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Quantum Yield

  1. May 21, 2010 #1
    So, I'm trying to work out a method of calculating quantum yields for some samples that contain fluorescent dyes.

    I've dug around and found a procedure for how to do it, which was essentially the same as doing an extinction coefficient (take a sample, run its fluorescence and absorbance, dilute it, run it again, continue until you have at least six data points) in that you end up with a graph and its slope that you can plug into an equation.

    But where I'm having trouble here is that to get an accurate quantum yield, you need to cross-calibrate two different standards. I'm using anthracene (0.27 in ethanol) and Rhodamine B (0.49 in ethanol).

    The equation for doing this is;

    [tex]\Phi_{X}=\Phi_{ST}\frac{Grad_{X}}{Grad_{ST}}\frac{\eta_{X}^{2}}{\eta_{ST}^{2}}[/tex]

    Where 'GradX' is the sample's slope and 'GradST' is the standard's slope. The Etas are for the refractive index of the solvent, which shouldn't be an issue as I've run my standards in ethanol as that was what I could find the literature values for anthracene and Rhodamine B in.

    My understanding of what I need to do at this point is thus;

    For Anthracene, I use the literature value for Rhodamine B's quantum yield for [tex]\Phi_{ST}[/tex], then use my slope for Anthracene as GradX and my slope for Rhodamine B for GradST.

    Then do the reverse for Rhodamine B. What I feel I'm missing here is the standard's slope, GradST. It feels odd that would also come from my data. But I don't know what else I could possibly use.

    Anyone know how to do a cross-calibration for something like this properly? The literature has been little help, and my advisor hasn't been able to work out the issue either.

    Also, I have not been getting garbage numbers entirely, they just are quite different from the literature values. Doing my cross-calibration, I get a QY of 0.346 for anthracene (compare to 0.27 lit value) and 0.38 for Rhodamine B (compare to 0.49 lit value).

    Here is the exact passage from the instructions I have been following as well;


    I'm putting this here as it isn't homework, though the moderators may feel free to move it to the homework section if they feel it is more appropriate for that forum still.
     
    Last edited: May 21, 2010
  2. jcsd
  3. Jun 24, 2010 #2
    The way I have always measured quantum yields is to use :
    [tex]\Phi[/tex]unk = [tex]\Phi[/tex]std * (Iunk /Aunk )*(Istd /Astd )*([tex]\eta[/tex]unk /[tex]\eta[/tex]std )2
    where [tex]\Phi[/tex] is the quantum yield, I is the integrated emission intensity, A is the absorbance at the excitation wavelength, and [tex]\eta[/tex] is the refractive index of the solvent
    Measure an emission spectrum of the standard sample and the unknown sample, keeping all settings (like excitation wavelength, integration time, slit widths etc.) the same. Apply whatever instrument corrections are necessary. Integrate the area under both emission curves and use in the formula above. For the most accurate results choose a standard that emits in the same wavelength range as your unknown and make the absorbance of the standard and unknown samples at the excitation wavelength between .1 and .2 (optically dilute)
     
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