Ion Trap Mass Spectrometry for Kinetic Studies of Stable Isotope Labeled Vitamin A at Low Enrichments

摘要

The role of β-carotene in chemoprevention of cancers and other chronic diseases generated controversy when sub-populations taking β-carotene supplements showed increased mortality in clinical trails. Determination of the dynamics ofβcarotene in individual human subjects has emerged as a high priority. Stable isotope labeled β-carotene tracers can be employed to determine rates of conversion to retinol (vitamin A), but tracer doses must be small to minimize perturbation of endogenous retinoid and carotenoid pools. In such cases, ratios of labeled tracer/endogenous retinol are Often low, and quantitative analysis at enrichments of <1 mol % are unreliable owing to ion--molecule reactions mt generate tons at me same mass as the labeled tracer even when no tracer is present. The current study demonstrates improved gas chromatography/mass spectrometry quantification of retinol-d↓(4) and unlabeled retinol, as their tert-butyldimethylsilyl ethers, at low enrichments using an ion trap mass spectrometer operated in selected ion storage mode. Electron ionization of analyte takes place in the ion trap using conditions that eject ions outside the range m/z 390-420, and molecular ions at m/z 400 and 404 from retinol and retinol-d↓(4) are quantified. Using this approach, unlabeled retinol yields a signal close to values calculated from natural isotopic abundances (～0.13%), whereas several quadrupole instruments operated using selected ion monitoring yielded 2-5 times greater signal when no labeled retinol was present.