Surface-enhanced Raman spectroscopy of soft-landed polyatomic ions and molecules

Surface-enhanced Raman spectroscopy (SERS) was used to detect and characterize polyatomic cations and molecules that were electrosprayed into the gas phase and soft-landed in vacuum on plasma-treated silver substrates. Organic dyes such as crystal violet and Rhodamine B, the nucleobase cytosine, and nucleosides cytidine and 2'-deoxycytidine were immobilized by soft landing on plasma-treated metal surfaces at kinetic energies ranging from near thermal to 200 eV. While enhancing Raman scattering 10(5)-10(6)-fold, the metal surface effectively quenches the fluorescence that does not interfere with the Raman spectra. SERS spectra from submonolayer amounts of soft-landed compounds were sufficiently intense and reproducible to allow identification of Raman active vibrational modes for structure assignment. Soft-landed species appear to be microsolvated on the surface and bound via ion pairing or pi-complexation to the Ag atoms and ions in the surface oxide layer. Comparison of spectra from soft-landed and solution samples indicates that the molecules survive soft landing without significant chemical damage even when they strike the surface at hyperthermal collision energies.     

Photoelectron spectroscopy and secondary ion mass spectrometry characterization of diamond-like carbon films

Photoelectron spectroscopy at different photon energies was used to optimize the photoexcitation cross section for valence band study of diamond-like hydrogenated carbon films. The electronic states of diamond-like carbon (DLC) were studied by synchrotron radiation photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. The valence band spectra measured at different excitation energies show the gradual emergence of the p-π band in relation to the sample annealing and ion bombardment amorphization. The p-π band of the annealed DLC was characterized by localized p_z states whilst the formation of the amorphous carbon surface was accompanied by appearance of the delocalized p_z states in the gap. Secondary ion mass spectrometry and thermal desorption spectroscopy showed that sample annealing was accompanied by hydrogen content decrease.     

Detection, characterization, and screening of heme-binding molecules by mass spectrometry for malaria drug discovery

Drug screening for antimalarials uses heme biocrystallization inhibition methods as an alternative to parasite cultures, but they involve complex processes and cannot detect artemisinin-like molecules. The described method detects heme-binding compounds by mass spectrometry, using dissociation of the drug-heme adducts to evaluate putative antiplasmodial activity. Applied to a chemical library, it showed a good hit-to-lead ratio and is an efficient early stage screening for complex mixtures like natural extracts.     

Interaction potential of nonpolar polyatomic molecules

The extension of the (12-7)-pair potential model to nonpolar polyatomic molecules is proposed. It is demonstrated that this model consistently fits diverse experimental data.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 38, No. 4, pp. 639–643, April, 1980.     

Quantitative X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry characterization of the components in DNA

The great diversity of techniques to synthesize and use DNA microarrays has made them extremely flexible for a variety of applications. This flexibility also has made standardization difficult, leading to problems comparing data from these different systems. In this work, we use the surface science techniques of X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) to analyze the components of DNA. The atomic ratios of the components of nucleobases, nucleosides, and nucleotides were characterized by XPS. The chemical shifts in the high-resolution XPS spectra allow for their relatively easy resolution. The unique positive and negative ions from the nucleobases, nucleosides, and nucleotides in their TOF-SIMS spectra were identified. This information was used to build a comprehensive table of all of the molecular ions. These standard spectra of DNA components can be used to predict the relative amounts of the bases within more complex molecules either by univariate analysis (i.e., by relating the base molecular ions to the sugar fragment ions within the nucleotides) or by multivariate analysis (e.g., principal component analysis). Our preliminary examination of four oligonucleotides shows promising results in that we can distinguish between two oligomers of similar composition using univariate and multivariate analysis, although additional studies are needed to expand this method to more complex oligomers.     

Processing of giant graphene molecules by soft-landing mass spectrometry

The processability of giant (macro)molecules into ultrapure and highly ordered structures at surfaces is of fundamental importance for studying chemical, physical and biological phenomena, as well as their exploitation as active units in the fabrication of hybrid devices. The possibility of handling larger and larger molecules provides access to increasingly complex functions. Unfortunately, larger molecules commonly imply lower processability due to either their low solubility in liquid media or the occurrence of thermal cracking during vacuum sublimation. The search for novel strategies to process and characterize giant building blocks is therefore a crucial goal in materials science. Here we describe a new general route to process, at surfaces, extraordinarily large molecules, that is, synthetic nanographenes, into ultrapure crystalline architectures. Our method relies on the soft-landing of ions generated by solvent-free matrix-assisted laser desorption/ionization (MALDI). The nanographenes are transferred to the gas phase, purified and adsorbed at surfaces. Scanning tunnelling microscopy reveals the formation of ordered nanoscale semiconducting supramolecular architectures. The unique flexibility of this approach allows the growth of ultrapure crystalline films of various systems, including organic, inorganic and biological molecules, and therefore it can be of interest for technological applications in the fields of electronics, (bio)catalysis and nanomedicine.     

A method for the analysis of low-mass molecules by MALDI-TOF mass spectrometry

We report a novel method termed matrix suppressed laser desorption/ionization to improve the analysis of low-mass molecules by MALDI-TOF mass spectrometry. In this method, the surfactant of cetrimonium bromide (CTAB) is added to the conventional matrix of alpha-cyano-4-hydroxycinnamic acid solution to prepare the MALDI samples. During the MALDI process, the presence of CTAB could substantially or even completely suppress the matrix-related ion background. As a result, very clean mass spectra can be routinely obtained in the low-mass range. In addition, the presence of CTAB can significantly improve the mass resolution of low-mass molecules. It is seen that high-quality spectra were routinely obtained at a matrix/CTAB ratio of 1000:1. This method has been successfully used to analyze a variety of low-mass molecules.     

Direct current glow discharge mass spectrometry for elemental characterization of polymers

A direct current glow discharge mass spectrometer has been used for a novel application, the sputtering and subsequent analysis of polymers. This was made possible by the application of a secondary cathode, a tantalum diaphragm placed in front of the nonconducting sample. Different types of polymers were measured (polytetrafluoroethylene, polycarbonate, and poly(vinyl chloride)). Important to note is that the mass spectra obtained are predominantly characterized as atomic, a major difference from the radio frequency GDMS spectra of polymers reported earlier. This facilitates quantitative elemental analysis for several reasons.     

Comprehensive gas chromatography-time-of-flight mass spectrometry using soft and selective photoionization techniques

The hyphenation of gas chromatography and mass spectrometry (GC/MS) revolutionized organic analysis. In GC/MS coupling, usually electron impact ionization is applied, and molecules are identified by their fragment pattern. Although mass spectrometry in principle is a separation method, it is used predominantly as a spectrometric technique. However, if soft (i.e., fragmentation-free) ionization techniques are applied, the inherent separation character of MS is emphasized, which has similarities to a GC boiling point separation. By combining polar column GC separation and fast soft ionization time-of-flight mass spectrometry technology, a comprehensive separation of complex petrochemical samples can be obtained (GC x MS approach). Compounds of comparable physical-chemical properties are characteristically grouped together in a two-dimensional retention time-m/z representation. This resembles the separation characteristics of comprehensive two-dimensional gas chromatography (GC x GC) and, thus, represents a novel multidimensional separation approach. In this work, a gas chromatograph equipped with a polar separation column was coupled to a home-built laser ionization time-of-flight mass spectrometer. Laser-based, single-photon ionization was used for universal soft ionization and resonance-enhanced multiphoton ionization for selective ionization of aromatic compounds. A novel capillary-jet inlet system was used for the coupling. Multidimensional comprehensive analysis of complex petrochemical hydrocarbon samples using gas chromatography coupled to mass spectrometry with soft and selective photo ionization sources is first demonstrated.     

Combed single DNA molecules imaged by secondary ion mass spectrometry

Studies of replication, recombination, and rearrangements at the level of individual molecules of DNA are often limited by problems of resolution or of perturbations caused by the modifications that are needed for imaging. The Combing-Imaging by Secondary Ion Mass Spectrometry (SIMS) (CIS) method helps solve these problems by combining DNA combing, cesium flooding, and quantitative imaging via the NanoSIMS 50. We show here that CIS can reveal, on the 50 nm scale, individual DNA fibers labeled with different, nonradioactive isotopes and, moreover, that it can quantify these isotopes so as to detect and measure the length of one or more short nucleic acid fragments associated with a longer fiber.     

Surface characterization of lipid/chitosan nanoparticles assemblies, using X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry

Chitosan/tripolyphosphate nanoparticles are promising drug delivery systems, which show excellent capacity for protein entrapment and improvement of mucosal peptide absorption. We have recently developed a new drug delivery system consisting of assemblies formed between preformed chitosan nanoparticles and phospholipids (dipalmitoylphosphatidylcholine and dimiristoylphosphatidylglycerol) which are endogenous to the lung. These assemblies are prepared by lipid film hydration with a nanoparticles suspension. The aim of this work was to elucidate the architecture of these structures using sensitive surface analysis techniques such as X-ray photoelectron spectroscopy and static time-of-flight secondary ion mass spectrometry, as well as to determine their physicochemical characteristics. The combination of zeta potential measurements with the results obtained by X-ray photoelectron spectroscopy and static time-of-flight secondary ion mass spectrometry, demonstrated that a complete lipid coating of the nanoparticles can be achieved using a lipid film formed by both dipalmitoylphosphatidylcholine and dimiristoylphosphatidylglycerol, this way conferring to the lipid film a strong negative charge, which favors the interaction with the positively charged nanoparticles. Therefore, the major role of electrostatic interactions as driving forces to control the organisation of the lipid/nanoparticles assemblies was clearly evident. The implications of these findings for the structural organisation of the assemblies, for their in vitro behaviour, as well as for their mechanism of formation are discussed.     

Characterization of polyether and polyester polyurethane soft blocks using MALDI mass spectrometry

Selective degradation reactions combined with MALDI analysis have been applied for molecular weight (MW) determination of polyether and polyester polyurethane (PUR) soft blocks. Selective degradation allows recovery of the polyols, and direct observation of the soft block oligomer distribution is possible for the first time by using MALDI. Ethanolamine is applied for polyether PUR degradation. MALDI analysis indicates that the recovered polytetrahydrofuran (pTHF) MW distribution is nearly identical to the unreacted pTHF material. Reduction in the ethanolamine reaction time allows observation of oligomer ions containing the diisocyanate linkage, which provide identification of the diisocyanate. Ethanolamine is not used for polyester PUR's degradation because the ester bonds will be cleaved. Therefore, phenylisocyanate is applied for polyester PUR degradation. Polybutylene adipate (pBA) oligomers were directly observed in the MALDI spectra of the degraded pBA-PUR samples. Comparison of the degraded pBA-PUR oligomer distribution with the unreacted pBA material indicates that low-mass oligomers are less abundant in the degraded pBA-PURs. Oligomer ions containing the diisocyanate linkage are also observed in the spectrum, providing a means for identifying the diisocyanate used for PUR syntheses. Size-exclusion chromatography (SEC) was combined with MALDI to provide accurate MW determination. Narrow MW fractions of the degraded and unreacted polyols were collected and analyzed by MALDI. This method allows precise calibration of the SEC chromatogram. The SEC-MALDI results provide significantly larger Mw and PD values than MALDI alone. Using SEC-MALDI, it was determined that the PD indexes of the pTHF and pBA samples are larger than the assumed values, which are based on the polyol synthesis reactions. The combination of selective degradation with SEC-MALDI, using either ethanolamine or phenylisocyanate, is a viable method for polyurethane polyol characterization.     

In vivo nanoelectrospray for the localization of bioactive molecules in plants by mass spectrometry

The method for the localization of bioactive molecules in plants is highly needed since it provides a fundamental prerequisite for understanding their physiological and ecological functions. Here, we propose a simple method termed in vivo nanoelectrospray for the localization of bioactive molecules in plants without sample preparation. A capillary is partly inserted into the plant to sample liquid from a highly located region, and then, a high voltage is applied to the plant to generate an electrospray from the capillary tip for mass spectrometry analysis. Using this method, bioactive molecules such as saccharides, glycoalkaloids, flavonoids, organic acids, and glucosinolates (GLs) are detected in the target regions of living plants or fresh fruits. Original information for endogenous chemicals including liable molecules in plant can be obtained. A sketchy three-dimensional distribution of glycoalkaloids in a cherry tomato has been obtained. The present work provides a powerful tool for the study of bioactive molecules in a living plant by mass spectrometry.     

Application of mass spectrometry to the characterization of polymers

Recent advances in soft ionization techniques for mass spectrometry of polymeric materials make it possible to determine the mass of intact molecular ions exceeding 1 × 10⁶ Da. Developments in high resolution mass spectrometers have additionally led to impressive advances in our ability to characterize polymers. The entire molecular mass distribution of a polymer sample can be accurately measured. From the molecular mass, the molecular formulae and information regarding polymer composition and end-groups can be deduced. The two techniques which have received the most attention are matrix-assisted laser desorption/ionization and electrospray ionization. In recent work, these techniques have been combined with chromatographic separations, and a series of mass spectra are acquired for each fraction of the distribution. This simplifies the analysis by reducing the number of components present in each mass spectrum, and additionally improves quantitation.     

Combining ion mobility spectrometry, mass spectrometry, and photoelectron spectroscopy in a high-transmission instrument

We have developed a novel instrument that combines ion mobility spectrometry, mass spectro-metry, and photoelectron spectroscopy. The instrument couples an electrospray ion source, a high-transmission ion mobility cell based on ion funnels, a quadrupole mass filter, and a time-of-flight (magnetic bottle) photoelectron spectrometer operated with a pulsed detachment laser. We show that the instrument can resolve highly structured anion arrival time distributions and at the same time provide corresponding photoelectron spectra-using the DNA oligonucleotide ion [dC(6) - 5H](5-) as a test case. For this multianion we find at least four different, noninterconverting isomers (conformers) simultaneously present in the gas phase at room temperature. For each of these we record well-resolved and remarkably different photoelectron spectra at each of three different detachment laser wavelengths. Two-dimensional ion mobility/electron binding energy plots can be acquired with an automated data collection procedure. We expect that this kind of instrument will significantly improve the capabilities for structure determination of (bio)molecular anions in the gas phase.     

Pulsed radiofrequency glow discharge time-of-flight mass spectrometry for nanostructured materials characterization

Progress in the development of advanced materials strongly depends on continued efforts to miniaturizing their structures; thus, a great variety of nanostructured materials are being developed nowadays. Metallic nanowires are among the most attractive nanometer-sized materials because of their unique properties that may lead to applications as interconnectors in nanoelectronic, magnetic, chemical or biological sensors, and biotechnological labels among others. A simple method to develop self-ordered arrays of metallic nanowires is based on the use of nanoporous anodic alumina (NAA) and self-assembled nanotubular titanium dioxide membranes as templates. The chemical characterization of nanostructured materials is a key aspect for the synthesis optimization and the quality control of the manufacturing process. In this work, the analytical potential of pulsed radiofrequency glow discharge with detection by time-of-flight mass spectrometry (pulsed rf-GD-TOFMS) is investigated for depth profile analysis of self-assembled metallic nanostructures. Two types of nanostructured materials were successfully studied: self-assembled NAA templates filled with arrays of single metallic nanowires of Ni as well as arrays of multilayered Au/FeNi/Au and Au/Ni nanowires and nanotubular titanium dioxide templates filled with Ni nanowires, proving that pulsed rf-GD-TOFMS allows for fast and reliable depth profile analysis as well as for the detection of contaminants introduced during the synthesis process. Moreover, ion signal ratios between elemental and molecular species (e.g., (27)Al(+)/(16)O(+) and (27)Al(+)/(32)O(2)(+)) were utilized to obtain valuable information about the filling process and the presence of possible leaks in the system.     

Radio frequency glow discharge mass spectrometry for the characterization of bulk polymers

A radio frequency (rf)-powered glow discharge (GD) atomization/ionization source is utilized to determine the applicability of the technique for direct polymer analysis. A series of PTFE-based polymers are studied to assess their fingerprint mass spectra and to distinguish each sample by its differing base peaks and relative peak intensities. A parametric study with respect to discharge gas pressure and rf power is conducted to evaluate their respective roles in the sputtering process as well as possible ionization mechanisms. The results of the GD sputtering processes are examined by scanning electron micrographs of a sputtered PTFE surface. Excellent discharge stabilization characteristics (<3 min) were observed in temporal response curves. Internal stability with respect to signal intensity is observed to be <5% RSD for samples of different thicknesses. Finally, the ability to obtain depth profiles of layered samples was demonstrated for the case of a Cu layer deposited on a PTFE substrate.     

Gel permeation chromatography coupled to fourier transform mass spectrometry for polymer characterization

We report an on-line coupling of gel permeation chromatography (GPC) to Fourier transform mass spectrometry (FTMS) using a modified commercial electrospray ionization (ESI) interface. Selected oligomer profiles for the sodiated (1+ through 5+ charge states) oligomer ions of a narrow-molecular-weight poly(methyl methacrylate) were generated and used for obtaining a calibration curve. Using the MS-generated calibration curve and the refractive index response for quantification, an accurate molecular weight distribution was calculated and showed an excellent agreement with the value specified by the supplier. GPC/ESI/FTMS also allowed for an unequivocal end-group determination and characterization of a secondary distribution due to the formation of cyclic reaction products. We analyzed a glycidyl methacrylate/butyl methacrylate copolymer with a broad molecular weight distribution, where fractionation and high resolving power were required for adequate characterization. Molecular weight distribution data showed the advantage of coupling high-resolution MS and GPC to overcome the difficulty of analyzing polydisperse polymers with MS alone.