It is known that the retention of solute compounds on reversed-phase stationary phases decreases when pure water is used as a mobile phase. Nagae and Enami [5,6] investigated the influence of the length of the alkyl group of the silica gel packing on the degree of the decrease in the retention time of thymine and sodium nitrite. They found that triacontylsilyl (C )-silica gels showed less decrease of the retention time in comparison with octadecylsily silica gels and octylsilyl-silica gels. Therefore, we selected the Develosil C30-UG-5 as a separation column.