Salting-Out Assisted Liquid-Liquid Extraction for UPLC-MS/MS Determination of Thyroxine and Steroid Hormones in Human Serum and Fish Plasma

Measuring the level of steroid and thyroxine hormones is key to understanding organism health conditions. Liquid chromatography coupled with tandem mass spectrometry has become the method of choice for such hormone analyses in clinical laboratories. Detection of hormones at low levels typically requires a time-consuming sample preparation, such as liquid-liquid extraction followed by solvent evaporation and re-solubilization of the sample extract. Instead, we applied salting-out assisted liquid-liquid extraction (SALLE) for the extraction of thyroxine, testosterone, cortisone, and cortisol from human serum and fish plasma samples. SALLE allowed direct injection of sample extracts. Sodium chloride and ammonium sulfate were evaluated as salting-out reagents together with four different organic solvents. High extraction recovery and reduced matrix interference were achieved by using ammonium sulfate together with 10% methanol in acetonitrile. Limits of quantification were in the range of 0.1-0.2 ng/mL and signal responses were linear (R2 > 0.997) up to at least 100 ng/mL for all hormones. The method was applied for hormone measurements in fish plasma. In conclusion, SALLE combines the simplicity of crude protein precipitation with the high analyte enrichment of a liquid-liquid extraction. Here we have presented it as a novel sample preparation method for clinical laboratories where mass spectrometry is utilized in the field of endocrinology.