Determination of guanfacine in biological fluids by electron-capture GLC.

A quantitative electron-capture GLC method with good sensitivity is described for the determination of guanfacine in human plasma and urine. By condensing its amidino group with hexafluoroacetylacetone, guanfacine is converted to a pyrimidino derivative with better GLC properties than the parent drug. This method allows the determination of guanfacine in plasma and urine at concentrations as low as 0.5 ng/ml and was applied successfully to measurement of plasma levels in humans after therapeutic dosing.