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Stability of ascorbic acid in solutions stored in autosampler vials.

Ascorbic acid (AA) is one of the most important water-soluble antioxidants in biological systems. It is believed to be essential for scavenging free radicals, thereby reducing the mutation rate of DNA and preventing the development of diseases associated with gene mutations (1). AA plays an integral role in the synthesis of connective tissue and reduces biochemical cofactors and substrates (1). Serum concentrations of AA reflect the availability of AA for biochemical processes in humans and investigated animals (1). Our laboratory has been studying the precision and accuracy of serum AA concentration measurements performed by the medical research community (2). We have prepared several stable reference materials, as well as the stable Standard Reference Material (SRM) 970, Ascorbic Acid in Human Serum, for evaluating laboratory performance in the measurement of AA in human serum (2). We have distributed these materials in several international collaborative studies. These studies indicated that the interlaboratory precision is poorer than expected (relative SD, 10–30%). However, our laboratory studies on SRM 970 and other standard materials indicate that it is possible to routinely measure AA accurately with a high degree of precision (2).

Recently, we observed a significant decrease in the concentration of AA in our calibration solutions during the course of several experiments. We now document that the inside surface of glassware may contain materials that measurably degrade AA within a short period of time (<24 h). This is particularly true for autosampler vials used to hold samples for chromatography. This degradation may also occur in tubes and vials used in the collection and processing of blood and serum samples. It may well be that this type of degradation accounts for a substantial amount of the observed interlaboratory variation. …