I understand the purpose behind infrared spectroscopy and mass spectroscopy, but UV-VIS doesn't make much sense to me.

Through research, I see that it has to do with electron orbitals, and it has a relation to Beer's Law.

But what exactly can I learn about a molecule from its absorbance/transmittance of UV/Visible light?

  • 4
    $\begingroup$ Emission and absorption in the visible and UV can uniquely identify the material in question. $\endgroup$
    – Jon Custer
    Jun 12 '15 at 22:05
  • 1
    $\begingroup$ @JonCuster How so? I mean if I have an unkown compound and run it with a UV VIS spectrophotometer, wouldn't I have to go through thousands of other compounds to see if the spectra match to identify the sample? $\endgroup$ Jun 12 '15 at 22:08
  • $\begingroup$ And you don't have to do that for IR and mass spec? $\endgroup$
    – Jon Custer
    Jun 12 '15 at 22:09
  • $\begingroup$ It's another way to identify chemicals $\endgroup$
    – user15489
    Jun 12 '15 at 22:11
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    $\begingroup$ Often, IR energies match to vibrational states in a molecule. Vis/UV will probe electronic states. You might very well need both to identify something. Consider how useful color vision is to rapidly distinguishing, say, chlorophyll from xylophyll. All the different phosphor materials have different absorption and emission lines, lifetimes, etc... Often, just as for IR or mass spec, you will need to use some other info to help you out. $\endgroup$
    – Jon Custer
    Jun 12 '15 at 22:18

The broad absorption bands of the electronic transitions observed in UV/Vis spectroscopy/spectrophotometry, as well as the myriad types of electronic transitions that might lead to absorption at a given wavelength, indeed make the technique much less powerful for identification of unknown compounds as compared to, e.g., IR, NMR, microwave, and mass$^*$ spectroscopies.

Jon Custer is correct in his comment to the question that UV/Vis spectra can provide key information in identifying unknown species. However, I know of only two approaches to obtaining predictions of a given UV/Vis spectrum:

  • Experimental spectra of compounds suspected to be structurally related to the unknown species
  • Quantum chemical computation of the electronic transition spectra of candidate species

The former approach requires the relevant experimental data to exist; the latter is not trivial to perform accurately. Both suffer from significant uncertainties: How similar is a reference experimental spectrum to that of the unknown species? Can I trust that my quantum computation will give realistic band locations and strengths?

In my experience, the strength of UV/Vis lies in its facility for inexpensive, convenient, routine, non-destructive quantitation of known solutes, with known absorption spectra and/or extinction coefficient(s) at given wavelength(s). The following list of strengths is almost certainly incomplete:

  • The instrumentation and consumables are relatively inexpensive.

  • No auxiliary cooling, gas supply, etc. are required.

  • Preparation and analysis of fixed-volume samples is trivial: pour liquid into cuvette; place cuvette into instrument; measure.

  • Continuous analysis of dynamic systems (e.g., chromatographic eluates and ongoing reactions) is possible using a flow cell.

    • A simple example is described here, unfortunately paywalled.
    • A wide variety of configurations is presented at a commercial vendor here.
      (No endorsement intended. There are numerous other suppliers out there; I chose this one because it had a good variety shown in schematic.)
  • Interference from most solvents is minimal, particularly in the visible range.

  • Beer's Law in its linear form has a relatively high dynamic range for most UV/Vis-active solutes$-$that is, the concentration where absorbance deviates from linearity is much greater than the minimum concentration detectable:

$$ \frac{C_{\max}}{C_{\min}} \gg 1 $$

  • Another implication of Beer's Law is that interference(s) from a small number of absorbing co-solutes are relatively straightforward to handle, either by measuring at isobestic points or by measuring multiple wavelengths and solving the linear absorbance equations for the unknown concentrations. For example, if two species $\ce{X}$ and $\ce{Y}$ are present and absorbances are measured at wavelengths $\lambda_1$ and $\lambda_2$, the concentrations can be found by solving the following two expressions:

$$ A_{\lambda_1} = \epsilon_{\ce{X},\lambda_1}\cdot C_\ce{X} + \epsilon_{\ce{Y},\lambda_1}\cdot C_\ce{Y} $$ $$ A_{\lambda_2} = \epsilon_{\ce{X},\lambda_2}\cdot C_\ce{X} + \epsilon_{\ce{Y},\lambda_2}\cdot C_\ce{Y} $$

  • The steady-state nature of the measurements allows for 'deep' examination of a particular sample.

    • The full UV/Vis spectrum for a given volume of sample can be measured, giving a rich "fingerprint" of the particular solution and the solutes in it
    • The absorbance at a particular wavelength can be tracked over time, allowing for detailed kinetic analyses
  • At least in the visible range, the quality of the substance being measured is directly perceptible to the human senses. This is actually quite viscerally satisfying, to see that a sample anticipated to contain significantly more of a solute does indeed produce a noticeably stronger coloration of the solution.

While IR and microwave spectroscopies share some of the above advantages, they are far less suited for analysis of solutes, as the solvent absorbance is often much more likely to confound measurements. Further, the instrumentation and sample preparation are generally much more involved.

Mass spec is terrifically expensive and complicated, is an intrinsically destructive method, and is less well suited for direct quantitation since there is no way to know a priori what fraction of the injected sample is actually making it to the detector and thus careful calibration is required.

NMR, though non-destructive, also requires quite expensive equipment that is complicated to operate properly, and interpretation of the spectra requires quite a bit of practice.

I know too little about EPR/ESR, EXAFS, XANES, and others to comment on them here. I believe the above strengths hold in comparison to those methods as well, however.

$^*$ Technically, MS is mass spectrometry, not spectroscopy, since the measurement is of discrete ion impacts on a detector, as opposed to absorbance of portions of the electromagnetic spectrum by a detector.


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