What do UV or PDA detectors measure?
(To see the more detailed answer, click here) Most people would answer “absorbance”, because detector output is displayed in absorbance units. In fact, however, UV detectors do not measure absorbance, they compute absorbance. What they measure is transmittance, the fraction of the incident light that is transmitted through the sample (absorbance is the negative logarithm of transmittance). This may seem like a “distinction without a difference”, but it has a major bearing on detector sensitivitity issues. When we describe a sensitive detector as being able to work at low absorbance, this means it is working at high transmittance. In effect, the detector must measure a small difference between two large values. Other detector types, such as fluorescence, make absolute measurements of small values (e.g., the amount of light emitted at a certain wavelength) — a much easier task. This is part of the reason why these detectors are often more sensitive than UV detectors.
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