Effect of pH on the oxidation of paralytic shellfish poisoning toxins for analysis by liquid chromatography

The effect of pH on the oxidation of individual PSP toxins using both perio date and peroxide oxidations was studied. It was found that the optimum pH for individual toxins varied considerably. For periodate oxidations, pH 8.3 produced the maximum yield of fluorescent products for neosaxitoxin and GT X1/GTX4 while the non-hydroxylated toxins (saxitoxin, GTX2/GTX3, decarbamoy l saxitoxin, GTX5) showed optimum pHs from about pH 10-11.5. Neosaxitoxin a nd GTX1/ GTX4 did not produce significant fluorescent oxidation products wi th peroxide oxidation at any of the pHs studied (pH 8.2-12.8). The non-hydr oxylated toxins all showed optimum pHs above pH 12 with peroxide oxidation. Yields of fluorescent products of these toxins decreased substantially at pHs below pH 12. Neosaxitoxin and GTX1/GTX4 each produced three product pea ks at pH 8.2 with periodate oxidation. There was no pH where these toxins p roduced predominantly a single oxidation product. Decarbamoyl saxitoxin alw ays produced two oxidation products with both oxidation reactions at the pH s studied. However, the relative yields of the products changed with pH. At low pH the second eluting product predominated, while at higher pH values the first eluting product predominated. This pattern was observed for both oxidation reactions. The other non-hydroxylated toxins produced mainly sing le unique products with both oxidation reactions over the pH range studied. No single pH was found optimum for the oxidation of both hydroxylated and non-hydroxylated toxins without a significant compromise in yield of oxidat ion products. This has implications for the post column oxidation liquid ch romatographic methods, since small changes in pH of the post column oxidant can both positively and negatively affect the yields of oxidation products of toxin mixtures leading to increased error in the subsequent quantitatio n of these compounds. (C) 2001 Elsevier Science B.V. All rights reserved.