Ultrasonication-assisted spray ionization mass spectrometry for on-line monitoring of organic reactions

A straightforward on-line monitoring of organic reactions by ultrasonication-assisted spray ionization mass spectrometry (UASI MS) is demonstrated in this work.     

Vibrating tip spray ionization mass spectrometry for direct sample analysis

In this work, a vibrating tip spray ionization source was developed for direct mass spectrometric analysis of raw samples under voltage‐free condition. A solid tip was mounted on a vibrator, and the solid tip was placed on the front of MS inlet. Liquid, viscous, and bulk solid samples could be directly loaded on the tip‐end surface, and then a drop of solvent at microliter level was subsequently loaded on the tip for dissolution and extraction of analytes, and a vibrator was then started to atomize and ionize the analytes under ambient condition. We demonstrated vibrating tip spray mass spectrometry in various applications, including food safety, pharmaceutical analysis, and forensic science. Furthermore, in situ analysis of biological tissues and in vivo analysis of living plants were conveniently performed, due to voltage‐free. Different vibration frequencies and solvent compositions were investigated. The analytical performances, including sensitivity, reproducibility, and linear range, were investigated. The ionization process and mechanism were also discussed in this work.     

High-frequency AC electrospray ionization source for mass spectrometry of biomolecules

A novel high-frequency alternating current (AC) electrospray ionization (ESI) source has been developed for applications in mass spectrometry. The AC ESI source operates in a conical meniscus mode, analogous to the cone-jet mode of direct current (DC) electrosprays but with significant physical and mechanistic differences. In this stable conical-meniscus mode at frequencies greater than 50 kHz, the low mobility ions, which can either be cations or anions, are entrained within the liquid cone and ejected as droplets that eventually form molecular ions, thus making AC ESI a viable tool for both negative and positive mode mass spectrometry. The performance of the AC ESI source is qualitatively shown to be frequency-dependent and, for larger bio-molecules, the AC ESI source produced an ion signal intensity that is an order of magnitude higher than its DC counterpart.     

Nanoparticle-based mass spectrometry for the analysis of biomolecules

Nanoparticles (NPs) are useful as matrixes for the analyses of several types of biomolecules (including aminothiols, peptides, and proteins) and for mass spectrometric imaging through surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS), mainly because of their large surface area, strong absorption in the ultraviolet-near-infrared region, and ready functionalization. Metallic NPs, metal oxide NPs, and semiconductor quantum dots, unmodified or functionalized with recognition ligands, have a strong affinity toward analytes; therefore, they allow the enrichment of biomolecules, leading to improved sensitivity with minimal matrix interference in their mass spectra. SALDI-MS using NPs overcomes the two major problems commonly encountered in matrix-assisted laser desorption/ionization mass spectrometry: the presence of "sweet spots" and the high background signals in the low-mass region. In this tutorial review, we discuss the roles played by the nature, size, and concentration of the NPs, the buffer composition, and the laser energy in determining the sensitivity and mass ranges for the analytes. We describe internal standard SALDI-MS methods that allow the concentrations of analytes to be determined with low variation (relative standard deviations: <10%) and we highlight how the simplicity, sensitivity, and reproducibility of SALDI-MS approaches using various NPs allow the analyses of proteins and small analytes and the imaging of cells.     

A general soft ionization method for mass spectrometry: Resonance-enhanced multi-photon ionization of biomolecules

We show here that ‘double-barrelled’ laser desorption/resonance-enhanced multi-photon ionization mass spectrometry has many advantages over traditional single-step desorption/ionization techniques, particularly if combined with Reflectron-Time-of-Flight mass analysers. We demonstrate the effectiveness of this technique for large labile biomolecules, namely native chlorophylls, porphyrins and a peptide, as examples of structural and mass analysis.     

MPAI (mass probes aided ionization) method for total analysis of biomolecules by mass spectrometry.

We have designed and synthesized various mass probes, which enable us to effectively ionize various molecules to be detected with mass spectrometry. We call the ionization method using mass probes the "MPAI (mass probes aided ionization)" method. We aim at the sensitive detection of various biological molecules, and also the detection of bio-molecules by a single mass spectrometry serially without changing the mechanical settings. Here, we review mass probes for small molecules with various functional groups and mass probes for proteins. Further, we introduce newly developed mass probes for proteins for highly sensitive detection.     

Direct detection of large fat-soluble biomolecules in solution using membrane inlet mass spectrometry and desorption chemical ionization

This paper presents the first membrane inlet mass spectrometry system capable of detecting large biomolecules, such as testosterone (M(r) 288), testosterone acetate (M(r) 330) and alpha-tocopherol (M(r) 430, vitamin E). The result was obtained using a home-made chemical ionization ion source with a thermostated tubular silicone membrane mounted right in the centre of a methane CI plasma. The liquid sample was flushed through the inside of the membrane for a period of 20-25 min, where the analyte diffused into the membrane. Following this trapping period the analyte was released from the membrane into the mass spectrometer by the combined action of heat radiation from the filament and charge transfer from the chemical ionization plasma. As a result of this stimulated desorption a good desorption peak was obtained as the analyte vaporized out of the membrane. Retinol (M(r) 286, vitamin A), cholecalciferol (M(r) 384, vitamin D3) and cholesterol (M(r) 386) were also detected. However, these compounds (all containing a long hydrocarbon chain and being aliphatic alcohols) did not give a protonated molecule. They gave a series of cluster ions with the dominant located 20 mass units below the molecular ion. The detection limits of the new desorption chemical ionization MIMS technique were at low or sub-micromolar concentrations (high ppb levels) and the reproducibility was within 20%, when the area of the desorption peak was used for quantitation.     

Derivatization of small biomolecules for optimized matrix-assisted laser desorption/ionization mass spectrometry

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) is a powerful tool for the measurement of low molecular mass compounds of biological interest. The limitations for this method are the volatility of many analytes, possible interference with matrix signals or bad ionization or desorption behavior of the compounds. We investigated the application of well-known and straightforward one-pot derivatization procedures to circumvent these problems. The derivatizations tested allow the measurement and the labeling of alcohols, aldehydes and ketones, carboxylic acids, alpha-ketocarboxylic acids and amines.     

Gold nanoparticles as assisted matrices for the detection of biomolecules in a high-salt solution through laser desorption/ionization mass spectrometry

Citrate-capped gold nanoparticles (AuNPs) serve as matrices for the determination of biomolecules in a high-salt solution through matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). In the case of using 2,5-dihydroxybenzoic acid (2,5-DHB) as a matrix, the signal intensities of neutral steroids were severely suppressed in a high-salt solution. A high concentration of NaCl caused the formation of the sodium adduct ions during the desorption/ionization process, resulting in a decrease of the signal intensities of the protonated ions. In comparison, by applying AuNP-assisted LDI-TOF-MS, the signal intensities of neutral steroids remained almost constant when the concentration of NaCl was increased to 500 mM. Because the use of citrate-capped AuNPs as matrices primarily offers alkali metal ion adducts, AuNP matrices have a higher tolerance to high NaCl concentrations relative to that of 2,5-DHB matrices. The relevant phenomena are also discovered in the case of analysis of neutral carbohydrate, monosialoganglioside, indolamine, and angiotensin I. The quantification of small molecules in a high-salt solution has been accomplished by AuNP-assisted LDI-TOF-MS coupled to a unique sample preparation, in which samples are deposited onto the sample plate before AuNPs. The present method has been further applied to the determination of urea, creatinine, uric acid, and glucose in a urine sample.     

Nonspecific interactions between proteins and charged biomolecules in electrospray ionization mass spectrometry

An investigation of the nonspecific association of small charged biomolecules and proteins in electrospray ionization mass spectrometry (ES-MS) is described. Aqueous solutions containing pairs of proteins and a small acidic or basic biomolecule that does not interact specifically with either of the proteins were analyzed by ES-MS and the distributions of the biomolecules bound nonspecifically to each pair of proteins compared. For the basic amino acid arginine and the peptide RGVFRR, nonequivalent distributions were measured in positive ion mode, but equivalent distributions were measured in negative ion mode. In the case of uridine 5′-diphosphate, nonequivalent distributions were measured in negative ion mode, but equivalent distributions observed in positive ion mode. The results of dissociation experiments performed on the gaseous ions of the nonspecific complexes suggest that the nonequivalent distributions result from differences in the extent to which the nonspecific complexes undergo in-source dissociation. To test this hypothesis, the distributions of nonspecifically bound basic molecules measured in the presence of imidazole, which protects complexes from in-source dissociation, were compared. In all cases, equivalent distributions were obtained. The results indicate that nonspecific binding of charged molecules to proteins during ES is a statistical process, independent of protein structure and size. However, the kinetic stabilities of the nonspecific interactions are sensitive to the nature of the protein ions. It is concluded that the reference protein method for correcting ES mass spectra for nonspecific ligand-protein binding can be applied to the analysis of ionic ligands, provided that in-source dissociation of the nonspecific interactions is minimized.     

Mesoporous tungsten titanate as matrix for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis of biomolecules

In this paper, mesoporous tungsten titanate (WTiO) with different nano-pore structures was utilized as matrix for the analysis of short peptides by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS). Effect of characteristic features of mesoporous matrices on laser desorption/ionization process was investigated. Experiments showed that the ordered two-dimensional and three-dimensional mesoporous matrices were superior in performance to the non-ordered WTiO matrix. The dramatic enhancement of signal sensitivity by the ordered mesoporous matrices can be reasonably attributed to the ordered structure, which facilitated the understanding on structure-function relationship in mesoporous cavity for laser desorption process of adsorbed biomolecules. With the ordered mesoporous matrix, the short peptides are successfully detected. The presence of trace alkali metal salt effectively increased the analyte ion yields and the MALDI-TOFMS using the inorganic mesoporous matrices displayed a high salt tolerance. The developed technique also showed a satisfactory performance in peptide-mapping and amino-acid sequencing analysis.     

Desorption sonic spray ionization for (high) voltage-free ambient mass spectrometry

Sonic spray ionization is shown to create a supersonic cloud of charged droplets able to promote efficient desorption and ionization of drugs directly from the surfaces of commercial drug tablets at ambient conditions. Compared with desorption electrospray ionization (DESI), desorption sonic spray ionization (DeSSI) is advantageous since it uses neither heating nor high voltages at the spray capillary. DeSSI therefore provides a more friendly environment in which to perform ambient mass spectrometry (MS). DeSSI-MS is herein evaluated for the analysis of drug tablets, and found to be, in general, as sensitive as DESI-MS. The (high) voltage-free DeSSI method provides, however, cleaner mass spectra with less abundant solvent cluster ions and with enough abundant analyte signal for tandem mass spectrometry (MS/MS). These features may therefore facilitate the DeSSI-MS detection of low molar mass components or impurities, or both. The higher-velocity supersonic DeSSI spray also facilitates matrix penetration thus providing more homogenous sampling and longer lasting ion signals.     

Biogenic aldehyde determination by reactive paper spray ionization mass spectrometry

Ionization of aliphatic and aromatic aldehydes is improved by performing simultaneous chemical derivatization using 4-aminophenol to produce charged iminium ions during paper spray ionization. Accelerated reactions occur in the microdroplets generated during the paper spray ionization event for the tested aldehydes (formaldehyde, n-pentanaldehyde, n-nonanaldehyde, n-decanaldehyde, n-dodecanaldehyde, benzaldehyde, m-anisaldehyde, and p-hydroxybenzaldehyde). Tandem mass spectrometric analysis of the iminium ions using collision-induced dissociation demonstrated that straight chain aldehydes give a characteristic fragment at m/z 122 (shown to correspond to protonated 4-(methyleneamino)phenol), while the aromatic aldehyde iminium ions fragment to give a characteristic product ion at m/z 120. These features allow straightforward identification of linear and aromatic aldehydes. Quantitative analysis of n-nonaldehyde using a benchtop mass spectrometer demonstrated a linear response over 3 orders of magnitude from 2.5 ng to 5 μg of aldehyde loaded on the filter paper emitter. The limit of detection was determined to be 2.2 ng for this aldehyde. The method had a precision of 22%, relative standard deviation. The experiment was also implemented using a portable ion trap mass spectrometer.     

Impact desolvation of electrosprayed microdroplets--a new ionization method for mass spectrometry of large biomolecules.

Impact desolvation of electrosprayed microdroplets (IDEM) is a new method for producing gas-phase ions of large biomolecules. Analytes are dissolved in an electrolyte solution which is electrosprayed in vacuum, producing highly charged micron and sub-micron sized droplets (microdroplets). These microdroplets are accelerated through potential differences approximately 5 - 10 kV to velocities of several km/s and allowed to impact a target surface. The energetic impacts vaporize the droplets and release desolvated gas-phase ions of the analyte molecules. Oligonucleotides (2- to 12-mer) and peptides (bradykinin, neurotensin) yield singly and doubly charged molecular ions with no detectable fragmentation. Because the extent of multiple charging is significantly less than in atmospheric pressure electrospray ionization, and the method produces ions largely free of adducts from solutions of high ionic strength, IDEM has some promise as a method for coupling to liquid chromatographic techniques and for mixture analysis. Ions are produced in vacuum at a flat equipotential surface, potentially allowing efficient ion extraction.     

Mass probe-assisted ionization method for total analysis of biomolecules with electrospray ionization-mass spectrometry

In this paper, we review the mass probes used for the derivation of a variety of biomolecules efficiently detected by the electrospray ionization-mass spectrometry and mass probe-assisted ionization method for total analysis and determination by consecutive detection with a single instrument. We describe mass probes for a variety of molecules including proteins, nucleobases, metallic cations, and other small molecules.