Some remarks on reported inconsistencies in the high-field EPR spectrum of DPPH

Previous conflicting reports concerning the EPR lineshape of commercially obtained 2,2'-diphenyl-1-picrylhydrazyl (DPPH) at frequencies above 200 GHz are investigated. The magnetic parameters of dilute DPPH in polystyrene film determined at 220 GHz are presented. It is demonstrated that commercially obtained crystalline samples of DPPH which produce a broad line at 220 GHz when used without purification do produce a narrow line at 220 GHz when recrystallized from carbon disulfide. This observation suggests that the reported discrepancies in the high-field EPR spectrum of DPPH may be reconciled without invoking chemical impurities or misidentification. It also suggests a standard preparation procedure for DPPH that should be employed if DPPH is to be used as a marker for high-field EPR.