The extinction coefficient, ε, is a physical constant that is a key component of Beer’s Law regarding the relationship between optical absorption and concentration; A = ε [conc]. By knowing the ε of an oligonucleotide one can readily convert the optical absorbance reading into concentration, and then into mass. The ε is based on the actual bending and vibration of the bonds and takes into account the absorbance of the individual bases and the effects of neighboring bases. The ε is derived from the exact nucleotide composition of the oligonucleotide so it is unique to every oligonucleotide sequence. The units for the ε are expressed normally as OD₂₆₀ units /μmole. TriLink calculates the ε of an oligonucleotide using the nearest neighbor model which is considered to be the most accurate method without actually determining the ε empirically, which requires a great deal of experimental effort.