Ambient Profiling of Phenolic Content in Tea Infusions by Matrix-Assisted Ionization in Vacuum

Variations in physicochemical characteristics, antioxidant activity, phenolic and volatile profiles, and sensory attributes of tea-flavored Chardonnay wine during bottle aging

A novel Chardonnay wine flavored with either green tea or black tea was subjected to bottle aging for 9 months, and the physicochemical properties, antioxidant capacity, total phenolic content, volatile content and sensory properties were monitored. There were 272 phenolic and non-phenolic compounds characterized in the aged Chardonnay wines, including newly formed 9, 1, 3 and 8 phenolic compounds and 10, 6, 1 and 6 non-phenolic compounds after aging for 1, 3, 6 and 9 months, respectively. For all the aged wines, catechin was determined as the most abundant phenolic compound, and epigallocatechin mainly contributed toward the antioxidant power. A total of 54 volatile compounds were identified in the aged Chardonnay wines, including 17 odor-active compounds. The aging process diminished floral and fruity odors, but intensified green odor. The consumer study revealed the highest consumer liking for 1% (w/v) black tea infused wine. This study revealed the quality and bioactivity of this novel flavored wine type during aging which is critical to understand the shelf-life and functionality of the product.

A tutorial: Laserspray ionization and related laser-based ionization methods for use in mass spectrometry

This Tutorial is to provide a summary of parameters useful for successful outcomes of laserspray ionization (LSI) and related methods that employ a laser to ablate a matrix:analyte sample to produce highly charged ions. In these methods the purpose of the laser is to transfer matrix-analyte clusters into the gas phase. Ions are hypothesized to be produced by a thermal process where emitted matrix:analyte gas-phase particles/clusters are charged and loss of matrix from the charged particles leads to release of the analyte ions into the gas phase. The thermal energy responsible for the charge-separation process is relatively low and not necessarily supplied by the laser; a heated inlet tube linking atmospheric pressure with the first vacuum stage of a mass spectrometer is sufficient. The inlet becomes the "ion source", and inter alia, pressure, temperature, and the matrix, which can be a solid, liquid, or combinations, become critical parameters. Injecting matrix:analyte into a heated inlet tube using laser ablation, a shockwave, or simply tapping, all produce the similar mass spectra. Applications are provided that showcase new opportunities in the field of mass spectrometry.

Profiling of In Vitro Bioaccessibility and Intestinal Uptake of Flavonoids after Consumption of Commonly Available Green Tea Types

The aim of this study was to profile the bioaccessibility and intestinal absorption of epicatechins and flavonols in different forms of green tea and its formulation: loose leaf tea, powdered tea, 35% catechins containing GTE, and GTE formulated with green tea-derived polysaccharide and flavonols (CATEPLUS™). The bioaccessibillity and intestinal absorption of epicatechins and flavonols was investigated by using an in vitro digestion model system with Caco-2 cells. The bioaccessibility of total epicatechins in loose leaf tea, powdered tea, GTE, and CATEPLUS™ was 1.27%, 2.30%, 22.05%, and 18.72%, respectively, showing that GTE and CATEPLUS™ had significantly higher bioaccessibility than powdered tea and loose leaf tea. None of the flavonols were detected in powdered tea and loose leaf tea, but the bioaccessibility of the total flavonols in GTE and CATEPLUS™ was 85.74% and 66.98%, respectively. The highest intestinal absorption of epicatechins was found in CATEPLUS™ (171.39 ± 5.39 ng/mg protein) followed by GTE (57.38 ± 9.31), powdered tea (3.60 ± 0.67), and loose leaf tea (2.94 ± 1.03). The results from the study suggest that formulating green tea extracts rich in catechins with second components obtained from green tea processing could enhance the bioavailability of epicatechins.

A Combination MAI and MALDI Vacuum Source Operational from Atmospheric Pressure for Fast, Robust, and Sensitive Analyses

Previously, vacuum matrix-assisted ionization (vMAI) was employed with matrix/analyte sample introduction into the vacuum of a mass spectrometer on a probe sample introduction device. Low attomole detection was achieved, while no carryover was observed even for concentrated samples. Here, we report a new vacuum ionization source designed to duplicate the sensitivity and robustness of probe device while providing fast multisample introduction to vacuum and rapid sequential ionization. Exposure of a sample to the vacuum of the mass spectrometer provides spontaneous ionization of volatile as well as nonvolatile analytes without the need for external energy input. However, the novel source design described herein, in addition to vMAI, can employ a laser to obtain vacuum matrix-assisted laser desorption/ionization (vMALDI). In particular, ionization by vMAI or vMALDI is achieved by using the appropriate matrix. Switching between ionization modes is accomplished in a few seconds. We present results demonstrating the utility of the two ionization methods in combination to improve the molecular analyses of sample composition. In both ionization modes, multiple samples can be sequentially and rapidly acquired to increase throughput in MS. With the prototype source, samples were acquired in as little as 1 s per sample. Exchanging multisample plates can be accomplished in as little as 2 s, suggesting low-cost high-throughput automation when properly developed.

Matrix-Assisted Ionization of Molecular Uranium Species

Matrix-assisted ionization (MAI) demonstrates high sensitivity for a variety of organic compounds; however, few studies have reported the application of MAI for the detection and characterization of inorganic analytes. Trace-level uranium analysis is important in the realms of nuclear forensics, nuclear safeguards, and environmental monitoring. Traditional mass spectrometry methods employed in these fields require combinations of extensive laboratory chemistry sample preparation and destructive ionization methods. There has been recent interest in exploring ambient mass spectrometry methods that enable timely sample analysis and higher sensitivity than what is attainable by field-portable radiation detectors. Rapid characterization of uranium at nanogram levels is demonstrated in this study using MAI techniques. Mass spectra were collected on an atmospheric pressure mass spectrometer for solutions of uranyl nitrate, uranyl chloride, uranyl acetate, and uranyl oxalate utilizing 3-nibrobenzonitrile as the ionization matrix. The uranyl complexes investigated were detectable, and the chemical speciation was preserved. Sample analysis was accomplished in a matter of seconds, and limits of detection of 5 ng of uranyl nitrate, 10 ng of uranyl oxalate, 100 ng of uranyl chloride, and 200 ng of uranyl acetate were achieved. The observed gas-phase speciation was similar to negative-ion electrospray ionization of uranyl compounds with notable differences. Six matrix-derived ions were detected in all negative-ion mass spectra, and some of these ions formed adducts with the uranyl analyte. Subsequent analysis of the matrix suggests that these molecules are not matrix contaminants and are instead created during the ionization process.

An overview of biological applications and fundamentals of new inlet and vacuum ionization technologies

RATIONALE

The developments of new ionization technologies based on processes previously unknown to mass spectrometry (MS) have gained significant momentum. Herein we address the importance of understanding these unique ionization processes, demonstrate the new capabilities currently unmet by other methods, and outline their considerable analytical potential.

METHODS

The inlet and vacuum ionization methods of solvent-assisted ionization (SAI), matrix-assisted ionization (MAI), and laserspray ionization can be used with commercial and dedicated ion sources producing ions from atmospheric or vacuum conditions for analyses of a variety of materials including drugs, lipids, and proteins introduced from well plates, pipet tips and plate surfaces with and without a laser using solid or solvent matrices. Mass spectrometers from various vendors are employed.

RESULTS

Results are presented highlighting strengths relative to ionization methods of electrospray ionization (ESI) and matrix-assisted laser desorption/ionization. We demonstrate the utility of multi-ionization platforms encompassing MAI, SAI, and ESI and enabling detection of what otherwise is missed, especially when directly analyzing mixtures. Unmatched robustness is achieved with dedicated vacuum MAI sources with mechanical introduction of the sample to the sub-atmospheric pressure (vacuum MAI). Simplicity and use of a wide array of matrices are attained using a conduit (inlet ionization), preferably heated, with sample introduction from atmospheric pressure. Tissue, whole blood, urine (including mouse, chicken, and human origin), bacteria strains and chemical on-probe reactions are analyzed directly and, especially in the case of vacuum ionization, without concern of carryover or instrument contamination.

CONCLUSIONS

Examples are provided highlighting the exceptional analytical capabilities associated with the novel ionization processes in MS that reduce operational complexity while increasing speed and robustness, achieving mass spectra with low background for improved sensitivity, suggesting the potential of this simple ionization technology to drive MS into areas currently underserved, such as clinical and medical applications.

Toward understanding the ionization mechanism of matrix-assisted ionization using mass spectrometry experiment and theory

RATIONALE

Matrix-assisted ionization (MAI) mass spectrometry does not require voltages, a laser beam, or added heat to initiate ionization, but it is strongly dependent on the choice of matrix and the vacuum conditions. High charge state distributions of nonvolatile analyte ions produced by MAI suggest that the ionization mechanism may be similar to that of electrospray ionization (ESI), but different from matrix-assisted laser desorption/ionization (MALDI). While significant information is available for MAI using mass spectrometers operating at atmospheric and intermediate pressure, little is known about the mechanism at high vacuum.

METHODS

Eleven MAI matrices were studied on a high-vacuum time-of-flight (TOF) mass spectrometer using a 266 nm pulsed laser beam under otherwise typical MALDI conditions. Detailed comparisons with the commonly used MALDI matrices and theoretical prediction were made for 3-nitrobenzonitrile (3-NBN), which is the only MAI matrix that works well in high vacuum when irradiated with a laser.

RESULTS

Screening of MAI matrices with good absorption at 266 nm but with various degrees of volatility and laser energies suggests that volatility and absorption at the laser wavelength may be necessary, but not sufficient, criteria to explain the formation of multiply charged analyte ions. 3-NBN produces intact, highly charged ions of nonvolatile analytes in high-vacuum TOF with the use of a laser, demonstrating that ESI-like ions can be produced in high vacuum. Theoretical calculations and mass spectra suggest that thermally induced proton transfer, which is the major ionization mechanism in MALDI, is not important with the 3-NBN matrix at 266 nm laser wavelength. 3-NBN:analyte crystal morphology is, however, important in ion generation in high vacuum.

CONCLUSIONS

The 3-NBN MAI matrix produces intact, highly charged ions of nonvolatile compounds in high-vacuum TOF mass spectrometers with the aid of ablation and/or heating by laser irradiation, and shows a different ionization mechanism from that of typical MALDI matrices.

Novel ionization processes for use in mass spectrometry: 'Squeezing' nonvolatile analyte ions from crystals and droplets

Together with my group and collaborators, I have been fortunate to have had a key role in the discovery of new ionization processes that we developed into new flexible, sensitive, rapid, reliable, and robust ionization technologies and methods for use in mass spectrometry (MS). Our current research is focused on how best to understand, improve, and use these novel ionization processes which convert volatile and nonvolatile compounds from solids or liquids into gas-phase ions for analysis by MS using e.g. mass-selected fragmentation and ion mobility spectrometry to provide reproducible, accurate, and improved mass and drift time resolution. In my view, the apex was the discovery of vacuum matrix-assisted ionization (vMAI) in 2012 on an intermediate pressure matrix-assisted laser desorption/ionization (MALDI) source without the use of a laser, high voltages, or any other added energy. Only exposure of the matrix:analyte to the sub-atmospheric pressure of the mass spectrometer was necessary to initiate ionization. These findings were initially rejected by three different scientific journals, with comments related to 'how can this work?', 'where do the charges come from?', and 'it is not analytically useful'. Meanwhile, we and others have demonstrated analytical utility without a complete understanding of the mechanism. In reality, MALDI and electrospray ionization are widely used in science and their mechanisms are still controversially discussed despite use and optimization of now 30 years. This Perspective covers the applications and mechanistic aspects of the novel ionization processes for use in MS that guided us in instrument developments, and provides our perspective on how they relate to traditional ionization processes.