TG-DTA/FT-IR method for analyzing thermal decomposition mechanism of polyesters

An attempt to estimate the thermal decomposition mechanism of polymers using the simultaneous TG-DTA/FT-IR system was summarized. The library search of FT-IR spectra at various temperatures and of the subtraction spectrum obtained by subtracting the spectra at different temperatures were used to determine the types of evolved gases from poly(ethylene terephthalate) and poly(butylene terephthalate) at given stages of decomposition. The quantitative analysis of evolved gases was carried out using the specific gas profiles at the specific absorption band. The kinetic parameters were estimated from both TG and spectroscopic curves measured at various heating rates.     

Characterization of Polymers by Direct Pyrolysis/Chemical Ionization Mass Spectrometry

The characterization of polymers by pyrolysis directly in the ion source of a double focusing magnetic sector mass spectrometer, operating in the chemical ionization mode, is described. Pyrolysis is achieved by two different probe techniques. A low temperature, slow heating rate direct insertion probe (DIP) is used at 400°C, and a specifically constructed high temperature, fast heating rate, high temperature pyrolysis (HTP) probe is used at 1000°C. This probe is capable of achieving pyrolysis temperatures of 1200°C at controlled heating rates up to 20,000°C/s. The mass spectrometric analysis of the pyrolysis products was achieved under chemical ionization (CI) conditions utilizing methane, isobutane, and ammonia as reagent gases. Under CI conditions the molecular ions formed in the mass spectrometer show little tendency to fragment. The CI mass pyrograms are very simple, with each peak in the spectra ascribable to a particular component in the pyrolysis product mixture. The results of the two probe pyrolysis techniques are compared and the utility of each technique for the characterization of polymers is demonstrated using the vinyl polymers polymethyl methacrylate, polyvinyl chloride, and polystyrene.     

Identification of some polymeric materials by low-temperature pyrolysis mass spectrometry

The application of low-temperature pyrolysis mass spectrometry to the identification of some commercial and synthetic polymers using a direct insertion probe and electron ionization has been studied. The polymers were analyzed directly in the solid probe of a mass spectrometer within the ion source at 70 eV electron impact. The quadrupole mass spectrometer, equipped with a gas chromatogram software and library of low molecular weight compounds, was used to characterize the polymers by measuring the chemical structure of the repeat units and the end groups, and to examine the thermal degradation pathways. The polymers investigated show different degradation pathways. Investigation of sequences and determination of composition of copolymers were studied. Total ion pyrogram and spectrum subtractions were used to separate and measure spectra of pyrolysis steps at distinctly different temperatures.     

Vergleich von Bibliothekssuchverfahren für Steroidmassenspektren

Zusammenfassung 29 Bibliothekssuchverfahren wurden mit einer Bibliothek von 524 Steroidmassenspektren geprüft. Mit einer Stichprobe von 100 Spektren wurde die Wiedererkennung verrauschter Spektren in der Bibliothek abgeschätzt. Besonders rasche und erfolgreiche Verfahren sind mit binär codierten Spektren (konstante Intensitätsschwelle von 1–5% des Basispeaks) und mit reduzierten Spektren (1 oder 2 Peaks in Massenintervallen der Länge 7 oder 14) möglich. Mit diesen Verfahren wurden 93–100% der unbekannten Spektren durch das 1. Nachbarspektrum der Bibliothek richtig identifiziert. Die Wiedererkennungsrate verrauschter Spektren steht in engem Zusammenhang mit dem Informationsgehalt (Entropie) der codierten Bilbliothek.

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Comparison of library search methods for steroid mass spectraRecognition of noisy spectra in a library

29 library search methods have been tested with a library of 524 steroid mass spectra. A random sample of 100 spectra was used as unknown to estimate recognition of noisy spectra in the library. Binary encoded spectra with constant thresholds of 1–5% of base peak or reduced spectra with 1 (or 2) peaks in mass ranges of 7 (or 14) mass units enable quick and successful searches. 93–100% of the unknowns were identified correctly by the first neighbour spectra of the library. Recognition of noisy spectra is closely related to information content (entropy) of coded libraries.

     

On the risk of false positive identification using multiple ion monitoring in qualitative mass spectrometry: Large-scale intercomparisons with a comprehensive mass spectral library

Analysts involved in qualitative mass spectrometry have long debated the minimum data requirements for demonstrating that signals from an unknown sample are identical to those from a known compound. Often this process is carried out by comparing a few selected ions acquired by multiple ion monitoring (MIM), with due allowance for expected variability in response. In a few past experiments with electron-ionization mass spectrometry (EI-MS), the number of ions selected and the allowable variability in relative abundance were tested by comparing one spectrum against a library of mass spectra, where library spectra served to represent potential false positive signals in an analysis. We extended these experiments by carrying out large-scale intercomparisons between thousands of spectra and a library of one hundred thousand EI mass spectra. The results were analyzed to gain insights into the identification confidence associated with various numbers of selected ions. A new parameter was investigated for the first time, to take into account that a library spectrum with a different base peak than the search spectrum may still cause a false positive identification. The influence of peak correlation among the specific ions in all the library mass spectra was also studied. Our computations showed that (1) false positive identifications can result from similar compounds, or low-abundance peaks in unrelated compounds if the method calls for detection at very low levels; (2) a MIM method's identification confidence improves in a roughly continuous manner as more ions are monitored, about one order of magnitude for each additional ion selected; (3) full scan spectra still represent the best alternative, if instrument sensitivity is adequate. The use of large scale intercomparisons with a comprehensive library is the only way to provide direct evidence in support of these conclusions, which otherwise depend on the judgment and experience of individual analysts. There are implications for residue chemists who would rely on standardized confirmation criteria to assess the validity of a given confirmatory method. For example, standardized confirmation criteria should not be used in the absence of interference testing and rational selection of diagnostic ions.     

Interpretation Automatique des Spectres de Masse de Composes Organiques. Mise au Point

Computerized methods for interpretation of mass spectra are increasing and many papers have now been published. Three different approaches can be used to identify an unknown compound. These methods are reviewed and discussed. First, the heuristic technique which tries to simulate the reasoning of the chemist and deduce the formulae. The only information needed are the mass spectra, the empirical formulae and the n.m.r. spectra. Some promising results have been obtained but the method has important restrictions. Another approach represents the mass spectra as points in a hyperspace. By developing decision surfaces it is possible to classify an unknown compound. This is called pattern recognition and the different empirical methods for recognizing patterns in mass spectral data are explained and reported. The last and simplest technique is to match an unknown spectrum against a library of standard references. When the unknown spectrum is in the library, this approach gives the best results.     

Quality evaluation of tandem mass spectral libraries

Tandem mass spectral libraries are gaining more and more importance for the identification of unknowns in different fields of research, including metabolomics, forensics, toxicology, and environmental analysis. Particularly, the recent invention of reliable, robust, and transferable libraries has increased the general acceptance of these tools. Herein, we report on results obtained from thorough evaluation of the match reliabilities of two tandem mass spectral libraries: the MSforID library established by the Oberacher group in Innsbruck and the Weinmann library established by the Weinmann group in Freiburg. Three different experiments were performed: (1) Spectra of the libraries were searched against their corresponding library after excluding either this single compound-specific spectrum or all compound-specific spectra prior to searching; (2) the libraries were searched against each other using either library as reference set or sample set; (3) spectra acquired on different mass spectrometric instruments were matched to both libraries. Almost 13,000 tandem mass spectra were included in this study. The MSforID search algorithm was used for spectral matching. Statistical evaluation of the library search results revealed that principally both libraries enable the sensitive and specific identification of compounds. Due to higher mass accuracy of the QqTOF compared with the QTrap instrument, matches to the MSforID library were more reliable when comparing spectra with both libraries. Furthermore, only the MSforID library was shown to be efficiently transferable to different kinds of tandem mass spectrometers, including “tandem-in-time” instruments; this is due to the coverage of a large range of different collision energy settings—including the very low range—which is an outstanding characteristics of the MSforID library.     

Mass spectrometry fingerprinting coupled to National Institute of Standards and Technology Mass Spectral search algorithm for pattern recognition

A new analytical strategy based on mass spectrometry fingerprinting combined with the NIST-MS search program for pattern recognition is evaluated and validated. A case study dealing with the tracing of the geographical origin of virgin olive oils (VOOs) proves the capabilities of mass spectrometry fingerprinting coupled with NIST-MS search program for classification. The volatile profiles of 220 VOOs from Liguria and other Mediterranean regions were analysed by secondary electrospray ionization-mass spectrometry (SESI-MS). MS spectra of VOOs were classified according to their origin by the freeware NIST-MS search v 2.0. The NIST classification results were compared to well-known pattern recognition techniques, such as linear discriminant analysis (LDA), partial least-squares discriminant analysis (PLS-DA), k-nearest neighbours (kNN), and counter-propagation artificial neural networks (CP-ANN). The NIST-MS search program predicted correctly 96% of the Ligurian VOOs and 92% of the non-Ligurian ones of an external independent data set; outperforming the traditional chemometric techniques (prediction abilities in the external validation achieved by kNN were 88% and 84% for the Ligurian and non-Ligurian categories respectively). This proves that the NIST-MS search software is a useful classification tool.     

Interpretation niedrig aufgelöster Massenspektren durch Klassifikation mit Hilfe der Nachbarspektren aus einer Bibliothek

The Euclidean distance between mass spectra in a n-dimensional hyperspace is a good measure of chemical-structural similarity of low molecular organic substances. The KNN-method, well-known in pattern recognition, was used to find neighbour-spectra from a library which are close to the mass spectrum of an unknown substance. These neighbours are very suitable to determine some classes of molecular structures of the unknown substance, which need not be a member of the library. On an average the first neighbour has the highest classifying capability.     

Analysis of gas chromatography—mass spectrometry data by reverse library search and the detection of substances not in the library

Computer least-squares methods are used to conduct an automatic reverse search of a library of complete mass spectra for the analysis of the output data matrix from a g.c.—m.s. system. The search is limited to those mass spectra falling within a narrow retention index window about the retention index corresponding to each scan in the datamatrix. The least-squares estimated parameters obtained are proportional to the concentrations of the substances present at each scan and are displayed graphically as the profiles of resolved overlapping g.c. peaks. The precision of the parameters is estimated. The root mean square Values of the residuals of the least-squares fits are used successfully to indicate the presence of substances not in the library. Examples are given of the results produced with data from this laboratory. Possible elaborations and improvements of this computer program are discussed.     

Assessment of the pollutant elimination efficiency by gas chromatography/mass spectrometry, liquid chromatography-mass spectrometry and -tandem mass spectrometry. Comparison of conventional and membrane-assisted biological wastewater treatment processes

The elimination efficiency of advanced conventional biological wastewater treatment was compared to membrane-assisted biological wastewater treatment. The sum parameter analyses dissolved organic carbon (DOC) and chemical oxygen demand (COD) or substance-specific analyses such as gas chromatography combined with mass spectrometry, flow injection analysis (FIA-MS) and liquid chromatography (LC-MS) in combination with mass or tandem mass spectrometry (MS-MS) were applied to assess elimination of hardly eliminable compounds in both types of wastewater treatment plants (WWTP). Reduction of DOC and COD in wastewater treatment processes confirmed a favourable elimination efficiency. Substance-specific methods which were applied in addition permitted a qualitative and semi-quantitative assessment of elimination with a visual pattern recognition approach. In order to identify pollutants either the NIST library of electron impact mass spectra for unpolar compounds or the laboratory-made collision-induced dissociation spectra library for polar pollutants was used. To assess elimination efficiency FIA-MS in the selected ion monitoring mode (SIM) besides high selective substance-specific mass spectrometric techniques such as parent ion scans and neutral loss scans were used for quantification. Results proved that membrane-assisted treatment was more effective than advanced biological treatment. In both types of WWTPs predominantly unpolar pollutants were eliminated, while all effluents were dominated by polar compounds of anthropogenic and biogenic origin. These unpolar and polar compounds which had been identified as hardly eliminable are reported about. Quantitative results obtained by FIA-MS, LC-MS and MS-MS for the elimination of alkyl polyglycol ethers, nonylphenol ethoxylates and linear alkylbenzenesulfonic acids from wastewater are presented.     

Computer-aided identification of compounds by comparison of mass spectra

A new identity-orientated search procedure for mass spectral libraries (IDS) was developed by extending the similarity search system SISCOM. The aim of IDS is an exact identification of pure compounds and mixtures on the basis of their mass spectra. The concepts and methods applied, e.g., filtering, feature selection and optimization with pattern recognition, are described. Chracteristics of IDS are summarized and demonstrated for several examples.     

A combined forward—reverse library search system for the identification of low-resolution mass spectra

A combined forward—reverse library search routine for low-resolution mass spectra is described. The routine requires binary-coded spectra. Masses and peak intensities are used for spectral comparison. On the basis of three possible search strategies, this routine is adaptable to analytical problems. The program was tested for 25 000 spectra from the ISAS, MSDC and EPA mass spectra libraries. The program is written completely in FORTRAN IV.     

Comprehensive gas chromatography coupled to mass spectrometry for the separation of pesticides in a very complex matrix

The present research is focused on the development of a comprehensive two-dimensional gas chromatography–rapid scanning quadrupole mass spectrometric (GC x GC-qMS) methodology for the analysis of trace-amount pesticides contained in a complex real-world sample. Reliable peak assignment was carried out by using a recently developed, dedicated pesticide MS library (for comprehensive GC analysis), characterized by a twin-filter search procedure, the first based on a minimum degree of spectral similarity and the second on the interactive use of linear retention indices (LRI). The library was constructed by subjecting mixtures of commonly used pesticides to GC x GC-qMS analysis and then deriving their pure mass spectra and LRI values. In order to verify the effectiveness of the approach, a pesticide-contaminated red grapefruit extract was analysed. The certainty of peak assignment was attained by exploiting both the enhanced separation power of dual-oven GC x GC and the highly effective search procedure.     

The potential of composite pyrolysis gas chromatographic mass spectra in the study of lignocellulosics

The objective of this work was to evaluate the use of composite mass spectral (CMS) data from pyrolysis gas chromatography/mass spectrometry (PY–GC/MS) for lignocellulosic materials. Various forages, by-products and fiber fractions derived from them were examined as CMS by PY–GC/MS. The PY–GC/MS system consisted of a heated platinum filament, a capillary gas chromatograph and an ion trap detector (ITD) mass spectrometer operated under electron impact conditions. Mass spectra were then composited in several ways by summing all the mass spectra acquired within retention times corresponding to major product classes. CMS data were entered in a dedicated library and compared using the ITD library editor software. The usefulness of such a simple procedure for studies related to lignocellulose analysis, such as forage recognition, development of analytical methods and digestibility/maturity correlation, is discussed.     

A computer search system for similar organic compounds in carbon-13 nuclear magnetic resonance data files

From the ¹³C-NMR spectrum of an unknown compound, the system provides a list of compounds ranked according to their similarity to the unknown. The similarity is estimated in a three-dimensional feature space, rather than by direct match of all peaks. All the spectra in the library are first converted to pattern points in the feature space by a dimensionality-reduction method. Thus, the search for similar compounds is simplified to a search for points within a given distance from the point representing the unknown. The compounds listed can be offered as the result or used for further operations (match of carbon number and peak position) in order to get a more exact result. An auto-optimization option is included to provide efficiency and user convenience.     

Reliable identification of pesticides using linear retention indices as an active tool in gas chromatographic-mass spectrometric analysis

The present research is focused on the current development and employment of a dedicated pesticide mass spectral library, characterized by a double-filter search procedure: the first is based on the degree of spectral similarity and the second on chromatographic retention information (linear retention indices). The highly pure mass spectra contained in the library have been attained by subjecting mixtures of widely used pesticides to comprehensive GC-MS analysis. Linear retention indices for conventional GC-MS analyses were calculated by injecting the same compounds on a single-column GC-MS system. The effectiveness of this approach was verified by analysing a white wine sample spiked with 10 structurally similar pyrethroid compounds, through automated solid-phase microextraction-GC-MS.     

Pyrolysis and soft ionization mass spectrometry of aquatic/terrestrial humic substances and soils

The rapid, reproducible, chemical characterization of complex environmental materials such as plants, humic substances and whole soil can be performed by controlled thermal degradation. Except for drying and milling no pre-treatment of the samples is required. Biomacromolecular cleavage during a short degradation step directly in the ion source of a mass spectrometer results in the production of high-mass chemical subunits. Short reaction times and small amounts of sample favour the generation of large, thermal fragments, i.e., chemical building blocks, which can be identified and correlated with the structure of the polymeric biomaterials investigated. The principal aim is to monitor the primary, thermal fragmentation by high molecular ion intensities of the pyrolyzates and to avoid consecutive, mass spectrometric fragmentation as far as possible.For the detection and identification of the pyrolysis (Py) products, a combination with time-/temperature-controlled mass spectrometry (MS) is used. Typical heating rates are 0.2–10°C/s and the temperature range is 50–800°C. Soft ionization techniques such as field ionization (FI), field desorption, chemical ionization (CI) and, to some extent, fast atom bombardment are employed in the positive and negative modes. The results of direct Py-MS are supported by high-resolution mass measurements using electric or photographic detection and Curie-point pyrolysis in combination with gas chromatography-electron ionization/FI/CIMS and library searches for the identification of the pyrolysis products.Fingerprinting and time-resolved Py-MS of aquatic and terrestrial humic substances are reported. The methodology for the investigations of dynamic processes during the volatilization and thermal decomposition of these complex biomaterials is illustrated. Weight loss curves and the temperature function of accurate molecular weight averages for aquatic fulvic and humic acid are derived from the Py-FIMS data. Initial results on the differentiation of soil horizons in a moder profile by Py-FIMS and pattern recognition are presented. In particular, the chemometric evaluation appears promising for future Py-MS studies of humic substances and whole soils, but also for fossil fuels, synthetic polymers and food. In an integrated approach, the linking of conventional chemical and spectroscopic data with the high-mass signals in pyrolysis-mass spectra will be the focus of forthcoming work. Preliminary results for combining wet-chemical data with those of ¹³C nuclear magnetic resonance, Fourier transform infrared and electron spin resonance spectroscopy are put forward in this survey. Finally, initial results of pilot studies to detect biocides such as atrazine directly in soils using Py-FIMS are demonstrated.     

The role of pattern recognition in the computer-aided classification of mass spectra

The requirements for the use of pattern recognition techniques as an aid in the identification of chemical substances from their mass spectra are reviewed. Decision-tree pattern recognition is recommended as potentially satisfying these requirements. Examples of this approach using a large data base of mass spectra are provided.     

A quantitative measure of the reliability of searches of spectral libraries

A quantitative measure of library search reliability has been developed. Applications of the quantitative reliability metric (QRM) for evaluating the reliability of library searches for unknown target spectra and the use of this measure to detect the failure of a library search caused by noise, contaminant peaks and missing library spectra are discussed. The effects of noise and composite infrared spectra of mixtures on the QRM are examined for test sets of 561 infrared spectra. The QRM is also used to evaluate the performance of a search of an infrared library compressed by eigenvector projection.