Development of a liquid chromatography-tandem mass spectrometry method for plasma-free metanephrines with ion-pairing turbulent flow online extraction

Liquid chromatography-tandem mass spectrometry has become the preferred technology to measure unconjugated metanephrine and normetanephrine in plasma because of its high sensitivity and specificity over immunoassay and gas chromatography-mass spectrometry. In our earlier study, plasma metanephrines were extracted with offline ion-pairing solid-phase extraction and quantified by liquid chromatography-tandem mass spectrometry with porous graphitic carbon column based chromatography. In this study, we aim to automate the sample preparation with turbulent flow online extraction technology and maintain or improve the analytical performance previously achieved from the offline approach. The online extraction was done with a mixed-mode cation exchange turbulent flow chromatography column assisted with ion-pairing reagent and porous graphitic column was used for chromatographic separation. The total online extraction and analytical LC runtime was 12 min. This method was linear from 6.3 to 455.4 pg/mL for metanephrine; 12.6 to 954.5 pg/mL for normetanephrine with an accuracy of 80.6% to 93.5% and 80.9% to 101.7%, respectively. The lower limit of quantitation was 6.3 pg/mL for metanephrine and 12.6 pg/mL for normetanephrine. Inter-assay and intra-assay precision for metanephrine and normetanephrine at low and high concentration levels ranged from 2.0% to 10.5%. In conclusion, we have developed a fast and sensitive automated online turbulent flow extraction method for the quantitative analysis of plasma metanephrines. Ion-pairing reagent was necessary for the success of this method.