In-line fluorescence characterization of sodium fluorescein and application as tracer in residence time distribution...

Residence time distribution (RTD) characterization to validate continuous thermal processing systems at elevated temperatures is an important metric and particularly relevant for ultra-high temperature (UHT) and high-temperature short-time (HTST) treatments. Sodium fluorescein (SF) was investigated as a fluorescent tracer for RTD measurements at temperatures extending beyond previously reported ranges. Using a custom-built flow-through measurement system, fluorescence intensity and emission spectra were recorded from 65 - 136 °C in a stabilized pH 7.4 phosphate buffer. Linear thermal quenching ( ${\alpha }_S$ = -0.54%·K⁻¹, 95% CI: 0.554 to -0.535) and bathochromic shift (0.06 nm·K⁻¹) were quantified and no significant thermal degradation up to 145 °C was identified. Step-response experiments at 25 - 105 °C demonstrated that temperature-normalized fluorescence signals yield consistent cumulative distribution functions across the thermal range investigated, with mean residence times obtained using a semi-empirical beta-distribution model. The independently measured temperature sensitivity from RTD experiments ( ${\alpha }_{RTD}$ = -0.56%·K⁻¹) agreed well with spectral measurements. These results establish a quantitative framework for temperature-invariant RTD characterization in non-isothermal flow-through reactors with SF as tracer.