Microdroplet mass spectrometry: Accelerating reaction and application

Localization and Orientation of Dye Molecules at the Surface of a Levitated Microdroplet in Air Revealed by Whispering Gallery Mode Resonances

Microdroplets offer unique environments that accelerate chemical reactions; however, the mechanisms behind these processes remain debated. The localization and orientation of solute molecules near the droplet surface have been proposed as factors for this acceleration. Since significant reaction acceleration has been observed for electrospray- and sonic-spray-generated aerosol droplets, the analysis of microdroplets in air has become essential. Here, we utilized whispering gallery mode (WGM) resonances to investigate the localization and orientation of dissolved rhodamine B (RhB) in a levitated microdroplet (∼3 μm in diameter) in air. Fluorescence enhancement upon resonance with the WGMs revealed the localization and orientation of RhB near the droplet surface. Numerical modeling using Mie theory quantified the RhB orientation at 68° to the surface normal, with a small fraction randomly oriented inside the droplet. Additionally, low RhB concentrations increased surface localization. These results support the significance of surface reactions in the acceleration of microdroplet reactions.

Mass spectrometry imaging as a promising analytical technique for herbal medicines: an updated review

Herbal medicines (HMs) have long played a pivotal role in preventing and treating various human diseases and have been studied widely. However, the complexities present in HM metabolites and their unclear mechanisms of action have posed significant challenges in the modernization of traditional Chinese medicine (TCM). Over the past two decades, mass spectrometry imaging (MSI) has garnered increasing attention as a robust analytical technique that enables the simultaneous execution of qualitative, quantitative, and localization analyses without complex sample pretreatment. With advances in technical solutions, MSI has been extensively applied in the field of HMs. MSI, a label-free ion imaging technique can comprehensively map the spatial distribution of HM metabolites in plant native tissues, thereby facilitating the effective quality control of HMs. Furthermore, the spatial dimension information of small molecule endogenous metabolites within animal tissues provided by MSI can also serve as a supplement to uncover pharmacological and toxicological mechanisms of HMs. In the review, we provide an overview of the three most common MSI techniques. In addition, representative applications in HM are highlighted. Finally, we discuss the current challenges and propose several potential solutions. We hope that the summary of recent findings will contribute to the application of MSI in exploring metabolites and mechanisms of action of HMs.

Chemical Kinetics in Microdroplets

Micrometer-sized compartments play significant roles in driving heterogeneous transformations within atmospheric and biochemical systems as well as providing vehicles for drug delivery and novel reaction environments for the synthesis of industrial chemicals. Many reports now indicate that reaction kinetics are accelerated under microconfinement, for example, in sprays, thin films, droplets, aerosols, and emulsions. These observations are dramatic, posing a challenge to our understanding of chemical reaction mechanisms with potentially significant practical consequences for predicting the complex chemistry in natural systems. Here we introduce the idea of kinetic confinement, which is intended to provide a conceptual backdrop for understanding when and why microdroplet reaction kinetics differ from their macroscale analogs.

High-performance peptide and disulfide mapping by direct injection of intact proteins using on-line coupled UV-liquid chromatography microdroplet mass spectrometry (UVLC-MMS)

Microdroplet mass spectrometry (MMS), achieving ultra-fast enzyme digestion in the ionization source, holds great promises for innovating protein analysis. Here, in-depth protein characterization is demonstrated by direct injection of intact protein mixtures via on-line coupling MMS with capillary C4 liquid chromatography (LC) containing UV windows (UVLC-MMS) through an enzyme introduction tee. We showed complete sets of peptides of individual proteins (hemoglobin, bovine serum albumin, and ribonuclease A) in a mixture could be obtained in one injection. Such full (100%) sequence coverage, however, could not be achieved by conventional nanoLC-MS method using bottom-up approach with single enzyme. Moreover, direct injection of a chaperone α-crystalline (α-Cry) complex yielded identification of post-translational modifications including novel sites and semi-quantitative characterization including 3:1 stoichiometry ratio of αA- and αB-Cry sub-units and ∼1.4 phosphorylation/subunit on S45 (novel site) and S122 (main site) of αA-Cry, ∼0.7 phosphorylation/subunit on S19 (main site) and S45 of αB-Cry, as well as 100% acetylation on both N-termini of each subunits by matching the mass and retention time of the intact and its digested peptides. Furthermore, trifluoroacetic acid was able to be used in the mobile phase with UVLC-MMS to improve the separation of differentially reduced intact species and detectability of the droplet-digested products. This allowed us to completely map four disulfide linkages of ribonuclease A based on collision-induced dissociation of disulfide clusters, some of which would otherwise not be detected, preventing scrambling or shuffling errors arising from lengthy bulk solution digestion by the bottom-up approach. Integration of UVLC and MMS greatly improves droplet digestion efficiency and MS detection, enabling highly efficient workflow for in-depth and accurate protein characterization.

Real-time bottom-up characterization of protein mixtures enabled by online microdroplet-assisted enzymatic digestion (MAED)

Enzymatic digestion remains one of the "rate-determining steps" in the bottom-up analysis of proteins. However, by performing digestion in microdroplets generated from electrosonic spray, the reaction could be accelerated to a timescale lower than milliseconds. Here, we describe a simple and rapid online digestion platform named online microdroplet-assisted enzymatic digestion (MAED). It involves the integration of intact protein separation with enzymatic digestion in microdroplets. Via online MAED, various protein standards, including an antibody standard, were characterized in a bottom-up manner without prior digestion, and high sequence coverages were obtained. We further extended the application of online MAED to a more complex sample, mouse brain extract, where protein identifications were successfully yielded. Compared with the conventional bottom-up approach, a more comprehensive characterization could be obtained particularly for low molecular weight proteins. In short, we provide a rapid and alternative bottom-up analysis in a top-down fashion as well as a new possibility for microdroplet chemistry.

Development of mass spectrometry imaging techniques and its latest applications

Mass spectrometry imaging (MSI) is a novel molecular imaging technology that collects molecular information from the surface of samples in situ. The spatial distribution and relative content of various compounds can be visualized simultaneously with high spatial resolution. The prominent advantages of MSI promote the active development of ionization technology and its broader applications in diverse fields. This article first gives a brief introduction to the vital parts of the processes during MSI. On this basis, provides a comprehensive overview of the most relevant MS-based imaging techniques from their mechanisms, pros and cons, and applications. In addition, a critical issue in MSI, matrix effects is also discussed. Then, the representative applications of MSI in biological, forensic, and environmental fields in the past 5 years have been summarized, with a focus on various types of analytes (e.g., proteins, lipids, polymers, etc.) Finally, the challenges and further perspectives of MSI are proposed and concluded.