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Chemistry LibreTexts

Resolution

Because there are so many emission lines for elements other than those in the first two columns of the periodic table, one must worry about line overlaps and resolution of lines of interest from interfering lines of other elements. Lines may be broadened due to the finite lifetime of the excited state (natural linewidth), shortened lifetime due to collisions with other similar atoms (resonance broadening) or dissimilar atoms (collional broadening), the effects of electric fields (Stark broadening) or magnetic fields (Zeeman broadening), motion of atoms with respect to the detector (Doppler broadening), and of course combinations of broadening sources. The resolution of an emission spectrum can be no better than that set by the physical widths of emission lines. It can, of course, be much worse, depending on instrumental performance. This is readily simulated. Try combinations of Doppler Broadening, Instrumental Resolution, and line separation to see how line overlap appears. Interesting combinations are at least the following: for line separation = 0.2 nm:

  Doppler Width (nm)
Instrument Resolution (nm) 0.03/0.025 0.03/0.1 0.03/0.2
  0.06/0.025 0.06/0.1 0.06/0.2
  0.12/0.025 0.12/0.1 0.12/0.2

Questions to ask yourself:

  • How narrow must the instrument resolution be in order to resolve the shape of the spectral line?
  • How narrow must the instrument resolution be to see that there is more than one line?
  • If an instrument has a resolution of 0.1 nm, how many distinct lines can be resolved between 200 nm and 800 nm? If the typical transition element emits 200 lines in this region, how likely is it that spectral interferences can be avoided? If they can't be avoided, how can we ever do emission spectrochemical analysis? (Obviously, such measurements are done by the thousands daily, so there must be SOME solution that's been figured out!)

As if line broadening and instrumental resolution weren't complicated enough, atomic spectra have two more headaches awaiting the analyst, self absorption and self-reversal. Remember blackbody radiation? If we have only 1 atom in what is otherwise a vacuum, the atom can act independently of its surroundings, and the system is said to be optically thin. Put that same atom in the middle of 1023 identical atoms, and especially if it is in the interior of a liquid or solid, it must be in thermal equilibrium with its neighbors. Its individual spectral properties disappear, and the ensemble acts as a black body (or, if it emits more weakly than a black body, a grey body) with a featureless (or nearly featureless) spectrum. The intermediate case, where many atoms emit in an isothermal gas is said to be self-absorbed; if the center of the emission line emits with the radiance of a black body at the same temperature, adding atoms can not increase the intensity at line center. Instead the line broadens, acting like a black body over wider and wider ranges of the spectrum. The key here is that the broadening occurs in an ISOTHERMAL (all parts of the system the same temperature) arrangement.

In contrast, a hot source may (commonly!) be surrounded by a cloud of cold atoms (just as a hot candle is surrounded by cooler air). In this case, the cool atoms can absorb light emitted by the hot source. The extent of absorption depends on both the temperature difference and the concentration of cold atoms. The result is self-reversal, that is, an emission line whose center is lower in intensity that the wings of the line, as simulated in this Excel spreadsheet. Try various emission and absorption temperatures and watch the variety of line shapes that may occur. With a low resolution instrument, one can not see the details of line shape (just as we saw above), but the relationship between emitted light and the total amount of an analyte will depart from linearity (since some emitted light never diffuses out of the emission source). Is it any wonder that different matrices (i.e. solids vs. liquids, powders vs. crystals, inorganics vs. organics, or unknowns of a particular element in the presence of different sets of concomitants) give rise to subtly different working curves (emission as a function of concentration)? The astonishing thing is that in many situations, intensity is quite close to proportional to concentration, and so we can simply do elemental quantitative analysis.