Determination of Calcium in Apples by Resonance Rayleigh Scattering Method with Trypan Blue

Resonance Rayleigh scattering method with trypan blue was applied to the determination of calcium in apples. In Tris-hydrochloric acid buffer solution of pH 6.50, trypan blue reacts with calcium(Ⅱ) and cationic surfactant hexadecyl trimethyl ammonium bromide (CTMAB) to form a ternary ion association complex, which led to a distinctly enhanced resonance Rayleigh scattering (RRS) signal of the system. The maximum resonance Rayleigh scattering peak was located at 348 nm. The optimized conditions found were as follows:① amount of Tris-hydrochloric acid buffer solution of pH 6.50 was 2.00 mL; ② amount of 1.0×10^-4mol·L^-1 trypan blue solution was 2.50 mL; ③ amount of 4.00×10²mg·L^-1 CTMAB solution was 0.40 mL; ④ time of reaction was 25 min. Linear relationship between values of enhanced RRS intensity (ΔI_RRS) and mass concentration of calcium was obtained in the range of 0.007 0-0.080 0 mg·L^-1, with detection limit (3s/k) of 0.001 3 mg·L^-1. The proposed method was applied to the analysis of apple samples, giving values of recovery and RSDs (n=6) in the ranges of 98.3%-102% and 1.8%-2.3% respectively.