Liquid chromatographic determination of para-substituted N,N-dialkylaniline N-oxides

A high-performance liquid chromatographic method for the determination of p-substituted N,N-dialkylaniline N-oxides [N,N-dimethylaniline N-oxide (DMANO), N,N-dimethyl-p-toluidine N-oxide, N,N-diethylaniline N-oxide, N-ethyl-N-methylaniline N-oxide (EMANO), p-cyano-N,N-dimethylaniline N-oxide (pCNDMANO), and N-phenylpyrrolidine N-oxide (PPNO)] has been developed. It uses an octadecylsilica column, a mobile phase of methanol-phosphate buffer (pH 7.0, adjusted by triethylamine) and ultraviolet detection. N-Oxides were eluted in the order of hydrophilicity except for PPNO, which eluted between DMANO and EMANO. The number of theoretical plates and the detection limit under optimized conditions were between 2400 (DMANO) and 5400 (pCNDMANO) and between 2.3 microM (pCNDMANO) and 30 microM (PPNO), respectively. The concentration of N-oxides recovered from enzyme reaction mixtures by solid-phase extraction using Sep-Pak C18 (prior to chromatography) was also optimized with regard to the sensitivity and interference.