Problem-solving mass spectrometry

Separation of families by merely increasing the resolution evidently can not be used when the two chemical families have the same molecular formula. This is particularly true for naphthenes and olefins of the formula, C H2 , which also happen to have very similar fragmentation patterns. Resolution of these two molecular types is one of the problems not yet solved by mass spectrometry, despite the efforts of numerous laboratories motivated by the refiner s major interest in being able to make the distinction. Olefins are in fact abundantly present in the products from conversion processes. 

In the structure sections, labelled compounds have often been used to solve a spectroscopic problem involved in microwave (Section 4.04.1.3.2), nitrogen NMR (Section 4.04.1.3.5), IR (Section 4.04.1.3.7(i)) or mass spectrometry (Section 4.04.1.3.8). The synthesis usually involves non-radioactive compounds ( H, N) by classical methods that must be repeated several times in order to obtain good yields. 

An example of how information from fragmentation patterns can be used to solve structural problems is given in Worked Example 12.1. This example is a simple one, but the principles used are broadly applicable for organic structure determination by mass spectrometry. We ll see in the next section and in later chapters that specific functional groups, such as alcohols, ketones, aldehydes, and amines, show specific kinds of mass spectral fragmentations that can be interpreted to provide structural information. 

ESI mass spectra of mixtures are difficult to interpret, because each component produces ions with many different charge states. The most direct and reliable method to solve this problem is to use high-resolution MS and calculate the charge states by measuring the spacing of the isotope peaks. ESI mass spectrometry of (polymeric) mixtures with broad molecular weight distribution benefits from a prior separation that reduces the polydispersity of the analyte. 

Increasing reliance on mass spectrometry as the universal detector for GC has not solved all the problems of additive identification. Isomer identification is impossible (except for REMPI technology), but is hardly an issue in additive analysis. 

Laser desorption methods (such as LD-ITMS) are indicated as cost-saving real-time techniques for the near future. In a single laser shot, the LDI technique coupled with Fourier-transform mass spectrometry (FTMS) can provide detailed chemical information on the polymeric molecular structure, and is a tool for direct determination of additives and contaminants in polymers. This offers new analytical capabilities to solve problems in research, development, engineering, production, technical support, competitor product analysis, and defect analysis. Laser desorption techniques are limited to surface analysis and do not allow quantitation, but exhibit superior analyte selectivity. 

In the case of a total unknown it is a case of the more data, the better. Solving this sort of problem is like doing a jigsaw puzzle. You piece together information from a variety of sources to come up with a feasible structure. You then test that structure with more experiments to ensure you get a consistent answer. As a minimum you should consider COSY, HSQC, HMBC, 1D 13C. Don t forget-NMR is not the only technique so look at mass spectrometry (accurate mass in particular) and IR to help. 

How much detail does a student need to know and how much detail should a textbook then contain This is an almost unsolvable problem because of the diversity of students and their analytical needs. The majority of students will eventually move on into special fields in (bio)chemistry, molecular or systems biology or polymer chemistry. For them mass spectrometry will only be one of the commodities to help them solve their problems, which are defined by their field of activity, not the analytical technique. How much of the basics in mass spectrometry will they need to know Again, this depends on the problem at hand. For many a routine application of commercial instalments and the manufacturers manuals will suffice. However, if the problem is not routine the analytical technique cannot be either. Mass spectrometry is and, most probably, will remain a rather complex technique. To fully exploit its tremendous potential, but, equally important, to avoid its many pitfalls, a deeper understanding of the mechanisms and the technology will be mandatory. This book will, hopefully, help students to lay the basis for this expertise and, once the need arises, allow them to go back to the more specialized literature at a later time. It is in this sense that I hope this book will be a real help to many of them. 

The identification of these 123 compounds (see Table I) was made possible only by the synergistic application of several analytical techniques. For example, the very high concentrations of a few compounds in most of the samples (e.g., no. 6,10,46, 81), precluded identification of many of the minor components during GCMS analysis. This dynamic range problem was solved, at least qualitatively, by HPLC followed by mass spectrometry. 

A recent review [laj emphasizes the complexity of air pollutant samples and describes some of the techniques which have been recently developed for their analysis. Among these are a number of alternate ionization techniques for mass spectrometry. Positive and negative chemical ionization mass spectrometry (CIMSj are analytical tools which can provide significant assistance in solving these problems. Their application in a number of air pollution studies are discussed in this chapter. 

Mass spectrometry is not currently used in routine quality control (QC) but is placed in a research and development (R D) environment where it is used to solve specific problems arising from routine processes or in process development... 

Vargha was very progressive as far as the application of new techniques was concerned. He aided the introduction of the various chromatographic methods and the use of infrared (i.r.) and nuclear magnetic resonance spectroscopy and mass spectrometry in solving the various problems of structure determination. Despite the fact... 

In addition to sensory and physical properties, the content of certain typical components is determined. Problems concerning the natural, botanical, and geographical origins of these products are also solved by using modern chromatographic methods such as enantiomer separations [843-843c], and spectroscopic analytical techniques such as isotope ratio mass spectrometry (IRMS) [844-844b],... 

Characterization of noncovalent bonding of the proteins can also be done using MS. For example MALDI MS has been used in measurement of the molecular mass of the noncovalendy linked tetramer of glucose isomerase, a complex consisting of identical monomers of 43.1 kDa each. MALDI-TOFF peptide mass fingerprinting combined with electrospray tandem mass spectrometry can efficiently solve many complicated peptide protein analysis problems. 

At present, inorganic mass spectrometry is hardly applied at all in bionanotechnology, but it can be expected that increasing use will be made in future to solve problems that seem intractable. 

---