Recently, I am using reverse phase column to quantify serum vitamin D using Internal Standard, vitamin A acetate in mobile phase (80:20) methanol:water. However, the internal standard is subjected to decrease in peak area with more
impurities peaks just happened in a short time (within 2hr -1 day) after preparation.

Can anybody tell me whether this IS is so unstable in methanol or room light? If it is quite unstable, any alternatives be recommended which is more stable with similar retention time?