Inner filter effect mediated red-shift in synchronous and total synchronous fluorescence spectra as a tool to monitor quality of oils and petrochemicals

Abstract

In this study, we propose that the Inner filter effect (IFE) mediated concentration dependent red-shift (CDRS) can be used as a tool towards analysing optically dense multi-fluorophoric systems like oils and petrochemicals. It has been shown recently that as the optical density (OD) of the sample, at the excitation wavelength, reaches around 0.87–1 (the exact value is instrument specific), the IFE becomes significant and CDRS shows up; with further increase in OD, the CDRS effect is prominently observed. Moreover, the conventionally used fluorescence measurement procedures are not effective in analysing optically dense systems. It is reasonable to consider IFE as intrinsic part of such a system and use the IFE induced red-shift as a parameter towards analysing the system as a whole. This approach is also rational because IFE is directly related to the OD of the sample matrix. We have verified the proposed approach, using total synchronous fluorescence spectra (TSFS) and synchronous fluorescence spectra (SFS), considering three different kinds of optically dense multi-fluorophoric systems such as accelerated thermal aging of ester oil (Eoil; a potential transformer insulant), quantification of diesel-biodiesel and petrol-ethanol mixtures. The CDRS response with varying sample concentration is captured by linear regression tools like N-way partial least square (N-PLS), unfolded-PLS and PLS models. The proposed approach can invariably be applied to any optically dense multi-fluorophoric systems where CDRS is observed while using normal and synchronous fluorescence measurements.