Extending the solvent-free MALDI sample preparation method

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.

Mass spectrometry of polymers: A tutorial review

Ever since the inception of synthetic polymeric materials in the late 19th century, the number of studies on polymers as well as the complexity of their structures have only increased. The development and commercialization of new polymers with properties fine-tuned for specific technological, environmental, consumer, or biomedical applications requires powerful analytical techniques that permit the in-depth characterization of these materials. One such method with the ability to provide chemical composition and structure information with high sensitivity, selectivity, specificity, and speed is mass spectrometry (MS). This tutorial review presents and exemplifies the various MS techniques available for the elucidation of specific structural features in a synthetic polymer, including compositional complexity, primary structure, architecture, topology, and surface properties. Key to every MS analysis is sample conversion to gas-phase ions. This review describes the fundamentals of the most suitable ionization methods for synthetic materials and provides relevant sample preparation protocols. Most importantly, structural characterizations via one-step as well as hyphenated or multidimensional approaches are introduced and demonstrated with specific applications, including surface sensitive and imaging techniques. The aim of this tutorial review is to illustrate the capabilities of MS for the characterization of large, complex polymers and emphasize its potential as a powerful compositional and structural elucidation tool in polymer chemistry.

Application of surface-layer matrix-assisted laser desorption/ionization mass spectrometry imaging to pharmaceutical-loaded poly(ester urea) films

Polymer thin films are often used in transdermal patches as a method of continuous drug administration for patients with chronic illness. Understanding the drug segregation and distribution within these films is important for monitoring proper drug release over time. Surface-layer matrix-assisted laser desorption/ionization mass spectrometry imaging (SL-MALDI-MSI) is a unique analytical technique that provides an optical representation of chemical compositions that exist at the surface of polymeric materials. Solvent-free sublimation is employed for application of matrix to the sample surface, so that only molecules in direct contact with the matrix layer are detected. Here, these methodologies are utilized to visualize variations in drug concentration at both the air and substrate interface in pharmaceutical-loaded polymer films.

Applications of MALDI-TOF-MS in structural characterization of synthetic polymers

In recent years, matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) has been utilized to rapidly and precisely characterize the detailed molecular structures of synthetic polymers. This review summarizes recent progress regarding MALDI-TOF-MS for the characterization of synthetic polymers with a focus on specific important experimental aspects including sample preparation, the choice of matrix, the effects of cationizing agents and solvents, data processing and various applications. Finally, the recent trend of MALDI-TOF-MS development is discussed. We hope this review will be instructive for graduate students and junior users who need to use MALDI-TOF-MS as a necessary characterization technique for new synthetic polymers.

Design of Experiments for Matrix-Assisted Laser Desorption/Ionization of Amphiphilic Poly(Ethylene Oxide)-b-Polystyrene Block Copolymers

Matrix-assisted laser/desorption ionization (MALDI) has become a very popular ionization technique for mass spectrometry of synthetic polymers because it allows high throughput analysis of low amounts of sample while avoiding the complexity introduced by extensive multiple charging of electrospray ionization. Yet, fundamental mechanisms underlying this ionization process are not fully understood, so development of sample preparation methods remains empirical. Reliable prediction for the optimal matrix/analyte/salt system is indeed still not possible for homopolymers and it becomes even more challenging in the case of amphiphilic block copolymers where conditions dictated by one block are not compatible with MALDI requirements of the second block. In order to perform MALDI of copolymers composed of poly (ethylene oxide) (PEO) and polystyrene (PS) blocks, it was postulated here that experimental conditions suitable for both species would also be successful for PEO-b-PS. Accordingly, designs of experiments based on Quantitative Structure Activity Relationship (QSAR) analysis were first implemented, studying the influence of 19 matrices and 26 salts on the laser fluence requested for successful MALDI. This analysis first permitted to highlight correlations between the investigated 10 descriptors of matrices and salts and the analytical response, and then to construct models that permits reliable predictions of matrix/salt couples to be used for one or the other homopolymer. Selected couples were then used for MALDI of a PEO-b-PS copolymer but no general trend was observed: experimental conditions expected to work often failed whereas ionic adducts of the copolymer were clearly detected with some matrix/salt systems that were shown to badly perform for constituting homopolymers. Overall, this rules out the working assumption stating that the MALDI behavior of chains composed of PEO and PS segments should combine the behavior of the two polymeric species. Yet, although requiring a dedicated design of experiments, MALDI of the amphiphilic PEO-b-PS copolymer was achieved for the first time.

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.

Pyrolysis Vacuum-Assisted Plasma Ionization Ion Mobility-Mass Spectrometry for Insoluble Polymer Analysis

This Communication describes a new thermal desorption/pyrolysis vacuum-assisted plasma ionization (pyro-VaPI) ion source coupled to ion mobility-mass spectrometry (IM-MS) for insoluble polymer analysis. Pyro-VaPI combines a pyrolysis device, soft ambient plasma ionization, IM, and MS into a single platform for polymer analysis with minimal sample preparation. Nylons, a widely used and well-studied thermoplastic, were chosen to evaluate the pyro-VaPI performance. Six different nylon polymers were studied and characterized. With the application of IM-MS, two different isobars for the protonated cyclic dimers of 6-6, 6-9, 6-10, and 6-12 nylon and two isobars for the cyclic tetramer of nylon-6 were detected at 200 °C. These isobars were observed at different heating times, with the species drifting faster in the IM cell appearing several minutes after the slower drifting species. To the best of our knowledge, these isobaric dimers and tetramers have not been previously reported, indicating that pyro-VaPI IM-MS is a useful tool for the structural characterization of heated or pyrolyzed polymers.

Using solid-state nuclear magnetic resonance to rationalize best efficiency of 2,6-dihydroxybenzoic acid over other 2,X-dihydroxybenzoic acid isomers in solvent-free matrix-assisted laser desorption/ionization of poly(ethylene glycol)

RATIONALE

Among isomers of dihydroxybenzoic acid (DHB), 2,5-DHB is often the most efficient matrix in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) for a great variety of compounds. Yet, when performing solvent-free MALDI, 2,6-DHB yields better results for poly(ethylene glycol [PEG]). This intriguing feature is explored here using solid-state nuclear magnetic resonance (NMR).

METHODS

Ternary mixtures were prepared by grinding 2,X-DHB (X = 3-6), poly(ethylene glycol) (M_n = 2000 g mol^-1 ) and lithium fluoride (LiF) in a matrix/analyte/salt molar ratio of 50/1/10 for 16 min under a controlled atmosphere. After mixing, a few grains were applied to the MALDI target for MS analysis, whereas the major part of the ground sample was transferred into rotors to perform ¹³ C, ⁷ Li, and ¹⁹ F NMR experiments.

RESULTS

Lithiated PEG chains are mainly formed with 2,6-DHB in solvent-free MALDI, but their abundance increases with 2,3-DHB and 2,4-DHB when water uptake is favored by a humid atmosphere. Solid-state NMR shows that grinding 2,6-DHB-based samples in atmospheric conditions leads to a solid phase in which the matrix, PEG, and salt molecules exhibit a high mobility compared with systems involving other 2,X-DHB isomers. This mobile environment would favor (as a solvent) LiF dissociation and best promote PEG cationization.

CONCLUSIONS

Complementary data in ¹³ C, ⁷ Li, and ¹⁹ F NMR spectra are consistent with the formation of a solid phase of high mobility composed of 2,6-DHB, PEG, and the two salt components that ultimately favor the production of lithiated PEG chains.

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.

Investigations of the Equilibrium Conditions in Plumes of Laser Desorbed Sinapic Acid with Amino Acid Analytes: Influence of Sample Preparation and Matrix to Analyte Ratio

The equilibrium nature of a plume of laser desorbed material is examined through the application of a previously developed thermodynamic model to the ion signals observed in 337 nm MALDI mass spectra of mixtures of the matrix sinapic acid with the amino acids alanine, valine, isoleucine, and phenylalanine. Samples are prepared using both conventional dried-droplet and solvent-free methods for comparison. The relative yield of protonated amino acid is shown to increase as the amino acid gas-phase basicity increases for both sample preparation methods. Matrix gas phase basicity values extracted from the equilibrium plots are shown to be in good agreement ([M - H⁺]^• 876 kJ/mol and [M] 879 kJ/mol) with published experimental values supporting a mechanism wherein the protonated sinapic acid and/or the matrix radical cation act as the proton donor species. These experiments further reveal that there is a large difference in the extracted plume effective temperatures with the solvent-free method yielding lower effective temperatures as compared to the dried-droplet sample preparation, e.g., 552 K versus 1296 K, respectively, at M/A 1:1 (mole/mole). In addition, these experiments suggest that plume effective temperatures decrease as the relative amount of matrix deposited with the analyte increases, regardless of the sample preparation method. Cumulatively, these observations suggest that the crystalline solid allows more efficient transfer of the photoexcitation energy during the sample desorption step, as compared to the solvent-free sample, and/or collisional cooling is more effective for the plume of material desorbed from the solvent-free sample as compared to the conventional dried-droplet sample.

Subtle End Group Functionalization of Polymer Chains Drives Surface Depletion of Entire Polymer Chains

The surface of a blend of 6 kDa polystyrene and 6 kDa polystyrene functionalized with hydroxymethyl ends not only is depleted of the higher energy end groups but also is depleted of any segments belonging to the functionalized chains. This is demonstrated using the emerging technique of surface layer matrix-assisted laser desorption ionization time-of-flight mass spectrometry (SL-MALDI-ToF-MS), which detects entire chains that have any repeat unit at the outer surface, and requires no labeling. Detecting entire chains provides information about the relationship of chain functionalization to surface segregation behavior of entire chains. That the surface is depleted of interior segments of functionalized chains as well as of the ends is remarkable, since the functionality at the single chain end involves less than 0.5 wt % of the functionalized polymer chain.

Detection of Surface Enrichment Driven by Molecular Weight Disparity in Virtually Monodisperse Polymers

The preference for a shorter chain component at a polymer blend surface impacts surface properties key to application-specific performance. While such segregation is known for blends containing low molecular weight additives or systems with large polydispersity, it has not been reported for anionically polymerized polymers that are viewed, in practice, as monodisperse. Observations with surface layer matrix-assisted laser desorption ionization time-of-flight mass spectrometry (SL-MALDI-ToF-MS), which distinguishes surface species without labeling and provides the entire molecular weight distribution, demonstrate that entropically driven surface enrichment of shorter chains occurs even in low polydispersity materials. For 6 kDa polystyrene the number-average molecular weight (M_n) at the surface is ca. 300 Da (5%) lower than that in the bulk, and for 7 kDa poly(methyl methacryalate) the shift is ca. 500 Da. These observations are in qualitative agreement with results from a mean-field theory that considers a homopolymer melt with a molecular-weight distribution matched to the experiments.

Formation of Metal-Related Ions in Matrix-Assisted Laser Desorption Ionization

In a study of the metal-related ion generation mechanism in matrix-assisted laser desorption ionization (MALDI), crystals of matrix used in MALDI were grown from matrix- and salt-containing solutions. The intensities of metal ion and metal adducts of the matrix ion obtained from unwashed crystals were higher than those from crystals washed with deionized water, indicating that metal ions and metal adducts of the matrix ions are mainly generated from the surface of crystals. The contributions of preformed metal ions and metal adducts of the matrix ions inside the matrix crystals were minor. Metal adducts of the matrix and analyte ion intensities generated from a mixture of dried matrix, salt, and analyte powders were similar to or higher than those generated from the powder of dried droplet crystals, indicating that the contributions of the preformed metal adducts of the matrix and analyte ions were insignificant. Correlation between metal-related ion intensity fluctuation and protonated ion intensity fluctuation was observed, indicating that the generation mechanism of the metal-related ions is similar to that of the protonated ions. Because the thermally induced proton transfer model effectively describes the generation of the protonated ions, we suggest that metal-related ions are mainly generated from the salt dissolution in the matrix melted by the laser. Graphical Abstract ᅟ.

Matrix segregation as the major cause for sample inhomogeneity in MALDI dried droplet spots

The segregation in dried droplet MALDI sample spots was analyzed with regard to the matrix-to-sample ratio using optical microscopy, MALDI imaging mass spectrometry (MALDI MSI) and IR imaging spectroscopy. In this context, different polymer/matrix/solvent systems usually applied in the analysis of synthetic polymers were investigated. The use of typical matrix concentrations (10 mg mL⁻¹) in almost every case resulted in ring patterns, whereas higher concentrated matrix solutions always led to homogeneous sample spot layers. The data revealed that segregation is predominantly caused by matrix transport in the drying droplet, whereas polymer segregation seems to be only secondary.

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometric imaging of synthetic polymer sample spots prepared using ionic liquid matrices

RATIONALE

Polymer sample spots for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) prepared by the dried-droplet method often reveal ring formation accompanied by possible segregation of matrix and sample molecules as well as of the polymer homologs itself. Since the majority of sample spots are prepared by this simple and fast method, a matrix or sample preparation method that excludes such segregation has to be found.

METHODS

Three different ionic liquid matrices based on conventionally used aromatic compounds for MALDI-TOF MS were prepared. The formation of ionic liquids was proven by (1) H NMR spectroscopy. MALDI-Imaging mass spectrometry was applied to monitor the homogeneity.

RESULTS

Our results show a superior sample spot homogeneity using ionic liquid matrices. Spots could be sampled several times without visible differences in the mass spectra. A frequently observed loss of matrix in the mass spectrometer vacuum was not observed. The necessary laser irradiance was reduced, which resulted in less polymer fragmentation.

CONCLUSIONS

Ionic liquid matrices can be used to overcome segregation, a typical drawback of conventional MALDI dried-droplet preparations. Homogeneous sample spots are easy to prepare, stable in the MS vacuum and, thereby, improve the reproducibility of MALDI.

Probing Surface Concentration of Cyclic/Linear Blend Films Using Surface Layer MALDI-TOF Mass Spectrometry

Surface layer matrix-assisted laser desorption ionization time-of-flight mass spectrometry (SL-MALDI-TOF MS) is a powerful new surface sensitive technique to quantify the surface concentration of multicomponent polymer films with enrichment of one component at the surface. Its capabilities are demonstrated for the novel case of a blend of cyclic polystyrene with linear polystyrene, in which we find the composition of linear chains enriched at the surface after annealing, contrary to the expectation of a self-consistent field theory. The probing depth was confirmed to be monomolecular, which for these short chains is less than 2 nm, even though material at a much greater depth is removed by the analysis.

Solvent-based and solvent-free characterization of low solubility and low molecular weight polyamides by mass spectrometry: a complementary approach

RATIONALE

Polyamides (PA) belong to the most used classes of polymers because of their attractive chemical and mechanical properties. In order to monitor original PA design, it is essential to develop analytical methods for the characterization of these compounds that are mostly insoluble in usual solvents.

METHODS

A low molecular weight polyamide (PA11), synthesized with a chain limiter, has been used as a model compound and characterized by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). In the solvent-based approach, specific solvents for PA, i.e. trifluoroacetic acid (TFA) and hexafluoroisopropanol (HFIP), were tested. Solvent-based sample preparation methods, dried-droplet and thin layer, were optimized through the choice of matrix and salt. Solvent-based (thin layer) and solvent-free methods were then compared for this low solubility polymer. Ultra-high-performance liquid chromatography/electrospray ionization (UHPLC/ESI)-TOF-MS analyses were then used to confirm elemental compositions through accurate mass measurement.

RESULTS

Sodium iodide (NaI) and 2,5-dihydroxybenzoic acid (2,5-DHB) are, respectively, the best cationizing agent and matrix. The dried-droplet sample preparation method led to inhomogeneous deposits, but the thin-layer method could overcome this problem. Moreover, the solvent-free approach was the easiest and safest sample preparation method giving equivalent results to solvent-based methods. Linear as well as cyclic oligomers were observed. Although the PA molecular weights obtained by MALDI-TOF-MS were lower than those obtained by (1)H NMR and acido-basic titration, this technique allowed us to determine the presence of cyclic and linear species, not differentiated by the other techniques. TFA was shown to induce modification of linear oligomers that permitted cyclic and linear oligomers to be clearly highlighted in spectra.

CONCLUSIONS

Optimal sample preparation conditions were determined for the MALDI-TOF-MS analysis of PA11, a model of polyamide analogues. The advantages of the solvent-free and solvent-based approaches were shown. Molecular weight determination using MALDI was discussed.

Quantitative analysis of polymer additives with MALDI-TOF MS using an internal standard approach

MALDI-TOF MS is used for the qualitative analysis of seven different polymer additives directly from the polymer without tedious sample pretreatment. Additionally, by using a solid sample preparation technique, which avoids the concentration gradient problems known to occur with dried droplets and by adding tetraphenylporphyrine as an internal standard to the matrix, it is possible to perform quantitative analysis of additives directly from the polymer sample. Calibration curves for Tinuvin 770, Tinuvin 622, Irganox 1024, Irganox 1010, Irgafos 168, and Chimassorb 944 are presented, showing coefficients of determination between 0.911 and 0.990.

Quantitative analysis of polydisperse systems via solvent-free matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Quantitative analysis of partially soluble and insoluble polydisperse materials is challenging due to the lack of both appropriate standards and reliable analytical techniques. To this end, matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) incorporating a solvent-free sample preparation technique was investigated for the quantitative analysis of partially soluble, polydisperse, polycyclic aromatic hydrocarbon (PAH) oligomers. Molecular weight standards consisting of narrow molecular weight dimer and trimer oligomers of the starting M-50 petroleum pitch were produced using both dense-gas/supercritical extraction (DGE/SCE) and preparative-scale, gel permeation chromatography (GPC). The validity of a MALDI-based, quantitative analysis technique using solvent-free sample preparation was first demonstrated by applying the method of standard addition to a pitch of known composition. The standard addition method was then applied to the quantitative analysis of two insoluble petroleum pitch fractions of unknown oligomeric compositions, with both the dimer and trimer compositions of these fractions being accurately determined. To our knowledge, this study represents the first successful MALDI application of solvent-free quantitative analysis to insoluble, polydisperse materials.

The effects of molecular weight distribution and sample preparation on matrix-assisted laser desorption/ionization time-of-flight mass spectrometric analysis of petroleum macromolecules

To date there have been no systematic, quantitative investigations of the effect of sample preparation on the matrix-assisted laser desorption/ionization time-of-flight (MALDI) mass spectrometry response for polydisperse systems. To this end, the interrelationships between sample preparation, analyte molecular weight distribution (MWD) and solubility, and signal response were investigated for mixtures of alkylated polycyclic aromatic hydrocarbon (PAH) oligomers, the constituents of petroleum pitch that serve as precursors for advanced carbon materials. These PAH oligomers served as a useful analyte system for study, as their solvent solubilities decrease significantly with each increasing oligomeric unit. Molecular weight standards consisting of relatively pure dimer and trimer cuts of the starting M-50 petroleum pitch were produced using a dense-gas/supercritical extraction (DGE/SCE) technique and were then used to produce oligomeric mixtures of well-defined composition for study. Both traditional, solvent-based and newer, solvent-free sample preparation methods were evaluated, and their effects on both homogeneity and signal response were determined. While solvent-free sample preparation methods produced homogeneous samples and reproducible results regardless of the MWD of the analyte, solvent-based samples that contained more than one oligomeric cut produced non-homogeneous samples and poor reproducibilities. The differing solubilities of dimer, trimer, and tetramer oligomers in a given solvent (e.g., CS(2) or toluene) were found to be the cause of the inhomogeneities observed in solvent-based sample preparation. A quantitative analysis study performed with dimer/trimer mixtures over a wide range of compositions via solvent-free sample preparation indicates that linear, reproducible calibration curves can be generated and used to calculate the molecular composition of unknown dimer/trimer mixtures with confidence.

Investigating the effect of mixing ratio on molar mass distributions of synthetic polymers determined by MALDI-TOF mass spectrometry using design of experiments

It is well known that the mixing ratio affects the molar mass distribution of synthetic polymers determined by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). Surely, the molar mixing ratio determines whether a mass spectrum will be obtained or not. However, depending on the mass range, several effects such as multimer formation occur, which might be a source of errors in molar mass distribution calculations. In this study, the effect of mixing ratio was investigated for several synthetic polymers, including polystyrene (PS), poly(dimethylsiloxane) (PDMS), poly(ethylene glycol) (PEG), and poly(methyl methacrylate) (PMMA) using statistical designs of experiments. The 2(3) full factorial design was found to be suitable in the study of more than 1000 samples. The obtained MALDI mass spectra as well as the ANOVA statistics show that the mixing ratio affects the molar mass distribution. The optimal mixing ratio for a defined synthetic polymer depends on the studied combination (matrix, cationization reagent, solvent).

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.

Determining the time needed for the vortex method for preparing solvent-free MALDI samples of low molecular mass polymers

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is an important technique to characterize many different materials, including synthetic polymers. MALDI mass spectral data is used to determine the polymer average molecular weights, repeat units, and end groups. The development of the vortex method of solvent-free sample preparation showed that remarkably short mixing times could prepare samples that yielded high quality MALDI mass spectra. In this paper, we use microscopy images and MALDI mass spectra to evaluate the mixing time required by the vortex method to produce mass spectra for low molecular mass polymer samples. Our results show that mixing times of as little as 10 s can generate homogeneous thin films that produce high quality mass spectra with S/N approximately 100. In addition, ultrashort mixing times of only 2 s still produce samples with mostly smooth morphology and mass spectra with S/N approximately 10.

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.

Quantitative matrix-assisted laser desorption/ionization mass spectrometry

This review summarizes the essential characteristics of matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry (TOF MS), especially as they relate to its applications in quantitative analysis. Approaches to quantification by MALDI-TOF MS are presented and published applications are critically reviewed.

Using solvent-free sample preparation to promote protonation of poly(ethylene oxide)s with labile end-groups in matrix-assisted laser desorption/ionisation

Protonation is usually required to observe intact ions during matrix-assisted laser desorption/ionization (MALDI) of polymers containing fragile end-groups while cation adduction induces chain-end degradation. These polymers, generally obtained via living free radical polymerization techniques, are terminated with a functionality in which a bond is prone to homolytic cleavage, as required by the polymerization process. A solvent-free sample preparation method was used here to avoid salt contaminant from the solvent traditionally used in the dried-droplet MALDI procedure. Solvent-based and solvent-free sample preparations were compared for a series of three poly(ethylene oxide) polymers functionalized with a labile end-group in a nitroxide-mediated polymerization reaction, using 2,4,6-trihydroxyacetophenone (THAP) as the matrix without any added salt. Intact oligomer ions could only be produced as protonated molecules in solvent-free MALDI while sodium adducts of degraded polymers were formed from the dried-droplet samples. Although MALDI analysis was performed at the laser threshold, fragmentation of protonated macromolecules was still observed to occur. However, in contrast to sodiated molecules, dissociation of protonated oligomers does not involve the labile C--ON bond of the end-group. As the macromolecule size increased, protonation appeared to be less efficient and sodium adduction became the dominant ionization process, although no sodium salt was added in the preparation. Formation of sodiated degraded macromolecules would be dictated by increasing cation affinity as the size of the oligomers increases and would reveal the presence of salts at trace levels in the MALDI samples.

Negative-mode MALDI mass spectrometry for the analysis of pigments using tetrathiafulvalene as a matrix

Negative-mode matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) analysis of industrial pigments was performed using tetrathiafulvalene (TTF), an electron donating compound, as a matrix. Comparing with other matrices such as tetracyanoquinodimethane (TCNQ), dithranol, 2-hydroxy-6-nitroaniline (HNA), and harmine hydrochloride, the most abundant analyte ions were produced by the TTF as radical or deprotonated anions. In addition, fragment ions in the negative mode of operation are also found to be much less abundant than those in the positive mode of operation.

Imaging the morphology of solvent-free prepared MALDI samples

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry is an important technique to characterize many different materials, including synthetic polymers. MALDI mass spectral data can be used to determine the polymer average molecular weights, repeat units, and end groups. The development of solvent-free sample preparation methods has enabled MALDI to analyze insoluble materials and, interestingly, can provide higher-quality mass spectral data. Although the utility of solvent-free sample preparation for MALDI has been demonstrated, the reasons for its success are only now being discovered. In this study, we use microscopy tools to image samples prepared using solvent-free methods to examine the morphology of these samples. The samples are prepared using a simple vortex method. Our results show that the average particle size of typical MALDI matrices is reduced from their original tens to hundreds of micrometers to hundreds of nanometers. This size reduction of the matrix occurs in one minute using the vortex method. We also observe remarkably smooth and homogeneous sample morphologies for the laser to interrogate, especially considering the relatively crude methods used to prepare our samples.

The analysis of polystyrene and polystyrene aggregates into the mega dalton mass range by cryodetection MALDI TOF MS

Mass spectra of atactic polystyrene were collected into the mega-dalton mass range with a matrix-assisted laser desorption ionization time of flight (MALDI TOF) mass spectrometer, which incorporates a cryodetector comprised of an array of 16 superconducting tunnel junctions (STJ). The STJ cryodetector, theoretically, has no loss in signal response at any mass compared with the reduced signal found at high mass when using a conventional secondary-ionization detector. Since ion detection at high m/z is one of the fundamental limitations of mass spectrometry (MS), the cryodetector was used to explore the high m/z limit of the MALDI TOF technique for the analysis of two polymer types. Mass spectra were collected for polystyrene at Mn 170, 400, 900, and 2000 kDa and polymethyl methacrylate (PMMA) at Mn 62.6 kDa and 153.7 kDa. For polystyrene, the data showed a trend toward increased aggregation and charge state with mass. The Mn 2 MDa polystyrene data revealed a peak at m/z 2.2 MegaTh and a charge state analysis revealed that these ions were primarily polystyrene aggregates with a mass of approximately 4 MDa. This aggregate assignment was possible because the cryodetector response allows for the determination of a charge state up to about four. The contribution of each charge state for a selected peak can be determined in this fashion. This analysis revealed the preferential formation of doubly charged even-numbered aggregates over odd-numbered aggregates for high molecular mass polystyrene. A potential mechanism for the aggregation process for doubly charged species is discussed.

MALDI-TOF mass spectrometric identification of dyes and pigments

We have used MALDI-TOF mass spectrometry to characterize a selection of dyes from the Schweppe dye collection and pigments from the Tate Gallery collection. MALDI-TOF mass spectra of such samples are easily obtained and, through observation of both positive and negative ion spectra, provide a convenient, versatile method for dye characterization and identification. Such pairs of positive and negative ion spectra immediately distinguish between acidic and basic dyes and provide the characteristic mass of either the molecular ion or a simply related fragment ion. This approach is especially useful in situations where very small amounts of analyte are available, as in museum research and forensic analysis. In the case of textile dyes, we have carried out identification on material from single fibers and, with insoluble pigments, have begun to identify components of historically important pastel sticks from submicrogram samples.

Investigation of single-site zirconium azaborolinyl complexes by laser desorption ionization time of flight mass spectrometry

Laser desorption ionization mass spectrometry (LDI-MS) was performed on selected azaborolinyl zirconium complexes in order to study them as potential industrial polyolefin catalyst compounds. UV-vis absorption data in conjunction with negative and positive ionization analyses revealed the mechanistic workings of the compounds when such single-site catalysts function in olefin polymerization reactions. Results presented highlight the many benefits of LDI-MS in the study of single-site zirconium azaborolinyl catalyst complexes including minimal sample preparation, the absence of matrix/pH/solvation effects, a lower degree of fragmentation as compared to EI ionization, high sensitivity and the ability to observe interesting and unique gas phase chemistry including ring slippage, and distinctive metal coordination and oxidation states. The extreme lability of the chlorine ligands was apparent, as well as the reactivity of the resulting zirconium metal center as demonstrated by the appearance of numerous similar four-coordinate zirconium species along with di-chloro-bridged dimers. LDI-MS permits analysis at low molecular weights, improving spectral characterization of these zirconium complexes.