Understanding molecular absorption of light (UV-Vis spectroscopy)

In summary, the conversation discusses using a UV-Vis spectrometer to measure concentration and temperature of a supersaturated solution in order to experimentally verify the existence of boundary layers. The use of Beer's Law is also mentioned in relation to the temperature dependence of UV absorption spectra. The concept of hysteresis and its potential influence on the measurements is also brought up, as well as the use of a fluid-jacketed cell for more accurate results. The question of whether knowing the thickness of the sample is necessary to determine the absorption coefficient is also discussed.
  • #1
jonmohajer1
3
0
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/chemistry/tutorials/molspec/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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  • #2
Hi
I wanted to know how to find out absorption coefficient (alpha) of a uv-vis spectrum of thin film to determine energy gap of he material. Somewhere (http://www.sciencedirect.com/science/article/pii/S0925346709003656) it is written like if you know thickness then we can find out alpha. Is it compulsory to know thickness to know alpha?
 
  • #3
jonmohajer1 said:
Hi all,
[snip]

The temperature hysteresis is most likely due to the fact that the temperature in solution is not what is being measured. I.e. in heating and cooling the cell, there is a time lag. [I assume that what you are measuring is the temperature of the sample holder.] For really careful measurements, people will often use a fluid-jacketed cell with a large volume of fluid circulating through a constant-temperature bath. when changing temperatures, you should wait long enough until things have equilibrated (e.g. the spectrum stops changing).
 
  • #4
Deal with the carrion first: The "hysteresis" in the OP's measurements is most likely due to varying concentration in the sample cell as a result of "distilled" solvent hang-up in the plenum space above the light path. Stirring should fix that.
madhusoodan said:
Is it compulsory to know thickness to know alpha?
Yes.
 

1. What is UV-Vis spectroscopy and how does it work?

UV-Vis spectroscopy is a technique used to measure the absorption of light by molecules in the ultraviolet and visible regions of the electromagnetic spectrum. The sample is exposed to a range of wavelengths of light, and the amount of light absorbed is measured by a detector. The resulting spectrum can provide information about the electronic structure and concentration of molecules in the sample.

2. What are the applications of UV-Vis spectroscopy?

UV-Vis spectroscopy has a wide range of applications in various fields such as chemistry, biochemistry, environmental science, and materials science. It is commonly used to analyze the concentration of a substance in a solution, determine the purity of a compound, and study the kinetics of chemical reactions.

3. How does molecular structure affect UV-Vis absorption?

The electronic structure of a molecule plays a crucial role in its absorption of light. The arrangement of electrons in the molecule determines the energy levels that can be excited by light, and thus, the wavelengths of light that will be absorbed. Different functional groups and chemical bonds can lead to variations in absorption spectra, allowing for the identification and characterization of molecules.

4. How is UV-Vis spectroscopy used in quantitative analysis?

UV-Vis spectroscopy is widely used for quantitative analysis due to its high sensitivity and accuracy. The Beer-Lambert law, which states that the absorbance of a solution is directly proportional to the concentration of the absorbing species, is the basis for quantitative analysis using UV-Vis spectroscopy. By measuring the absorbance of a series of standard solutions with known concentrations, the concentration of an unknown sample can be determined.

5. Can UV-Vis spectroscopy be used to study reactions in real-time?

Yes, UV-Vis spectroscopy can be used to monitor reactions in real-time. By continuously measuring the absorbance of a solution at specific wavelengths, the changes in concentration of reactants and products can be tracked. This allows for the determination of reaction kinetics and can provide valuable insights into the mechanism of a reaction.

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