Reduction of organic dyes in matrix-assisted laser desorption/ionization and desorption/ionization on porous silicon

Hot Hydrogen Atom Irradiation of Protonated/Deprotonated Peptide in an Ion Trap Facilitates Fragmentation through Heated Radical Formation

Tandem mass spectrometry with fragmentation involving the reaction with hydrogen atoms is expected to be useful for the analysis of peptides and proteins. In general, hydrogen atoms preferentially react with odd-electron radicals. The attachment of hydrogen atoms to even-electron peptide ions is barely observed because of their low reaction rate. To date, only the methodology developed by our group has successfully induced the fragmentation of even-electron peptide ions by reacting with hydrogen atoms. In the present study, we focused on the temperature of the peptide ions and hydrogen atoms in an ion trap mass spectrometer to understand the mechanism of the corresponding reaction. Because the reaction between even-electron peptide ions and hydrogen atoms has a significant transition state barrier, the use of hot hydrogen atoms is required to initiate the reaction. The reaction contributes to increase the internal energy of the resultant peptide radicals because the heat of reaction and kinetic energy of the hydrogen atom are converted to the internal energy of the product. The resultant oxygen- and carbon-centered peptide radicals undergo radical-induced fragmentation with sub-picosecond and sub-millisecond time scales, respectively.

Enhanced thermal effect of plasmonic nanostructures confined in discoidal porous silicon particles

The design of plasmonic nanostructures could have many exciting applications since it enhances or provides valuable control over efficient energy conversion. A three-dimensional (3D) space is a realistic hotspot matrix harvesting a wide conversion that has been shown in zero-dimensional nanoparticles, one-dimensional linear structures, or two-dimensional films. A novel 3D plasmonic nanostructure platform consisting of plasmonic metal nanoparticles in discoidal porous silicon particles is used in this study. Plasmonic gold nanoparticles are anchored inside the discoidal porous silicon (DPS) particles by in situ reduction synthesis. The novel plasmonic nanostructures can tailor the plasmon band and overcome the instability of photothermal materials. The “trapping well” for the anchored nanoparticles in 3D space can result in a huge change of plasmonic band of metal nanoparticles to the near-IR region in a novel 3D geometry. A multifunctional scaffold, Au–DPS particle, composed of doxorubicin conjugated to poly-(l-glutamic acid) (pDOX), was developed for combinatorial chemo-photothermal cancer therapy. The therapeutic efficacy was examined in treatment of the A549 cell line under near-IR laser irradiation. The highly efficient photothermal conversion can also be demonstrated in the laser desorption/ionization time-of-flight mass spectrometry detection analysis. The limit of detection was obviously improved in the detection of angiotensin II, P14R, and ACTH fragments 18-39 peptides. Overall, we envision that Au–DPS particles may be used in ultrasensitive theranostics in the future.

A 3D plasmonic nanostructure with a tunable plasmon resonance band to the near IR region enabled ultrasensitive theranostics for enhanced thermal effect.

A novel laser desorption/ionization method using through hole porous alumina membranes

RATIONALE

A novel matrix-free laser desorption/ionization method based on porous alumina membranes was developed. The porous alumina membranes have a two-dimensional (2D) ordered structure consisting of closely aligned straight through holes of sub-micron in diameter that are amenable to mass production by industrial fabrication processes.

METHODS

Considering a balance between the ion generating efficiency and the mechanical strength of the membranes, the typical values for the hole diameter, open aperture ratio and membrane thickness were set to 200 nm, 50% and 5 μm, respectively. The membranes were coated with platinum on a single side that was exposed to the laser. Evaluation experiments were conducted on the feasibility of this membrane structure for an ionization method using a single peptide and mixed peptides and polyethylene glycol samples and a commercial matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometer in the positive ion mode.

RESULTS

Results showed a softness of ionization and no sweet spot nature. The capillary action of the through holes with very high aspect ratio enables several loading protocols including sample impregnation from the surface opposite to the laser exposure side.

CONCLUSIONS

The feasibility study indicates that the through hole porous alumina membranes have several advantages in terms of usefulness over the conventional surface-assisted laser desorption ionization (SALDI) methods. The proposed novel ionization method is termed Desorption Ionization Using Through Hole Alumina Membrane (DIUTHAME).

Application of dyes as doping agents in MALDI-MS matrices for the signal enhancement of proteins

Dyes can significantly influence the ionization of proteins, when used as dopants in conventional matrices used for Matrix Assisted Laser Desorption Ionization Mass Spectrometry (MALDI-MS).

Going beyond electrospray: mass spectrometric studies of chemical reactions in and on liquids

There has been a burst in the number and variety of available ionization techniques to use mass spectrometry to monitor chemical reactions in and on liquids. Chemists have gained the capability to access chemistry at unprecedented timescales, and monitor reactions and detect intermediates under almost any set of conditions. Herein, recently developed ionization techniques that facilitate mechanistic studies of chemical processes are reviewed. This is followed by a discussion of our perspective on the judicious application of these and similar techniques in order to study reaction mechanisms.

Monomer/dimer dependent modulation of reduction of the cationic dye methylene blue in negatively charged nanolayers as revealed by mass spectrometry

Mass spectrometric approach to differentiation of monomer or dimer form of cationic dye methylene blue adsorption at negatively charged nanolayers is proposed.

Laser desorption/ionization mass spectrometric analysis of folic acid, vancomycin and Triton® X-100 on variously functionalized carbon nanotubes

RATIONALE

Carbon nanotubes (CNTs) have been ascertained to constitute versatile assisting matrices for laser desorption/ionization mass spectrometric analysis of different molecules. The functionalization thereof can lead to obtaining laser desorption/ionization assisting surfaces that would allow the detection of molecules at lower concentration and produce spectra with a better signal-to-noise ratio.

METHODS

Pristine, -OH and -COOH functionalized multi-walled CNTs were obtained from commercial suppliers. Gallic or sinapinic acid was attached covalently to the CNT surfaces by forming an ester bond. Folic acid, vancomycin and Triton(®) X-100 were used as analytes to examine properties of these new assisting surfaces. Mass spectrometry analysis was conducted on a matrix-assisted laser desorption/ionization quadrupole time-of-flight (MALDIQTOF) mass spectrometer.

RESULTS

The functionalization of CNTs was confirmed with Fourier transform infrared (FTIR) spectroscopy. The obtained mass spectra revealed that all the assisting surfaces are capable of transferring energy to the analytes; moreover, the presence of carboxyl groups in the structures of CNTs highly enhances their ionization properties. Nevertheless, the presence of sinapinic acid on CNT surfaces does not increase their properties to absorb pulse laser energy.

CONCLUSIONS

The presented assisting surfaces are effective in LDI mass analysis of folic acid, vancomycin and Triton(®) X-100. The appropriate functionalization of CNTs can lead to the production of assisting surfaces that can become highly effective in the ionization of particular types of analytes.

The application of semiconductor quantum dots for enhancing peptide desorption, improving peak resolution and sensitivity of detection in matrix-assisted laser desorption/ionization (MALDI) mass spectrometry

The interest in quantum dots (QD) and a number of reported life sciences applications increased dramatically over the last decade. The popularity of QDs stems from better photostability, higher extinction -co-efficients, and unique optical properties such as superior light absorption. Here we report methods for improving matrix-assisted laser desorption/ionization desorption of crude tryptic protein digests by using CdSe/ZnS QDs. The addition of QDs to the matrix improves the signal-to-noise ratio, peak quality and increases the number of detected peptides and the overall sequence coverage.

High irradiance laser ionization orthogonal time-of-flight mass spectrometry: a versatile tool for solid analysis

This article reviews the development of and applications for high irradiance laser ionization orthogonal time-of-flight mass spectrometry (LI-O-TOFMS). LI-O-TOFMS has solved the bottleneck problems in traditional high irradiance laser ionization mass spectrometry, which allows the instrument to acquire explicit spectra with high resolution. A buffer-gas-assisted ion source effectively reduces the kinetic energy of the ions and suppresses the multiply charged ion interference. The pulse train data acquisition technique was applied to reduce the spectrum interference from multiply charged ions and polyatomic ions according to the temporal profiles of different ion packets in the repelling region. Relatively high laser irradiance (≥10(10) W/cm(2)) is preferable for achieving uniform relative sensitivities for different elements in the samples of different matrices. LI-O-TOFMS has been used in the standardless, semiquantitative analysis of solids, which is proved to be a fast and convenient technique for solid sample analysis. By increasing the laser irradiance and reducing the buffer gas pressure, the determination of nonmetallic elements in solids can also be achieved without losing spectral explicity. Recent applications, such as elemental analysis of a single egg cell and acquiring elemental, fragmental, and molecular information of chemicals, were given to demonstrate the potential of the new technique. All of these results reveal that LI-O-TOFMS is an advanced tool in the elemental analysis of solids in terms of modern mass spectrometry.

Nanomaterial-assisted laser desorption ionization for mass spectrometry-based biomedical analysis

Nanomaterials have been widely used to assist laser desorption ionization of biomolecules for mass spectrometry analysis. Compared with classical matrix-assisted laser desorption ionization, strategies based on nanomaterial-assisted ionization generate a clean background, which is of great benefit for the qualitative and quantitative analysis of small biomolecules, such as therapeutic and diagnostic molecules. As label-free platforms, they have successfully been used for high-throughput enzyme activity/inhibition monitoring and also for tissue imaging to map in situ the distribution of peptides, metabolites and drugs. In addition to widely used porous silicon nanomaterials, gold nanoparticles can be easily chemically modified by thiol-containing compounds, opening novel interesting perspectives. Such functionalized nanoparticles have been used both as probes to extract target molecules and as matrices to assist laser desorption ionization for developing new enzyme immunoassays or for studying DNA hybridization. More recently, semiconductor nanomaterials or quantum dots acting as photosensitive centers to induce in-source redox reactions for proteomics and to investigate biomolecule oxidation for metabolomics have been shown to offer new analytical strategies.

Use of TOF-SIMS to study adsorption and loading behavior of methylene blue and papain in a nano-porous silicon layer

TOF-SIMS was applied to study the cross-sectional distribution of methylene blue and papain in porous silicon layers. Elemental and molecular information were used to study their distributions in the porous region and the chemistry of their adsorption. Methylene blue (MW = 284 Da) penetrated to the base to the pores. Positive ions (SiCH(3)(+)) suggest methylene blue binds to the substrate via its methyl groups. Negative fragments (SiOSH(3)(-) and SiO(2)SCH(-)) also suggested chemisorption via O bridging of the substrate Si and methylene blue S. The larger Papain molecule (23,406 Da) distributed itself in a similar manner to methylene blue demonstrating larger molecules can be effectively incorporated into such pore structures.

Electrochemical aspects of electrospray and laser desorption/ionization for mass spectrometry

Soft-ionization methods, namely electrospray ionization and laser desorption/ionization, are widely used to transfer large molecules as intact gas-phase ions either from a solution or from a solid substrate. During both processes, in-source electrochemical and photoelectrochemical reactions occur. These electrode reactions, which take place at interfaces, play important roles in influencing the ionization products, but they have received little attention. We show that having good control over both types of electrochemical reactions can lead to new analytical applications. Examples include online tagging by grafting of mass tags and in-source photooxidation of peptides.

Quantum dots improve peptide detection in MALDI MS in a size dependent manner

Laser Desorption Ionization Mass Spectrometry employs matrix which is co-crystallised with the analyte to achieve "soft ionization" that is the formation of ions without fragmentation. A variety of matrix-free and matrix-assisted LDI techniques and matrices have been reported to date. LDI has been achieved using ultra fine metal powders (UFMPs), desorption ionisation on silicon (DIOS), sol-gel assisted laser desorption/ionization (SGALDI), as well as with common MALDI matrices such as 2,5-dihydroxy benzoic acid (DHB), 3,5-dimethoxy-4-hydroxycinnamic acid (SA), α-cyano-4-hydroxycinnamic acid (CHCA) to name a few. A variety of matrix additives have been shown to improve matrix assisted desorption, including silicon nanowires (SiNW), carbon nanotubes (CNT), metal nanoparticles and nanodots. To our knowledge no evidence exists for the application of highly fluorescent CdSe/ZnS quantum dots to enhance MALDI desorption of biological samples. Here we report that although CdSe/ZnS quantum dots on their own can not substitute matrix in MALDI-MS, their presence has a moderately positive effect on MALDI desorption, improves the signal-to-noise ratio, peak quality and increases the number of detected peptides and the overall sequence coverage.

Effect of urea surface modification and photocatalytic cleaning on surface-assisted laser desorption ionization mass spectrometry with amorphous TiO2 nanoparticles

We have investigated the effect of urea surface modification and the photocatalytic cleaning on surface-assisted laser desorption ionization mass spectrometry (SALDI-MS) with amorphous TiO2 nanoparticles for the reduction of the background noise and the improvement of the sensitivity. In the use of nanoparticles of high surface area, chemical background signals arising from ambient environments and organic contaminants can frequently be serious problems below 500 Da, possibly reducing the advantages of the matrix-free approach. In this study, removal of contaminants and enhanced SALDI efficiency were easily achieved with UV irradiation via the photocatalyst effect of TiO2 before SALDI-MS measurements. The surface cleaning achieved by the UV photocatalytic procedure reduced the background noise and increased the peak intensities of peptides. In addition, we found that urea surface modification of TiO2 nanoparticles increased the performance of the TiO2-SALDI-MS. (1) The urea-surface modification of TiO2 made it possible to produce proton-adduct forms without citrate buffer, resulting in low background noises below 500 Da, in contrast to the essential use of a citrate buffer in the bare TiO2-SALDI-MS. (2) The detection sensitivity of angiotensin I increased to 0.3 fmol with the urea-surface modification, as compared to the use of bare TiO2 nanoparticles (6 fmol). The urea-TiO2 could ionize proteins of more than 20,000 Da such as trypsinogen (600 fmol). (3) The urea modification of TiO2 had the advantage of selective detection of phosphopeptides without sample clean up, or prefractionation in tryptic digest products of bovine hemoglobin.

Chemically modified porous silicon for laser desorption/ionization mass spectrometry of ionic dyes

Desorption/ionization on silicon (DIOS) mass spectra of model ionic dyes methylene blue (MB+Cl-) and methyl orange (Na+MO-) were studied using p+ type-derived porous silicon (PS) free layers. As-prepared PS (PS-H), the PS thermally oxidized at 300 degrees C (PS-OX), PS with chemically grafted cation-exchanging alkylsulfonic acid (PS-SO(3)H) and anion-exchanging propyl-octadecyldimethylammonium chloride (PS-ODMA+Cl-) groups was tested as ionization platforms. Two mechanisms of the methylene blue desorption/ionization were found: (1) the formation of [MB + H]+* ion due to the reduction/protonation of MB+, which is predominant for PS-H and PS-OX platforms and (2) direct thermal desorption of the MB+ cation, prevailing for PS-SO3H. The fragmentation of the cation is significantly suppressed in the latter case. The samples of PS-SO3H and PS-ODMA+ Cl- efficiently adsorb the dyes of the opposite charge from their solutions via the ion-exchange. Consequent DIOS MS studies allow to detect only low fragmented ions (MB+ and MO-, respectively), demonstrating the potential of the ion-exchange adsorption combined with DIOS MS for the analysis of ionic organic compounds in solutions.

Study on the redox reactions for organic dyes and S-nitrosylated peptide in electrospray droplet impact

Reduction of analytes in ionization processes often obscures the determination of molecular structure. The reduction of analytes is found to take place in various desorption/ionization methods such as fast atom bombardment (FAB), secondary ion mass spectrometry (SIMS), matrix-assisted laser desorption/ionization (MALDI) and desorption ionization on porous silicon (DIOS). To examine the extent of the reduction reactions taking place in electrospray droplet impact (EDI) processes, reduction-sensitive dyes and S-nitrosylated peptide were analyzed by EDI. No reduction was observed for methylene blue. While methyl red has a lower reduction potential than methylene blue, the reduction product ions were detected. For S-nitrosylated peptide, protonated molecule ion [M + H](+) and NO-eliminated molecular ion [M - NO + H](+*) were observed but reduction reactions are largely suppressed in EDI compared with that in MALDI. As such, the analytes examined suffer from little reduction reactions in EDI.

Halohydrination of epoxy resins using sodium halides as cationizing agents in MALDI-MS and DIOS-MS

Halohydrination of epoxy resins using sodium halides as cationizing agents in matrix-assisted laser desorption/ionization (MALDI) and desorption ionization on porous silicon mass spectrometry (DIOS-MS) were investigated. Different mass spectra were observed when NaClO(4) and NaI were used as the cationizing agents at the highest concentration of 10.0 mM, which is much higher than that normally used in MALDI-MS. MALDI mass spectra of epoxy resins using NaI revealed iodohydrination to occur as epoxy functions of the polymers. The halohydrination also occurred using NaBr, but not NaCl, due to the differences in their nucleophilicities. On the basis of the results of experiments using deuterated CD(3)OD as the solvent, the hydrogen atom source was probably ambient water or residual solvent, rather than being derived from matrices. Halohydrination also occurred with DIOS-MS in which no organic matrix was used; in addition, reduction of epoxy functions was observed with DIOS. NaI is a useful cationizing agent for changing the chemical form of epoxy resins due to iodohydrination and, thus, for identifying the presence of epoxy functions.

Sensitivity of redox reactions of dyes to variations of conditions created in mass spectrometric experiments

Redox behaviour of four imidazophenazine dye derivatives under mass spectrometric conditions of matrix-assisted laser desorption/ionization (MALDI), laser desorption/ionization (LDI) from metal and graphite surface, electrospray, low temperature secondary ion mass spectrometry (LT SIMS) and fast atom bombardment (FAB) was studied and distinctions in the reduction-dependent spectral patterns were analyzed from the point of view of different quantities of protons and electrons available for reduction in different techniques. The reduction products [M + 2H](+*), [M + 3H](+) and M(-*), [M + H](-) were observed in the positive and negative ion modes, respectively, which permitted to suggest independent occurrence of reduction and protonation/deprotonation processes. LDI from graphite substrate was the only technique that allowed us to obtain abundant negative ions of all dye derivatives. The yield of field ionization (FI) or field desorption (FD) mechanism to ion formation under LDI from rough graphite surface has been addressed. The sensitivity of reduction of the dyes to variation of reduction-initiating agents confirms high redox activity of the dyes essential for their functioning in natural and artificial systems.

Surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) of low molecular weight organic compounds and synthetic polymers using zinc oxide (ZnO) nanoparticles

We have developed surface-assisted laser desorption/ionization mass spectrometry using zinc oxide (ZnO) nanoparticles with anisotropic shapes (ZnO-SALDI-MS). The mass spectra showed low background noises in the low m/z, i.e. less than 500 u region. Thus, we succeeded in SALDI ionization on low molecular weight organic compounds, such as verapamil hydrochloride, testosterone, and polypropylene glycol (PPG) (average molecular weight 400) without using a liquid matrix or buffers such as citric acids. In addition, we found that ZnO-SALDI has advantages in post-source decay (PSD) analysis and produced a simple mass spectrum for phospholipids. The ZnO-SALDI spectra for synthetic polymers of polyethylene glycol (PEG), polystyrene (PS) and polymethylmethacrylate (PMMA) showed the sensitivity and molecular weight distribution to be comparable to matrix-assisted laser desorption/ionization (MALDI) spectra with a 2,5-dihydroxybenzoic acid (DHB) matrix. ZnO-SALDI shows good performance for synthetic polymers as well as low molecular weight organic compounds.

Small-Molecule Analysis with Silicon-Nanoparticle-Assisted Laser Desorption/Ionization Mass Spectrometry

Silicon nanopowder (5-50 nm) was applied as a matrix for the analysis of small molecules in laser desorption/ionization mass spectrometry. In contrast with conventional matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry, the matrix background interference in the low mass range was significantly reduced. Effects of the particle size and sample preparation procedures on the background mass spectra and the analyte signal intensity have been investigated, and an optimized powder and sample preparation protocol was established. Several surface characterization tools have been applied as well. Both positive mode and negative mode laser desorption/ionization have been applied to different analytes including drugs, peptides, pesticides, acids, and others. Detection limits down to the low femtomole per microliter levels were achieved for propafenone and verapamil drugs. The method developed was found relatively tolerant to salt contamination, which allowed the direct analysis of morphine and propaphenone in untreated urine and triazine herbicides in a soil extract. The new silicon-nanoparticle-assisted laser desorption ionization method was found to be highly selective, which may be due to analyte-dependent precharging in solution, prior to vacuum laser desorption. Some aspects of the charge-transfer mechanism have been studied and discussed. In comparison with standard MALDI matrixes, the silicon nanopowder requires much lower laser fluence (contributing to a reduced background) has much better surface homogeneity, and is more tolerant to salt interference, which makes it an easily applicable practical tool at a potentially low cost.

Matrix-free methods for laser desorption/ionization mass spectrometry

Matrix assisted laser desorption/ionization (MALDI) is a soft ionization mass spectrometric method that has become a preeminent technique in the analysis of a wide variety of compounds including polymers and proteins. The main drawback of MALDI is that it is difficult to analyze low molecular weight compounds (<1,000 m/z) because the matrix that allows MALDI to work interferes in this mass range. In recent years there has been considerable interest in developing laser desorption/ionization (LDI) techniques for the analysis of small molecules. This review examines the approaches to matrix-free LDI mass spectrometry including desorption/ionization on silicon (DIOS), sol-gels, and carbon-based microstructures. For the purposes of this review matrix-free methods are defined as those that do not require matrix to be mixed with the analyte and therefore does not require co-crystallization. The review will also examine mechanisms of ionization and applications of matrix-free LDI-MS.

Evaluation of the reduction of imidazophenazine dye derivatives under fast-atom-bombardment mass-spectrometric conditions

Satellite [M + 2](+*) and [M + 3](+) peaks accompanying the common peak of the protonated molecule [M + H](+) that are known to indicate the occurrence of a reduction process were observed in the fast atom bombardment (FAB) mass spectra of imidazophenazine dye derivatives in glycerol matrix. The distribution of the abundances in the [M + nH](+) peak group varied noticeably for different derivatives. This indicated different levels of the reduction depending on the different structure variations of the studied molecules. In the search for correlations between the mass spectral pattern and the structural features of the dyes, ab initio HF/6-31++G** quantum chemical calculations were performed. They revealed that the abundances of the [M + 2](+*) and [M + 3](+) ions show growth proportional to the decrease of the energy of the lowest unoccupied molecular orbital, i.e. proportional to the increase of the electron affinity of the dye molecule. A method for rapid screening of reductive properties of sets of dye derivatives on the basis of the FAB mass spectral data is discussed.

Minimum proton affinity for efficient ionization with atmospheric pressure desorption/ionization on silicon mass spectrometry

We performed a systematic study using a set of compounds with different proton affinities (PAs) on the ionization in atmospheric pressure desorption/ionization on silicon mass spectrometry (AP-DIOS-MS). The compounds studied included various aromatic molecules of different sizes. The PAs of these compounds were calculated using ab initio and hybrid density functional theory calculations at the B3LYP/6-31G(d) level of theory. We observed that only compounds with relatively high PAs above a threshold value of 920-950 kJ/mol were efficiently ionized as protonated molecules under AP-DIOS conditions and produced very clean mass spectra.

Oxidation of Ferrocene Derivatives in Desorption/Ionization on Porous Silicon

In matrix-assisted laser desorption/ionization (MALDI), the true molecular structures of some analytes are not represented by the observed ions due to a redox reaction. In earlier reports, electron transfer from analyte to chemical matrix has been proposed for the oxidation of ferrocene derivatives in MALDI. To address such a redox phenomenon in laser desorption/ionization processes, two ferrocene derivatives, FcCH2CH2Fc and FcCH2NMe2 [Fc:(CsHs)Fe(CsH4)], were analyzed by a matrix-free method, desorption/ionization on porous silicon (DIOS). The oxidized species, Fc+CH2NMe2 and FcCH2CH2Fc+, were detected in the DIOS mass spectra. The results suggested that electron transfer from the analytes to the sample target occurs during the ionization process.