Accurate and High-Resolution Particle Mass Measurement Using a Peak Filtering Algorithm.

Charge detection quadrupole ion trap mass spectrometry (CD-QIT MS) is an effective way of achieving the mass analysis of microparticles with ultrahigh mass. However, its mass accuracy and resolution are still poor. To enhance the performance of CD-QIT MS, the resolution Rpeak of each peak in the mass spectra resulting from an individual particle was assessed, and a peak filtering algorithm that can filter out particle adducts and clusters with a lower Rpeak was proposed. By using this strategy, more accurate mass information about the analyzed particles could be obtained, and the mass resolution of CD-QIT MS was improved by nearly 2-fold, which was demonstrated by using the polystyrene (PS) particle size standards and red blood cells (RBCs). Benefiting from these advantages of the peak filtering algorithm, the baseline separation and relative quantification of 3 and 4 µm PS particles were achieved. To prove the application value of this algorithm in a biological system, the mass of yeast cells harvested at different times was measured, and it was found that the mixed unbudded and budded yeast cells, which otherwise would not be differentiable, were distinguished and quantified with the algorithm.

Desorption Separation Ionization Mass Spectrometry (DSI-MS) for Rapid Analysis of COVID-19.

During the coronavirus disease 2019 (COVID-19) pandemic, which has witnessed over 772 million confirmed cases and over 6 million deaths globally, the outbreak of COVID-19 has emerged as a significant medical challenge affecting both affluent and impoverished nations. Therefore, there is an urgent need to explore the disease mechanism and to implement rapid detection methods. To address this, we employed the desorption separation ionization (DSI) device in conjunction with a mass spectrometer for the efficient detection and screening of COVID-19 urine samples. The study encompassed patients with COVID-19, healthy controls (HC), and patients with other types of pneumonia (OP) to evaluate their urine metabolomic profiles. Subsequently, we identified the differentially expressed metabolites in the COVID-19 patients and recognized amino acid metabolism as the predominant metabolic pathway involved. Furthermore, multiple established machine learning algorithms validated the exceptional performance of the metabolites in discriminating the COVID-19 group from healthy subjects, with an area under the curve of 0.932 in the blind test set. This study collectively suggests that the small-molecule metabolites detected from urine using the DSI device allow for rapid screening of COVID-19, taking just three minutes per sample. This approach has the potential to expand our understanding of the pathophysiological mechanisms of COVID-19 and offers a way to rapidly screen patients with COVID-19 through the utilization of machine learning algorithms.

Pocket-Size Wireless Nanoelectrospray Ionization Mass Spectrometry for Metabolic Analysis of Salty Biofluids and Single Cells.

Analysis of volume-limited biological samples such as single cells and biofluids not only benefits clinical purposes but also promotes fundamental research in life sciences. Detection of these samples, however, imposes strict requirements on measurement performance because of the minimal volume and concentrated salts of the samples. Herein, we developed a self-cleaning nanoelectrospray ionization device powered by a pocket-size "MasSpec Pointer" (MSP-nanoESI) for metabolic analysis of salty biological samples with limited volume. The self-cleaning effect induced by Maxwell-Wagner electric stress helps with keeping the borosilicate glass capillary tip free from clogging and thus increasing salt tolerance. This device possesses a high sample economy (about 0.1 µL per test) due to its pulsed high voltage supply, sampling method (dipping the nanoESI tip into analyte solution), and contact-free electrospray ionization (ESI) (the electrode does not touch the analyte solution during ESI). High repeatable results could be acquired by the device with a relative standard deviation (RSD) of 1.02% for voltage output and 12.94% for MS signals of caffeine standard. Single MCF-7 cells were metabolically analyzed directly from phosphate buffered saline, and two types of untreated cerebrospinal fluid from hydrocephalus patients were distinguished with 84% accuracy. MSP-nanoESI gets rid of the bulky apparatus and could be held in hand or put into one's pocket for transportation, and it could operate for more than 4 h without recharge. We believe this device will boost scientific research and clinical usage of volume-limited biological samples with high-concentration salts in a low-cost, convenient, and rapid manner.

High Speed Mass Measurement of a Single Metal-Organic Framework Nanocrystal in a Paul Trap.

Mass spectrometry (MS) has emerged as an excellent tool for the characterization of metal-organic frameworks (MOFs) based on the characteristic metal ions and organic ligands. Mass measurement of intact MOF nanocrystals, however, remains a challenge for MS technology. Here, we reported the development of a probe particles based charge detection-quadrupole ion trap mass spectrometry (probe CD-QIT MS) method, where charge detection and mass measurement of a single MOF nanocrystal were achieved under the assistance of probe particles of micrometer size. As a validation of the method, the masses of a series of polystyrene (PS) size standards from 493 nm to 1.6 µm were measured with 3 µm PS particles as probes, and the measured masses were found to match well with their certified masses. Then, charge detections and mass analysis of single ZIF-8 and GOx@ZIF-8 with a size around 600 nm were achieved successfully. The method presented here demonstrates simplicity, high speed, and accuracy. Notably, it allows quantitative measurement of the amount of immobilized GOx enzyme by using the mass difference between ZIF-8 and GOx@ZIF-8. In addition, based on the determined mass, the size analysis of these MOF particles with irregular shape was carried out and demonstrated to be complementary to transmission electron microscopy (TEM).

Biofluids Metabolic Profiling Based on PS@Fe3O4-NH2 Magnetic Beads-Assisted LDI-MS for Liver Cancer Screening.

Liver cancer (LC) is the third frequent cause of death worldwide, so early diagnosis of liver cancer patients is crucial for disease management. Herein, we applied NH2-coated polystyrene@Fe3O4 magnetic beads (PS@Fe3O4-NH2 MBs) as a matrix material in laser desorption/ionization mass spectrometry (LDI-MS). Rapid, sensitive, and selective metabolic profiling of the native biofluids was achieved without any inconvenient enrichment or purification. Then, based on the selected m/z features, LC patients were discriminated from healthy controls (HCs) by machine learning, with the high area under the curve (AUC) values for urine and serum assessments (0.962 and 0.935). Moreover, initial-diagnosed and subsequent-visited LC patients were also differentiated, which indicates potential applications of this method in early diagnosis. Furthermore, among these identified compounds by FT-ICR MS, the expression level of some metabolites changed from HCs to LCs, including 29 and 12 characteristic metabolites in human urine and serum samples, respectively. These results suggest that PS@Fe3O4-NH2 MBs-assisted LDI-MS coupled with machine learning is feasible for LC clinical diagnosis.

Profiling of Urine Carbonyl Metabolic Fingerprints in Bladder Cancer Based on Ambient Ionization Mass Spectrometry.

The diagnosis of bladder cancer (BC) is currently based on cystoscopy, which is invasive and expensive. Here, we describe a noninvasive profiling method for carbonyl metabolic fingerprints in BC, which is based on a desorption, separation, and ionization mass spectrometry (DSI-MS) platform with N,N-dimethylethylenediamine (DMED) as a differential labeling reagent. The DSI-MS platform avoids the interferences from intra- and/or intersamples. Additionally, the DMED derivatization increases detection sensitivity and distinguishes carboxyl, aldehyde, and ketone groups in untreated urine samples. Carbonyl metabolic fingerprints of urine from 41 BC patients and 41 controls were portrayed and 9 potential biomarkers were identified. The mechanisms of the regulations of these biomarkers have been tentatively discussed. A logistic regression (LR) machine learning algorithm was applied to discriminate BC from controls, and an accuracy of 85% was achieved. We believe that the method proposed here may pave the way toward the point-of-care diagnosis of BC in a patient-friendly manner.

Laser Desorption/Ionization Mass Spectrometry Imaging: A New Tool to See through Nanoscale Particles in Biological Systems.

Understanding the fate of nanoscale particles (NPs) in biological systems is significant with the increasing risk for human exposure. Recent research endeavors in laser desorption/ionization mass spectrometry imaging (LDI-MSI) have enriched the toolbox for evaluation of NPs' behavior in biological tissues, especially in aspects including sub-organ bio-distribution, clearance, quantification and surface chemistry variation analysis. In recognition of the potential for advancement in LDI MSI, this concept provides a brief overview of recent research works in LDI MSI for NPs, illustrates new applications that demonstrate the superiority of this technique, and highlights a series of perspectives and directions to move the field forward.

Pocket-Size "MasSpec Pointer" for Ambient Ionization Mass Spectrometry.

Current ambient ionization sources for mass spectrometry (MS) are typically connected to gas cylinders, high-voltage supply, injection pump, and other accessory equipment, which hinder the popularization of MS in the field of on-site detection. Here, we developed a wireless pocket-size "MasSpec Pointer" (weights 65 g) based on arc discharge powered by a 3.7 V polymer Li battery for ambient ionization MS. A high voltage of 5600 V and 20 kHz was generated from the boost coil to penetrate air and form a plasma. The relative standard deviation (RSD) of the high-voltage pulses is 3.8%, leading to a stable discharge and a good quantification performance. A mini diaphragm pump was used to cool the plasma from ∼600 to ∼40 °C and to blow the plasma into a jet, which facilitates sampling. MasSpec Pointer can work well at both positive- and negative-ion modes without any modification and can quickly test gaseous, liquid, or solid samples. The limit of detection of this device for atrazine (an agrochemical) is lower than 0.1 ng/mL. MasSpec Pointer has shown its ability to pinpoint the double-bond location of fatty acid isomers without derivatization reagents or light illumination. Agrochemicals from the surface of an apple and daily chemicals from the surface of a finger were detected successfully using MasSpec Pointer coupled with a miniature mass spectrometer. We believe the "point-and-shoot" device coupled with mini-MS brings the hope for an age of detecting chemicals on-site by nonprofessionals.

Development of capillary-paper spray for small-molecule analysis in complex samples.

We develop a capillary-paper spray (CPS) ion source which allows for sample separation in the capillary and enables rapid and sensitive paper spray (PS) mass spectrometry (MS) analysis of biofluids. The CPS employs a glass capillary to load liquid analytes, vertically standing at the rear of the PS. To further reduce the matrix effect, a nitrocellulose filter membrane is placed between the glass tube and chromatography paper to absorb proteins and other macromolecules, which is beneficial for the detection of the small molecules. Compared with the normal PS method, the CPS method markedly improves spray stability and prolongs analysis duration, and also generates significantly better signal intensities during the analysis of drugs, thus indicating its potential for clinical use. As a proof of concept, quantitative analysis of drugs (metformin hydrochloride and berberine hydrochloride) in serum is performed.

Mass Spectrometry Imaging Reveals In Situ Behaviors of Multiple Components in Aerosol Particles.

The inhalation of atmospheric particles is deleterious to human health. However, as a complex mixture, tracing the behaviors of multiple components from real aerosol particles is crucial but unachievable by the existing methods. Here, taking advantage of the intrinsic fingerprints of elemental carbon (EC) and organic carbon (OC) in carbonaceous aerosol (CA) upon laser irradiation, we proposed a label-free mass spectrometry imaging method to visualize and quantify the deposition, translocation and component variation of CA in organs. With this method, the heterogeneous deposition, clearance and release behavior of CA in lung, that more OC was released in parenchyma and OC was cleared faster than EC, was observed. The translocation of CA to extrapulmonary organs including kidney, liver, spleen and even brain was also verified and quantified. By comparing the ratio of OC to EC, an organ-specific release behavior of OC from CA during circulation was revealed. In orthotopic lung and liver tumor, OC was found to penetrate more into tumor foci than EC. This technique provides deeper information for understanding the systemic health effects of aerosol particles.

Ultrafast Photocatalytic Reaction Screening by Mass Spectrometry.

Here we report a semiconductor-assisted laser desorption/ionization mass spectrometry (SA-LDI MS) platform to monitor photocatalytic reactions online and apply it for ultrafast reaction screening. In this method, we use photocatalytic nanomaterials as the substrate for LDI and then initiate and monitor the reactions simultaneously. The features of our method include the following: (i) It has a reaction acceleration effect: only seconds are needed in our interfacial reactions vs hours in conventional bulk phase. (ii) The reaction trend in our system agrees with that in bulk phase. (iii) By adding a stable analogue of reactant as internal standard, a quantification of the reaction can be achieved. (iv) The sensitivity is high: for 500 amol of reactant, the photocatalytic reaction can still be initiated and detected. This platform has advantages in ultrafast reaction screening (e.g., screening of nine catalysts requires 24 h by the UPLC-MS system but only 10 min by SA-LDI MS). Furthermore, the high specificity of MS enables the screening of catalytic selectivity of A-TiO2 nanoparticles for a methyl red (MR) and acid yellow (AY) mixture, whose absorption wavelengths are overlapped and thus cannot be discriminated by conventional optical methods. Furthermore, by using SA-LDI MS, we also monitored reductive debrominations during the degradation process of polybrominated diphenyl ethers (PBDEs), which is a type of important pollutant that is difficult to degrade and detect in liquid phase, and the photocatalytic reduction of CO2. Overall, SA-LDI MS realizes ultrafast photocatalytic reaction screening for the first time and provides practical analytical value in the field of catalyst screening.

Mass, Size, and Density Measurements of Microparticles in a Quadrupole Ion Trap.

The physical properties of microparticles, such as mass, size, and density, are critical for their functions. The comprehensive characterization of these physical parameters, however, remains a fundamental challenge. Here, we developed a particle mass spectrometry (PMS) methodology for determining the mass, size, and density of microparticles simultaneously. The collisional cross-section (CCS) and mass spectrometry (MS) measurements were performed in a single quadrupole ion trap (QIT), and the two modes can be switched easily by tuning the electric and gas hydrodynamic fields of the QIT. The feasibility of the method was demonstrated through a series of monodispersed polystyrene (PS) and silica (SiO2) particle standards. The SiO2/polypyrrole core-shell particles were also successfully characterized, and the measured results were verified by using conventional methods.

A Miniature Particle Mass Spectrometer.

Microparticles play important roles in our life. Besides chemical compositions and morphology, the size of microparticles will also decide their behavior in the environment or in organisms. Weighing the mass of microparticles by mass spectrometry is a useful method to characterize their size. In this technical note, a miniature particle mass spectrometer with an aerodynamic desorption/ionization ion source has been developed. We used a compact main control board to produce an ac voltage for trapping and ejecting the particles. The sampling process and data acquisition were also controlled by this board. We utilized this instrument to measure polystyrene spheres, silica particles, and mice red blood cells. Mass distributions of these particles were obtained rapidly with good accuracy.

Direct identification of forensic body fluids by MALDI-MS.

The rapid identification of human body fluids is meaningful for forensic casework. However, current methods suffer from several limitations such as poor sensitivity, time consumption and big sample consumption. Herein, we developed a mass spectrometry method to distinguish human body fluids (blood, semen, urine, sweat, and saliva) based on small molecular regions with no pretreatment, microliter sample consumption and high throughput. A highly sensitive and high salt-tolerance matrix N-(1-naphthyl)ethylenediamine dihydrochloride (NEDC) was used to efficiently detect metabolites in complex humoral environment. Some characteristic small metabolic molecules such as heme, hemin, creatinine, phosphate acid, uric acid, citric acid and lactic acid were identified and served as potential biomarkers to differentiate different body fluid types. Further principal component analysis (PCA) was performed to cluster the body fluid samples and three principal components allowed 75% clustering of all body fluid types. Blind testing revealed that nine out of ten unknown body fluid samples could be correctly classified into their corresponding group. This novel method can efficiently differentiate five body fluids with minimal interferences due to the storage time (less than 12 months) and carrier materials (cotton, fabric and tissue). The whole process from sampling to recording of mass spectra of body fluids can be finished in less than 10 minutes. We believe that this developed strategy has significant implications for rapid and effective human body fluid screening in forensic casework.

Heat-Induced Rearrangement of the Disulfide Bond of Lactoglobulin Characterized by Multiply Charged MALDI-TOF/TOF Mass Spectrometry.

Disulfide bonds are an important post-translational modification of proteins and play a significant role in stabilizing protein structure. While both mass spectrometry-based bottom-up and top-down proteomics are widely used in the identification of disulfide linkages, the top-down approach can avoid potential information loss of disulfide linkage occurring in the bottom-up analysis. In the present work, we applied matrix-assisted laser desorption/ionization tandem Time-of-Flight (MALDI-TOF/TOF) mass spectrometry to investigate the heat-induced disulfide rearrangement of ß-lactoglobulin (ß-LG). Since ß-LG (18 kDa) is too large for common TOF/TOF analysis, we use 2-nitrophloroglucinol (2-NPG) as a matrix to generate multiply charged proteins by MALDI. Fragmentation of doubly charged protein ions yields characteristic triplet peaks of disulfide bonds. We found that the characteristic fragments derived from the heterolytic cleavage of disulfide bonds decreased sharply when the incubation temperature of ß-LG solution reached the critical point of 75 °C. These results indicate that the disulfide linkage between C160 and C66 has been broken during the heating process and, probably, new disulfide formed. In conclusion, our work highlights the analytical value of the multiply charged MALDI-TOF/TOF method in the identification of larger proteins (>12 kDa) and disulfide-containing proteins.

Differentiation and Relative Quantitation of Disaccharide Isomers by MALDI-TOF/TOF Mass Spectrometry.

Saccharide isomer differentiation has been a challenge in glycomics, as the lack of technology to decipher fully the diverse structures of compositions, linkages, and anomeric configurations. Several mass spectrometry-based methods have been applied to the discrimination of disaccharide isomers, but limited quantitative analyses have been reported. In the present study, MALDI-LIFT-TOF/TOF has been investigated to differentiate and relatively quantify underivatized glucose-containing disaccharide isomers that differ in composition, connectivity or configuration. N-(1-naphthyl)ethylenediamine dihydrochloride (NEDC) was used as a highly sensitive matrix without matrix interferences in low mass range, thus yielding intense chloride-attached disaccharide ions [M + Cl]-, which could be fragmented to give diagnostic characteristic fragment patterns for distinguishing these isomers. Three different types of disaccharide isomers were successfully relatively quantified in a binary mixture using the specific product ion pairs. Finally, this method was utilized to identify and relatively quantify two disaccharide isomers in Medicago leaf (maltose and sucrose) without numerous preparation steps. In general, this method is a fast, effective, and robust method for rapid differentiation and quantitation of disaccharide isomers in complex medium.

N-Phenyl-2-naphthylamine as a Novel MALDI Matrix for Analysis and in Situ Imaging of Small Molecules.

Due to its strong ultraviolet absorption, low background interference in the small molecular range, and salt tolerance capacity, N-phenyl-2-naphthylamine (PNA) was developed as a novel matrix in the present study for analysis and imaging of small molecules by matrix-assisted laser desorption/ionization mass spectrometry time-of-fight (MALDI-TOF MS). The newly developed matrix displayed good performance in analysis of a wide range of small-molecule metabolites including free fatty acids, amino acids, peptides, antioxidants, and phospholipids. In addition, PNA-assisted LDI MS imaging of small molecules in brain tissue of rats subjected to middle cerebral artery occlusion (MCAO) revealed unique distributions and changes of 89 small-molecule metabolites including amino acids, antioxidants, free fatty acids, phospholipids, and sphingolipids in brain tissue 24 h postsurgery. Fifty-nine of the altered metabolites were identified, and all the changed metabolites were subject to relative quantitation and statistical analysis. The newly developed matrix has great potential application in the field of biomedical research.

Utilizing a Mini-Humidifier To Deposit Matrix for MALDI Imaging.

MALDI mass spectrometry imaging (MALDI-MSI) is a powerful tool to study endogenous metabolites. The process of matrix deposition is crucial for a high-quality imaging result. Commercial instruments for matrix deposition are expensive. Low-cost methods like airbrushing will generate matrix crystals that are too large for high-spatial-resolution imaging. Sublimation may cause some compounds to go undetected because of the lack of solvent. Herein, we utilized a mini-humidifier, costing less than 5 dollars, to deposit matrix for MALDI-MSI. Compared with Imageprep, a commercialized instrument, our device based on the humidifier provided higher sensitivity and much smaller matrix crystals with diameters of less than 10 µm. High-quality ion images with 10 µm spatial resolution were obtained using our method. The enhancement of sensitivity by the humidifier could provide a sufficient amount of ions to perform tandem mass imaging. We also performed MALDI-MS/MS imaging to separate two lipids in mouse brain.

Laser Cleavable Probes-Based Cell Surface Engineering for in Situ Sialoglycoconjugates Profiling by Laser Desorption/Ionization Mass Spectrometry.

Cell-surface sialoglycoconjugates (sialoglycoproteins and sialoglycolipids) play important roles in cell-cell interactions and related tumor metastasis process. Although there have been some analytical methods to evaluate the sialoglycoconjugates, an effective method providing both qualitative and quantitative information is still deficient. Here we establish an extraction-free, sensitive, and high-throughput platform to realize in situ detection of the cell-surface sialoglycoconjugates on various cell lines, e.g., cancer and normal cells by laser desorption/ionization mass spectrometry (LDI MS). In this proposal, azide groups were introduced into the ends of cell-surface sialoglycoconjugates by the biorthogonal method, and then the sialoglycoconjugates were armed with a laser-cleavable probe (Tphsene) through click chemistry. We can easily get the probes signal under laser irradiation, which reflected the presence of cell-surface sialoglycoconjugates. Different cell lines were discriminated simultaneously, and the LDI relative quantification agreed with fluorescent results. Besides, a linear quantitation relationship in the range of 100 fmol to 100 pmol was obtained with a designed and synthesized internal standard (phTsane) added. A detection limit of 5 fmol was obtained with good reproducibility. Based on the quantitative and high-throughput ability, we conducted pharmacodynamics study of drug (tunicamycin) on cancer cells. In addition, we found the tag was safe from sweet-spot effect of matrix adding. The simultaneous detection of sialoglycoconjugates and metabolites was therefore achieved. We believe that this laser cleavable probes-based cell-surface engineering for sialoglycoconjugates platform means great significance to diagnosis, prognosis, and therapeutic purposes. Besides, this strategy can be applied to other glycoconjugates which is hard to detect and the related disease processes when more corresponding chemically modified sugar substrates and exact biorthogonal reactions are developed.

Mass Measurement of Single Intact Nanoparticles in a Cylindrical Ion Trap.

Accurate nanoparticle mass characterization is a challenging task, especially at a single particle level. To solve this problem, a strategy for the mass measurement of single intact nanoparticle was proposed. A microscopy-based ion trap mass spectrometer was built up. To improve the detection sensitivity, a cylindrical ion trap with transparent conductive end-caps was used to increase the transmission of scattered light, and a vacuum ultraviolet lamp was used to increase the charge state of the isolated nanoparticle. By detecting the scattered light of the isolated nanoparticle, a series of secular frequencies were obtained, from which the corresponding mass-to-charge ratio of the nanoparticle was calculated. Finally, a Labview program was used to help deduce the charge state and absolute mass of the individual nanoparticle. Masses of gold nanoparticles with different sizes were accurately examined, which are (5.08 ± 0.44) × 10(7) Da for 20 nm, (3.55 ± 0.34) × 10(8) Da for 40 nm, and (1.22 ± 0.14) × 10(9) Da for 60 nm, respectively. The mass of MOFs with irregular shapes was also determined, which is (6.48 ± 1.08) × 10(9) Da. This method can provide the mass information on nanomaterials, thus opens up new possibility of characterizing nanoparticles at the single particle level.