Ion libraries

Provided that the compounds analysed in positive mode are not known, the selected and fragmented compound ions (cf. Fig. 2.5.7(b)—(d)) after computer aided product ion library search were described as AE (Fig. 2.5.7(b) and (c)) and polyglycol amines... 

By the application of API-FIA-MS-MS(+/—) to the detergent formulation the main ingredients could be identified in product ion mode. MS-MS spectra available in a product ion library were used for comparison. So AE compounds with their [M + NH4]+ ions were... 

While fast atom bombardment (FAB) [66] and TSI [25] built up the basis for a substance-specific analysis of the low-volatile surfactants within the late 1980s and early 1990s, these techniques nowadays have been replaced successfully by the API methods [22], ESI and APCI, and matrix assisted laser desorption ionisation (MALDI). In the analyses of anionic surfactants, the negative ionisation mode can be applied in FIA-MS and LC-MS providing a more selective determination for these types of compounds than other analytical approaches. Application of positive ionisation to anionics of ethoxylate type compounds led to the abstraction of the anionic moiety in the molecule while the alkyl or alkylaryl ethoxylate moiety is ionised in the form of AE or APEO ions. Identification of most anionic surfactants by MS-MS was observed to be more complicated than the identification of non-ionic surfactants. Product ion spectra often suffer from a reduced number of negative product ions and, in addition, product ions that are observed are less characteristic than positively generated product ions of non-ionics. The most important obstacle in the identification and quantification of surfactants and their metabolites, however, is the lack of commercially available standards. The problems with identification will be aggravated by an absence of universally applicable product ion libraries. 

Figure 3. Top and middle, CAD MS/MS fragment ion library spectra of 0- and Qf-ionone, respectively bottom, CAD MS/MS fragment ion...

Figure 6. Top, MS/MS fragment ion library spectrum of lactic acid standard and bottom, MS/MS fragment ion spectrum of lactic acid emanating from the hand.

The application of MS/MS or in-source-CID for identification of pollutants brought problems, since no product ion libraries were available. Product ion spectra either had to be interpreted for compound identification or standard comparison had to be performed provided that standards were available. To overcome this disadvantage, analysts prepared their own libraries suitable for the instrument generated on or for instruments of the same type. Attempts at generating mass spectral libraries for polar compounds determined by API methods were reported and the results obtained with their application in real environmental samples were discussed [291, 292]. The generation of IT-MS product ion spectra and their use for identification provided the most promising results so far. The preparation of a universally applicable product ion library for the identification of polar compounds, i.e. of a library which could work with various equipments for identification, still remains wishful thinking. 

These characteristic absorption regions called group frequencies allow the analyst to detect the different elemental patterns and from them to reconstruct the molecule either by dej duct ion or by comparison with library reference spectra. The libraries contaih severaY hundred thousand spectra. 

Electron ionization occurs when an electron beam crosses an ion source (box) and interacts with sample molecules that have been vaporized into the source. Where the electrons and sample molecules interact, ions are formed, representing intact sample molecular ions and also fragments produced from them. These molecular and fragment ions compose the mass spectrum, which is a correlation of ion mass and its abundance. El spectra of tens of thousands of substances have been recorded and form the basis of spectral libraries, available either in book form or stored in computer memory banks. 

Finally, note that the ions produced by the combined inlet and ion sources, such as electrospray, plasmaspray, and dynamic FAB, are normally molecular or quasi-molecular ions, and there is little or none of the fragmentation that is so useful for structural work and for identifying compounds through a library search. While production of only a single type of molecular ion may be useful for obtaining the relative molecular mass of a substance or for revealing the complexity of a mixture, it is often not useful when identification needs to be done, as with most general analyses. Therefore,... 

When a mass spectrum has been acquired by the spectrometer/computer system, it is already in digital form as m/z values versus peak heights (ion abundances), and it is a simple matter for the computer to compare each spectrum in the library with that of the unknown until it finds a match. The shortened search is carried out first, and the computer reports the best fits or matches between the unknown and spectra in the library. A search of even 60,000 to 70,000 spectra takes only a few seconds, particularly if transputers are used, thus saving the operator a great deal of time. Even a partial match can be valuable because, although the required structure may not have been found in the library, it is more than likely that some of the library compounds will have stractural pieces that can be recognized from a partial fit and so provide information on at least part of the structure of the unknown. 

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. 

Eda and Kurth applied a similar solid-phase combinatorial strategy for synthesis of pyridinium, tetrahydropyridine, and piperidine frameworks as potential inhibitors of vesicular acetylcholine transporter. One member of the small library produced was prepared from amino-functionalized trityl resin reacting with a 4-phenyl Zincke salt to give resin-bound product 62 (Scheme 8.4.21). After ion exchange and cleavage from the resin, pyridinium 63 was isolated. Alternatively, borohydride reduction of 62 led to the 1,2,3,6-tetrahydropyridine 64, which could be hydrogenated to the corresponding piperidine 65. 

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. 

Figure 16.2 is the mass spectrum of propylene glycol and shows the presence of an abundant m/z 45 ion. A library search will provide strong evidence that this compound is propylene glycol. Preparation of a TMS derivative will confirm this assignment. 

Even though a good fit is not obtained, the library search may indicate the structural type. Review the characteristic fragment pathways of the suspected structural type in Part II of this book, and check Part III to determine if the ions observed and neutral losses correspond to the suggested structural type. 

If you frequently analyze pesticides, obtain the latest edition of Mass Spectrometry of Pesticides and Pollutants (Safe and Hutzinger. Boca Raton, FL, CRC Press). This book, combined with the list of most abundant ions (Table 25.1) and/or a computer library search, will be sufficient to identify most commercial pesticides. Also, see Chapters 17, 26, and 27. 

If the belt moves too quickly, in relation to the rate of deposition, sample will not be deposited on all parts of the belt. This results in the production of an uneven total-ion-current (TIC) trace and a distortion of the mass spectra obtained, with consequent problems in interpretation, particularly if library searching is employed. 

The reaction was first tested with these substances as ligands but the organic molecule, in the absence of any added metal ion, proved to be the most enantioselective catalyst (library 1 19% ee vs. less than 13% ee for the best metal catalyst). The effects of selective variations of the amino acid nature and of the salicylidene moiety on the diamine structure were investigated for urea and thiourea derivatives via HTS (library 2 48 urea compounds and... 

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