Second‐order Scattering and Frequency Doubling Scattering Spectra of Thallium(III)‐Methotrexate System and Its Analytical Application

In pH 4.9 Britton‐Robinson buffer solution, methotrexate (MTX) reacted with thallium(III) to form a 3:1 chelate. This resulted in great enhancement of second‐order scattering (SOS) spectra and frequency doubling scattering (FDS) spectra and appearance of new SOS and FDS spectra. Their maximum wavelengths were located at 520 and 390 nm, respectively. The increments of scattering intensities (ΔI) were directly proportional to the concentrations of MTX in the ranges of 0.022–2.0 µg·mL^?1 (SOS method) and 0.008–2.5 µg·mL^?1 (FDS method). The methods exhibited high sensitivities. The detection limits for MTX were 7.4 ng·mL^?1 (SOS method) and 2.3 ng·mL^?1 (FDS method), respectively. The optimum conditions of the reaction, the influencing factors and the effects of coexisting substances were investigated. A highly sensitive, simple and fast method for the determination of MTX has been developed. The method can be applied satisfactorily to the determination of MTX in human serum samples. In this work, the charge distribution of MTX was calculated by a CNDO quantum chemistry method. In addition, the reaction mechanism was discussed.