Development of mobile phases containing high concentrations of UV–Vis absorbable components for ion chromatographic separation of anions with indirect photometric detection.

A new method of how to use high concentrations of UV–Vis absorbable components in mobile phases for ion chromatographic separation of anions with indirect photometric detection (IPD) was developed. The UV–Vis absorption curves, the signal strengths at selected wavelengths, and the response ranges at different wavelengths of three organic salts, sodium benzoate, sodium 4-nitrobenzoate and sodium 3,5-dinitrobenzoate were detected using a photodiode array detector. Mobile phases containing these three organic salts at concentrations as high as 20 mM were used for ion chromatographic separations of inorganic anions, such as F⁻, Cl⁻, NO₂⁻, Br⁻, NO₃⁻, and SO₄²⁻. Separation characteristics detected at different wavelengths such as, the baseline stability, signal to noise levels shown as detection limits, linearity, etc., were investigated. It is found that the best wavelength used for IPD may not be the maximum or near the maximum absorption wavelength, but definitely not the non-absorptive wavelength of the eluent. Therefore, the best wavelength should be optimized. With the optimized wavelength at 385 nm, anions of sub-nanomoles were able to be quantitatively detected, and a good linear relationship with a correlation coefficients, (r > 0.9998), within the sample ranges from 0.5 to 200 ng was available. By using the optimized wavelength, the threshold of the detector will no longer be a strict restriction factor of using high concentrations of UV–Vis absorbable components. Consequently, columns with different ion exchange capacities can be used for separations with IPD. Furthermore, because the separation system can be fast re-equilibrated after sample injection using high concentrations of monovalent ion eluent, the separation efficiency can be highly improved. This method is promising for developing mobile phases for other chromatographic separations with IPD. [ABSTRACT FROM AUTHOR]