Understanding molecular absorption of light (UV-Vis spectroscopy)


by jonmohajer1
Tags: beer's law, hysteresis, spectroscopy, uv-vis, vis
jonmohajer1
jonmohajer1 is offline
#1
Jan31-13, 09:26 AM
P: 3
Hi all,
I'm relatively new to the forum, but excited to be a part of a community discussing physics, as I lack company to really discuss my research at my place of work (university), as I am a physicist working solely amongst chemical engineers.

I am using a UV-Vis spectrometer, with the eventual goal to monitor in-situ crystal growth, to measure concentration and temperature of a supersaturated solution at various distances with respect to the growing crystal / solution interface, in an attempt to experimentally verify the existence of 'boundary layers'.

In the process, I have got side tracked and am focussing on trying to understand the temperature dependence of the UV absorption spectra.

I am currently recording spectra of well below saturation solutions of caffeine and paracetamol in distilled water.

Absorption is known to vary linearly with concentration, given by Beer's law (A = εbc , where A is Absorption (dimensionless, a ratio of detected light intensities), ε the absorptivity, normally acquired through calibration using known solute concentrations, considered constant for a given λ, at a given temperature, b is the pathlength (constant), c is the solute concentration.) For a fixed concentration, I have found Abs to vary with temperature, in different ways for different molecules. Hence temperature could be incorporated into Beer's Law by making ε some function of T (species dependent).

But it is not that simple, as in addition to this varying of Abs with T, I have also noted, repeatedly, what could only be described as some kind of hysteresis phenomenon; i.e. some effect on the history of the system on later measurements. I have noticed that in heating the solution (sealed, to avoid solvent evaporation) through a range of temperatures, a particular series of Abs values are obtained at the peak λ, while spectra are recorded again on cooling the solution back down, the data points (Abs at peak λ for a given T) do not fall on the same trend. It is almost always the case, that with each repeated heating, then cooling there is an increase in the Abs at a given T.

Hence it seems insufficient to say that A is a function of only T and c, but also additional variables, whose effect I don't really understand. Rate / direction of heat transfer of the system? Time? What is happening over time to alter the ability of fully dissolved solute molecules to absorb light of a fixed λ?

I have a lot of questions and have quite frankly got a bit lost in holding a coherent fundamental picture of what's going on, so would appreciate any sense someone could talk into me. What is the physical manifestation of heat? Motion, kinetic energy, and in motion the emission of EM radiation (ala blackbody), typically a lot of infra red, not much UV, as UV is assosciated with electronic transitions as opposed to transitions in vibrational states. That is not to say that UV-Vis spectroscopy does not 'see' vibrational states, as they appear overlaid in the broadness of peaks, through their fine structure composed of a number of possible transitions (http://teaching.shu.ac.uk/hwb/chemis...pec/nrglev.gif) that average out to the mean 'E_(n+1) - E_n' equivalent photon λ.

I have also suspected that it is the method of heating, via a cell controlled by the Peltier effect that might influence the performance of the lamp / detector of the spectrometer. Is this possible? I seem to get more stable results when not using the temperature control.

Okay I have a lot more I need clarification with, or just someone else to bounce ideas around with, but don't want to drag on too much, so should probably wrap it up for now and just see if anyone out there thinks they might be up for getting some discussion going on this.

Essentially it's a fundamental question of the interaction of light with matter, that I think calls on;
  • Occupancy of electron energy states (can Fermi-Dirac statistics be incorporated into the absorptivity factor in Beer's Law?)
  • The origin of 'chromophores', and insight to the transformation of bonding over time; possibly due to degradation/transformation by UV light.
  • Heat transfer, and its effect on a molecule's ability to absorb UV light
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