Laser electrospray mass spectrometry minimizes ion suppression facilitating quantitative mass spectral response for multicomponent mixtures of proteins

Long term online desalting analysis of MS/LC-MS using thermal assisted recrystallization ionization

Mass spectrometric analysis of non-volatile salts containing samples remains challenging due to salt-induced ion suppression and contamination. This challenge is even more pronounced for a liquid chromatography-mass spectrometry analysis, where the accumulation of salts in the transmission system poses an ongoing problem. In this study, a novel thermal assisted recrystallization ionization mass spectrometry (TARI-MS) device was developed to achieve efficient on-line desalting and prolonged analysis of saline samples. The core component of this device was a heated plate positioned between the electrospray unit and the MS inlet. The desalting mechanism was demonstrated as the spontaneous separation of target molecules from salts during the "crystallization" process. After optimization, the angle between the nebulizer and the heated plate was 45°; the distance between the front end of the heated plate and the MS inlet was 2 mm; the distance between the front edge of the heated plate and the center of the sample spray projected onto the heating plate was 3 mm; the distance between the emitter of nebulizer and the heated plate was 3 mm. TARI-MS realized direct analysis of eight drugs dissolved in eight commonly used non-volatile salts solutions (up to 0.5 mol/L). The high sensitivity, repeatability, linearity, accuracy, and intra- and inter-day precision of TARI-MS confirm its reliability as a robust tool for the analysis of saline samples. Furthermore, TARI-MS allowed continuous analysis of salty eluates of LC for up to nearly 1 h without maintenance and verified the feasibility of LC-MS analysis through detecting a five-drug mixture and a crude aripiprazole product. Finally, six impurities in the crude aripiprazole product were successfully detected by LC-TARI-MS. The established method holds promise for applications across academic and pharmaceutical domains.

Spatially resolved absolute quantitation in thin tissue by mass spectrometry

Mass spectrometry (MS) has become the de facto tool for routine quantitative analysis of biomolecules. MS is increasingly being used to reveal the spatial distribution of proteins, metabolites, and pharmaceuticals in tissue and interest in this area has led to a number of novel spatially resolved MS technologies. Most spatially resolved MS measurements are qualitative in nature due to a myriad of potential biases, such as sample heterogeneity, sampling artifacts, and ionization effects. As applications of spatially resolved MS in the pharmacological and clinical fields increase, demand has become high for quantitative MS imaging and profiling data. As a result, several varied technologies now exist that provide differing levels of spatial and quantitative information. This review provides an overview of MS profiling and imaging technologies that have demonstrated quantitative analysis from tissue. Focus is given on the fundamental processes affecting quantitative analysis in an array of MS imaging and profiling technologies and methods to address these biases.Graphical abstract.

LASER DESORPTION/ABLATION POSTIONIZATION MASS SPECTROMETRY: RECENT PROGRESS IN BIOANALYTICAL APPLICATIONS

Lasers have long been used in the field of mass spectrometric analysis for characterization of condensed matter. However, emission of neutrals upon laser irradiation surpasses the number of ions. Typically, only one in about one million analytes ejected by laser desorption/ablation is ionized, which has fueled the quest for postionization methods enabling ionization of desorbed neutrals to enhance mass spectrometric detection schemes. The development of postionization techniques can be an endeavor that integrates multiple disciplines involving photon energy transfer, electrochemistry, gas discharge, etc. The combination of lasers of different parameters and diverse ion sources has made laser desorption/ablation postionization (LD/API) a growing and lively research community, including two-step laser mass spectrometry, laser ablation atmospheric pressure photoionization mass spectrometry, and those coupled to ambient mass spectrometry. These hyphenated techniques have shown potentials in bioanalytical applications, with major inroads to be made in simultaneous location and quantification of pharmaceuticals, toxins, and metabolites in complex biomatrixes. This review is intended to provide a timely comprehensive view of the broadening bioanalytical applications of disparate LD/API techniques. We also have attempted to discuss these applications according to the classifications based on the postionization methods and to encapsulate the latest achievements in the field of LD/API by highlighting some of the very best reports in the 21st century. © 2020 John Wiley & Sons Ltd.

Characterizing and alleviating ion suppression effects in atmospheric pressure matrix-assisted laser desorption/ionization

RATIONALE

As a powerful ambient ion source, atmospheric pressure (AP) matrix-assisted laser desorption/ionization (MALDI) enables direct analysis at atmospheric pressure/temperature and minimal sample preparation. With the increasing usage of AP-MALDI sources with Orbitrap instruments, systematic characterization of the extent of ion suppression effect (ISE) in AP-MALDI-Orbitrap mass spectrometry imaging (MSI) is desirable. Recently, a new low-pressure MALDI platform has been introduced that reportedly provided better sensitivity. While extensive research efforts have been devoted to improving spatial resolution, fewer studies focused on the characterization and sensitivity improvement of these MALDI platforms that, coupled with high-resolution Orbitraps, provide powerful strategy for MSI.

METHODS

We compared the analytical performance of AP and low-pressure (subatmospheric) MALDI sources to study the effect of pressure control in the ion source. Using a model peptide/protein mixture, we systematically evaluated the factors influencing ISE. Furthermore, the effect of laser spot size was evaluated through tissue imaging analysis of lipids and neuropeptides. The effects of ion suppression and laser spot size have also been examined by comparing the number of identified molecular species during MSI analysis.

RESULTS

Several key operating parameters including source pressure, laser energy, laser repetition rate, and microscopic slide coating materials were optimized to minimize the ISE. Under the optimal conditions, the subatmospheric AP-MALDI-Orbitrap platform with high spatial and mass spectral resolution enabled significantly improved coverage of several lipid and neuropeptide families in the MS analysis of mouse brain tissue sections.

CONCLUSIONS

The new SubAP-MALDI source coupled with an Orbitrap mass spectrometer was established as a viable platform for in situ endogenous biomolecular analysis with increased sensitivity compared with conventional AP-MALDI sources as evidenced by the confident identification of neuropeptides from mouse brain imaging analyses. The alleviated ISE was key to substantial performance improvement due to optimized intermediate pressure conditions and better ion collection by the ion funnel.

Quantification of Protein-Ligand Interactions by Laser Electrospray Mass Spectrometry

Laser electrospray mass spectrometry (LEMS) measurement of the dissociation constant (K_d) for hen egg white lysozyme (HEWL) and N,N',N″-triacetylchitotriose (NAG₃) revealed an apparent K_d value of 313.2 ± 25.9 μM for the ligand titration method. Similar measurements for N,N',N″,N″'-tetraacetylchitotetraose (NAG₄) revealed an apparent K_d of 249.3 ± 13.6 μM. An electrospray ionization mass spectrometry (ESI-MS) experiment determined a K_d value of 9.8 ± 0.6 μM. In a second LEMS approach, a calibrated measurement was used to determine a K_d value of 6.8 ± 1.5 μM for NAG₃. The capture efficiency of LEMS was measured to be 3.6 ± 1.8% and is defined as the fraction of LEMS sample detected after merging with the ESI plume. When the dilution is factored into the ligand titration measurement, the adjusted K_d value was 11.3 μM for NAG₃ and 9.0 μM for NAG₄. The calibration method for measuring K_d developed in this study can be applied to solutions containing unknown analyte concentrations. Graphical Abstract.

Ion suppression effect in desorption electrospray ionization and electrospray ionization mass spectrometry

RATIONALE

Although it is claimed that desorption electrospray ionization (DESI) causes less ion suppression effects than electrospray ionization (ESI), a related investigation with quantification measurement of ion suppression effects is absent. Herein, a comparative analysis of ion suppression effects between DESI and ESI was conducted, with the aiming of quantitatively studying the ion suppression effect.

METHODS

CTVA mixtures, a constant concentration of tioconazole with varied concentrations of atenolol, were analyzed by ESI and DESI. The ion suppression effect was characterized by the signal loss of tioconazole in the mixture compared to the signal intensity of tioconazole without interference (denoted as (I_single - I_mixture )/I_single ). According to the variations in the experimental conditions (such as flow rate, solvent composition, substrate material, capillary inner diameter, sheath gas velocity), ion suppression effects in DESI and ESI were compared.

RESULTS

With the increasing flow rate, the ion suppression effect in DESI became weaker, while the opposite trend was obtained for ESI. As for capillary inner diameter, a smaller inner diameter resulted in weaker ion suppression effects in DESI and ESI. The solvent composition affected the ion suppression effect, and the PTFE substrate presented the weakest ion suppression effect among the five substrate materials.

CONCLUSIONS

Though the ion suppression effect in DESI and ESI was shown to relate to experimental conditions, DESI had less effect than ESI under the same experimental conditions in most cases. Moreover, DESI displayed stronger matrix-tolerant ability than ESI which is also attributed to the weaker ion suppression effect.

Assessment of Reproducibility of Laser Electrospray Mass Spectrometry using Electrospray Deposition of Analyte

A nonresonant, femtosecond (fs) laser is employed to desorb samples of Victoria blue deposited on stainless steel or indium tin oxide (ITO) slides using either electrospray deposition (ESD) or dried droplet deposition. The use of ESD resulted in uniform films of Victoria blue whereas the dried droplet method resulted in the formation of a ring pattern of the dye. Laser electrospray mass spectrometry (LEMS) measurements of the ESD-prepared films on either substrate were similar and revealed lower average relative standard deviations for measurements within-film (20.9%) and between-films (8.7%) in comparison to dried droplet (75.5% and 40.2%, respectively). The mass spectral response for ESD samples on both substrates was linear (R² > 0.99), enabling quantitative measurements over the selected range of 7.0 × 10^-11 to 2.8 × 10^-9 mol, as opposed to the dried droplet samples where quantitation was not possible (R² = 0.56). The limit of detection was measured to be 210 fmol. Graphical Abstract ᅟ.

Isolating Protein Charge State Reduction in Electrospray Droplets Using Femtosecond Laser Vaporization

Charge state distributions are measured using mass spectrometry for both native and denatured cytochrome c and myoglobin after laser vaporization from the solution state into an electrospray (ES) plume consisting of a series of solution additives differing in gas-phase basicity. The charge distribution depends on both the pH of the protein solution prior to laser vaporization and the gas-phase basicity of the solution additive employed in the ES solvent. Cytochrome c (myoglobin) prepared in solutions with pH of 7.0, 2.6, and 2.3 resulted in the average charge state distribution (Z_avg) of 7.0 ± 0.1 (8.2 ± 0.1), 9.7 ± 0.2 (14.5 ± 0.3), and 11.6 ± 0.3 (16.4 ± 0.1), respectively, in ammonium formate ES solvent. The charge distribution shifted from higher charge states to lower charge states when the ES solvent contained amines additives with higher gas-phase basicity. In the case of triethyl ammonium formate, Z_avg of cytochrome c (myoglobin) prepared in solutions with pH of 7.0, 2.6, and 2.3 decreased to 4.9 (5.7), 7.4 ± 0.2 (9.6 ± 0.3), and 7.9 ± 0.3 (9.8 ± 0.2), respectively. The detection of a charge state distribution corresponding to folded protein after laser vaporized, acid-denatured protein interacts with the ES solvent containing ammonium formate, ammonium acetate, triethyl ammonium formate, and triethyl ammonium acetate suggests that at least a part of protein population folds within the electrospray droplet on a millisecond timescale. Graphical Abstract ᅟ.

Internal energy deposition for low energy, femtosecond laser vaporization and nanospray post-ionization mass spectrometry using thermometer ions

The internal energy of p-substituted benzylpyridinium ions after laser vaporization using low energy, femtosecond duration laser pulses of wavelengths 800 and 1042 nm was determined using the survival yield method. Laser vaporization of dried benzylpyridinium ions from metal slides into a buffered nanospray with 75 μJ, 800 nm laser pulses resulted in a higher extent of fragmentation than conventional nanospray due to the presence of a two-photon resonance fragmentation pathway. Using higher energy 800 nm laser pulses (280 and 505 μJ) led to decreased survival yields for the four different dried benzylpyridinium ions. Analyzing dried thermometer ions with 46.5 μJ, 1042 nm pulse-bursts resulted in little fragmentation and mean internal energy distributions equivalent to nanospray, which is attributable to the absence of a two-photon resonance that occurs with higher energy, 800 nm laser pulses. Vaporization of thermometer ions from solution with either 800 nm or 1042 nm laser pulses resulted in comparable internal energy distributions to nanospray ionization.

Increasing protein charge state when using laser electrospray mass spectrometry

Femtosecond (fs) laser vaporization is used to transfer cytochrome c, myoglobin, lysozyme, and ubiquitin from the condensed phase into an electrospray (ES) plume consisting of a mixture of a supercharging reagent, m-nitrobenzyl alcohol (m-NBA), and trifluoroacetic acid (TFA), acetic acid (AA), or formic acid (FA). Interaction of acid-sensitive proteins like cytochrome c and myoglobin with the highly charged ES droplets resulted in a shift to higher charge states in comparison with acid-stable proteins like lysozyme and ubiquitin. Laser electrospray mass spectrometry (LEMS) measurements showed an increase in both the average charge states (Zavg) and the charge state with maximum intensity (Zmode) for acid-sensitive proteins compared with conventional electrospray ionization mass spectrometry (ESI-MS) under equivalent solvent conditions. A marked increase in ion abundance of higher charge states was observed for LEMS in comparison with conventional electrospray for cytochrome c (ranging from 19+ to 21+ versus 13+ to 16+) and myoglobin (ranging from 19+ to 26+ versus 18+ to 21+) using an ES solution containing m-NBA and TFA. LEMS measurements as a function of electrospray flow rate yielded increasing charge states with decreasing flow rates for cytochrome c and myoglobin.

Direct analysis of intact biological macromolecules by low-energy, fiber-based femtosecond laser vaporization at 1042 nm wavelength with nanospray postionization mass spectrometry

A fiber-based laser with a pulse duration of 435 fs and a wavelength of 1042 nm was used to vaporize biological macromolecules intact from the condensed phase into the gas phase for nanospray postionization and mass analysis. Laser vaporization of dried standard protein samples from a glass substrate by 10 Hz bursts of 20 pulses having 10 μs pulse separation and <50 μJ pulse energy resulted in signal comparable to a metal substrate. The protein signal observed from an aqueous droplet on a glass substrate was negligible compared to either a droplet on metal or a thin film on glass. The mass spectra generated from dried and aqueous protein samples by the low-energy, fiber laser were similar to the results from high-energy (500 μJ), 45-fs, 800-nm Ti:sapphire-based femtosecond laser electrospray mass spectrometry (LEMS) experiments, suggesting that the fiber-based femtosecond laser desorption mechanism involves a nonresonant, multiphoton process, rather than thermal- or photoacoustic-induced desorption. Direct analysis of whole blood performed without any pretreatment resulted in features corresponding to hemoglobin subunit-heme complex ions. The observation of intact molecular ions with low charge states from protein, and the tentatively assigned hemoglobin α subunit-heme complex from blood suggests that fiber-based femtosecond laser vaporization is a "soft" desorption source at a laser intensity of 2.39 × 10(12) W/cm(2). The low-energy, turnkey fiber laser demonstrates the potential of a more robust and affordable laser for femtosecond laser vaporization to deliver biological macromolecules into the gas phase for mass analysis.

Determination of internal energy distributions of laser electrospray mass spectrometry using thermometer ions and other biomolecules

The internal energy distributions for dried and liquid samples that were vaporized with femtosecond duration laser pulses centered at 800 nm and postionized by electrospray ionization-mass spectrometry (LEMS) were measured and compared with conventional electrospray ionization mass spectrometry (ESI-MS). The internal energies of the mass spectral techniques were determined by plotting the ratio of the intact parent molecular features to all integrated ion intensities of the fragments as a function of collisional energy using benzylpyridinium salts and peptides. Measurements of dried p-substituted benzylpyridinium salts using LEMS resulted in a greater extent of fragmentation in addition to the benzyl cation. The mean relative internal energies, <E(int)> were determined to be 1.62 ± 0.06, 2.0 ± 0.5, and 1.6 ± 0.3 eV for ESI-MS, dried LEMS, and liquid LEMS studies, respectively. Two-photon resonances with the laser pulses likely caused lower survival yields in LEMS analyses of dried samples but not liquid samples. In studies with larger biomolecules, LEMS analyses of dried samples from glass showed a decrease in survival yield compared with conventional ESI-MS for leucine enkephalin and bradykinin of ~15% and 11%, respectively. The survival yields for liquid LEMS analyses were comparable to or better than ESI-MS for benzylpyridinium salts and large biomolecules.

Ambient femtosecond laser vaporization and nanosecond laser desorption electrospray ionization mass spectrometry

Recent investigations of ambient laser-based transfer of molecules into the gas phase for subsequent mass spectral analysis have undergone a renaissance resulting from the separation of vaporization and ionization events. Here, we seek to provide a snapshot of recent femtosecond (fs) duration laser vaporization and nanosecond (ns) duration laser desorption electrospray ionization mass spectrometry experiments. The former employs pulse durations of <100 fs to enable matrix-free laser vaporization with little or no fragmentation. When coupled to electrospray ionization, femtosecond laser vaporization provides a universal, rapid mass spectral analysis method requiring no sample workup. Remarkably, laser pulses with intensities exceeding 10(13) W cm(-2) desorb intact macromolecules, such as proteins, and even preserve the condensed phase of folded or unfolded protein structures according to the mass spectral charge state distribution, as demonstrated for cytochrome c and lysozyme. Because of the ability to vaporize and ionize multiple components from complex mixtures for subsequent analysis, near perfect classification of explosive formulations, plant tissue phenotypes, and even the identity of the manufacturer of smokeless powders can be determined by multivariate statistics. We also review the more mature field of nanosecond laser desorption for ambient mass spectrometry, covering the wide range of systems analyzed, the need for resonant absorption, and the spatial imaging of complex systems like tissue samples.