Solvent selection for matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometric analysis of synthetic polymers employing solubility parameters

The principle relating to the selection of a proper matrix, cationization reagent, and solvent for matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOF MS) of synthetic polymers is still a topic of research. In this work we focused on the selection of a suitable MALDI solvent. Polystyrene PS7600 and poly(ethylene glycol) PEG4820 were analyzed by MALDI‐TOF MS using various solvents which were selected based on the Hansen solubility parameter system. For polystyrene (PS), dithranol was used as the matrix and silver trifluoroacetate as the cationization reagent whereas, for poly(ethylene glycol) (PEG), the combination of 2,5‐dihydroxybenzoic acid and sodium trifluoroacetate was used for all experiments. When employing solvents which dissolve PS and PEG, reliable MALDI mass spectra were obtained while samples in non‐solvents (solvents which are not able to dissolve the polymer) failed to provide spectra. It seems that the solubility of the matrix and the cationization reagent are less important than the polymer solubility. Copyright © 2010 John Wiley & Sons, Ltd.     

Combining MALDI‐2 and transmission geometry laser optics to achieve high sensitivity for ultra‐high spatial resolution surface analysis

A transmission geometry optical configuration allows for smaller laser spot size to facilitate high‐resolution matrix‐assisted laser/desorption ionization (MALDI) mass spectrometry. This increase in spatial resolution (ie, smaller laser spot size) is often associated with a decrease in analyte signal. MALDI‐2 is a post‐ionization technique, which irradiates ions and neutrals generated in the initial MALDI plume with a second orthogonal laser pulse, and has been shown to improve sensitivity. Herein, we have modified a commercial Orbitrap mass spectrometer to incorporate a transmission geometry MALDI source with MALDI‐2 capabilities to improve sensitivity at higher spatial resolutions.     

9,10‐Diphenylanthracene as a matrix for MALDI‐MS electron transfer secondary reactions

The most common secondary‐ionization mechanism in positive ion matrix‐assisted laser desorption/ionization (MALDI) involves a proton transfer reaction to ionize the analyte. Peptides and proteins are molecules that have basic (and acidic) sites that make them susceptible to proton transfer. However, non‐polar, aprotic compounds that lack basic sites are more difficult to protonate, and creating charged forms of this type of analyte can pose a problem when conventional MALDI matrices are employed. In this case, forming a radical molecular ion through electron transfer is a viable alternative, and certain matrices may facilitate the process. In this work, we investigate the performance of a newly developed electron‐transfer secondary reaction matrix: 9,10‐diphenylanthracene (9,10‐DPA). The use of 9,10‐DPA as matrix for MALDI analysis has been tested using several model compounds. It appears to promote ionization through electron transfer in a highly efficient manner as compared to other potential matrices. Thermodynamic aspects of the observed electron transfers in secondary‐ionization reactions were also considered, as was the possibility for kinetically controlled/endothermic, electron‐transfer reactions in the MALDI plume. Copyright © 2012 John Wiley & Sons, Ltd.     

Online deposition of nano‐aerosol for matrix‐assisted laser desorption/ionization mass spectrometry

An online nano‐aerosol sample deposition method for matrix‐assisted laser desorption/ionization (MALDI) mass spectrometry is described in which matrix and analyte particles between 50 and 500 nm are aerodynamically focused onto a tight spot, ca. 200 µm in diameter, on the target plate under vacuum. MALDI analysis of the target is performed without additional sample preparation. The method is evaluated with insulin as the analyte and alpha‐cyano‐4‐hydroxycinnamic acid (CHCA) as the matrix. Two preparation modes are compared with conventional dried‐droplet deposition: mixture deposition where a single layer is deposited consisting of particles that contain both matrix and analyte, and layered deposition where an underlayer of matrix particles and an overlayer of analyte particles are deposited separately. Desalting is performed by adding ammonium sulfate to the solution used to generate the matrix aerosol. With mixture deposition, the optimum matrix‐to‐analyte mole ratio is about 500:1 compared with 5000:1 for the conventional dried‐droplet method. With layered deposition, the thicknesses of the matrix and analyte layers are more important determinants of the analyte signal intensity than the matrix‐to‐analyte mole ratio. Analyte signal intensities are independent of matrix layer thickness above 200 nm, and the optimum analyte signal is obtained with an analyte layer thickness of about 100 nm. Copyright © 2009 John Wiley & Sons, Ltd.     

磷酸铵盐作为基质添加物促进磷酸化肽在MALDI-MS分析中的离子化效率

本文建立了以磷酸铵盐为添加剂的基质新系统,增强了磷酸化肽在MALDI正离子模式下的离子化。系统地考察了不同的磷酸盐以及不同的盐浓度对磷酸化肽离子化效率的影响。考察了两种适合于磷酸化肽离子化的基质类型2,5-二羟基苯甲酸和2,4,6-三羟基苯乙酮。用2,5-二羟基苯甲酸作为基质时,当加入10 mM 磷酸氢二铵时,磷酸化蛋白质β-casein的磷酸肽 48FQ[pS]EEQQQTEDELQDK63的离子化效率可以增强5-8倍,当加入10 mM磷酸二氢胺时,磷酸肽的离子化效率可以增强3-4倍。用2,4,6-三羟基苯乙酮作为基质时,当加入5mM磷酸氢二铵时,磷酸化肽的离子化效率比文献报道的最有利于磷酸化肽离子化的基质体系增强了2倍。并探讨了铵根离子和磷酸根离子促进磷酸化肽在MALDI的正离子模式下离子化效率的机理。     

Quantitative analyses of fullerene and polycyclic aromatic hydrocarbon mixtures via solvent‐free matrix‐assisted laser desorption/ionization mass spectrometry

We explore the feasibility of reliable quantitative matrix‐assisted laser desorption/ionization mass spectrometry (MALDI‐MS) analyses via solvent‐free sample preparation, as this procedure provides the unique convenience of being applicable also to insoluble samples. As quantitative MALDI measurements are even more complicated for species ionized by cation attachment, we investigated model systems, such as polycyclic aromatic hydrocarbons (PAHs) and fullerenes, which undergo photoionization and do not require additional cationizing salts. Our quantitative approach rests upon applying the standard‐addition method in MALDI for the quantitative characterization of binary mixtures. Two different systems are tested. Set 1 is composed of hexakis(dodecyl)hexabenzocoronene and hexakis(dodecyl)hexaphenylbenzene, which represent the product and precursor of a cyclodehydrogenation reaction, and Set 2 is a mixture of C60 and C70 fullerenes. In Set 1, severe anomalies could be detected due to a strong influence of the matrix/analyte ratio on the correlation between signal intensity and analyte amount. This can be related to the strong intermolecular interactions among the hexabenzocoronene (HBC) aromatic cores hampering the desorption step and to intermolecular charge transfers, which influence the ionization probability. Minor interferences to the quantitative MALDI characterization are encountered in the analysis of C60 and C70 fullerenes. The spherical shapes of C60 and C70 buckyballs prevent strong aggregation. Thus, no molecule‐dependent anomalies in their desorption‐photoionization behaviour are recognized. Copyright © 2008 John Wiley & Sons, Ltd.     

Matrix suppression and analyte suppression effects of quaternary ammonium salts in matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry: an investigation of suppression mechanism

In the matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI TOF MS) analysis of some quaternary ammonium salts (QASs), very clean spectra of the quaternary ammonium ions were recorded with a strong matrix suppression effect (MSE). The QASs also showed a considerable analyte suppression effect (ASE). It was demonstrated that the MSE and ASE of the QASs can be explained well by the cluster ionization model. According to this model, MALDI ions are formed from charged matrix/analyte clusters. Various analyte ions and matrix ions might coexist in the cluster, and they will compete for the limited number of net charges available. If enough quaternary ammonium ions are present in the cluster, they will take away the net charges, thus resulting in the MSE and ASE. Our results also suggest that ‘the cluster ionization model’ is not in conflict with ‘the theory of ionization via secondary gas‐phase reactions’. The initial MALDI ions produced from charged matrix/analyte clusters will collide with other molecules or ions in the MALDI plume. Depending on the properties of the initial ions and the composition of the MALDI plume, secondary gas‐phase reactions might result from these collisions. The final ions observed are the combined results of ‘cluster ionization’ and ‘ionization via secondary gas‐phase reactions’. Copyright © 2009 John Wiley & Sons, Ltd.     

Solid state nuclear magnetic resonance as a tool to explore solvent-free MALDI samples

Solid-state Nuclear magnetic resonance (NMR) was used here to explore structural characteristics of samples to be subjected to matrix-assisted laser desorption/ionization (MALDI) and prepared without the use of any solvent. The analytical systems scrutinized in NMR were mixtures of a 2,5-dihydroxybenzoic acid (2,5-DHB) matrix and caesium fluoride (CsF), used as the cationization agent in synthetic polymer MALDI mass analysis, at different molar ratios (1:1, 5:1, and 10:1). Complementary information could be obtained from ¹³C, ¹³³Cs, and ¹⁹F NMR spectra. Grinding the matrix together with the salt in the solid state was shown to induce a strong modification in the molecular organization within the MALDI sample. The evidenced mechano-induced reactions allow strong interactions between the matrix and the cation, up to the formation of a salt, and only occur in the presence of some water molecules. Addition of a poly(ethylene oxide) polymer as the analyte did not further modify the observed molecular organizations. Although relative matrix and salt concentrations in the scrutinized samples were unusual for MALDI analysis, mass spectra of good quality could be obtained and revealed that cation attachment on polymers during the MALDI process is not a matrix-independent event since a lower ionization efficiency was obtained from highly organized solid samples, mostly consisting of 2,5-DHB caesium salt species.     

A statistical design of experiments for optimizing the MALDI-TOF-MS sample preparation of polymers. An application in the assessment of the thermo-mechanical degradation mechanisms of poly (ethylene terephthalate)

The sample preparation procedure for MALDI-TOF MS of polymers is addressed in this study by the application of a statistical Design of Experiments (DoE). Industrial poly (ethylene terephthalate) (PET) was chosen as model polymer. Different experimental settings (levels) for matrixes, analyte/matrix proportions and concentrations of cationization agent were considered. The quality parameters used for the analysis were signal-to-noise ratio and resolution. A closer inspection of the statistical results provided the study not only with the best combination of factors for the MALDI sample preparation, but also with a better understanding of the influence of the different factors, individually or in combination, to the signal. The application of DoE for the improvement of the MALDI measure of PET stated that the best combination of factors and levels was the following: matrix (dithranol), proportion analyte/matrix/cationization agent (1/15/1, V/V/V), and concentration of cationization agent (2 g L(-1)). In a second part, multiple processing by means of successive injection cycles was used to simulate the thermo-mechanical degradation effects on the oligomeric distribution of PET under mechanical recycling. The application of MALDI-TOF-MS showed that thermo-mechanical degradation primarily affected initially predominant cyclic species. Several degradation mechanisms were proposed, remarking intramolecular transesterification and hydrolysis. The ether links of the glycol unit in PET were shown to act as potential reaction sites, driving the main reactions of degradation.     

On‐chip solid‐phase extraction pre‐concentration/focusing substrates coupled to atmospheric pressure matrix‐assisted laser desorption/ionization ion trap mass spectrometry for high sensitivity biomolecule analysis

Atmospheric pressure matrix‐assisted laser desorption/ionization (AP‐MALDI) has proven a convenient and rapid method for ion production in the mass spectrometric (MS) analysis of biomolecules. AP‐MALDI and electrospray ionization (ESI) sources are easily interchangeable in most mass spectrometers. However, AP‐MALDI suffers from less‐than‐optimal sensitivity due to ion losses during transport from the atmosphere into the vacuum of the mass spectrometer. Here, we study the signal‐to‐noise ratio (S/N) gains observed when an on‐chip dynamic pre‐concentration/focusing approach is coupled to AP‐MALDI for the MS analysis of neuropeptides and protein digests. It was found that, in comparison with conventional AP‐MALDI targets, focusing targets showed (1) a sensitivity enhancement of approximately two orders of magnitude with S/N gains of 200–900 for hydrophobic substrates, and 150–400 for weak cation‐exchange (WCX) substrates; (2) improved detection limits as low as 5 fmol/µL for standard peptides; (3) significantly reduced matrix background; and (4) higher inter‐day reproducibility. The improved sensitivity allowed successful tandem mass spectrometric (MS/MS) sequencing of dilute solutions of a derivatized tryptic digest of a protein standard, and enabled the first reported AP‐MALDI MS detection of neuropeptides from Aedes aegypti mosquito heads. Copyright © 2009 John Wiley & Sons, Ltd.     

Inline pneumatically assisted atmospheric pressure matrix‐assisted laser desorption/ionization ion trap mass spectrometry

Atmospheric pressure (AP) matrix‐assisted laser desorption/ionization (MALDI) is known to suffer from poor ion transfer efficiencies as compared to conventional vacuum MALDI (vMALDI). To mitigate these issues, a new AP‐MALDI ion source utilizing a coaxial gas flow was developed. Nitrogen, helium, and sulfur hexafluoride were tested for their abilities as ion carriers for a standard peptide and small drug molecules. Nitrogen showed the best ion transport efficiency, with sensitivity gains of up to 1900% and 20% for a peptide standard when the target plate voltage was either continuous or pulsed, respectively. The addition of carrier gas not only entrained the ions efficiently but also deflected background species and declustered analyte–matrix adducts, resulting in higher absolute analyte signal intensities and greater signal‐to‐noise (S/N) ratios. With the increased sensitivity of pneumatically assisted (PA) AP‐MALDI, the limits of detection of angiotensin I were 20 or 3 fmols for continuous or pulsed target plate voltage, respectively. For analyzing low‐mass analytes, it was found that very low gas flow rates (0.3–0.6 l min^?1) were preferable owing to increased fragmentation at higher gas flows. The analyte lability, type of gas, and nature of the extraction field between the target plate and mass spectrometer inlet were observed to be the most important factors affecting the performance of the in‐line PA‐AP‐MALDI ion source. Copyright © 2010 John Wiley & Sons, Ltd.     

Matrix-dependent cationization in MALDI mass spectrometry

The matrix dependence in cationization processes, the competition between cationization and protonation and the question of whether gas-phase cation transfer or attachment of free cations dominates in matrix-assisted laser desorption/ionization mass spectrometry were studied. Two different sample preparation methods were employed, the dried-droplet sample preparation and a mixture of solid matrix, analyte and salt. The latter ensures that the formation of cation adducts takes place in the gas phase. By monitoring the suppression of matrix signals for different matrices, it was found that matrices with high gas-phase metal ion binding energies require high analyte concentrations for matrix suppression to occur. By comparing the mass spectra obtained using sinapinic acid or sinapinic methyl ester as a matrix, a correlation between cationization and deprotonation of matrix molecules was found. It is also demonstrated that attachment of free gas-phase cations, rather than cation transfer from the cationized matrix, is the predominant process in cationization.     

Molecular weight analysis of water‐soluble poly(phenylene ethynylene)s using MALDI‐TOF MS

Molecular weights of seven poly(phenylene ethynylene)‐based water‐soluble conjugated polyelectrolytes (CPEs) obtained through Sonogashira coupling are determined by matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOF MS). A standard sample preparation protocol is developed to characterize the seven CPEs using 2,5‐dihydroxybenzoic acid as the matrix (M) and AgTFA as the cationization reagent (CR). High‐quality MALDI mass spectra are obtained at volume mixing ratios (CPE/M/CR) of 5/5/1 for anionic polymers (P1–P4) and 5/50/1 for cationic polymers (P5–P7). Molecular weight, molecular weight distribution, and end‐group information are analyzed. The effects of molecular weight of CPEs on optical and quenching properties are also studied. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2537–2543     

Aquatic fulvic acid as a matrix for matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometric analysis

Aquatic fulvic acids (AFAs) are demonstrated to be effective matrices for the analysis of various polar compounds (ranging from 100–1500 Da) by matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOFMS). The efficiency of AFA as a matrix is shown for a wide range of test compounds, including a number of carbohydrates, cyclodextrins and peptides, with typical detection limits of ～10 µg mL^?1. The propensity of AFA to enhance ionization through protonation of peptides, and formation of sodium and potassium adducts of carbohydrates and polyethylene glycol, was noted. Differences were observed in the performances of the two AFA matrices used, a Suwannee River, International Humic Substances Society (IHSS) standard and a locally extracted fulvic acid (LFA). For example, in the analysis of carbohydrate standards, the use of the LFA matrix typically doubled the analyte ion signal intensities and resulted in signal‐to‐noise (S/N) ratios that were 2–4 times better than when the Suwannee River AFA matrix was used. AFA was also used in the analysis of real‐world samples without extraction or purification; cantaloupe juice and acetaminophen tablets were analyzed, and glucose and acetaminophen could easily be identified as respective components. When lower concentrations of fulvic acid were used in the presence of sugars, a reversal of roles was observed in which the sugars functioned as the matrix and significantly enhanced ionization of the AFA components, while ions associated with the sugars themselves were suppressed or absent. Effective as a matrix for a variety of analytes and widely available, AFA is an attractive environmentally friendly choice for use in MALDI applications. Copyright © 2006 John Wiley & Sons, Ltd.     

On‐target separation of analyte with 3‐aminoquinoline/α‐cyano‐4‐hydroxycinnamic acid liquid matrix for matrix‐assisted laser desorption/ionization mass spectrometry

3‐Aminoquinoline/α‐cyano‐4‐hydroxycinnamic acid (3AQ/CHCA) is a liquid matrix (LM), which was reported by Kumar et al. in 1996 for matrix‐assisted laser desorption/ionization (MALDI) mass spectrometry. It is a viscous liquid and has some advantages of durability of ion generation by a self‐healing surface and quantitative performance. In this study, we found a novel aspect of 3AQ/CHCA as a MALDI matrix, which converges hydrophilic material into the center of the droplet of analyte‐3AQ/CHCA mixture on a MALDI sample target well during the process of evaporation of water derived from analyte solvent. This feature made it possible to separate not only the buffer components, but also the peptides and oligosaccharides from one another within 3AQ/CHCA. The MALDI imaging analyses of the analyte‐3AQ/CHCA droplet indicated that the oligosaccharides and the peptides were distributed in the center and in the whole area around the center of 3AQ/CHCA, respectively. This 'on‐target separation' effect was also applicable to glycoprotein digests such as ribonuclease B. These features of 3AQ/CHCA liquid matrix eliminate the requirement for pretreatment, and reduce sample handling losses thus resulting in the improvement of throughput and sensitivity. Copyright © 2012 John Wiley & Sons, Ltd.     

芳香环状聚膦酸酯齐聚物的基质辅助激光解吸电离飞行时间质谱分析

用基质辅助激光解吸电离飞行时间质谱方法对一系列芳香环状聚膦酸酯低聚物进行了结构分析。比较了不同基质及阳离子剂对芳香环状聚膦酸酯分析结果的影响。1,8,9-蒽三酚基质仅对含有羰基基团聚膦酸酯环状齐聚物分析有效,而视黄酸基质则对所有聚膦酸酯环状剂聚物有效,是这类新型芳香环状齐聚物的适宜基质。环状聚膦酸酯齐聚物的阳离子齐分析表明,氯化锂是这种环状齐聚物的适宜的阳离子添加剂。     

The cationization mechanism study of novel oligo (<Emphasis Type="Italic">p</Emphasis>-phenyleneethynylene)s

Two series of new oligo(p-phenyleneethynylene)s (OPEs) O1-O4 and O5-O8, which have been proven to be one of the chief classes of molecules mainly used as the wires and other potential backbones of molecular electronic devices, have been synthesized by stepwise synthetic approach. The characterization of these oligomers was performed on MALDI TOF MS. Different cationization salts have been applied to investigate the ionization processes of these series of oligomers under MALDI conditions. The experimental results show that these oligomers display a strong tendency to undergo radical cationization and varied ionization efficiency with different cationization agents attributable to their difference in cationic diameters. Furthermore, we found that these two series of oligomers differed in ionization properties because of their different end-groups even when the same cationization agent was used.     

A study of gas-phase cationization in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

A specially constructed split sample probe was used to unequivocally demonstrate that gas-phase cationization occurs within the desorption plume during a matrix-assisted laser desorption/ionization experiment. Two separate samples were prepared for analysis: on side A, a mixture of poly(ethylene glycol) (PEG) 1500 analyte and 2,5-dihydroxybenzoic acid (DHB) matrix, and on side B a mixture of DHB matrix and lithium hydroxide (LiOH), the cationization reagent. Analysis of the data showed that when the ionization laser was focused on the split (so that both sides were illuminated), Li(+)-cationized PEG peaks were observed. Since the PEG analyte did not come into contact with Li(+) in either the solution or solid phase, the only possibility for the observed cationization was a reaction in the gas phase. Due to the difficulty in completely removing the adventitious cations (Na(+) and K(+)) present in DHB and on sample surfaces, gas-phase cationization could not be demonstrated to be either the only or most important mechanism operating in the MALDI experiment.     

A solvent‐free matrix application method for matrix‐assisted laser desorption/ionization imaging of small molecules

Matrix application continues to be a critical step in sample preparation for matrix‐assisted laser desorption/ionization (MALDI) mass spectrometry imaging (MSI). Imaging of small molecules such as drugs and metabolites is particularly problematic because the commonly used washing steps to remove salts are usually omitted as they may also remove the analyte, and analyte spreading is more likely with conventional wet matrix application methods. We have developed a method which uses the application of matrix as a dry, finely divided powder, here referred to as dry matrix application, for the imaging of drug compounds. This appears to offer a complementary method to wet matrix application for the MALDI‐MSI of small molecules, with the alternative matrix application techniques producing different ion profiles, and allows the visualization of compounds not observed using wet matrix application methods. We demonstrate its value in imaging clozapine from rat kidney and 4‐bromophenyl‐1,4‐diazabicyclo(3.2.2)nonane‐4‐carboxylic acid from rat brain. In addition, exposure of the dry matrix coated sample to a saturated moist atmosphere appears to enhance the visualization of a different set of molecules. Copyright © 2010 John Wiley & Sons, Ltd.     

The improved performance of MALDI‐TOF MS on the analysis of herbal saponins by using DHB‐GO composite matrix

Matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOF MS) is an excellent analytical technique for rapid analysis of a variety of molecules with straightforward sample pretreatment. The performance of MALDI‐TOF MS is largely dependent on matrix type, and the development of novel MALDI matrices has aroused wide interest. Herein, we devoted to seek more robust MALDI matrix for herbal saponins than previous reported, and ginsenoside Rb1, Re, and notoginsenoside R1 were used as model saponins. At the beginning of the present study, 2,5‐dihydroxybenzoic acid (DHB) was found to provide the highest intensity for saponins in four conventional MALDI matrices, yet the heterogeneous cocrystallization of DHB with analytes made signal acquisition somewhat “hit and miss.” Then, graphene oxide (GO) was proposed as an auxiliary matrix to improve the uniformity of DHB crystallization due to its monolayer structure and good dispersion, which could result in much better shot‐to‐shot and spot‐to‐spot reproducibility of saponin analysis. The satisfactory precision further demonstrated that minute quantities of GO (0.1 μg/spot) could greatly reduce the risk of instrument contamination caused by GO detachment from the MALDI target plate under vacuum. More importantly, the sensitivity and linearity of the standard curve for saponins were improved markedly by DHB‐GO composite matrix. Finally, the application of detecting the Rb1 in complex biological sample was exploited in rat plasma and proved it applicable for pharmacokinetic study quickly. This work not only opens a new field for applications of DHB‐GO in herbal saponin analysis but also offers new ideas for the development of composite matrices to improve MALDI MS performance.