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Supercritical fluid extraction-supercritical fluid chromatography of saliva: Single quadrupole MS monitoring of caffeine for gastric emptying studies†.

Saliva is an attractive sampling matrix for measuring various endogenous and exogeneous substances but requires sample treatment prior to chromatographic analysis. Exploiting supercritical CO2 for both extraction and chromatography simplifies sample preparation, reduces organic solvent consumption, and minimizes exposure to potentially infectious samples, but has not yet been applied to oral fluid. Here, we demonstrate the feasibility and benefits of on-line supercritical fluid extraction coupled to supercritical fluid chromatography and single-quadrupole MS for monitoring the model salivary tracer caffeine. A comparison of 13 C- and 32 S-labeled internal standards with external standard calibration confirmed the superiority of stable isotope-labeled caffeine over non-analogous internal standards. As proof of concept, the validated method was applied to saliva from a magnetic resonance imaging study of gastric emptying. After administration of 35 mg caffeine via ice capsule, salivary levels correlated with magnetic resonance imaging data, corroborating caffeine's usefulness as tracer of gastric emptying (R2 = 0.945). In contrast to off-line methods, on-line quantification required only minute amounts of organic solvents and a single manual operation prior to on-line bioanalysis of saliva, thus demonstrating the usefulness of CO2 -based extraction and separation techniques for potentially infective biomatrices. This article is protected by copyright. All rights reserved.