Improved Multifrequency Phase-Modulation Method That Uses Rectangular-Wave Signals to Increase Accuracy in Luminescence Spectroscopy

We propose a novel multifrequency phase-modulation method for luminescence spectroscopy that uses a rectangular-wave modulated excitation source with a short duty cycle. It is used for obtaining more detailed information about the luminescence system: the information provided by different harmonics allows estimating a model for describing the global frequency response of the luminescent system for a wide range of analyte concentration and frequencies. Additionally, the proposed method improves the accuracy in determination of the analyte concentration. This improvement is based on a simple algorithm that combines multifrequency information provided by the different harmonics of the rectangular-wave signal, which can be easily implemented in existing photoluminescence instruments by replacing the excitation light source (short duty cycle rectangular signal instead of sinusoidal signal) and performing appropriate digital signal processing after the transducer (implemented in software). These claims have been demonstrated by using a well-known oxygen-sensing film coated at the end of an optical fiber [a Pt(II) porphyrin immobilized in polystyrene]. These experimental results show that use of the proposed multifrequency phase-modulation method (1) provides adequate modeling of the global response of the luminescent system (R-2 > 0.9996) and (2) decreases the root-mean-square error in analytical determination (from 0.1627 to 0.0128 kPa at 0.5 kPa O-2 and from 0.9393 to 0.1532 kPa at 20 kPa O-2) in comparison with a conventional phase-modulation method based on a sinusoidally modulated excitation source (under equal luminous power conditions).