ION-TRAP MASS-SPECTROMETRY FOR KINETIC-STUDIES OF STABLE-ISOTOPE LABELED VITAMIN-A AT LOW ENRICHMENTS

The role of beta-carotene in chemoprevention of cancers and other chro nic diseases generated controversy when subpopulations taking beta-car otene supplements showed increased mortality in clinical trails. Deter mination of the dynamics of beta-carotene in individual human subjects has emerged as a high priority, Stable isotope labeled beta-carotene tracers can be employed to determine rates of conversion to retinol (v itamin A), but tracer doses must be small to minimize perturbation of endogenous retinoid and carotenoid pools. In such cases, ratios of lab eled tracer/endogenous retinol are often low, and quantitative analysi s at enrichments of <1 mol % are unreliable owing to ion-molecule reac tions that generate ions at the same mass as the labeled tracer even w hen no tracer is present. The current study demonstrates improved gas chromatography/mass spectrometry quantification of retinol-d(4) and un labeled retinol, as their tert-butyldimethylsilyl ethers, at low enric hments using an ion trap mass spectrometer operated in selected ion st orage mode, Electron ionization of analyte takes place in the ion trap using conditions that eject ions outside the range m/z 390-420, and m olecular ions at m/z 400 and 404 from retinol and retinol-d(4) are qua ntified. Using this approach, unlabeled retinol yields a signal close to values calculated from natural isotopic abundances (similar to 0.13 %), whereas several quadrupole instruments operated using selected ion monitoring yielded 2-5 times greater signal when no labeled retinol w as present.