I am measuring the fluorescence of Ethidium Bromide during the efflux process using various bacterial strains. Its total uptake is different in different organism but I need to compare all. So I have gone through literature which describe arbitrary unit are more preferential as FLU vary from instrument to instrument. Please let me know the method to convert FLU to Arbitrary Units.
Fluorescence intensity measurements are always in arbitrary units (FLU, RFU, etc all mean the same thing). You can compare the intensity measurements between samples using arbitrary units as long as all the measurement parameters are the same.
To expand a bit on Adam's answer, quite often, the Y-axis of an emission spectrum is given in “arbitrary units.” Absorption values have specific units, namely absorbance or optical density which are independent of instrument parameters such as lamp intensity, slitwidths, PMT voltages and amplifier gains, since the measurements are always referred to the intensity of the incident light. In the case of fluorescence, however, the observed fluorescence intensity will depend upon how much light illuminates the sample – which is a function of the lamp power, the excitation slitwidth, and the efficiency of the excitation monochromator. The observed intensity will also depend on the emission slitwidth, the efficiency of the emission monochromator, the PMT voltage, the details of the electronic amplification (e.g. analog versus photon counting) and the particular optical arrangement of the instrument, i.e., the focal length of the lenses and even the nature of the cuvette utilized, i.e., pathlength. These parameters vary from instrument to instrument, and sometimes even from measurement to measurement. Thus, even though the signal being recorded may actually be the number of photons detected, these counts will vary with extrinsic instrumentation parameters in addition to the fluorophore concentration and photophysical parameters (e.g., extinction coefficient and quantum yield) and hence the descriptor “arbitrary” is utilized. It is also common to see the fluorescence intensity normalized to unity, i.e., the maximum is set to “1.”
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