Relationship between batch age, methylnicotinate concentration and relative nicotinic acid content of 1 M aqueous solutions of methylnicotinate. Batches of 1 M methylnicotinate were stored at 4°C for increasing lengths of time prior to analysis by HPLC. Filled circles, plotted on the right axis, show the measured concentration of methylnicotinate in each batch. Open circles, plotted on the left axis, show the percentage of nicotinic acid relative to the total methylnicotinate plus nicotinic acid concentration in each batch. Values shown are the mean of quadruplicate determinations; bars indicate the standard deviation. The 'best-fit' linear regression line for the correlation between batch age and nicotinic acid content is indicated by a solid line; the dotted lines indicate the 95% confidence intervals for this line based on the available data.