Uncertainty in Measurement of Absorbance

In absorption spectroscopy, precision is limited by indeterminate errors—primarily instrumental noise—introduced when measuring absorbance. Precision is generally worse for low absorbances where P0PT, and for high absorbances when PT approaches 0. We might expect, therefore, that precision will vary with transmittance. The illustration below shows the percent relative uncertainty as a function of absorbance for three categories of indeterminate error: (A) uncertainty in reading the percent transmittance from the spectrometer’s scale and from thermal noise; (B) noise in photomultiplier and photodiode detectors; and (C) fluctuations in source intensity and variations in pathlength due to positioning of sample cell within the spectrometer. The dashed lines correspond to the minimum uncertainty for curve A (absorbance of 0.4343) and for curve B (absorbance of 0.963).


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