Balance of xanthophylls molecular and protonated molecular ions in electrospray ionization

This paper reports the chemical evidence of the balance between radical molecular ions and protonatedmolecules of xanthophylls (an oxygen-containing carotenoid) with a conjugated pi-system (polyene) and oxygen as a heteroatom in ESI and HRESI mass spectrometry. The ionization energy of neutral xanthophylls was calculated by semi-empirical methods. The results were compared with those previously published for carotenoids and retinoids, which have also been shown in ESI-MS to form M+center dot and [M + H](+), respectively. This study demonstrates, for the first time, the correlation of an extended conjugation and the presence of oxygen in the formation and balance of M+center dot or [M + H](+) for the carotenoids, neoxanthin, lutein, violaxanthin and zeaxanthin. Copyright (c) 2005 John Wiley & Sons, Ltd.