Characterization of synthetic polymers by MALDI-TOF/MS: investigation into new methods of sample target preparation and consequence on mass spectrum finger print

Quantitative MALDI-TOF Mass Spectrometry of Star-Shaped Polylactides Based on Chromatographic Hyphenation

The end groups of three- and four-arm star-shaped polylactides (PLA) with trimethylolpropane and pentaerythritol core structures were functionalized with acetic acid. Reaction products with different degrees of functionalization were analyzed by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. Additional gradient elution liquid adsorption chromatography (GELAC) measurements were performed to determine the degree of functionalization. This technique enabled clear separation and sufficient quantification of the formed species. These chromatographic data could be used inversely to quantify mass spectrometric results, which are usually biased by the unknown ionization probabilities of different polymer end group structures. Our results showed that, in this particular case, the peak intensity in the MALDI-TOF mass spectra can be used to semiquantitatively determine the degree of functionalization in incompletely functionalized multiarm PLA.

DART isotope dilution high resolution mass spectrometry and 19F-NMR detection of fluorotelomeric alcohols in hydrolyzed food contact paper

Fluorotelomer-based acrylate polymers and surfactants used to grease-proof food contact paper (FCP) are potential sources of dietary exposure to perfluoroalkyl substances (PFAS). Food contact substances (FCS) containing polyfluorinated long-chains (≥C8) were voluntarily removed by their manufacturers from the U.S. market in 2011 due to health concerns and largely replaced with FCSs containing short-chain (≤C7) PFAS. In 2020, FDA findings of potential biopersistence of 6:2 FTOH (CF3(CF2)5CH2CH2OH) similarly prompted an additional voluntarily phase-out of FCSs containing 6:2 FTOH by their manufacturers that was completed by the end of 2023. To monitor the phase-out process, a screening method was developed to detect FCPs containing ester-linked polyfluorinated pendant chains. Direct Analysis in Real Time-Isotope Dilution-High Resolution Mass Spectrometry (DART-ID-HRMS) enabled rapid semi-quantitative detection of 6:2 FTOH in FCP saponification reaction headspace without requiring sample concentration or chromatography. 19F-NMR analysis confirmed 6:2 FTOH pendant chain identity and detection dependence on saponification. The speed and specificity of this approach arise from ester saponification in the presence of stable isotopically labeled 6:2 FTOH; high FTOH differential volatility relative to nonfluorinated matrix, and the facile production of FTOH gas-phase anions (e.g., [M + O2]·-, [M-H + CO2]-) under ambient ionization conditions. The efficiency of this simple workflow makes it well-suited for monitoring the phase-out of FCS containing ester-linked polyfluorinated chains from the U.S. marketplace.

Solvent-free sample preparation for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of polymer blends

RATIONALE

Solvent-free sample preparation offers some advantages over solvent-based techniques, such as improved accuracy, reproducibility and sensitivity, for matrix-assisted laser desorption/ionization (MALDI) analysis. However, little or no information is available on the application of solvent-free techniques for the MALDI analysis of polymer blends.

METHODS

Solvent-free sample preparation by ball milling was applied with varying sample-to-matrix ratios for MALDI time-of-flight mass spectrometry analysis of various polymers, including polystyrenes, poly(methyl methacrylate)s and poly(ethylene glycol)s. The peak intensity ratios were compared with those obtained after using the conventional dried droplet sample preparation method. In addition, solvent-assisted milling was also applied to improve sample homogeneities.

RESULTS

Depending on the sample preparation method used, different peak intensity ratios were found, showing varying degrees of suppression of the signal intensities of higher mass polymers. Ball milling for up to 30 min was required to achieve constant intensity ratios indicating homogeneous mixtures. The use of wet-assisted grinding to improve the homogeneity of the blends was found to be disadvantageous as it caused partial degradation and mass-dependent segregation of the polymers in the vials.

CONCLUSIONS

The results clearly show that solvent-free sample preparation must be carefully considered when applied to synthetic polymer blends, as it may cause additional problems with regard to homogeneity and stability of the blends.

Characterization of balsam sulfide via pyrolysis-gas chromatography-mass spectrometry/sulfur chemiluminescence detection and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Balsam sulfide, produced by the reaction of turpentine/rosin and sulfur, has been used as one of the raw materials of liquid gold to decorate ceramics and tableware with thin gold film for more than 100 years. The characterization of balsam sulfide is still insufficient because of its compositional complexity. In this study, balsam sulfide was characterized using pyrolysis-gas chromatography (Py-GC)-mass spectrometry (MS) and Py-GC with sulfur chemiluminescence detection (SCD) as well as matrix-assisted laser desorption/ionization time-of-flight MS (MALDI-TOFMS). Py-GC-MS/SCD analyses of balsam sulfide and its model samples revealed that the low molecular weight reaction products were mainly composed of compounds of one α-pinene unit reacted with 1-3 sulfur atoms. In the analysis of the high molecular weight components by MALDI-TOFMS, the products of two or three α-pinene units crosslinked by sulfur atoms were observed. It was found that dehydrogenation reaction proceeded gradually with the increase in the reaction time.

Characterization of styrene/methyl methacrylate/n-butyl acrylate terpolymer with Kendrick mass defect analysis in electrospray ionization-ion mobility spectrometry-mass spectrometry

RATIONALE

Synthesis of copolymer is one of the major approaches to establish sophisticated polymers. In copolymer analysis in mass spectrometry (MS), an increase in the number of monomer unit species and/or the polymer molecular weight results in complex mass spectra, and a method is required to solve this problem.

METHODS

In this study, electrospray ionization-ion mobility spectrometry-mass spectrometry was employed to analyze styrene/methyl methacrylate/n-butyl acrylate (St/MMA/nBA) terpolymers via Kendrick mass defect (KMD) analysis. An expanded Fourier transform-based noise reduction method for the copolymer was developed to improve the decrease in the signal/noise ratio observed in the higher m/z region.

RESULTS

Low-mass terpolymers (0.7 kDa) were identified by comparing the observed and theoretical results using a two-dimensional KMD plot. For the 1.5 kDa terpolymer, the signal overlap, of which KMD value was not matched with the theoretical one, was interpreted by the shift from the theoretical value in the KMD plot. For the 3.0 kDa terpolymers, the compositional candidates were determined by the prediction based on the compositional information of low-mass terpolymers previously analyzed. The 4.5 kDa terpolymer was interpreted after the expanded noise filtering.

CONCLUSIONS

The terpolymers, whose molecular weight was up to 4.5 kDa, were successfully characterized at a molecular level. The dependency of the St/MMA/nBA composition on molecular weight was observed; that is, the nBA content decreased with an increase in the molecular weight.

High-performance micro/nanoplastics characterization by MALDI-FTICR mass spectrometry

Micro/nanoplastics (MNPs) are widespread environmental pollutants that cause high health risks. However, high heterogeneity in particle sizes and chemical compositions of MNPs make their accurate characterization extremely challenging. Herein, we established a matrix-assisted laser desorption ionization-Fourier transform ion cyclotron resonance mass spectrometry (MALDI-FTICR MS) strategy for the unambiguous characterization of different types of MNPs with high performance, including polystyrene, polyethylene glycol terephthalate, polyamide, polymethyl methacrylate, acrylonitrile butadiene styrene copolymer, and polycarbonate. The MNP sample preparation and detection conditions were systematically optimized by using response surface methodology, and the MS detection signal-to-noise ratios were improved 1.5 times on average. The ultrahigh mass resolution of FTICR MS is crucial to the unambiguous elucidation of MNP structures. We demonstrate that this MS strategy is highly efficient in the characterization of polymer constitutions of environmental MNPs derived from foam, bottles, cable ties, and compact discs, providing a promising tool for MNP detection and safety evaluation.

End group analysis of poly(methylmethacrylate)s using the most abundant peak in electrospray ionization-ion mobility spectrometry-tandem mass spectrometry and Fourier transform-based noise filtering

RATIONALE

We recently developed the characterization method for synthetic polymers weighing more than a few tens of kilodalton using electrospray ionization-ion mobility spectrometry-tandem mass spectrometry, in which the m/z value of the most abundant peak was used for characterization. However, the identification of the most abundant peak from the isotopic peaks was often difficult due to the background noise.

METHODS

Here, we employed a noise reduction method using Fourier transform (FT) filtering. In the power spectrum obtained using FT of the mass spectrum of the multiple charged analytes, the significant signals in the low-frequency region and at frequency z are observed for the analytes of z charges. When the signals in both regions were used for inversed FT (i.e., the signals in other regions were zero padded), a noise-filtered mass spectrum was obtained.

RESULTS

In the analysis of poly(methylmethacrylate)s weighing 13-17 kDa, mass spectra without noise filtering with relatively high-intensity noise (than signal) were complicated to identify the most abundant peak. On the contrary, the most abundant peak was clearly identified from the mass spectra after FT-based noise filtering, and end group composition was estimated successfully.

CONCLUSIONS

The proposed FT-based noise filtering for the mass spectrum is effective to characterize multiply charged synthetic polymers weighing more than a few tens of kilodalton using electrospray ionization-ion mobility spectrometry-tandem mass spectrometry.

Optimized MALDI-TOF MS Strategy for Characterizing Polymers

In recent years, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) plays an essential role in the analysis of polymers. To acquire a more reliable strategy for polymer profiling, we characterized four representative polymers including polyethylene glycol 6000, polyvinylpyrrolidone K12, polymer polyol KPOP-5040, and polyether polyol DL-4000. The preparation methods of these four polymer samples have been optimized from six aspects, including matrix, cationization reagent, solvent, mixing ratio of cationization reagent to polymer, mixing ratio of matrix to polymer, and laser intensity. After investigating the effects of seven commonly used matrices on the ionization efficiency of four polymers, trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene] malononitrile (DCTB) was found to be the only matrix suitable for the analysis of all the four polymers. Our experimental results suggested that different polymers showed a certain preference for different cationization reagents. For example, the polymer polyol KPOP-5040 was suitable for sodium iodide as the cationization reagent, while polyvinylpyrrolidone K12 was more suitable for silver trifluoroacetate (AgTFA). For the choice of solvent, tetrahydrofuran is a reagent with rapid evaporation and a wide range of dissolution which can achieve the best results for the analysis of four polymers. The optimized method was successfully applied to the identification of DSPE-PEG-NH₂ with different polymerized degrees. This MALDI-TOF strategy potentially provided the supplementary function through the polymer's application in biomedical and visible probing.

A general purpose MALDI matrix for the analyses of small organic, peptide and protein molecules

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) has been widely applied for the analysis of large biomolecules. The emergence of inorganic material substrates and new organic matrices extends the use of MALDI MS for small molecule analyses. However, there are usually preferred matrices for different types of analytes. Here, an organic compound, 4-hydroxy-3-nitrobenzonitrile, was found to be a general purpose matrix for the analyses of small organic, peptide and protein molecules. In particular, 4-hydroxy-3-nitrobenzonitrile has a strong UV absorption property, and it provides a clean background in the low mass range. Its analytical performances as a UV-laser matrix were demonstrated for different types of analytes, including organic drugs, peptides, proteins, mouse brain tissue and bacteria. Compared with commercial matrices, this new matrix has better performances when analyzing small molecules, such as drugs, peptides and lipids, while it has similar performances when analyzing proteins.

2-Cyano-3-(2-thienyl)acrylic Acid as a New MALDI Matrix for the Analysis of a Broad Spectrum of Analytes

A low-cost synthetic 2-cyano-3-(2-thienyl)acrylic acid (CTA) is developed as a new MALDI matrix for the analysis of various classes of compounds such as lipids (e.g., fatty acids), peptides, proteins, saccharides, natural products (i.e., iridoids), PEGs, and organometallics in the positive-ion mode. The difficulty in the analysis of high molecular mass PEGs was overcome by using CTA as matrix even at low concentrations. Both high molecular mass proteins and peptides were successfully analyzed using CTA. The mass spectra of all of the studied analytes with CTA showed high signal-to-noise (S/N) ratios and spectral resolutions when compared to other conventional matrices such as SA, DHB, DT, and HCCA. However, in the case of peptide analysis with CTA, the resulting mass spectra are found to be similar to that of the well-established HCCA matrix. On the basis of the physicochemical properties of the analytes, the CTA works as a proton/cation or electron-transfer matrix. It proves that the CTA can be used as a common matrix for the analysis of majority classes of analytes instead of using a specific matrix for the particular class of analytes. Further, the CTA provides an advantage in the analysis of unknown samples as it rules out ambiguity in the selection of particular matrix and it may also offer a complete profile of the tissue surface in the MALDI-imaging experiments.

Recent developments in pre-treatment and analytical techniques for synthetic polymers by MALDI-TOF mass spectrometry

A great deal of effort has been expended to develop accurate means of determining the properties of synthetic polymers using matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS). Many studies have focused on the importance of sample pre-treatment to obtain accurate analysis results. This review discusses the history of synthetic polymer characterization and highlights several applications of MALDI-TOF MS that recognize the importance of pre-treatment technologies. The subject area is of significance in the field of analytical chemistry, especially for users of the MALDI technique. Since the 2000s, many such technologies have been developed that feature improved methods and conditions, including solvent-free systems. In addition, the recent diversification of matrix types and the development of carbon-based matrix materials are described herein together with the current status and future directions of MALDI-TOF MS hardware and software development. We provide a summary of processes used for obtaining the best analytical results with synthetic polymeric materials using MALDI-TOF MS.

Universal Sample Preparation Unlocking Multimodal Molecular Tissue Imaging

A new tissue sample embedding and processing method is presented that provides downstream compatibility with numerous different histological, molecular biology, and analytical techniques. The methodology is based on the low temperature embedding of fresh frozen specimens into a hydrogel matrix composed of hydroxypropyl methylcellulose (HPMC) and polyvinylpyrrolidone (PVP) and sectioning using a cryomicrotome. The hydrogel was expected not to interfere with standard tissue characterization methods, histologically or analytically. We assessed the compatibility of this protocol with various mass spectrometric imaging methods including matrix-assisted laser desorption ionization (MALDI), desorption electrospray ionization (DESI) and secondary ion mass spectrometry (SIMS). We also demonstrated the suitability of the universal protocol for extraction based molecular biology techniques such as rt-PCR. The integration of multiple analytical modalities through this universal sample preparation protocol offers the ability to study tissues at a systems biology level and directly linking results to tissue morphology and cellular phenotype.

Structure-based engineering of heparinase I with improved specific activity for degrading heparin

BACKGROUND

Heparinase I from Pedobacter heparinus (Ph-HepI), which specifically cleaves heparin and heparan sulfate, is one of the most extensively studied glycosaminoglycan lyases. Enzymatic degradation of heparin by heparin lyases not only largely facilitates heparin structural analysis but also showed great potential to produce low-molecular-weight heparin (LMWH) in an environmentally friendly way. However, industrial applications of Ph-HepI have been limited by their poor yield and enzyme activity. In this work, we improve the specific enzyme activity of Ph-HepI based on homology modeling, multiple sequence alignment, molecular docking and site-directed mutagenesis.

RESULTS

Three mutations (S169D, A259D, S169D/A259D) exhibited a 50.18, 40.43, and 122.05% increase in the specific enzyme activity and a 91.67, 108.33, and 75% increase in the yield, respectively. The catalytic efficiencies (k_cat/K_m) of the mutanted enzymes S169D, A259D, and S169D/A259D were higher than those of the wild-type enzyme by 275, 164, and 406%, respectively. Mass spectrometry and activity detection showed the enzyme degradation products were in line with the standards of the European Pharmacopoeia. Protein structure analysis showed that hydrogen bonds and ionic bonds were important factors for improving specific enzyme activity and yield.

CONCLUSIONS

We found that the mutant S169D/A259D had more industrial application value than the wild-type enzyme due to molecular modifications. Our results provide a new strategy to increase the catalytic efficiency of other heparinases.

Poly(ethylene glycol)-Based Peptidomimetic "PEGtide" of Oligo-Arginine Allows for Efficient siRNA Transfection and Gene Inhibition

While a wide range of experimental and commercial transfection reagents are currently available, persistent problems remain regarding their suitability for continued development. These include the transfection efficiency for difficult-to-transfect cell types and the risks of decreased cell viability that may arise from any transfection that does occur. Therefore, research is now turning toward alternative molecules that improve the toxicity profile of the gene delivery vector (GDV), while maintaining the transfection efficiency. Among them, cell-penetrating peptides, such as octa-arginine, have shown significant potential as GDVs. Their pharmacokinetic and pharmacodynamic properties can be enhanced through peptidomimetic conversion, whereby a peptide is modified into a synthetic analogue that mimics its structure and/or function, but whose backbone is not solely based on α-amino acids. Using this technology, novel peptidomimetics were developed by co- and postpolymerization functionalization of substituted ethylene oxides, producing poly(ethylene glycol) (PEG)-based peptidomimetics termed "PEGtides". Specifically, a PEGtide of the poly(α-amino acid) oligo-arginine [poly(glycidylguanidine)] was assessed for its ability to complex and deliver a small interfering ribonucleic acid (siRNA) using a range of cell assays and high-content analysis. PEGtide-siRNA demonstrated significantly increased internalization and gene inhibition over 24 h in Calu-3 pulmonary epithelial cells compared to commercial controls and octa-arginine-treated samples, with no evidence of toxicity. Furthermore, PEGtide-siRNA nanocomplexes can provide significant levels of gene inhibition in "difficult-to-transfect" mouse embryonic hypothalamic (mHypo N41) cells. Overall, the usefulness of this novel PEGtide for gene delivery was clearly demonstrated, establishing it as a promising candidate for continued translational research.

Persistent Multiexcitons from Polymers with Pendent Pentacenes

Singlet fission has emerged as a key mechanism of exciton multiplication in organic chromophores, generating two triplet excitons from a single photon. Singlet fission is typically studied in crystalline films or in isolated dimers. Here, we investigate an intermediate regime where through-space interactions mediate singlet fission and triplet pair recombination within isolated polymer chains. Specifically, we investigate how appending pentacenes to a polynorbornene backbone can lead to macromolecules that take advantage of through-space π-π interactions for fast singlet fission and rapid triplet pair dissociation. Singlet fission in these systems is affected by molecular dynamics, and triplet-triplet recombination is a geminate process where the rate of recombination scales with molecular-weight. We find that these pendent pentacene polymers yield free triplets with lifetimes that surpass those of crystalline chromophores in both solution as isolated polymers and in thin films.

Electrospray ionization-ion mobility spectrometry-high resolution tandem mass spectrometry with collision-induced charge stripping for the analysis of highly multiply charged intact polymers

Polymers with large molecular weight are difficult to interpret using electrospray ionization-mass spectrometry (ESI-MS) due to the generation of various highly multiply charged analytes. Although ESI-ion mobility spectrometry (IMS)-MS is effective in reducing the complexity of the mass spectrum, this approach is insufficient for analyzing highly multiply charged polymers. In this study, we propose a method combining tandem mass spectrometry (quadrupole and high-resolution time-of-flight MS, QMS/TOFMS), IMS, and collision-induced charge stripping (CICS) for analyzing large intact polymers (∼40 kDa), which are highly multiply charged. The number of charges can be estimated from a Fourier transform power spectrum of a mass spectrum when the charge number is less than approximately 20. Interpretations of the spectra of poly(ethylene oxide)s (PEOs) weighing 20 kDa, poly(methyl methacrylate)s weighing 22 kDa, and methoxy-PEO-maleimide weighing 40 kDa were successfully demonstrated with isotope level and polymerization degree level separations, respectively. In the proposed method, a mixture can be analyzed for relatively small (a few kDa) and large (a few tens of kDa) polymers simultaneously without any sample pretreatment.

Quasi-monodisperse polymer libraries via flash column chromatography: correlating dispersity with glass transition

Quasi-monodisperse (Đ = 1.005–1.040) poly(methyl) acrylate (PMA) polymer libraries are obtained via flash column chromatography separation of disperse (Đ = 1.130) PMA. The influence of dispersity on the glass transition is investigated.

Tough and Elastic α-Tricalcium Phosphate Cement Composites with Degradable PEG-Based Cross-Linker

Dual setting cements composed of an in situ forming hydrogel and a reactive mineral phase combine high compressive strength of the cement with sufficient ductility and bending strength of the polymeric network. Previous studies were focused on the modification with non-degradable hydrogels based on 2-hydroxyethyl methacrylate (HEMA). Here, we describe the synthesis of suitable triblock degradable poly(ethylene glycol)-poly(lactide) (PEG-PLLA) cross-linker to improve the resorption capacity of such composites. A study with four different formulations was established. As reference, pure hydroxyapatite (HA) cements and composites with 40 wt% HEMA in the liquid cement phase were produced. Furthermore, HEMA was modified with 10 wt% of PEG-PLLA cross-linker or a test series containing only 25% cross-linker was chosen for composites with a fully degradable polymeric phase. Hence, we developed suitable systems with increased elasticity and 5⁻6 times higher toughness values in comparison to pure inorganic cement matrix. Furthermore, conversion rate from α-tricalcium phosphate (α-TCP) to HA was still about 90% for all composite formulations, whereas crystal size decreased. Based on this material development and advancement for a dual setting system, we managed to overcome the drawback of brittleness for pure calcium phosphate cements.

Citric Acid Capped Iron Oxide Nanoparticles as an Effective MALDI Matrix for Polymers

A new matrix-assisted laser desorption ionization (MALDI) mass spectrometry matrix is proposed for molecular mass determination of polymers. This matrix contains an iron oxide nanoparticle (NP) core with citric acid (CA) molecules covalently bound to the surface. With the assistance of additives, the particulate nature of NPs allows the matrix to mix uniformly with polar or nonpolar polymer layers and promotes ionization, which may simplify matrix selection and sample preparation procedures. Several distinctively different polymer classes (polyethyleneglycol (PEG), polywax/polyethylene, perfluoropolyether, and polydimethylsiloxane) are effectively detected by the water or methanol dispersed NPCA matrix with NaCl, NaOH, LiOH, or AgNO₃ as additives. Furtheremore, successful quantitative measurements of PEG1000 using polypropylene glycol 1000 as an internal standard are demonstrated. Graphical Abstract ᅟ.

Synthesis of poly(ethylene oxide) approaching monodispersity

Polydispersity in polymers hinders fundamental understanding of their structure-property relationships and prevents them from being used in fields like medicine, where polydispersity affects biological activity. The polydispersity of relatively short-chain poly(ethylene oxide) [(CH2CH2O2)n; PEO] affects its biological activity, for example, the toxicity and efficacy of PEOylated drugs. As a result, there have been intensive efforts to reduce the dispersity as much as possible (truly monodispersed materials are not possible). Here we report a synthetic procedure that leads to an unprecedented low level of dispersity. We also show for the first time that it is possible to discriminate between PEOs differing in only 1 ethylene oxide (EO) unit, essential in order to verify the exceptionally low levels of dispersity achieved here. It is anticipated that the synthesis of poly(ethylene oxide) approaching monodispersity will be of value in many fields where the applications are sensitive to the distribution of molar mass.

Matrix-assisted laser desorption/ionization sample preparation optimization for structural characterization of poly(styrene-co-pentafluorostyrene) copolymers

The influence of the sample preparation parameters (the choice of the matrix, matrix:analyte ratio, salt:analyte ratio) was investigated and optimal conditions were established for the MALDI time-of-flight mass spectrometry analysis of the poly(styrene-co-pentafluorostyrene) copolymers. These were synthesized by atom transfer radical polymerization. Use of 2,5-dihydroxybenzoic acid as matrix resulted in spectra with consistently high ion yields for all matrix:analyte:salt ratios tested. The optimized MALDI procedure was successfully applied to the characterization of three copolymers obtained by varying the conditions of polymerization reaction. It was possible to establish the nature of the end groups, calculate molecular weight distributions, and determine the individual length distributions for styrene and pentafluorostyrene monomers, contained in the resulting copolymers. Based on the data obtained, it was concluded that individual styrene chain length distributions are more sensitive to the change in the composition of the catalyst (the addition of small amount of CuBr2) than is the pentafluorostyrene component distribution.

A perspective on MALDI alternatives-total solvent-free analysis and electron transfer dissociation of highly charged ions by laserspray ionization

Progress in research is hindered by analytical limitations, especially in biological areas in which sensitivity and dynamic range are critical to success. Inherent difficulties of characterization associated with complexity arising from heterogeneity of various materials including topologies (isomeric composition) and insolubility also limit progress. For this reason, we are developing methods for total solvent-free analysis by mass spectrometry consisting of solvent-free ionization followed by solvent-free gas-phase separation. We also recently constructed a novel matrix-assisted laser desorption ionization (MALDI) source that provides a simple, practical and sensitive way of producing highly charged ions by laserspray ionization (LSI) or singly charged ions commonly observed with MALDI by choice of matrix or matrix preparation. This is the first ionization source with such freedom-an extremely powerful analytical 'switch'. Multiply charged LSI ions allow molecules exceeding the mass-to-charge range of the instrument to be observed and permit for the first time electron transfer dissociation fragment ion analysis.

Automated solvent-free matrix deposition for tissue imaging by mass spectrometry

The ability to analyze complex (macro) molecules is of fundamental importance for understanding chemical, physical, and biological processes. Complexity may arise from small differences in structure, large dynamic range, as well as a vast range in solubility or ionization, imposing daunting tasks in areas as different as lipidomics and proteomics. Here, we describe a rapid matrix application that permits the deposition of matrix-assisted laser desorption/ionization (MALDI) matrix solvent-free. This solvent-free one-step automatic matrix deposition is achieved through vigorous movements of beads pressing the matrix material through a metal mesh. The mesh (20 mum) produces homogeneous coverage of <12 microm crystals (DHB, CHCA matrixes) in 1 min, as determined by optical microscopy, permitting fast uniform coverage of analyte and possible high-spatial resolution surface analysis. Homogenous tissue coverage of <5 microm sized crystals is achieved using a 3 microm mesh. Solvent-free MALDI analysis on a time-of-flight (TOF) mass analyzer of mouse brain tissue homogenously covered with CHCA matrix subsequently provides a homogeneous response in ion signal intensity. Total solvent-free analysis (TSA) by mass spectrometry (MS) of tissue sections is carried out by applying the MALDI matrix solvent-free for subsequent ionization and gas phase separation for decongestion of complexity in the absence of any solvent using ion mobility spectrometry (IMS) followed by MS detection. Isobaric compositions were well-delineated using TSA by MS.

Surface modification of upconverting NaYF4 nanoparticles with PEG-phosphate ligands for NIR (800 nm) biolabeling within the biological window

We present a technique for the replacement of oleate with a PEG-phosphate ligand [PEG = poly(ethylene glycol)] as an efficient method for the generation of water-dispersible NaYF(4) nanoparticles (NPs). The PEG-phosphate ligands are shown to exchange with the original oleate ligands on the surface of the NPs, resulting in water-dispersible NPs. The upconversion intensity of the NPs in aqueous environments was found to be severely quenched when compared to the original NPs in organic solvents. This is attributed to an increase in the multiphonon relaxations of the lanthanide excited state in aqueous environments due to high energy vibrational modes of water molecules. This problem could be overcome partially by the synthesis of core/shell NPs which demonstrated improved photophysical properties in water over the original core NPs. The PEG-phosphate coated upconverting NPs were then used to image a line of ovarian cancer cells (CaOV3) to demonstrate their promise in biological application.

Surface analysis by imaging mass spectrometry

A review of four MS-based techniques available for molecular surface imaging is presented. The main focus is on the commercially available mass spectrometry imaging techniques: secondary ion mass spectrometry (SIMS), matrix assisted laser desorption ionization mass spectrometry (MALDI-MS), desorption electrospray ionization mass spectrometry (DESI-MS) and laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS). A short historical perspective is presented and traditional desorption ionization techniques are also briefly described. The four techniques are compared mainly with respect to their usage for imaging of biological surfaces. MALDI is evaluated as the most successful in life sciences and the only technique usable for imaging of large biopolymers. SIMS is less common but offers superior spatial lateral resolution and DESI is considered to be an emerging alternative approach in mass spectrometry imaging. LA-ICP ionization is unbeatable in terms of limits of detection but does not provide structural information. All techniques are considered extremely useful, representing a new wave of expansion of mass spectrometry into surface science and bioanalysis. A minireview with 121 references.

Transglutaminase-mediated PEGylation of proteins: direct identification of the sites of protein modification by mass spectrometry using a novel monodisperse PEG

Poly(ethylene glycol) (PEG) has been widely used to prolong the residence time of proteins in blood and to decrease their immunogenicity and antigenicity. A drawback of this polymer lies in its polydispersity that makes difficult the identification of the sites of protein modification. This is a mandatory requirement if a PEGylated protein should be approved as a drug. Here, a fast and reliable method is proposed to characterize proteins conjugated at the level of glutamine (Gln) residues using microbial transglutaminase (TGase). The novelty resides in the use of a monodisperse Boc-PEG-NH(2) for the derivatization that allows the direct identification of the sites of PEGylation by electrospray ionization mass spectrometry (ESI-MS). The procedure has been tested on three model proteins, namely, human granulocyte colony-stimulating factor, human growth hormone, and horse heart apomyoglobin. The Gln residues linked to the polymer chain were easily identified by ESI-MS and tandem MS analyses, demonstrating the advantage of using a monodisperse polymer in combination with mass spectrometry for an easy characterization of conjugated proteins. Interestingly, the PEGylation reaction led to the production only of mono- and bis-derivative products, indicating that the TGase-mediated PEGylation can be extremely selective and thus very useful for the derivatization of protein drugs.

Imaging mass spectrometry for examining localization of polymeric composition in matrix-assisted laser desorption/ionization samples

The localization of polymeric composition in samples prepared for matrix-assisted laser desorption/ionization (MALDI) analysis has been investigated by imaging mass spectrometry. Various matrices and solvents were used for sample spot preparation of a polybutyleneglycol (PBG 1000). It was shown that in visibly homogeneous spots, prepared using the 'dried droplet' method, separation between matrix and polymer takes place. Moreover, using alpha-cyano-4-hydroxycinnamic acid (CCA) as matrix and methanol as solvent molecular mass separation of the polymer homologues in the spots was detectable. In contrast to manually spotted samples, dry spray deposition results in homogeneous layers showing no separation effects.

A rapid, sensitive and effective quantitative method for simultaneous determination of cationic surfactant mixtures from river and municipal wastewater by direct combination of single-drop microextraction with AP-MALDI mass spectrometry

A rapid, simple, sensitive, and effective quantitative method for simultaneous determination of cationic surfactants (CS(+)) from river and municipal wastewater by direct combination of single-drop microextraction (SDME) with atmospheric pressure (AP)-MALDI mass spectrometry has been successfully demonstrated without the requirements of tedious sample pre- or post-treatment or separation by high-performance liquid chromatography (HPLC), gas chromatography (GC), and capillary electrophoresis (CE). This quantitative method can greatly enhance the signal-to-noise ratio for analysis of small molecules of CS(+) owing to the strong suppression of matrix ions by the analytes. In addition, SDME assisted in the isolation and preconcentration of CS(+) from water samples, which could effectively reduce the background interferences from the matrices present in waste and river water. The SDME conditions were optimized for achieving high extraction efficiency of CS(+) from aqueous samples, in terms of solvent selection, stirring speed, extraction time, exposure volume of acceptor phase, and salt addition. The enrichment factors for CS(+) were found to be 40-64-folds for 7 min of extraction time with no salt addition and at room temperature. This method was found to yield a linear calibration curve in the concentration range from 50 to 1500 microg/l CS(+) with a limit of detection (LOD) of 10 microg/l. The relative recoveries in river and municipal wastewater were found to be 93.8-103.6% and 91.0-98.7%, respectively. These results indicate that the combination of SDME with AP-MALDI/MS is effective for the simultaneous determination of CS(+) from river and municipal wastewater. In addition, a comparison of enrichments and LOD values for this method with hollow-fiber liquid phase microextraction (HF-LPME) was also demonstrated. The present approach is easy to operate, rapid, sensitive, and suitable for high-throughput of analysis.

Structural characterization of native high-methoxylated pectin using nuclear magnetic resonance spectroscopy and ultraviolet matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Comparative use of 2,5-dihydroxybenzoic acid and nor-harmane as UV-MALDI matrices

The successful analysis by ultraviolet matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (UV-MALDI-TOF MS) of native and hydrolyzed high-methoxylated pectin samples is described. In order to find the optimal conditions for UV-MALDI-TOF MS analysis several experimental variables were studied such as: different UV-MALDI matrices (nor-harmane, 2,5-dihydroxybenzoic acid), sample preparation methods (mixture, sandwich), inorganic salt addition (doping salts, NaCl, KCl, NH(4)Cl), ion mode (positive, negative), linear and reflectron mode, etc. nor-Harmane has never been used as a UV-MALDI matrix for the analysis of pectins but its use avoids pre-treatment of the sample, such as an enzymatic digestion or an acid hydrolysis, and there is no need to add salts, making the analysis easier and faster. This study suggested an alternative way of analyzing native high-methoxylated pectins, with UV-MALDI-TOF MS, by using nor-harmane as the matrix in negative ion mode. The analysis by (1)H and (13)C nuclear magnetic resonance (NMR) spectroscopy of the native and hydrolyzed pectin is also briefly described.

Solvent-free MALDI-MS: developmental improvements in the reliability and the potential of MALDI in the analysis of synthetic polymers and giant organic molecules

A dry sample preparation strategy was previously established as a new method for matrix assisted laser desorption/ionization mass spectrometry (MALDI-MS), so-called solvent-free MALDI-MS. In this contribution, we examine systems that have been shown problematic with conventional solvent-based MALDI approaches. Problems frequently encountered are solubility, miscibility, and segregation effects during crystallization as a result of unfavorable analyte and matrix polarities. In all cases studied, solvent-free MALDI-MS simplified the measurement and improved the analysis. Solvent-free MALDI-MS enables more reliable results in well-known problematic systems such as polydimethylsiloxane with its segregation effects. However, even in highly compatible analyte/matrix systems such as polystyrene and dithranol, there were undesirable suppression effects when employing THF as solvent. Generally, the solvent-free method allows for more homogeneous analyte/matrix mixtures as well as higher shot-to-shot and sample-to-sample reproducibility. As a result, less laser power has to be applied, which yields milder MALDI conditions, reduced background signals, and provides better resolution of the analyte signals. Solvent-free MALDI-MS proved valuable for the characterization of nanosized material, e.g., fullereno-based structures, which indicated having an increased fragmentation-susceptibility. New analyte/matrix combinations (e.g., polyvinylpyrrolidone/dithranol) are accessible independent of solubility and compatibility in common solvents. An improved quantitation potential is recognized (e.g., insoluble polycyclic aromatic hydrocarbon against soluble dendrite precursor). The rapid and easy measurement of industrial products demonstrates the solvent-free method capable for improved throughput analysis of a variety of compounds (e.g., poly(butylmethacrylate) diol) in routine industrial analysis. Hence, this new MALDI method leads to qualitative and quantitative improvements, making it a powerful tool for analytical purposes, which may also prove to be valuable in future automation attempts.

Hydrocarbon polymer analysis by external MALDI fourier transform and reflectron time of flight mass spectrometry

The traditional solvent-based matrix-assisted laser desorption ionization (MALDI) preparation method has been used to analyze nonpolar polymers of various molecular weights. High resolution silver cationized oligomers with masses of up to 12 KDa were measured using 9.4 tesla Fourier transform mass spectrometry (FTMS) with an external ionization source. It was observed that when time-of-flight mass spectrometry was used, the spectra of polyethylene polymers showed abundant low mass fragment ions. However, these fragments were absent from the FTMS spectra.

End-labeled free-solution electrophoresis of DNA

DNA is a free-draining polymer. This subtle but "unfortunate" property of highly charged polyelectrolytes makes it impossible to separate nucleic acids by free-flow electrophoresis. This is why one must typically use a sieving matrix, such as a gel or an entangled polymer solution, in order to obtain some electrophoretic size separation. An alternative approach consists of breaking the charge to friction balance of free-draining DNA molecules. This can be achieved by labeling the DNA with a large, uncharged molecule (essentially a hydrodynamic parachute, which we also call a drag-tag) prior to electrophoresis; the resulting methodology is called end-labeled free-solution electrophoresis (ELFSE). In this article, we review the development of ELFSE over the last decade. In particular, we examine the theoretical concepts used to predict the ultimate performance of ELFSE for single-stranded (ssDNA) sequencing, the experimental results showing that ELFSE can indeed overcome the free-draining issue raised above, and the technological advances that are needed to speed the development of competitive ELFSE-based sequencing and separation technologies. Finally, we also review the reverse process, called free-solution conjugate electrophoresis (FSCE), wherein uncharged polymers of different sizes can be analyzed using a short DNA molecule as an electrophoretic engine.