Three sample-sparing techniques to estimate the molar absorption coefficient of luminescent dyes

Luminescent dyes are commonly modified to improve their solubility, permeability, or spectral properties. However, changing the chemical structure influences the absorption of light and thus the compound-specific molar absorption coefficient (ε), which also confounds the compound's concentration in solution. The accurate determination of the molar absorption coefficient of new luminescent molecules is labor intensive and challenging when a limited amount of material is available for testing. To address this problem, we developed three techniques combined with UV-Vis spectrophotometry to closely approximate the molar absorption coefficient of various light-emitting dyes. The first technique uses Electrospray Mass Spectrometry to obtain a high-resolution incorporation ratio of a dye-labeled protein. The second approach utilizes covalent linking of the unknown dye to a dye with a known absorption coefficient. In the third method, we used fluorescence correlation spectroscopy to determine the fluorophore concentration in solution. We test each method with well-characterized fluorescent dyes and an uncharacterized chemilumiphore. Each technique produced calculated absorption coefficients comparable to the published reference values, although each presented unique limitations that reduced accuracy under certain conditions. Nevertheless, the techniques could be incorporated into current compound evaluation workflows and require only a small amount of sample, two significant advantages over traditional methods for characterizing new luminescent compounds.