Target factor analysis of infrared spectra of multicomponent mixtures

The target factor analysis (t.f.a.) error criteria discussed earlier are applied to Fourier-transform infrared spectra in order to determine, without any prior knowledge of the experimental error, the number and identities of components in a series of related multicomponent mixtures. The t.f.a. method is also used for quantitative analysis and is compared to regression analysis, which force-fits the data and does not compensate for an impurity of small but measurable quantity. In addition, it is shown how an impurity in a single mixture can be detected, identified, and quantified. An actual experimental example is presented, using solutions of o-xylene, m-xylene, p-xylene, and ethylbenzene in cyclohexane, with chloroform as an impurity.     

Molecular weight adjustment to probability based matching of unknown mass spectra

For an unknown mass spectrum of lower molecular weight, there should be relatively fewer compounds in the reference file giving similar mass spectra. Testing this with the Probability Based Matching (PBM) algorithm, for a specific degree of match a reference compound of low molecular weight has a higher probability of being correct. Modifying the predicted reliability values for such matches statistically improves the overall performance of PBM appreciably at high reliability values.     

Chemical interpretation of differences in pyrolysis—mass spectra of simulated mixtures of biopolymers by factor analysis with graphical rotation

A method is described for the separation of a set of pyrolysis-mass spectra of simulated mixtures into subspectra for each of the components and for calculation of their relative concentrations. The technique is based on factor analysis and graphical rotation and is evaluated on a data set, constructed by linear combination of pyrolysis-mass spectra of three biopolymers.     

Identification of components in mixtures by a mathematical analysis of mass spectral data

Mathematical techniques for the identification of components in mixtures from the mass spectra of a series of related mixtures are described. The approach is analogous to library search methods in that spectra from a reference collection are compared with a multidimensional unknown. Searches are conducted with a library file containing approximately 17000 mass spectra. Results for the analyses of several mixtures are reported, to illustrate the effectiveness of the method.     

Probability based matching of mass spectra. Rapid identification of specific compounds in mixtures

An automated system is described for computer examination of the mass spectrum of an unknown mixture for the presence of a specific compound. The probability that the compound is present is indicated as a ‘confidence index’ K; 2^K should signify the average number of compounds, selected at random, whose mass spectra would have to be examined to find data which match the target spectrum to the same degree as does the unknown. The value of K is determined by the uniqueness U of the m/e value, the abundance A, and the criterion of abundance agreement W of the matching peaks, and of the compound concentration D in the sample. Application to spectra from a variety of compounds, including illicit drugs, indicates that the system has high sensitivity and selectivity, and that the value of K is a useful index of the reliability of the identification. Initial testing of this system has utilized an automated quadrupole mass spectrometer under control of a dedicated microcomputer with sample introduction through a flash evaporator and membrane separator.     

Computer-assisted library search system for identification of unknown mass spectra

The library search system described identifies a single component or two components in the unknown pure or mixture mass spectra by comparing them with a large data base of reference spectra. A preliminary search is based on the spectral interpretation and the main search is based on the probability of peak appearance. The performance of this system was tested on 254 pure spectra and 88 mixture spectra. The percentages of successful search by using the NIH/EPA/MSDC data base were 75% for the pure spectra, and 63% for both the first and second components in mixture spectra. The percentages of successful search improved to 94% for the first components of the mixture spectra and 77% for both first and second components of the mixture spectra, when conditions for measurement of reference spectra were the same as those for the unknown spectra.     

Comparison of algorithms and databases for matching unknown mass spectra

The most used algorithms for the identification of electron-ionization mass spectra are INCOS and probability based matching (PBM). For unknown spectra of high purity, ～75% of rank 1 answers are correct for both algorithms, matched against the National Institute of Standards and Technology 62,235 spectrum database. With matching criteria that retrieve 50% of the possible correct answers from the Wiley 228,998 spectrum database, 54% of the PBM and 42% of the INCOS answers are correct; for 85% purity unknowns, 48% and 27% are correct. For an unknown spectrum of two compounds, neither was reported in the first three INCOS answers; eight of the first ten PBM answers identify both components.     

Quantitation of isomeric ion mixtures using collisional activation mass spectra

Recently Bass and Bowers have criticized methods for calculation of isomeric composition of gaseous ions based on mass spectra from collisionally activated dissociations (CAD). They fault values from this laboratory on the proportion of benzyl and tropylium ions calculated by two methods from product ion abundances. However, a careful remeasurement of the relevant yields of CAD product ions shows that neither calculation method affected the result outside experimental error, as the ion yields for at least 14 higher energy dissociations are insensitive to both isomeric identity and internal energy. Reducing the [C₇H₇]⁺ internal energy by 1.5 eV does give a 25% reduction in the ion yield for the lowest energy process, supporting the accepted recommendation that these be omitted for quantitative spectral comparisons. A calculation error was made in one of our previous reports on [C₇H₇]⁺, and corrected isomeric composition values are presented. The disparate values from recalculation by Bass and Bowers of other data from this laboratory on [C₂H₄O]^+˙ ions are shown here to be consistent with the large experimental error of those measurements; the medians of their values are actually near the values originally reported. In direct contrast to their assertions, we find no evidence that previous calculation methods have produced misleading conclusions, or that the assumption of linear superposition of CAD spectra in ion mixtures has yielded unreliable results.     

Molecular ions in spark source mass spectra of sulphide-graphite mixtures

Summary Molecular Ions in Spark Source Mass Spectra of Sulphide-Graphite Mixtures Investigations of the intensities of CS₂ ⁿ⁺ ions (n=1,2,3) as a function of sparking voltage, electrode gap, accelerating voltage and electrode composition were carried out using two different mass spectrometers and various sulphide-graphite mixtures.The molecular ion intensity was found to increase with increasing electrode gap and sparking voltage. In these experiments, no CS₂ ³⁺ ions were observed. The intensity of CS₂ ³⁺ ions relative to S⁺ ions was less than 9·10^–9.     

Principal component analysis of mass spectra of peptides generated from the tryptic digestion of protein mixtures

Principal component analysis (PCA) has been used to analyse mass spectral peptide profiles obtained from the enzymatic digestion of standard protein mixtures. Scores and loadings plots clearly revealed peptide fragments that differentiated one protein mixture from another. Peptide map search results identified with a high degree of certainty any additional proteins in these mixtures. As a proof-of-concept this methodology was applied to hepatic protein mixtures obtained from rats treated with two hepatotoxic compounds: methapyriline and SB-219994. Liver proteins were extracted, pre-separated by one-dimensional polyacrylamide gel electrophoresis, subjected to tryptic digestion and analysed by mass spectrometry. Two up-regulated proteins, glutathione S-transferase with methapyrilene and peroxisomal bifunctional enzyme with SB-219994, were identified in this manner.     

Multi‐component analysis: blind extraction of pure components mass spectra using sparse component analysis

The paper presents sparse component analysis (SCA)‐based blind decomposition of the mixtures of mass spectra into pure components, wherein the number of mixtures is less than number of pure components. Standard solutions of the related blind source separation (BSS) problem that are published in the open literature require the number of mixtures to be greater than or equal to the unknown number of pure components. Specifically, we have demonstrated experimentally the capability of the SCA to blindly extract five pure components mass spectra from two mixtures only. Two approaches to SCA are tested: the first one based on ?₁ norm minimization implemented through linear programming and the second one implemented through multilayer hierarchical alternating least square nonnegative matrix factorization with sparseness constraints imposed on pure components spectra. In contrast to many existing blind decomposition methods no a priori information about the number of pure components is required. It is estimated from the mixtures using robust data clustering algorithm together with pure components concentration matrix. Proposed methodology can be implemented as a part of software packages used for the analysis of mass spectra and identification of chemical compounds. Copyright © 2009 John Wiley & Sons, Ltd.     

The fast atom bombardment mass spectra of glucosinolates and glucosinolate mixtures

The behaviour of glucosinolates and desulphoglucosinolates under fast atom bombardment conditions has been investigated. Relatively little fragmentation was noted in the mass spectra of these compounds. The positive ion fast atom bombardment mass spectra of glucosinolates exhibited abundant cationized and protonated molecula ions, thus complementing the previously reported electron impact and chemical ionization mass spectra of these compounds. Desulphoglucosinolates yield only the protonated molecular ion. Under negative ion conditions the spectra of glucosinolates were dominated by the molecular anion and this has been used as a means of analysing a crude plant extract containing a complex mixture of glucosinolates.     

Identification of unknown compounds using data bases and computer simulation of mass spectra

The possibility of identification based on the comparison of experimental electron-ionization mass spectrum of an unknown (in our case, model) compound with the mass spectra of the candidate compounds generated by the Mass Frontier software has been demonstrated by the example of three model compounds. The structural isomers of the identified substances found in the ChemExper database have been used as the candidate compounds. The candidate substances have been ranged by the degree of similarity between their simulated mass spectra and the experimental mass spectrum of the unknown compound. The mass spectra have been compared on the basis of the algorithm used in the NIST MS Search standard search system. In all three cases, the sought-after structure has been indicated as the most probable one of all the candidate structures.     

Typing of unknown microorganisms based on quantitative analysis of fatty acids by mass spectrometry and hierarchical clustering

Rapid identification of unknown microorganisms of clinical and agricultural importance is not only critical for accurate diagnosis of infections but also essential for appropriate and prompt treatment. We describe here a rapid method for microorganisms typing based on quantitative analysis of fatty acids by iFAT approach (Isotope-coded Fatty Acid Transmethylation). In this work, lyophilized cell lysates were directly mixed with 0.5M NaOH solution in d3-methanol and n-hexane. After 1 min of ultrasonication, the top n-hexane layer was combined with a mixture of standard d0-methanol derived fatty acid methylesters with known concentration. Measurement of intensity ratios of d3/d0 labeled fragment ion and molecular ion pairs at the corresponding target fatty acids provides a quantitative basis for hierarchical clustering. In the resultant dendrogram, the Euclidean distance between unknown species and known species quantitatively reveals their differences or shared similarities in fatty acid related pathways. It is of particular interest to apply this method for typing fungal species because fungi has distinguished lipid biosynthetic pathways that have been targeted for lots of drugs or fungicides compared with bacteria and animals. The proposed method has no dependence on the availability of genome or proteome databases. Therefore, it is can be applicable for a broad range of unknown microorganisms or mutant species.     

Principal component analysis of the infrared spectra of mixtures

Principal component analysis of the infrared spectra of a series of related mixtures is used to determine the number of compounds present. The use of empirical error estimates makes it possible to determine correctly the number of components even when the spectra of the individual compounds are very similar.     

Spectrophotometric analysis of mixtures of two components with extensively or completely overlapping spectra by the H-point standard additions method

Summary This paper discusses the requirements and scope of application of the H-Point Standard Additions Method (HPSAM) for resolving binary mixtures. Various mixtures of dyes with extensively or completely overlapping spectra were assayed with accurate, precise results. The method thus developed was also applied to the spectrophotometric resolution of binary mixtures of phenol and o-cresol, the absorption bands of which lie in similar positions and feature similar absorptivities.     

一种新的目标转换因子分析算法用于混合物红外光谱中纯组分光谱的解析

本文针对运用目标转换因子分析从混合物红外光谱中解析出纯组分光谱的有关限制,进行了有效的改进。把抽象正交特征光谱与以单一向量为初始向量的目标转换因子分析相结合,提出了一种新的算法,结果令人满意。     

Phase-titration analysis of ternary mixtures containing mutually miscible components

A method, based on phase-titration, for the analysis of ternary mixtures composed of mutually miscible components is described. One of the components is determined by an independent chemical or physical means and its concentration in the original mixture then brought to a fixed value by adding a calculated amount of one of the three components. The resultant mixture is then titrated to the turbidimetric end-point with a titrant which is immiscible with one of the components and the composition of the sample is computed from a calibration curve. The method is illustrated by its application to benzene-methanol-acetic acid and benzene-toluene-acetic acid mixtures.     

Gas chromatography/mass spectrometry analysis of components of pyridine temperature-programmed desorption spectra from surface of copper-supported catalysts

The method of pyridine temperature-programmed desorption (TPD) was applied for the measurement of acid properties of in situ reduced copper catalysts on silicate support. A thermal-conductivity detector (TCD) was used for the detection of TPD spectra of pyridine. The combination of flame-ionization detector and thermal conductivity detector shows that the region of TPD spectrum with the peak maxima T_MAX1 = 350 °C is a superposition of the TCD response on spectra of desorbed pyridine, water and carbon dioxide, desorbing simultaneously from the catalyst surface. The method for the elimination of H₂O and CO₂ on the layer of NaOH was tested and the pure TPD spectrum of pyridine was obtained. The exact determination of pyridine concentration allows to estimate the amount of weak and medium acid centers of the catalyst. The gas chromatography with the mass spectroscopy (GC–MS) analyses was used for the interpretation of high temperature region of the pyridine TPD spectra (T_MAX2 = 620 °C). It was found that pyridine bonded on the strong acid centers is decomposed to N₂ and CO under very high temperature. The available chromatographic method for the separation of components present in pyridine TPD spectrum in the high-temperature region was suggested. The method for the quantification of strong acidity of copper-supported catalyst was found.