Stable isotope dilution method using thermospray liquid chromatography/mass spectrometry for quantification of daily cortisol production in humans

A thermospray high‐performance liquid chromatography/mass spectrometry method for the separation and quantification of cortisol in human serum has been developed. The technique does not require derivatization, allows for both qualitative and quantitative determinations, provides increased specificity not available from conventional high‐performance liquid chromatography, and has a detection limit of 5 pmol on‐column. This isotope dilution mass spectrometry method, using d₃‐cortisol as an internal standard, allows precise determinations even at low isotopic ratios (2%–5 mole%). Evidence that this technique can be applied to the quantification of serum cortisol and to the determination of daily cortisol production in humans is presented.