The following chemicals were obtained from various suppliers: o-phthalaldehyde (OPA), disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium carbonate from Nacalai tesque (Kyoto, Japan); glycine and N-acetylcysteine (NAC) from Sigma (St. Louis, MO, USA); citric acid, sodium citrate, and sodium polyacrylate from Wako (Osaka, Japan); Triton X100 and ethanol (95%) from Sigma (St. Louis, MO, USA); alcohol from Shimakyu’s Pure Chemicals (Osaka, Japan); and hydrochloric acid from Union Chemical Works Ltd. (Taichung, Taiwan). All analytical-grade reagents were used without further purification. Ultra-pure water with a resistivity greater than 18.2 MΩ cm was prepared using a Direct-Q gradient system (Millipore, Milford, MA, USA).
Buffers were prepared by dissolving the following chemicals in deionized water and storing them at room temperature as the stock: borate buffer (0.1 M, pH 8–10 with NaOH), phosphate buffer (0.1 M, pH 6–8 with NaOH), and citrate buffer (0.1 M, pH 4–6 with NaOH). The 20% OPA reagent was prepared by mixing 2 g of OPA in 10 mL of 99% ethanol. The 1% OPA reagent was freshly prepared by dissolving 0.1 g OPA in 10 mL of ultra-pure water and then diluting it to 0.6, 0.5, 0.4, 0.3, and 0.2%.
Buffers were prepared by dissolving the following chemicals in deionized water and storing them at room temperature as the stock: borate buffer (0.1 M, pH 8–10 with NaOH), phosphate buffer (0.1 M, pH 6–8 with NaOH), and citrate buffer (0.1 M, pH 4–6 with NaOH). The 20% OPA reagent was prepared by mixing 2 g of OPA in 10 mL of 99% ethanol. The 1% OPA reagent was freshly prepared by dissolving 0.1 g OPA in 10 mL of ultra-pure water and then diluting it to 0.6, 0.5, 0.4, 0.3, and 0.2%.