Determination of steroid hormones by membrane inlet mass spectrometry and desorption chemical ionization

Abstract

This paper presents an optimized version of the recently introduced desorption chemical ionization membrane inlet mass spectrometry (DCI-MIMS) system (Analyst, 2000, 125, 211) for the detection of large fat-soluble biomolecules. Two modifications were made to the DCI-MIMS system: (1) the methane chemical ionization (CI) gas was replaced by isobutene and (2) better ceramic fittings for the ion source were prepared in order to allow for a higher CI gas pressure. The two modifications improved the CI spectra considerably by eliminating the presence of large unexplained clusters of ions, and the detection limits were improved by an order of magnitude, reaching intermediate nanomolar concentrations (low ppb levels) for many steroids. As a demonstration of the technique, the concentrations of the synthetic female sex hormones ethynylestradiol and levonorgestrel in birth control pills were determined. The pills were simply mixed with 20 ml of water, shaken and then passed through the membrane inlet without further treatment. The membrane effectively eliminated interferences from the additives in the pill and good characteristic CI spectra of the two hormones were obtained. Using multiple ion monitoring we obtained quantitative data with a high degree of reproducibility (about 7%) for the extraction/analysis procedure.