Assay methods for determining purity of a sample of a sterol are provided. These methods include providing an HPLC eluent including a sterol, other compounds related to the sterol and a mobile volatile phase; generating an aerosol of liquid droplets from the HPLC eluent; drying the liquid droplets to obtain residue particles of the sterol; contacting the residue particles of the sterol with an ion stream which applies a size-dependent electrical charge to each of the residue particles to generate an electrical signal and measuring the electrical signal to determine the purity of the sterol in the sample. The sterol can be OXY133 or OXY133 monohydrate.