Spectroscopy Lab Conceptual Question

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Discussion Overview

The discussion revolves around the use of absorbance maxima versus minima in spectroscopy assays, particularly in the context of UV-Vis spectroscopy. Participants explore the implications of using different absorbance values for quantitative analysis and the reliability of detecting analytes.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants question the rationale behind using absorbance maxima for assays, seeking clarification on the potential issues with using absorbance minima.
  • One participant suggests that reliability in detection could depend on taking multiple absorbance measurements and comparing them to a reference value.
  • Another participant explains that using maximum absorbance is preferred for quantitative analysis because it corresponds to wavelengths with higher absorptivity, which enhances reliability.
  • Concerns are raised regarding the signal-to-noise ratio when using absorbance minima, indicating that this could compromise the quality of the analysis.
  • Participants note that high absorbance intensities can also lead to problems, particularly when scattering is involved.

Areas of Agreement / Disagreement

Participants express differing views on the appropriateness of using absorbance minima, with some supporting the use of maxima for reliability in quantitative analysis. The discussion remains unresolved regarding the implications of using minima.

Contextual Notes

Participants reference the need for a high signal-to-noise ratio and the ideal conditions for absorbance measurements, but specific assumptions and definitions regarding absorbance values and their implications are not fully clarified.

Who May Find This Useful

This discussion may be useful for students and professionals in chemistry and analytical science, particularly those interested in spectroscopy techniques and quantitative analysis methods.

jxs919
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Can someone explain why an absorbance maxium is always used for an assay. What problems would occur if an absorbance minimum were used to get an Absorbance spectrum of a solution?

thanks.
 
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What is your analysis for this question with relevance to the reliability in the detection of the analyte as well as the reliability (precision) involved in the quantitative analysis?
 
hmmm I'm not sure what yu're askin but i think the reliability in the detection of the analysis would be based on taking the absorbance of the solution several times and then comparing the values to a referenced value... right ?

i just don't understand why we have to use a max absorbance value...
why can't we do the minimum value...

help me out some more here ..
 
By "minimum" absorbance value, I'm assuming that there is another absorbance peak which has the smallest of the intensities, right? And, does this pertain to UV-Vis spectroscopy?

Quantitative analysis, such as when constructing a beer's law based calibration plot, is done using the maximum absorbance value, in particular, because it works well with the wavelength peak that has a high absorptivity at that wavelength. For the latest uv-vis spectrometers you want the absorbance peak to be between 1.0 and 3.0 absorbance units, and the peak should be pretty broad, assuming that all peaks have an ideal shape, the one with the highest intensity will be more reliable. The position of the peaks can shift at times, with broader peaks, the corresponding change in the intensity value will be less dramatic.
 
ok yea i see what you mean now. awesome. thanks for your help.
 
jxs919 said:
Can someone explain why an absorbance maxium is always used for an assay. What problems would occur if an absorbance minimum were used to get an Absorbance spectrum of a solution?
You'd compromise the signal to noise ratio.
 
Yeah, and that too, although this can also be a problem with high absorbance intensities when scattering is involved.
 

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