3.6. Stability of the caffeine RM
To assess the stability of the caffeine RM, a sample was stored at 4°C and at 20°C, and then measured three times by HPLC at three time points (0 months, 2 months, and 6 months) at each temperature. The measurement results shown in Table 5 were plotted as a function of time and the regression lines were calculated to check for significant trends, possibly indicating degradation of the material, as shown in Figure 11, Figure 12.