Investigations of β‐carotene radical cation formation in infrared matrix‐assisted laser desorption electrospray ionization (IR‐MALDESI)

• Published in: Rapid communications in mass spectrometry 2021-08, Vol.35 (16), p.e9133-n/a
• Main Authors: Bagley, M. Caleb, Muddiman, David C.
• Format: Article
• Language: English
• Subjects: Ascorbic acid; beta Carotene - analysis; beta Carotene - chemistry; Carotene; Cations; Cations - analysis; Cations - chemistry; Desorption; Electrospraying; Ethanol; Ice cover; Imaging techniques; Infrared analysis; Infrared lasers; Ionization; Ions; Mass spectrometry; Particulates; Protonation; Reducing agents; Scientific imaging; Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization - methods; Spectroscopy; Tandem Mass Spectrometry; Thermal degradation
• ISSN: 0951-4198; 1097-0231
• DOI: 10.1002/rcm.9133

Rationale Radical cationization of endogenous hydrocarbons in cherry tomatoes was previously reported using infrared matrix‐assisted laser desorption electrospray ionization (IR‐MALDESI), a mass spectrometry imaging technique that operates at ambient conditions and requires no sample derivatization. Due to the surprising nature of this odd‐electron ionization, subsequent experiments were performed on β‐carotene to determine the amount of radical cationization across different sampling conditions. Methods β‐Carotene was analyzed across a variety of sample states using IR‐MALDESI followed by Orbitrap mass spectrometric analysis: first, as a standard in ethanol in a well plate; second, as particulates on printer paper; and third, as particulates covered by an ice matrix. These techniques were also performed with a β‐carotene standard either in solution with a reducing agent (ascorbic acid) or with ascorbic acid in the electrospray solution. Results Tandem mass spectrometry confirmed the presence of the radical cation of β‐carotene by comparing fragments against NIST and METLIN databases. It was always analyzed as a radical cation when sampled from solution, where ascorbic acid increased radical cation abundance when in solution with β‐carotene. Mixed‐mode ionization between radical cationization and proton adduction was observed from dried particulates using IR‐MALDESI. Conclusions There are several potential mechanisms for β‐carotene radical cationization prior to IR‐MALDESI analysis, with multiphoton ionization, thermal degradation, and/or reaction with oxygen appearing to be the most logical explanations. Furthermore, although not the primary cause, changing certain aspects of sample conditions can result in significant mixed‐mode ionization with competing protonation.