Spectral interpretation

Mass-spectral peaks correspond to both the original molecule and fragments derived from it. In addition to the masses of these peaks, the empirical formula 

The following list contains some factors to consider when interpreting mass spectra. To make best use of fhis summary, the interested reader should consult the text by McLafferty and Turecek, which is considered by many to be the best resource to learn mass spectral interpretation. 

Using the considerations described above, identify the molecular ion. 

If possible, determine the elemental composition for M and other important peaks using isotopic abundances. In particular, look for isotope peaks from M + 2 elements like Cl, Br, S, and Si (Table 8.36 and Fig. 8.54). If you are able to establish a molecular formula, calculate rings + tt bonds  

An even-electron ion, with no unpaired electrons, will have a fractional value. If halogens are present, they are counted as hydrogens. 

Is the molecular weight odd If so, fhis indicates an odd number of nitrogen atoms (for organic molecules). 

For structural determination, it is necessary to observe mass-spectrometric fragment ions that are characteristic of the structure of the ion from which they were formed. Whether and how fragmentation occurs is dependent on the ion s internal energy. Although considered a soft ionization technique, FAB imparts 

Most of the positive-ion fragments can be rationalized as having similar structures whether the charge-bearing species is a proton or an alkali metal cation, most commonly a sodium ion. However, while B ions derived from protonated molecules can be considered to have the structure of an oxonium ion (Fig. 29a), B ions from sodiated molecules are proposed to be produced by an elimination reaction which generates a C- l-C-2 double bond, with the sodium ion imparting the charge157 (Fig. 29b). 

In addition, it should be noted that, inconveniently, the bond through which a glycan is attached to the peptide in a glycopeptide (whether N- or O-linked) is 

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In recent decades, much attention has been paid to the application of artificial neural networks as a tool for spectral interpretation (see, e.g.. Refs. [104, 105]). The ANN approach app]ied to vibrational spectra allows the determination of adequate functional groups that can exist in the sample, as well as the complete interpretation of spectra. Elyashberg [106] reported an overall prediction accuracy using ANN of about 80 % that was achieved for general-purpose approaches. Klawun and Wilkins managed to increase this value to about 95% [107]. 

Lee, T.A., A Beginner s Guide to Mass Spectral Interpretation, Wiley, Chichester, U.K., 1998. 

Nuclear Magnetic Resonance Spectroscopy. Nmr is a most valuable technique for stmeture determination in thiophene chemistry, especially because spectral interpretation is much easier in the thiophene series compared to benzene derivatives. Chemical shifts in proton nmr are well documented for thiophene (CDCl ), 6 = 7.12, 7.34, 7.34, and 7.12 ppm. Coupling constants occur in well-defined ranges J2-3 = 4.9-5.8 ... 

In conclusion, SSIMS spectra provide not only evidence of all the elements present, but also detailed insight into molecular composition. Quasimolecular ions can be desorbed intact up to 15000 amu, depending on the particular molecule [3.17] and on whether an effective mechanism of ionization is present. Larger molecules can be identified from fragment peak patterns which are characteristic of the particular molecules. If the identity of the material being analyzed is completely unknown, spectral interpretation can be accomplished by comparing the major peaks in the spectrum with those in a library of standard spectra. 

Maximum benefit from Gas Chromatography and Mass Spectrometry will be obtained if the user is aware of the information contained in the book. That is, Part I should be read to gain a practical understanding of GC/MS technology. In Part II, the reader will discover the nature of the material contained in each chapter. GC conditions for separating specific compounds are found under the appropriate chapter headings. The compounds for each GC separation are listed in order of elution, but more important, conditions that are likely to separate similar compound types are shown. Part II also contains information on derivatization, as well as on mass spectral interpretation for derivatized and underivatized compounds. Part III, combined with information from a library search, provides a list of ion masses and neutral losses for interpreting unknown compounds. The appendices in Part IV contain a wealth of information of value to the practice of GC and MS. 

The GC separations, derivatization procedures, mass spectral interpretation, structure correlations, and other information presented in this book were collected or experimentally produced over the length of a 30-year career (F.G.K.) in GC/MS. It has not been possible to reference all sources therefore, in the acknowledgments, we thank those persons whose work has significantly influenced this publication. 

II. GC Conditions, Derivatization, and Mass Spectral Interpretation of Specific Compound Types 43... 

The book is divided into four parts. Part I, The Fundamentals of GC/MS, includes practical discussions on GC/MS, interpretation of mass spectra, and quantitative GC/MS. Part II, GC Conditions, Derivatization, and Mass Spectral Interpretation of Specific Compound Types, contains chapters for a variety of compounds, such as acids, amines, and common contaminants. Also included are GC conditions, methods for derivatization, and discussions of mass spectral interpretation with examples. Part III, Ions for Determining Unknown Structures, is a correlation of observed masses and neutral losses with suggested structures as an aid to mass spectral interpretation. Part IV, Appendices, contains procedures for derivatization, tips on GC operation, troubleshooting for GC and MS, and other information which are useful to the GC/MS user. Parts I to III also contain references that either provide additional information on a subject or provide information about subjects not covered in this book. 

There are many ways to interpret mass spectra. Frequently, prior knowledge or the results from a library search dictate the method. The proceeding is a brief description of an approach to mass spectral interpretation that is especially useful when little is known about the compounds in the sample. 

Computer Techniques McLafferty (Ref 63) has pointed out that the usefulness of elemental composition information increases exponentially with increasing mass, since the number of elemental combinations with the same integral mass becomes larger. There are compilations of exact masses and elemental compositions available (Refs 12a, 13 18a). Spectral interpretation will be simplified in important ways if elemental compositions of all but, the smallest peaks are determined. Deriving the elemental compositions of several peaks in a spectrum is extremely laborious and time-consuming. However, with the availability of digital computers such tasks are readily performed. A modern data acquisition and reduction system with a dedicated online computer can determine peak centroids and areas for all peaks, locate reference peaks, interpolate between them to determine the exact masses of the unknown peaks, and find within minutes elemental compositions of all ions in a spectrum (Refs 28b 28c)... 

The n.m.r. sjjectra of typical reaction mixtui es from homolytic aromatic substitution are very complex. To simplify the problem of spectral interpretation, perdeuteriobenzoyl peroxide has been used together with a symmetrically trisubstituted aromatic substrate, such as... 

Derivatives play an essential part in almost all f.a.b.-m.s. studies of carbohydrates. They facilitate spectral interpretation (see Sections IV-VI), improve sensitivity (see Section 11,6), permit the analysis of salty samples (see later), allow unambiguous sequencing (see Section V,2), confirm the presence of cyclic structures (see Section VI,S), enable spectra to be obtained from very large molecules (see Section III,4), and help in the location of O-acylated residues in oligosaccharides (see Section V,S). 

Recent attention has focused on MS for the direct analysis of polymer extracts, using soft ionisation sources to provide enhanced molecular ion signals and less fragment ions, thereby facilitating spectral interpretation. The direct MS analysis of polymer extracts has been accomplished using fast atom bombardment (FAB) [97,98], laser desorption (LD) [97,99], field desorption (FD) [100] and chemical ionisation (Cl) [100]. 

MS High sensitivity (low pg) High selectivity (MSn mode) Spectral interpretation [54]... 

C 1-NMR spectroscopy is the method of choice for determining the molecular structure of polymers in solution [230]. Polyolefin 13C NMR is mainly quantitative ID 1-NMR multiple pulse techniques are used for spectral interpretation. The resolution obtained in 13C NMR spectra of LDPE is an order of magnitude larger than in the corresponding 1H-NMR spectra... 

When considering libraries of spectra for identification purposes, the effect of sample preparation on spectral characteristics is also important. Two FUR sampling methods have been adopted for IR analysis of TLC eluates in the presence of a stationary phase, namely DRIFTS [741] and PAS [742], of comparable sensitivity. It is to be noted that in situ TLC-PA-FTIR and TLC-DRIFT spectra bear little resemblance to KBr disc or DR spectra [743,744]. This hinders spectral interpretation by fingerprinting. For unambiguous identification, the use of a reference library consisting of TLC-FTIR spectra of adsorbed species is necessary. 

Wavelength database libraries of >32000 analytical lines can be used for fast screening of the echellogram. Such databases allow the analyst to choose the best line(s) for minimum interferences, maximum sensitivity and best dynamic range. Further extension of the wavelength range (from 120 to 785 nm) is desirable for alkali metals, Cl, Br, Ga, Ge, In, B, Bi, Pb and Sn, and would allow measurement of several emission lines in a multivariate approach to spectral interpretation [185]. 

Auger electron spectroscopy is preferred over XPS where high spatial resolution is required, although the samples need to be conducting and tolerant to damage from the electron beam. Many oxides readily decompose under electron radiation, and this may give rise to difficulty in spectral interpretation, and this has restricted the application of AES in the field of catalysis. 

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