Can spectrophotometers or colorimeters be used to measure fluorescence?

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Document ID TE7896


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Published Date

Published Date 08/01/2019
Can spectrophotometers or colorimeters be used to measure fluorescence?
Spectrophotometers and colorimeters compared to fluorometers and spectrofluorometers.
In fluorescence testing, the instrument passes a specific wavelength of light called the excitation wavelength through the sample. Some of the light is absorbed and some is transmitted. Some or all of the light absorbed is "re-emitted" at a different wavelength of light called the emission wavelength. What is measured is the amount of light that gets emitted.
In the case of spectrophotometers and colorimeters, the detector on the instrument doesn't differentiate between what wavelengths hit the detector. The detector is at 180° from the light source. The wavelength of light that gets sent through the sample is fixed in colorimeters and adjustable in spectrophotometers, but the wavelength detected by the detector isn't differentiated. This is because the color filter is between the light source and the sample, and not between the sample and the detector. In a fluroescent sample, the transmitted light and the emitted light will both be picked up by the detector equally. Essentially the emitted light will cause a positive bias on the percent transmittance (%T) and a negative interference on the absorbance (ABS). They are set up to measure the amount of light absorbed by the sample, not light emitted by the sample.
What would be needed for these types of applications is a fluorometer, which has a color filter between the the light source and the sample, as well as a light filter between the sample and the detector (which is at 90° from the light source), so that none of the transmitted light hits the detector.

Most fluorometers are like colorimeters where the wavelengths are fixed, although spectrofluorometers do exist where the two filters can be set to different wavelengths independently.
Hach carries only one fluorometer called the BW680 (Product # 2037700)  which is for measuring Chlorophyll-a in ballast water. The wavelengths are fixed, and the instrument is not programmable.