Determination of Acephate in Ginseng by Visible Diffuse Reflectance Spectroscopy with Enrichment on Nylon Membrane

Powdered ginseng sample (about 1 g) was extracted ultrasonically with 7 mL of acetone for 10 min, and the extract was separated and concentrated to a volume of 1.0 mL. 4 mL of water, 1 mL of H₂SO₄ and 5 mL of 40 g·L^-1 K₂S₂O₈ solution were added, and the solution was heated to incipient boiling for 30 min, while keeping its volume between 25 to 30 mL. After cooling, acidity of the solution was adjusted to pH 5-8 with 100 g·L^-1 NaOH solution. The solution was transferred to a 50 mL volumeric flask, and 2.0 mL of 26 g·L^-1 ammonium molybdate solution and 1.0 mL of 100 g·L^-1 ascorbic acid solution were added and diluted to 50.0 mL with water. Let the solution to stand for 10 min. 5.0 mL of the solution were taken and filtered through nylon membrane by suction at a pressure of 0.07 MPa, during the process of which the "Mo blue" was adsorbed on the membrane. At the end of filtration, the membrane was taken and dried in air, and then the reflection absorbance of "Mo blue" on the membrane was measured by visible-diffuse reflectance spectrometer. It was shown that linear relationship between values of reflection absorbance and respective mass concentrations of acephate was obtained in the range of 0.5 to 5.0 mg·L^-1 with detection limit (3s) of 0.18 mg·L^-1. Using blank ginseng sample as matrix, recovery was tested by standard addition method, giving results of recovery in the range of 94.0% to 104%, and values of RSDs (n=5) found were in the range of 1.4% to 5.7%.