Search

A matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) interlaboratory comparison was conducted on mixtures of synthetic polymers having the same repeat unit and closely matching molecular mass distributions but with different end groups. The interlaboratory comparison was designed to see how well the results from a group of experienced laboratories would agree on the mass fraction, and molecular mass distribution, of each polymer in a series of binary mixtures. Polystyrenes of a molecular mass near 9000 u were used. Both polystyrenes were initiated with the same butyl initiator; however, one was terminated with -H (termed PSH) and the other was terminated with -C[H.sub.2]C[H.sub.2]0H (termed PSOH). End group composition of the individual polymers was checked by MALDI-TOF MS and by nuclear magnetic resonance (NMR). Five mixtures were created gravimetrically with mass ratios between 95:5 and 10:90 PSOH/PSH. Mixture compositions where measured by NMR and by Fourier transform infrared spectrometry (VF-IR). NMR and FT-IR were used to benchmark the performance of these methods in comparison to MALDI-TOF MS. Samples of these mixtures were sent to any institution requesting it. A total of 14 institutions participated. Analysis of variance was used to examine the influences of the independent parameters (participating laboratory, MALDI matrix, instrument manufacturer, TOF mass separation mode) on the measured mass fractions and molecular mass distributions for each polymer in each mixture. Two parameters, participating laboratory and instrument manufacturer, were determined to have a statistically significant influence. MALDI matrix and TOF mass separation mode (linear or reflectron) were found not to have a significant influence. Improper mass calibration, inadequate instrument optimization with respect to high signal-to-noise ratio across the entire mass range, and poor data analysis methods (e.g., baseline subtraction and peak integration) seemed to be the greatest obstacles in the correct application of MALDI-TOF MS to this problem. Each of these problems can be addressed with proper laboratory technique.

A mathematical algorithm is presented that locates and calculates the area beneath peaks from real data using only reproducible mathematical operations and no user-selected parameters. It makes no assumptions about peak shape and requires no smoothing or preprocessing of the data. In fact, it is shown that for matrix-assisted laser desorption time-of-flight mass spectra noise exists at all frequency ranges making the smoothing of data without distortion of peak areas impossible. The algorithm is based on a time-series segmentation routine that reduces the data set to groups of three strategic points where each group defines the beginning, center, and ending of each peak located. The peak areas are found from the strategic points using a commonplace polygonal area calculation routine. Peaks with statistically insignificant height or area are then discarded. The performance of the algorithm is demonstrated on a polystyrene mass spectrum with varying degrees of noise added either mathematically or experimentally. An on-line implementation of the method, termed MassSpectator, for public use can be found at www.nist.gov/maldi.

Polyethylene is one of the most important industrial polymers and is also one of the most challenging polymers to be characterized by mass spectrometry. We have developed a substrate-assisted laser desorption/ionization (LDI) mass spectrometric method for polyethylene analysis. In this method, cobalt, copper, nickel, or iron metal powders are used as a sample substrate and silver nitrate is used as the cationization reagent. Using a conventional UV LDI time-of-flight mass spectrometer, intact oligomer ions having masses up to 5000 u can be detected. Cobalt is found to produce spectra with the highest signal-to-noise ratio and the lowest level of fragmentation. Cobalt powder size is shown to have some effect on the spectra produced. The best results are obtained with the use of cobalt powders with diameters ranging from 30 to 100 [micro]m. Fragmentation cannot be totally eliminated, but the fragment ion peaks can be readily discerned from the intact polyethylene ions in the substrate-assisted LDI spectrum. Thus, the average molecular masses of low-mass polyethylene samples can be determined by using this method. A rapid heating model is used to account for the effectiveness of using the coarse metal powders to assist the analysis of intact polyethylene molecules by LDI.

A study is conducted on the corrosive nature of iron-zinc alloys which are electroplated on mild steel, in aerated and deaerated quadrupuly distilled water using valence band x-ray photoelectron spectroscopy. Results indicate that the 17% iron alloy forms a protective zinc oxide surface.

Secondary ion mass spectrometry is the most used mass spectrometric technique for the surface analysis of molecular solids composed of organic, polymeric and biomolecular species. By increasing the projectile ion mass and kinetic energy, the secondary ion yields increase due to improved momentum transfer to the surface.

The absolute molecular orientational distribution of the pendant phenyl groups at the free surface of polystyrene (PS) thin films on oxidized silicon substrates is studied using vibrationally resonant sum frequency generation (VR-SFG). Results show that at the PS-air interface, the pendant phenyl groups are oriented away from the film.

The method of preparation and methods of analysis of a narrow distribution polystyrene of ~7 ku used in an interlaboratory comparison of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) of synthetic polymers is described. Size exclusion chromatography was used to measure the polystyrene sample variability. Fourier transform infrared spectroscopy and MALDI-TOF-MS were used to analyze end groups on the polymer. The polystyrene was analyzed by MALDI-TOF-MS and classical methods of polymer characterization. The number ([M.sub.n]) and mass ([M.sub.w]) average of the molecular mass distribution (MMD) determined by the classical methods (light scattering and NMR) were compared with those obtained by MALDI-TOF-MS. Agreement between classical methods to obtain the moments of the MMD and the MALDI is found to be good overall. However, all the experimental values obtained by MALDI fell below the classical values. A discussion of why these values are lower is included. We discuss the statistical analysis of the data from the interlaboratory comparison conducted by NIST, which includes data from 23 different laboratories. Analysis of variance is used to examine the influences of the independent parameters (laboratory, matrix, instrument manufacturer, instrument mode) on the data. The parameters, laboratory and instrument manufacturer, were determined to have an influence on the MMD, where matrix and instrument mode were found not to have a significant influence.

A mass spectral library search algorithm that identifies compounds that differ from library compounds by a single "inert" structural component is described. This algorithm, the Hybrid Similarity Search, generates a similarity score based on matching both fragment ions and neutral losses. It employs the parameter DeltaMass, defined as the mass difference between query and library compounds, to shift neutral loss peaks in the library spectrum to match corresponding neutral loss peaks in the query spectrum. When the spectra being compared differ by a single structural feature, these matching neutral loss peaks should contain that structural feature. This method extends the scope of the library to include spectra of "nearest-neighbor" compounds that differ from library compounds by a single chemical moiety. Additionally, determination of the structural origin of the shifted peaks can aid in the determination of the chemical structure and fragmentation mechanism of the query compound. A variety of examples are presented, including the identification of designer drugs and chemical derivatives not present in the library. [ABSTRACT FROM AUTHOR]

MassSpectator is a novel suite of numerical algorithms that accurately locates and calculates the area beneath peaks from real mass spectral data by using only reproducible mathematical operations. It requires no user-selected parameters but does require a background spectrum to determine instrumental noise. Such a fully automated algorithm is useful for rapid and repeatable processing of mass spectral data containing hundreds of peaks. MassSpectator represents a first attempt to create an unbiased method using time series segmentation to analyze mass spectral data.

The mass spectral library of the Scientific Working Group for the Analysis of Seized Drugs (SWGDRUG) is the most comprehensive free reference database of its kind in the world. It provides reliable mass spectra for identification of seized drugs, their metabolites, and related forensic compounds when using gas chromatography/mass spectrometry (GC/MS). The SWGDRUG library (version 3.13) contains spectra for 3598 compounds. All spectra are evaluated by the Mass Spectrometry Data Center (MSDC) at the National Institute of Standards and Technology (NIST). Over the past few years, new evaluation methods aided by improved software have been developed. First, all chemical information, such as chemical structure and name, is confirmed. Second, the product ions in each spectrum are verified to match the compound structure using the NIST MS Interpreter software tool. Subsequently, the mass spectra are compared to the same or similar compounds across six different mass spectral reference libraries using three distinct library search methods. Additionally, the NIST Artificial Intelligence Retention Indices (AIRI) software is used to help confirm the corresponding compounds of spectra, especially for those without molecular ions. Low-quality and incorrect spectra are rejected for inclusion in the library.

This work presents methods for identifying and then creating a mass spectral library for disulfide-linked peptides originating from the NISTmAb, a reference material of the humanized IgG1k monoclonal antibody (RM 8671). Analyses involved both partially reduced and non-reduced samples under neutral and weakly basic conditions followed by nanoflow liquid chromatography tandem mass spectrometry (LC-MS/MS). Spectra of peptides containing disulfide bonds are identified by both MS1 ion and MS2 fragment ion data in order to completely map all the disulfide linkages in the NISTmAb. This led to the detection of 383 distinct disulfide-linked peptide ions, arising from fully tryptic cleavage, missed cleavage, irregular cleavage, complex Met/Trp oxidation mixtures, and metal adducts. Fragmentation features of disulfide bonds under low-energy collision dissociation were examined. These include (1) peptide bond cleavage leaving disulfide bonds intact; (2) disulfide bond cleavage, often leading to extensive fragmentation; and (3) double cleavage products resulting from breakages of two peptide bonds or both peptide and disulfide bonds. Automated annotation of various complex MS/MS fragments enabled the identification of disulfide-linked peptides with high confidence. Peptides containing each of the nine native disulfide bonds were identified along with 86 additional disulfide linkages arising from disulfide bond shuffling. The presence of shuffled disulfides was nearly completely abrogated by refining digest conditions. A curated spectral library of 702 disulfide-linked peptide spectra was created from this analysis and is publicly available for free download. Since all IgG1 antibodies have the same constant regions, the resulting library can be used as a tool for facile identification of "hard-to-find" disulfide-bonded peptides. Moreover, we show that one may identify such peptides originating from IgG1 proteins in human serum, thereby serving as a means of monitoring the completeness of protein reduction in proteomics studies. Data are available via ProteomeXchange with identifier PXD023358.

We describe the creation of a mass spectral library of acylcarnitines and conjugated acylcarnitines from the LC-MS/MS analysis of six NIST urine reference materials. To recognize acylcarnitines, we conducted in-depth analyses of fragmentation patterns of acylcarnitines and developed a set of rules, derived from spectra in the NIST17 Tandem MS Library and those identified in urine, using the newly developed hybrid search method. Acylcarnitine tandem spectra were annotated with fragments from carnitine and acyl moieties as well as neutral loss peaks from precursors. Consensus spectra were derived from spectra having similar retention time, fragmentation pattern, and the same precursor m / z and collision energy. The library contains 157 different precursor masses, 586 unique acylcarnitines, and 4 332 acylcarnitine consensus spectra. Furthermore, from spectra that partially satisfied the fragmentation rules of acylcarnitines, we identified 125 conjugated acylcarnitines represented by 987 consensus spectra, which appear to originate from Phase II biotransformation reactions. To our knowledge, this is the first report of conjugated acylcarnitines. The mass spectra provided by this work may be useful for clinical screening of acylcarnitines as well as for studying relationships among fragmentation patterns, collision energies, structures, and retention times of acylcarnitines. Further, these methods are extensible to other classes of metabolites.

Dust samples from sintering and detonation coating hard-metal processes were characterized, compared, and contrasted for morphology, composition, and generation of hydroxyl radicals. Inhalation of respirable hard-metal (sintered carbide) dusts from hard-metal processes is known to cause fibrotic and asthmatic lung disease. Scanning electron microscopy/energy-dispersive X-ray analysis was used for morphology, composition, and elemental distribution. An electron spin resonance (ESR) spin trapping technique was used to detect hydroxyl radical generation. Samples were incubated with air-saturated buffer solutions containing a spin trap and analyzed by ESR for the presence of *OH in solution. Postdetonation coating samples often had surface contamination of Co on the WC particles, as shown by elemental mapping of individual particles; this was not evident in predetonation samples or unsintered materials in this study. ESR measurements show that both detonation-gun materials were capable of generating *OH , while the WC, cobalt, and presintered mixture did not produce detectable amounts of *OH radicals. The DMPO/*OH adduct formation was apparently facilitated by Fe-mediated reactions for predetonation dusts, and by Fe-mediated site-specific reactions for postdetonation dusts. The overspray materials from the detonation-gun process produced 9-fold more *OH radicals than the predetonation coating mixture. Overall, this study indicates there are substantial differences between postdetonation materials and both predetonation and unsintered hard-metal process materials with respect to morphology, elemental distribution, and *OH radical generation reactions and that these differences may be important in the toxic potential of those materials.