Determination of Iron in Powder Food by Fluorescence Spectrophotometry with Phenanthroline

Based on the principle that the complex formed by phenanthroline and Fe²⁺ weakens the fluorescence intensity of phenanthroline in tris(hydroxymethyl)methyl aminomethane solution, the method mentioned by the title was proposed. 1.000 0 g of the sample was taken, and 8.00 mL of nitric acid and 5.00 mL of 30% (mass fraction) hydrogen peroxide solution were added. The mixture was settled overnight, and then heated to near dryness. 5.00 mL of nitric acid and 5.00 mL of perchloric acid were added, and the mixture was continuously heated to near dryness. After 2.50 mL of 1.0 mol·L⁻¹ nitric acid solution was added for dissolution, the volume of the mixed solution was made up to 25 mL by water. A certain volume of sample solution was taken(1.00 mL for protein powder solid beverage sample, 1.50 mL for infant formula milk powder sample, and 2.00 mL for flour sample), and 1.00 mL of 50.0 g·L⁻¹ hydroxylamine hydrochloride solution was added. The mixed sample solution was settled for 2 min, and 4.00 mL of 5.00×10⁻⁴ mol·L⁻¹ phenanthroline working solution was added. After mixing well, 2.50 mL of 1.0 mol·L⁻¹ tris(hydroxymethyl)methyl aminomethane solution was added and the volume of the mixed sample solution was made up to 25 mL by water, settling for 10 min. The fluorescence intensity of the system at 363 nm was measured at the excitation wavelength of 323 nm. It was shown that linear relationship between the decrease in fluorescence intensity of phenanthroline and mass concentration of Fe³⁺ was found within 1.344 0 mg·L⁻¹, with detection limit (3s/k) of 0.009 mg·L⁻¹. The method was applied for the determination of iron in powder food, and RSDs (n=5) of background values ranged from 1.7% to 7.6%. Test for recovery was made by the standard addition method, giving results in the range of 94.6%－99.9%, with RSDs (n=5) of the determined values in the range of 1.5%－4.1%. After F-test and t-test analysis, there was no significant difference among these methods, including this method, UV-Vis spectrophotometry and flame atomic absorption spectrometry, but the sensitivity of this method was higher.