# How can I calculate as exactly as possible the factor for the photometric determination of concentration of oligonucleotides?

In general, the following applies: F = 1: (e x l) where F = factor [(µg/ml)-1 cm-1], e = molar extinction coefficient [M-1 cm-1], l = optical layer thickness of cuvette [cm]. For small molecules like oligonucleotides, for example, the correct extinction coefficient is determined from the base composition. As the concentration of oligonucleotides is commonly reported as mmol/liter, a millimolar extinction coefficient (E) is conventionally used in the Beer-Lambert equation by means of: E = A (15.3) + G (11.9) + C (7.9) + T (9.3) A, G, C and T here stand for the number of corresponding bases in this oligonucleotide, the numbers in brackets for the molar extinction coefficient of any deoxynucleotide at pH 7.0. Source: Sambrook et. al. Molecular Cloning (2001).Third Edition, A8.

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