for various pharmaceutical and biomedical research projects. A specific high performance liquid chromatography-coulometric electrochemical detection method, for the assay of COMT activity was developed by measuring the formation of normetanephrine from norepinephrine. The chromatographic separation was achieved on a C 18 reversed phase column with a mobile phase consisting of 10 mM sodium dihydrogen phosphate buffer, 4 mM sodium 1-octanesulfonate, 0.17 mM ethylenediaminetetra-acetic acid disodium salt, 6 % methanol and 4 % acetonitrile (pH ± 4.0). The detection of normetanephrine was achieved through electrochemical detection, with a coulometric cell potential setting of +450 mV. The flow rate was at 1 ml/min and the total run time was 45 min. The method was validated according to validation guidelines (Shabir 2006; European Medicines Agency 2011; US FDA 2018). The method was found to be linear (R² > 0.99) over the analytical range (100 to 2500 ng/ml) for all the analytes. All the other validation parameters (sensitivity, precision, accuracy, recovery and stability) were acceptable and within range. The method was applied for the determination of COMT activity in rat liver homogenate test samples. The known selective COMT inhibitor entacapone was used as test inhibitor. The results confirmed the ability of entacapone to inhibit COMT activity by decreasing the production of all the metabolites of norepinephrine.