Optical pH sensing using spectral analysis

Spectral analysis by chemometric methods is applied to optical pH sensing with aminofluorescein as fluorescent indicator. This method is superior to the ratiometric dual-wavelength detection when the pH value influences the spectrum in a complex way. In the case of aminofluorescein the range of measurement is extended compared to the ratiometric method. The pH value can be calculated from the fluorescence spectra of the indicator independently of the absolute intensity and of the temperature of the sample. At high indicator concentrations the fluorescence spectra are strongly influenced by inner filter effects and the error of the pH prediction is significantly smaller than in a dilute solution. The reduction of the fluorescence spectra to a few data points shows that the chemometric pH determination does not require high-resolution spectral sensors.