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Study Notes: Analytical Wavelength

The analytical wavelength for quantitative analysis is normally lamdamax ie the wavelength corresponding to an absorption peak.

Absorption spectra with one peak

There are several reasons for this choice.

Absorbance per unit concentration of analyte is greatest at lambdamax - this provides maximum sensitivity. That is to say, the higher the absorbance, the lower the concentration that the analysis is able to reliably measure.

Consider the comparison (below) of using the analytical wavelength at lambdamax and at half lambdamax - the analyte concentration is some value called ‘x’. If absorbances cannot be read below 0.1 because of the potential for substantial error, you can see in this example that lambdamax enables the analyte concentration to be determined down to at least x/8 whereas this concentration falls below the 0.1 absorbance threshold at half lambdamax. The higher the absorbance per unit of concentration (highest at lambdamax), the more sensitive the analysis.

Analytical wavelength at lambda max and at half lambda max

The absorbance curve is normally flat (or plateaus out) around lambdamax and this circumstance leads to optimum precision and accuracy. There can be some spectrophotometer error in the setting of lambdamax but without there being significant change in the value of the absorbance. On the other hand, if the analytical wavelength is selected from a point on the side of a peak, the same error in wavelength setting can have a significant impact on absorbance. Error in the wavelength setting of a spectrophotometer translates to imprecision/inaccuracy in the calculated concentrations.

A peak on a spectrum showing that variation in wavelength setting leads to relatively small errors in absorbance at lambda max compared to much larger errors at wavelength settings on the shoulder of the peak.

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