Sodium nitrite

Sodium nitrite (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) aqueous solution was added to a constant-pressure funnel at a molar ratio of 4.25:1, and the Sodium nitrite solution was added to a three-mouth flask dropwise to react with sodium bisulfite (Sinopharm Chemical Reagent Co., Ltd.) at 90°C. After a 90-minute isothermal reaction, the esterification agent sodium trisulfonate (N(SO₃Na)₃) was prepared.29 The pH was adjusted to 7 with 1 mol/L of sodium hydroxide solution at 40°C; then, SH (Liyang Biomaterials Co., China) was added (nNaNO₂/mSH = 6/403 mol/g), and the mixture was magnetically agitated for 6 hours. The solution was washed with anhydrous ethanol and then dialyzed for 72 hours. Finally, SHS was obtained by isothermal drying. The reaction steps are shown in Figure 1.

Ethyleneglycol, anhydrous ethanol, sodium acetate anhydrous, iron(iii) chloride hexahydrate, tetraethyl orthosilicate (TEOS), concentrated ammonium aqueous solution (25 wt%), sodium citrate, silver nitrate, sodium nitrite, hydrochloric acid, sodium carbonate, acetonitrile, potassium chloride and sodium chloride were purchased from the Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Salbutamol (SAL), sulfanilic acid, 2,2′-benzidinedisulfonic acid, clenbuterol hydrochloride, l-lysine monohydrate, aminoacetic acid, glycerol and phosphate-buffered saline (PBS, pH 7.4) contained NaCl (100 mM) and 10 mM Na₂HPO₄/KH₂PO₄, were bought from Aladdin Reagent Co. Ltd. (Shanghai, China). Deionized water (Millipore) with a resistivity of 18 MΩ cm was used in all experiments.
2.39 mg SAL was dissolved in 10 mL methanol (1.0 × 10⁻³ M) and kept in the dark at 4 °C as stock solution. Then its standard solution was obtained by diluting the stock solution to the desired concentration with distilled water.

Three fresh chili peppers including linear, millet and bullet peppers were harvested and collected from Weinan, Shanxi Province, Kunming, Yunnan Province, and Zhoukou, Henan Province, China, separately. The fresh linear, millet and bullet peppers were labeled as XF, XMF, and ZF. While the dried linear, millet and bullet peppers were labeled as XD, XMD, and ZD. To ensure the uniformity of physical characteristics of samples, the chili peppers with the same size and maturity were carefully selected and stored at 4 ℃ until utilization. Sodium hydroxide, sodium molybdate dihydrate, methanol, anhydrous ethanol, hydrochloric acid, glucose, sodium nitrite and the internal standard 2-octanol (purity > 99 %) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Capsaicin (HPLC grade) was provided by Shanghai Yuanye Biotechnology Co., Ltd. (Shanghai, China). Ascorbic acid was obtained from ANPEL Laboratory Technologies Inc. (Shanghai, China). All of the other reagents were of analytical grade or above.