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

Abstract

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.

Directly Mapping the Spatial Distribution of Organic Compounds on Mineral Rock Surfaces by DESI and LAESI Mass Spectrometry Imaging

“…This may not fully explain hydrocarbon ionization in LAESI experiments because the compounds do not flow through the solvent–ESI emitter interface but are rather ablated from the sample surface by the IR laser beam. Thus, other mechanisms, such as thermal or photodegradation, reaction with oxygen, or field-assisted photoionization, may be involved. , Nevertheless, these data indicate the possibility of using LAESI to detect hydrocarbons in rocks without any sample pretreatment, paving the way to skip the laborious sample preparation steps required in conventional hydrocarbon analysis techniques that use gas chromatography.…”

“…Thus, other mechanisms, such as thermal or photodegradation, reaction with oxygen, or field-assisted photoionization, may be involved. 50,51 Nevertheless, these data indicate the possibility of using LAESI to detect hydrocarbons in rocks without any sample pretreatment, paving the way to skip the laborious sample preparation steps required in conventional hydrocarbon analysis techniques that use gas chromatography.…”

Tolyporphins–Exotic Tetrapyrrole Pigments in a Cyanobacterium—A Review

“…Process 6 is for the detection of a dominant tolyporphin (e.g., tolyporphin A) in an unadulterated cyanobacterial culture. The technique known as IR-MALDESI-FTMS ( Figure 40 ) is a recently developed analytical method that affords accurate mass measurements in the open air of samples that typically have undergone little or no preparation [ 145 , 146 ]. The IR-MALDESI-FTMS acronym stands for infrared matrix-assisted laser desorption electrospray ionization–Fourier-transform mass spectrometry.…”

“…In short, the present technique does not yet appear suitable for application in widespread screening of diverse cyanobacterial cultures, unless the search is aimed at identifying substantial producers of a specific tolyporphin (i.e., of known mass). IR-MALDESI-MS has been used to measure analytes in plants, including Artemisia annua [ 147 ], Arabidopsis thaliana seedlings [ 148 ], fungal ( Magnaporthe oryzae )-infected barley [ 149 ], and maize [ 150 ]; in plant products, including cucumbers [ 151 ] and cherry tomatoes [ 152 ]; and in surrogate samples related to plants, including β-carotene [ 146 ]. The IR-MALDESI-FTMS method remains under rapid development.…”