The fluorescence ratiometric method involves the use of two wavelengths of light to excite the pH indicator of the sensing films. One of the two excitation wavelengths is at the isosbestic point of the indicator (Ex. λ1) where both the protonated and deprotonated forms of the indicator excite with equal efficiency, while the second wavelength exclusively excites the deprotonated form of the indicator (Ex.λ2).
The pH of the solution is determined from a ratio of the emission signals collected under illumination at each of the two excitation wavelengths. This ratio is independent of indicator concentration giving the approach immunity to calibration drift arising from photobleaching.
Available ranges of operation
pH Low – 3.0 to 6.5
pH Mid – 6.0 to 9.0