Sensitive trace-first liquid chromatographic determination of 4-aminopyridine in 3,4-diaminopyridine. 
For the treatment of human neuromuscular diseases, 3,4-diaminopyridine (DAP) is six to ten times more effective than 4-aminopyridine (AP), but only half as convulsant and toxic.  Therefore, there is a need for the determination of AP in DAP.  With only conventional equipment, high-pressure liquid chromatography can be used for the extremely sensitive detection of a trace contaminant under one condition: that is, the trace must be eluted before the major component it contaminates.  Prior elution presents a trace peak in a fully exploitable form that is maximally efficient and maximally observable.  This has already been demonstrated with a Pirkle-concept chiral stationary phase for determination of a chiral trace.  However, its application to determination of a nonchiral trace with a reversed phase has not previously been reported.  Such an application is reported here.  In this demonstrative study, selectivity and loading capability were iteratively improved.  Ion pairing with dodecanesulfonate maximized selectivity.  It was again shown that using a less concentrated sample in greater volume maximizes loading capability without obscuring the peak of the trace.  Eventually, the ability to detect 0.005% AP in DAP was demonstrated.  Whether that sensitivity might be improved still more, perhaps with a larger column, was not established.