Imaging Laser-Induced Fluorescence Detection at the Taylor Cone of Electrospray Ionization Mass Spectrometry

Combining amperometry and mass spectrometry as a dual detection approach for capillary electrophoresis

Dual detection concepts (DDCs) are becoming more and more popular in analytical chemistry. In this work, we describe a novel DDC for capillary electrophoresis (CE) consisting of an amperometric detector (AD) and a mass spectrometer (MS). This detector combination has a good complementarity as the AD exhibits high sensitivity, whereas the MS provides excellent selectivity. Both detectors are based on a destructive detection principle, making a serial detector arrangement impossible. Thus, for the realization of the DDC, the CE flow was divided into two parts with a flow splitter. The DDC was characterized in a proof-of-concept study with ferrocene derivates and a nonaqueous background electrolyte. We could show that splitting the CE flow was a suitable method for the instrumental realization of the DDC consisting of two destructive detectors. By lowering the height of the AD compared to the MS, it was possible to synchronize the detector responses. Additionally, for the chosen model system, we confirmed that the AD was much more reproducible and had lower limits of detection (LODs) than the MS. The LODs were identical for the DDC and the single-detection arrangements, indicating no sensitivity decrease due to the CE flow splitting. The DDC was successfully applied to determine the drug and doping agent trimetazidine.

Cryogenic Fluorescence Spectroscopy of Ionic Fluorones in Gaseous and Condensed Phases: New Light on Their Intrinsic Photophysics

Fluorescence spectroscopy of gas-phase ions generated through electrospray ionization is an emerging technique able to probe intrinsic molecular photophysics directly without perturbations from solvent interactions. While there is ample scope for the ongoing development of gas-phase fluorescence techniques, the recent expansion into low-temperature operating conditions accesses a wealth of data on intrinsic fluorophore photophysics, offering enhanced spectral resolution compared with room-temperature measurements, without matrix effects hindering the excited-state dynamics. This perspective reviews current progress on understanding the photophysics of anionic fluorone dyes, which exhibit an unusually large Stokes shift in the gas phase, and discusses how comparison of gas- and condensed-phase fluorescence spectra can fingerprint structural dynamics. The capacity for temperature-dependent measurements of both fluorescence emission and excitation spectra helps establish the foundation for the use of fluorone dyes as fluorescent tags in macromolecular structure determination. We suggest ideas for technique development.

High sensitivity glycomics in biomedicine

Many analytical challenges in biomedicine arise from the generally high heterogeneity and complexity of glycan- and glycoconjugate-containing samples, which are often only available in minute amounts. Therefore, highly sensitive workflows and detection methods are required. In this review mass spectrometric workflows and detection methods are evaluated for glycans and glycoproteins. Furthermore, glycomic methodologies and innovations that are tailored for enzymatic treatments, chemical derivatization, purification, separation, and detection at high sensitivity are highlighted. The discussion is focused on the analysis of mammalian N-linked and GalNAc-type O-linked glycans.

Liquid chromatography and capillary electrophoresis in glycomic and glycoproteomic analysis

Glycosylation is one of the most significant and abundant post-translational modifications in cells. Glycomic and glycoproteomic analyses involve the characterization of oligosaccharides (glycans) conjugated to proteins. Glycomic and glycoproteomic analysis is highly challenging because of the large diversity of structures, low abundance, site-specific heterogeneity, and poor ionization efficiency of glycans and glycopeptides in mass spectrometry (MS). MS is a key tool for characterization of glycans and glycopeptides. However, MS alone does not always provide full structural and quantitative information for many reasons, and thus MS is combined with some separation technique. This review focuses on the role of separation techniques used in glycomic and glycoproteomic analyses, liquid chromatography and capillary electrophoresis. The most important separation conditions and results are presented and discussed.

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Formation of Gaseous Peptide Ions from Electrospray Droplets: Competition between the Ion Evaporation Mechanism and Charged Residue Mechanism

The transfer of peptide ions from solution into the gas phase by electrospray ionization (ESI) is an integral component of mass spectrometry (MS)-based proteomics. The mechanisms whereby gaseous peptide ions are released from charged ESI nanodroplets remain unclear. This is in contrast to intact protein ESI, which has been the focus of detailed investigations using molecular dynamics (MD) simulations and other methods. Under acidic liquid chromatography/MS conditions, many peptides carry a solution charge of 3+ or 2+. Because of this pre-existing charge and their relatively small size, prevailing views suggest that peptides follow the ion evaporation mechanism (IEM). The IEM entails analyte ejection from ESI droplets, driven by electrostatic repulsion between the analyte and droplet. Surprisingly, recent peptide MD investigations reported a different behavior, that is, the release of peptide ions via droplet evaporation to dryness which represents the hallmark of the charged residue mechanism (CRM). Here, we resolved this conundrum by performing MD simulations on a common model peptide (bradykinin) in Rayleigh-charged aqueous droplets. The primary focus was on pH 2 conditions (bradykinin solution charge = 3+), but we also verified that our MD strategy captured pH-dependent charge state shifts seen in ESI-MS experiments. In agreement with earlier simulations, we found that droplets with initial radii of 1.5-3 nm predominantly release peptide ions via the CRM. In contrast, somewhat larger radii (4-5 nm) favor IEM behavior. It appears that these are the first MD data to unequivocally demonstrate the viability of peptide IEM events. Electrostatic arguments can account for the observed droplet size dependence. In summary, both CRM and IEM can be operative in peptide ESI-MS. The prevalence of one over the other mechanism depends on the droplet size distribution in the ESI plume.

Applications of capillary electrophoresis for biopharmaceutical product characterization

Capillary electrophoresis (CE), after being introduced several decades ago, has carved out a niche for itself in the field of analytical characterization of biopharmaceutical products. It does not only offer fast separation, high resolution in miniaturized format, but equally importantly represents an orthogonal separation mechanism to high-performance liquid chromatography. Therefore, it is not surprising that CE-based methods can be found in all major pharmacopoeias and are recommended for the analysis of biopharmaceutical products during process development, characterization, quality control, and release testing. Different separation formats of CE, such as capillary gel electrophoresis, capillary isoelectric focusing, and capillary zone electrophoresis are widely used for size and charge heterogeneity characterization as well as purity and stability testing of therapeutic proteins. Hyphenation of CE with MS is emerging as a promising bioanalytical tool to assess the primary structure of therapeutic proteins along with any impurities. In this review, we confer the latest developments in capillary electrophoresis, used for the characterization of critical quality attributes of biopharmaceutical products covering the past 6 years (2015-2021). Monoclonal antibodies, due to their significant share in the market, have been given prioritized coverage.

Fluorescence-Based Detection of the Desolvation Process of Protein Ions Generated in an Aqueous Electrospray Plume

A new experimental setup to study laser-induced fluorescence from analytes at different locations in an electrospray plume has been developed. The high fluorescence collection efficiency (∼2%) of the setup, along with a sensitive charge coupled device (CCD) detector, enabled the study of low ion concentrations (down to ∼fM) in the plume. The use of small electrospray tip inner diameters (<1 μm) facilitated the fast desolvation of gaseous protein ions in an aqueous electrospray plume. Fluorescence spectra were acquired from specific locations along the plume axis in different aqueous electrospray plumes with three different analytes: a rhodamine dye and two proteins (ubiquitin and apomyoglobin) labeled with rhodamine dyes. To confirm the presence of gaseous ions, pure gas-phase fluorescence spectra were acquired in the vacuum of a modified ion trap mass spectrometer. These spectra were used to fit to confirm the presence of gaseous species in the corresponding spectra obtained from the electrospray plume. This study shows that with small inner diameter spray capillaries, gaseous protein ions generated at atmospheric pressure in an electrospray plume can be detected with fluorescence-based techniques. Fluorescence measurements can be used to study their structure in the electrospray plume, and the dynamics as they transition from solution to the gas phase and in the early stages after desolvation from charged droplets. Other techniques can also be applied to further study gaseous biomolecular structures under ambient conditions immediately after desolvation.

A simple and highly sensitive masking fluorescence detection system for capillary array electrophoresis and its application to food and medicine analysis

A high sampling rate, good stability, high throughput masking fluorescence detection system with easy positioning of each channel for capillary array electrophoresis was prepared and studied. A special mask combined with convex lenses was designed to modulate signals, without using any extra device to position each channel. The signal of each channel was detected by a photomultiplier tube, classified and saved by software. The design was used to evidently reduce the rotational vibration of optical components and to stabilize the system, so a high sampling rate was obtained by increasing the DC motor speed. To improve the optical system, optical fibers instead of conventional bulky optical components were used to transmit optical signal and to collect fluorescences in multiple directions, which greatly raised the sensitivity. Other important parameters including sampling rate, rotating speed and driven voltage laser diode (LDs) have also been investigated. Under optimal conditions, the performance of the detection system was evaluated. This novel system had a well-designed structure, and allowed independent multiple capillary operations and easy microanalysis. Its limit of detection for rhodamine 6G was 2.0 × 10^-2 µg/mL.