Photodegradation Illuminated
Authors:   , Groeneveld Iris 
Published: May 2023

The study of photodegradation processes concerns many fields, including those of cultural heritage, the food industry, and water purification. In each of these areas, different questions concerning photodegradation arise, but generally, they are related to either (i) the prevention of photodegradation aiming to avoid loss or change of properties, such as color, taste or smell, or (ii) the exploitation of photodegradation for removal of potentially harmful compounds in, e.g., drinking water. Studying light-induced degradation (LID) reactions is challenging and often it is difficult to establish a strong link between the degradation and the starting products. Several techniques and approaches for studying photodegradation had been developed previously, but these can be laborious and prone to errors. A solution to this could be found in a comprehensive, automated device that enables simultaneous sample irradiation of compounds in solution and chemical analysis in real-time and after photodegradation.

The need for such an analytical platform is clarified in Chapter 1. The ‘Toolbox for studying the Chemistry Of Light-induced Degradation’ (TooCOLD) project envisioned to develop an integrated device. It would encompass a light-exposure cell, allow in-situ spectroscopic monitoring of the irradiated sample and on-line coupling to liquid chromatography (LC) with diode array detection (DAD) and mass spectrometry (MS) for direct identification of degradation products formed during irradiation. This thesis describes the step-wise development of such an automated device employing a gas-permeable liquid-core waveguide (LCW) as a light-exposure cell, a sample handler, a spectrograph for in-situ absorption spectroscopy, and switching valves for coupling to LC-DAD-QTOFMS. The full device was applied to study the photodegradation of several compounds to evaluate and demonstrate its analytical performance.

Ideally, parameters that affect photodegradation should be known to aid the design of a system that can be used to study photodegradation in the broadest sense. Chapter 2 describes the many parameters that can influence the photodegradation of dyes and pigments in solution and on a substrate.

The light-exposure cell developed in this project was based on an LCW, which employs the principle of total internal reflection (TIR) to irradiate the sample from within in contrast to using perpendicular illumination. Chapter 3 reviews the different types of LCWs that are available and applicable as a photoreactor and for chemical analysis.

The analysis of complex mixtures resulting from photodegradation may be challenging as the chemical properties of the components can vary greatly. Chapter 4 describes the development of a generic LC-DAD method for the analysis of natural and synthetic dyes.

Chapter 5 describes the development and overall performance of a low-volume LID cell based on a gas-permeable LCW made of Teflon AF2400 connected to a spectrograph, allowing the collection of spectral data in real-time.

All of the abovementioned results were included in the design of a full prototype of an LID-cell based device. Chapter 6 describes the analytical performance of the fully automated system coupled to LC-DAD.

Chapter 7 describes the study to the effect of oxygen on the photodegradation of Riboflavin and EY using the final prototype of the device coupled to LC-DAD-QTOFMS.

The feasibility of Raman spectroscopy for studying changes in the molecular structure during irradiation was assessed in several exploratory subprojects, discussed in Chapter 8. Surface-enhanced Raman spectroscopy (SERS) using silver colloids was compared to on-chip SERS monitoring during the photodegradation of CV using leaning pillar chip substrates. Finally, carbon paper was used as a gas-permeable SERS substrate for the implementation inside a microfluidic device.

Finally, Chapter 9 contains conclusions on the work covered in Chapters 1 to 8, discussing several relevant aspects and providing perspectives on future use of the developed system, optimization strategies, and possible new application areas.