Bipolar mass spectrometry of labile coordination complexes, redox active inorganic compounds, and proteins using a glass nebulizer for sonic-spray ionization

Liquid chromatography coupled to Venturi easy ambient sonic spray ionization mass spectrometry

Venturi easy ambient sonic spray ionization (V-EASI) is a soft ambient ionization (AI) source that has the advantages of being suitable to the analysis of samples in solution (differently from the majority of AI sources), performing self-pumping, voltage- and heat-free ionization, and requiring minimum or no sample preparation. Since this ionization technique has not been fully explored, the present study provides a proof of principle of the coupling of liquid chromatography to mass spectrometry (LC-MS) using V-EASI as the interface. In order to test the performance of the developed LC-V-EASI-MS system, a quantification method for bixin, a natural dye from annatto (Bixa Orellana L.), which is known to be sensitive to the high voltage applied for electrospray ionization mass spectrometry (ESI-MS) analysis, was validated according to FDA criteria and tested in real samples. The analytical method was successfully applied and met the validation criteria, providing a detectability 10 times better than methods already reported to the quantification of bixin and no matrix effect was observed. Therefore, this proof of principle contributes to the continuous development of AI sources that represents the last great technological advance in MS towards becoming a miniaturized technique able to analyze samples closer to their actual state.

Sonic-spray introduction of liquid samples to hand-held Ion mobility spectrometry analyzers

Sonic spray vaporization of liquid samples for introduction to hand-held ion mobility spectrometry vapor analyzers.

Probing surfaces of atmospherically relevant organic particles by easy ambient sonic-spray ionization mass spectrometry (EASI-MS)

EASI-MS is a promising technique for probing the chemical structures of inhomogeneous airborne organic particles.

Both ambient and laboratory-generated particles can have a surface composition different from the bulk, but there are currently few analytical techniques available to probe these differences. Easy ambient sonic-spray ionization mass spectrometry (EASI-MS) was applied to solid, laboratory-generated particles with core–shell morphologies formed from a variety of dicarboxylic acids. The soft ionization facilitated parent peak detection for the two compounds, from which the depth probed could be determined from the relative signal intensities. Two different configurations of a custom-made nebulizer are reported that yield different probe depths. In the “orthogonal mode,” with the nebulizer ∼10 centimeters away from the particle stream and at a 90° angle to the MS inlet, evaporation of the nebulizer droplets forms ions before interaction with the particles. The probe depth for orthogonal mode EASI-MS is shown to be 2–4 nm in these particle systems. In the “droplet mode”, the nebulizer and particle streams are in close proximity to each other and the MS inlet so that the particles interact with charged liquid droplets. This configuration resulted in full dissolution of the particles and gives particle composition similar to that from collection on filters and extraction of the particles (bulk). These studies establish that EASI-MS is a promising technique for probing the chemical structures of inhomogeneous airborne organic particles.

Determination of chlorate, perchlorate and bromate anions in water samples by microbore reversed-phase liquid chromatography coupled to sonic-spray ionization mass spectrometry

RATIONALE

Sonic-spray ionization mass spectrometry (SSI-MS) has recently been shown to provide similar mass spectra to those generated by electrospray ionization mass spectrometry for a wide range of compounds, i.e. from small inorganic species to peptides, proteins and numerous other biomolecules. However, limited information about this new ionization technique, such as sensitivity, limit of detection and quantification accuracy, has been reported. In particular, its coupling to liquid chromatography needs further development and assessment, along with the introduction of a broad range of applications.

METHODS

A high-efficiency glass pneumatic nebulizer, used for decades for sample introduction in atomic spectrometry, was used for the SSI-MS analysis of chlorate (ClO₃^- ), perchlorate (ClO₄^- ) and bromate (BrO₃^- ) anions, following their separation using reversed-phase microbore high-performance liquid chromatography and tandem mass spectrometry (MS/MS) operated in selected reaction monitoring mode.

RESULTS

The developed and optimized microbore HPLC/SSI-MS/MS technique exhibited low limits of detection: 5.3 ng L^-1 for chlorate, 10 ng L^-1 for perchlorate and 33.7 ng L^-1 for bromate, and provided reliable and accurate measurements of chlorate concentrations in water samples as demonstrated when comparing it with Ion Chromatography-Conductivity Detection (IC-CD), the benchmark technique for ion quantitation.

CONCLUSIONS

This is the first time that the use of HPLC/SSI-MS/MS has been reported for the detection and quantitation of chlorate, perchlorate and bromate in water samples. In addition, the exceptionally low LODs achieved using SSI render the technique competitive with the established and dominating electrospray ionization technique. Here, we have demonstrated that a commercially available high-efficiency glass pneumatic nebulizer can also be used, without any further modification, as an efficient gas-phase ion source. Copyright © 2017 John Wiley & Sons, Ltd.

Monitoring Enzymatic Reactions in Real Time Using Venturi Easy Ambient Sonic-Spray Ionization Mass Spectrometry

We developed a technique to monitor spatially confined surface reactions with mass spectrometry under ambient conditions, without the need for voltage or organic solvents. Fused-silica capillaries immersed in an aqueous solution, positioned in close proximity to each other and the functionalized surface, created a laminar flow junction with a resulting reaction volume of ∼5 pL. The setup was operated with a syringe pump, delivering reagents to the surface through a fused-silica capillary. The other fused-silica capillary was connected to a Venturi easy ambient sonic-spray ionization source, sampling the resulting analytes at a slightly higher flow rate compared to the feeding capillary. The combined effects of the inflow and outflow maintains a chemical microenvironment, where the rate of advective transport overcomes diffusion. We show proof-of-concept where acetylcholinesterase was immobilized on an organosiloxane polymer through electrostatic interactions. The hydrolysis of acetylcholine by acetylcholinesterase into choline was monitored in real-time for a range of acetylcholine concentrations, fused-silica capillary geometries, and operating flow rates. Higher reaction rates and conversion yields were observed with increasing acetylcholine concentrations, as would be expected.

Food quality and authenticity screening via easy ambient sonic-spray ionization mass spectrometry

This review is the first to summarize a decade of studies testing the use of easy ambient sonic-spray ionization mass spectrometry (EASI-MS) and its several sister techniques, Venturi (V-EASI), thermal imprinting (TI-EASI) and Spartan (S-EASI) mass spectrometry in food quality control and authentication. Since minimal or no sample preparation is required, such ambient desorption/ionization techniques have been shown to provide direct, fast and selective fingerprinting characterization at the molecular level based on the pools of the most typical components. They have also been found to be applicable on intact, undisturbed samples or on simple solvent extracts. Fundamentals of EASI-MS and its sister techniques, including mechanisms, devices, parameters and strategies, as well as the many applications reported for food analysis, are summarized and discussed.

Microfluidic self-aspiration sonic-spray ionization chip with single and dual ionization channels for mass spectrometry

In consideration of the miniaturization, integration, and universal disadvantages of microfluidic chip-based ionization coupled with mass spectrometry, this study proposed a novel microfluidic self-aspiration sonic-spray ionization chip.

Use of 3-nitrobenzonitrile as an additive for improved sensitivity in sonic-spray ionization mass spectrometry

RATIONALE

Sonic-spray ionization (SSI) has been shown to produce gas-phase ions for a wide range of compounds, without the application of voltage or a laser. However, it remains to be shown that it can also provide similar sensitivities to those obtained by electrospray ionization mass spectrometry (ESI-MS).

METHODS

Here we report on an attempt to further improve the sensitivity of SSI-MS, more specifically a version of SSI that is referred to as Venturi easy ambient sonic-spray ionization (V-EASI) MS, by adding a signal-enhancing additive to the sample solution. The additive used is 3-nitrobenzonitrile (3-NBN), which has recently been used with success in a new ionization approach named matrix-assisted ionization vacuum. In order to conduct this study we have analyzed a range of compounds, including peptides, metalloproteins, and some organometalloids. During the V-EASI-MS analyses molecular ion and protonated molecule signal intensities as well as their corresponding signal-to-noise (S/N) ratios, obtained in the presence and absence of the 3-NBN, were compared.

RESULTS

The 3-NBN-assisted V-EASI-MS approach developed here provides significant improvement in sensitivity relative to conventional V-EASI-MS for almost all compounds tested. More specifically, for peptides a 1.6- to 4-fold enhancement was realized, for proteins the enhancements were from 2- to 5-fold, and for some metalloid species enhancements reached up to 10-fold. However, optimum additive concentration and ion transfer capillary temperature were found to be compound-dependent and thus require optimization in order for maximum enhancements to be achieved. In most cases the 3-NBN-assisted V-EASI-MS approach provides comparable sensitivities and S/N ratios to ESI-MS on the same ion trap mass spectrometer.

CONCLUSIONS

The use of 3-NBN with V-EASI-MS gives rise to a novel 3-NBN-assisted MS technique, which has demonstrated considerable signal enhancement for most of the compounds analyzed, thus improving its competitiveness towards the well-established and dominating ESI-MS technique.

Characterisation of an inexpensive sonic spray ionisation source using laser-induced fluorescence imaging and mass spectrometry

A commercially available airbrush gun as a new source for spray ionisation is presented. It is best operated employing moderate stagnation pressures, resulting in a sonic gas flow. A mass spectrometric investigation on the amino acid Lysine and several peptides reveals that this inexpensive approach results in reproducible mass spectra. The ion patterns strongly resemble the results from other studies obtained with custom-made sonic spray vaporisers. The patterns also resemble the mass spectra recorded with electrospray devices. For a better understanding of the vaporisation process, the mass spectrometry experiments are accompanied by laser-induced fluorescence experiments. Inverse Abel Transform of the obtained fluorescence maps allows the determination of the full three-dimensional distribution of the spray cone. Furthermore, via exploitation of the solvatochromism of the used dye the solvation-state distribution can be visualised. In addition, expansion parameters, such as droplet size and velocity, are obtained by laser stroboscopy. The experiments demonstrate that the analyte hardly desolvates throughout the expansion. This indicates a subsequent vaporisation of the residual solvent in the intermediate pressure region of the mass spectrometer.