The combination of tunable excitation, multichannel (broadband) detection and computer control of spectrometer components allows us to combine multiple spectroscopic parameters into multivariate measurements with the enhanced selectivity needed to study complex systems. Many variable combinations are possible; our past work includes analyses utilizing fluorescence excitation-dependent broadband fluorescence measurements, temperature- and frequency-resolved broadband fluorescence measurements, and temperature-dependent Raman scattering. Current work focuses on applying time-resolved broadband UV-Vis absorbance and time- and frequency-resolved broadband fluorescence measurements to photo-induced reaction monitoring and characterization. The examples listed below illustrate the scope of this approach.
Fluorescence emission-decay (time- and frequency domain) measurements show great potential to provide great insight in process and reaction monitoring because they can provide better selectivity than their steady-state counterparts. Frequency domain measurements also provide several advantages in the analysis of complex kinetics because of the linearizing capabilities of the Fourier transform. The challenge is to reduce the acquisition time sufficiently to monitor reactions of interest in-situ.
The traditional method of UV-Vis analysis on reactions based on long reactions times or very quick reaction rates on the order of micro-millisenconds can be quite challenging. Flow UV-Vis provides the advantage of having inline measurements with variable data acquisition rates that can be adjusted based on the sample being analyzed in order to get a full timeline of the reaction as it's occurring. This allows an increase in the data acquisition of time sensitive reactions, leading to an improvement with graphical and numerical analysis with techniques such as reaction progress monitoring.
Component spectra isolated from temperature-dependent Raman scattering measurements of the gel-forming phospholipid mixtures reflect the phase state of the lipid fatty acid groups and organization of the associated networks of hydrogen bonded water. The spectra were consistent with the evolution of mixed phase bicelles (nanodisks) and worm-like aggregates, at low temperature to suspensions of entangled worm-like aggregates above the long fatty acid phase transition and perforated multi-lamellar aggregates with at higher temperatures.
Fluorescence correlation spectroscopy (FCS) measurements use spatiotemporal relationships in fluorescence intensities to measure fluorophore translational dynamics. Multicolor versions of these measurements have been limited to a few wavelengths and by distortions such as cross-talk (measuring green photons, for example, at the red photon detector). Recent work demonstrated simultaneous measurement of multiple fluorophores in polymer solutions and the utility of using numerical methods to avoid detector cross-talk.
