Measurement of Urinary Serotonin for Clinical Research, Using Mixed-Mode SPE and a High-Strength Silica PFP Column

Importance of Topic

Accurate measurement of urinary serotonin is critical in clinical research to assess serotonin metabolism, monitor neuroendocrine tumors, and evaluate psychiatric or gastrointestinal disorders. Traditional HPLC-ECD methods often face challenges with specificity and matrix interference. The integration of mixed-mode solid phase extraction (SPE) with UPLC-MS/MS using a high-strength silica pentafluorophenyl (HSS PFP) column offers enhanced selectivity, sensitivity, and throughput.

Objectives and Overview

This study aimed to develop and validate a robust, linear, and precise method for quantifying serotonin (5-HT) in human urine. By combining orthogonal sample pretreatment via weak-cation exchange (WCX) SPE with chromatographic separation on an HSS PFP column, the method seeks to minimize matrix effects and achieve baseline resolution of endogenous interferences.

Methodology and Sample Preparation

• Stock Solutions: 1.0 mg/mL serotonin in methanol/0.1% formic acid; 500 µg/mL D4-serotonin internal standard in 50:50 methanol/water with 0.1% formic acid.
• Sample Pretreatment: Acidify urine with 1.67% 6 N HCl, add 40 µL internal standard to 400 µL sample, adjust with 1 mL 0.5 M NH4CH3COO, and load onto conditioned Oasis WCX 96-well plate.
• Washes: 1 mL 20 mM NH4CH3COO and 1 mL methanol. Dry under vacuum for 30 s.
• Elution: Two 250 µL aliquots of 30:70 methanol/water with 5% formic acid; inject 5 µL onto UPLC-MS/MS.

Chromatographic and Mass Spectrometric Conditions

• UPLC System: ACQUITY UPLC I-Class with HSS PFP column (2.1 × 100 mm, 1.7 µm) at 30 °C; sample at 10 °C.
• Mobile Phase A: Water + 0.1% formic acid; B: Acetonitrile + 0.1% formic acid; gradient from 2% to 30% B over 2.5 min.
• MS/MS: Xevo TQD with positive ESI; MRM transitions 177.1 > 160 and 177.1 > 115 for serotonin, 181.1 > 164 for D4-IS; cone 22 V; collision energy 10–22 eV.

Main Results and Discussion

• Retention Time: Serotonin eluted at 2.24 min with sharp peak shape and baseline separation from endogenous compounds.
• Recovery and Matrix Effects: 80% extraction recovery and –19% matrix effect, demonstrating effective removal of interferents.
• Linearity and Calibration: Standard addition to pooled urine (endogenous 29.7 ng/mL) adjusted calibration range to 34.7–529.7 ng/mL; R2 = 0.9997.
• Accuracy and Precision: QC levels at 7.5, 75, and 300 ng/mL showed accuracies 97.3–99.9% and RSD < 2% across replicates (n = 6).

Instrumental Setup

• Oasis WCX 96-well SPE Plate, ACQUITY UPLC I-Class System, ACQUITY UPLC HSS PFP Column, Xevo TQD, MassLynx and TargetLynx Software.

Benefits and Practical Applications

The method delivers high throughput, minimal sample preparation time, and strong analytical robustness. It is well suited for clinical laboratories and research environments monitoring serotonin in patient urine samples, supporting studies in neurochemistry, endocrinology, and pharmacokinetics.

Future Trends and Applications

• Automation: Integration of automated SPE workflows and microelution plates to increase throughput.
• Expanded Biomarker Panels: Coupling with untargeted metabolomics to profile additional indoleamines.
• High-Resolution MS: Implementation of HRMS for structural confirmation and discovery applications.
• Portable Platforms: Development of field-deployable or point-of-care assays leveraging miniaturized LC-MS systems.

Conclusion

This validated SPE-UPLC-MS/MS approach for urinary serotonin quantitation offers excellent selectivity, linearity, and reproducibility. The orthogonal combination of WCX extraction and PFP chemistry ensures minimal matrix effects and reliable data, supporting its adoption in clinical research and diagnostic method development.