Additional Mass Spectral Information

Quite often a normal electron ionization mass spectrum appears insufficient for reliable analyte identification. In this case additional mass spectral possibilities may be engaged. For example, the absence of the molecular ion peak in the electron ionization spectrum may require recording another type of mass spectrum of this analyte by means of soft ionization (chemical ionization, field ionization). The problem of impurities interfering with the spectra recorded via a direct inlet system may be resolved using GC/MS techniques. The value of high resolution mass spectrometry is obvious as the information on the elemental composition of the molecular and fragment ions is of primary importance. 

At first glance the necessity to use collision activation or metastable ion spectra is less evident. However, these two methods allow, first of all, recording pure mass spectra, 

INTRODUCTION TO MASS SPECTRA INTERPRETATION ORGANIC CHEMISTRY 

Sometimes it is impossible to elucidate the structure of an ion by means of any possible types of mass spectra. In this case it is useful to obtain spectra of labeled analogs of these compounds. This approach is widely used in other physicochemical methods, as well as in classic chemistry. 

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Bombick et al. [3] presented a simple, low cost method for producing thermal potassium metal ions for use as Cl reagents. All studies were performed on a commercial gas chromatography-mass spectrometiy (GC-MS) system. Thermionic emitters of a mixture of silica gel and potassium salts were mounted on a fabricated probe assembly and inserted into the Cl volume of the ion source through the direct insertion probe inlet. Since adduct ions (also referred to as cationized molecular ions or pseudomolecular ion ) of the type (M + K)+ have been observed, molecular weight information is easily obtained. The method is adaptable to any mass spectrometer with a Cl source and direct inlet probe (DIP). In addition, the technique is compatible with chromatographic inlet systems, i.e., GC-MS modes, which will provide additional dimensions of mass spectral information. 

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. 

Mass spectrometric studies yield principally three types of information useful to the radiation chemist the major primary ions one should be concerned with, their reactions with neutral molecules, and thermodynamic information which allows one to eliminate certain reactions on the basis of endothermicity. In addition, attempts at theoretical interpretations of mass spectral fragmentation patterns permit estimates of unimolecular dissociation constants for excited parent ions. 

Principles and Characteristics Mass-spectral analysis methods may be either indirect or direct. Indirect mass-spectral analysis usually requires some pretreatment (normally extraction and separation) of the material, to separate the organic additives from the polymers and inorganic fillers. The mass spectrometer is then used as a detector. Direct mass-spectrometric methods have to compete with separation techniques such as GC, LC and SFC that are more commonly used for quantitative analysis of polymer additives. The principal advantage of direct mass-spectrometric examination of compounded polymers (or their extracts) is speed of analysis. However, quite often more information can be... 

One of the major problems has been to determine the site of attachment of the PAH to the base. Some information may be obtained directly from the nmr spectra eliminating certain points of attachment. As mentioned above, if the C-8 proton of guanine or adenine can be identified, then this cannot be the point of attachment of the carcinogen. Estimation of the pKa s of the adducts either by titration (108) or partition (110) has, however, provided additional valuable information. Mass spectral fragmentation patterns can be of help in determining the site of substitution as well as in determining which bases are involved in binding (108.111-113). Substantial advances have been made in recent years on the mass spectral analysis of involatile compounds and derivatization is not always essential (114-118). X-ray analysis of DNA adducts has, to date, only been applied to model systems (119-121). 

The NIOSH RTECS is the first non-spectroscopic CIS data base and has proven to be a very valuable addition to the CIS. Interest in the data base has been shown by many groups within EPA involved in the implementation of TSCA. For example, work is now underway to link spectral data with the NIOSH toxicity data so that as a result of a mass spectral identification, the EPA lab can quickly be informed if the chemical identified is toxic and hence requires immediate action. 

Therefore, the information obtained, through onsite GC/MS analysis about the identity of compounds present in a sample should be restricted to chemicals relevant to the aim of the inspection. This is achieved by operating the instrument in a specially developed blinded mode, which shows neither the chromatogram nor mass spectra during or after the chromatographic run. Additionally, if the analysis is conducted in blinded mode, the only available postprocessing software is a specifically developed on-site version of AMDIS (Automated Mass Spectral Deconvolution and Identification System). This software works only with the OCAD, which contains only compounds relevant to the CWC thus, it reports exclusively the presence of compounds for which spectra are in this database library. 

In problems of structure elucidation an NMR spectrum may provide useful, even vital data, but it is seldon the sole piece of information available. A knowledge of the source of the compound or its method of synthesis is frequently the single most important fact. In addition, the interpretation of the NMR spectrum is carried out with concurrent knowledge of other physical properties, such as elemental analysis from combustion or mass spectral studies, the molecular weight, and the presence or absence of structural features, as indicated by infrared or ultraviolet spectra or by chemical tests. Obviously, the procedure used for analyzing the NMR spectrum is highly dependent on such ancillary knowledge. 

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