Absorbance (A)
Definition
The base-10 logarithm of the ratio of incident to transmitted light intensity, A = log10(I0/I). Dimensionless. Linear with concentration in the working range; departs from linearity above roughly A = 1.5 because of stray light, detector saturation and analyte self-absorption.
- Formula
- A = log10(I0/I)
- Units
- Dimensionless
- Linear range
- Roughly up to A = 1.5
- Causes of nonlinearity
- Stray light, detector saturation, self-absorption
Common questions
Why does linearity break down above about A = 1.5?+
At high absorbance very little light reaches the detector, so stray light and detector noise make up a larger fraction of the signal, curving the calibration away from linear.
How does absorbance differ from transmittance?+
Transmittance is simply the fraction of light passed through the sample, while absorbance is its negative base-10 logarithm, a transform chosen because it is linear with concentration under the Beer-Lambert law.
What should an analyst do if a reading falls above the linear range?+
Dilute the sample and re-measure, since readings taken above the linear ceiling systematically underestimate the true concentration.
Related terms
- Bandpass and Slit Width
- The wavelength interval the monochromator delivers to the sample. Routine forensic UV-Vis runs at 1 nm or 2 nm bandpass; pharmacopoeial methods...
- Chromophore
- The molecular subunit responsible for a dye's colour, typically a conjugated double-bond system; chemical erasure destroys the chromophore while leaving the decolourised...
- Diode Array Detector (DAD)
- A UV-Vis detector that records the full 190 to 800 nm absorbance spectrum of every peak as it elutes, instead of a...
- Molar Absorptivity (ε)
- The proportionality constant in the Beer-Lambert law, in L mol⁻¹ cm⁻¹. A property of the analyte at a given wavelength, solvent and...
- Photomultiplier Tube (PMT)
- Cascaded electron-multiplication detector behind the monochromator slit on a dispersive scanning instrument. Reads one wavelength at a time at very high sensitivity.