Twenty milligram of rutin standard sample is precisely weighed and dissolved in the solution with 40% ethyl alcohol. Then, the solution was transferred to the volumetric flask of 100 mL prior to metered volume and shaking up. In this way, a solution with mass concentration of 0.2 mg rutin per milliliter is obtained. A series amount of the standard solution, including 0, 1.0 ml, 2.0 ml, 1.0 ml, 6.0 ml, 8.0 ml and 10.0 ml, are added into seven 25 ml volumetric flasks, respectively. Then 12.5 mL 40% ethanol solution, 0.5 mL (1:20) solution with mass fraction of 5%, are added in each of the volumetric flasks, and left for 5 min. The solution with the mass fraction of 10% was injected prior to shaking up. After left for 5 min, 4 mL sodium hydroxide solution (1:25) was added, and the solution was homogeneously mixed. Finally, 40% was added to calibrate the mixture, and then left for 10 min. With distilled water as blank control, the absorbance of the solution was measured at 510 nm by ultraviolet spectrophotometer. The Rutin standard curve was drawn with the rutin concentration as abscissa and absorbance the ordinate [2] .

The grass of Houttuynia cordata was dried, smashed and sieved to 60 mesh, prior to immersing into petroleum ether for 2 h (Figure 1). After the supernatant was discarded, petroleum ether was added to immerse the grass again, until the colorless supernatant was observed. Then the sample was dried with drying cabinet (70˚C). Four grams of the Houttuynia cordata powder was taken into return pipe of the Soxhlet extractor along with 120 ml 80%

ethanol solvent. Reflux extraction was carried out at 70˚C for 65 min [3] . Flavonoids filtrate was obtained by vacuum filtration from the extract. After five milliliter of flavonoids filtrate was fetched from the volumetric flask, a solution containing 5 percent of was added into the mixture before homogenously mixed and left for 5 min. Similarly, the solution of 10 percent was added, mixed and left for 1.5 min, before 4 mL 1 mol/L sodium hydroxide was mixed. In the end, 40% ethyl alcohol was used to calibrate, then it was left for 10 min. The flavonoids content was calculated by the method of determining absorbance using ultraviolet spectrophotometer.

According to the experimental process, the amount of extract could be indicated as follows:

The content of flavonoids:.

C is the concentration of the extract, mg/ mL; V is the volume of the extract, mL; M is the mass of the sample, g.

The absorbance of the Houttuynia cordata sample was tested to be 0.574, and the responding concentration was 0.048 mg/mL according to the standard curve. Therefore, the content of the flavonoids was 12 mg/g.

The filtrate was concentrated into a flavonoids-ethanol solution with mass concentration of 500 mg/L, in order to examine the antioxidant activity.

By using the prepared solution, a series of flavonoids sample solution were obtained, of which the concentration were 10 mg/L, 20 mg/L, 30 mg/L, 40 mg/L and 50 mg/L, respectively. Taking 1.0 ml different mass concentration of flavonoids sample solution, 3.0 mL (pH = 6.6, 0.2 mol/L) and 2.5 ml solution which contains 1% mass fraction of Fe (SCN) ₃ into a 10 ml centrifuge tube, the mixture was homogenously mixed and maintained at 60˚C for 25 min. Afterwards, the mixture was subject to cold water and amended with 2.5 mL 10% trichloroacetic acid, and the distilled water was used for calibration. After the centrifugation in a speed of 2500 r/min, 2.5 mL supernatant was taken to mix 0.1% 1.0 mL and distilled water. 15 min later, its absorbance was determined at 700 nm using ultraviolet spectrophotometer. Distilled water and vitamin C were used as reference and control, respectively.

By using certain amount of the standby solution, 10 ml flavonoids sample solution were prepared under different concentration (50 mg/L, 100 mg/L, 150 mg/L, 200 mg/L, 250 mg/L). For flavonoids samples of different mass concentration, 1.0 ml of the solution prepared above was injected into the five test tubes, respectively. Then 1.0 ml/L 10 mmol/L FeSO₄ and 1.0 ml 10 mmol/L salicylic acid-ethanol solution were added into the tube, which was consequently placed in a water bath at 37˚C for 35 min prior to taking out and calibration to the scale line with distilled water. Using ultraviolet spectrophotometer, the absorbance was measured at 510 nm with distilled water as a reference and vitamin C as control. The hydroxyl radical scavenging rate was calculated as follows [4] .

In the equation, Y represents the scavenging rate, A₀ denotes the absorbance of control, A_x is the absorbance of the sample after the flavonoid solution was added, and A_x0 is the absorbance of flavonoids solution with no H₂O₂.

By using certain amount of the standby solution, 10 ml flavonoids sample solution were prepared under different concentration (50 mg/L, 100 mg/L, 150 mg/L, 200 mg/L, 250 mg/L). For flavonoids samples of different mass concentration, 1.0 ml of the solution prepared above was injected into the five test tubes, respectively. Then 4.0 ml/L Tris (hydroxymethyl) metyl aminomethane- HCl buffer solution (pH = 8.2, 50 mmol/L) were added into the tube, which was consequently placed in a water bath at 25˚C for 25 min prior to the addition of 0.1 mL 25 mmol/L pyrogallic acid. After the mixing, the water bath was carried out again at 25˚C for 6 min. Then two droplets of 10 mol/L HCl solution were injected to terminate the reaction. Using ultraviolet spectrophotometer, the absorbance was measured at 325 nm with Tris (hydroxymethyl) metyl aminomethane-HCl buffer solution as a reference and vitamin C as control. The hydroxyl radical scavenging rate was calculated as follows [4] .

In the equation, Y represents the scavenging rate, A₀ denotes the absorbance of control, A_x is the absorbance of the sample after the flavonoid solution was added, and A_x0 is the absorbance of flavonoids solution with no pyrogallic acid.