[4] Dansyl chloride is unstable in dimethyl sulfoxide, which should never be used to prepare solutions of the reagent.

[5] The extinction coefficient of dansyl derivatives are important for measuring their concentration in solution.

But even for dansyl chloride, there are a variety of extinction coefficient values that have been reported.

Some of the values are used to estimate the extent of success in attempts to conjugate the dye to a protein.

This compound may be prepared by reacting the corresponding sulfonic acid with excess phosphorus oxychloride (POCl3) at room temperature.