The Development of Spectroscopy Continues

The German scientists R. W. Bunsen, born in 1811 and G. R. Kirchhoff, born in 1824, are regarded as the inventors of the spectroscope and the founders of spectral analysis. The year 1859 is generally mentioned as the starting year. The methods were, however, developed gradually and with contributions from many scientists and technicians. This has been mentioned earlier and will be further explained in the following sections. 

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At the research level, spectroscopy continues to flourish and is continually developing with occasional quantum leaps. For example, such a leap resulted from the development of lasers. Not all leaps provide suitable material for inclusion in an undergraduate text such as this. Flowever, even in the relatively short period of seven years since the third edition, there have been either new developments or consolidation of rather less recent ones, which are not only of the greatest importance but which can (1 hope ) be communicated at this level. 

An alternative approach is provided by NMR spectroscopy. Separate NMR signals can be in principle obtained for stable or short-lived diastereomeric derivatives of the enantiomeric mixtures, the intensities of which are correlated with the enantiomeric composition and their relative stereochemistry to the absolute configuration. For this reason, great effort has been continually devoted to the development of new chiral auxiliaries for NMR spectroscopy. The majority of these are dedicated to the chiral assay of molecules having polar functional groups. 

With the development of Fourier transform (FT) techniques in NMR spectroscopy (early 1970s), the first major advance in the NMR technology was made. A significant increase in the sensitivity, as compared to the conventional continuous wave method, resulted in the NMR spectroscopy of rare nuclei, particularly 13C NMR, which is essential for polymer studies. The 13C NMR analysis of swollen lightly crosslinked polymers was made possible. The relaxation measurements, based on the different pulse sequences, provided additional information on the network dynamics. 

As techniques for chemical analysis are used in continually smaller domains, experimental challenges for inherently insensitive methods such as NMR spectroscopy become increasingly severe. Among the various schemes to boost the intrinsic sensitivity of an NMR experiment, the development of small-volume RF probes has experienced a renaissance during the past decade. Commercial NMR probes now allow analyses of nanomole quantities in microliter volumes from natural product extracts and combinatorial chemical syntheses. Figure 7.3.1.9 illustrates the range of volumes that can be examined by NMR probes and accessories such as microsample tubes and inserts. With recently reported advances in sample preconcentration for microcoil NMR analysis [51], dilute microliter-volume samples can now be concentrated into nanoliter-volume... 

Atomic absorption remained the technique of choice until relatively recently. However, with the introduction of plasma sources, atomic emission, in the form of inductively coupled plasma spectroscopy, has made a comeback. This development is now receiving historical attention, and was the subject of a symposium held in 1999. Papers discussed atomic emission analysis prior to 1950,206 the fact that emission techniques developed continuously, even in the period when absorption methods were dominant,207 and the development of the plasma sources on which the new techniques depend.208 Also discussed was the powerful hyphenated technique of ICP-MS,209 and the history of one of the leading manufacturers of atomic emission instruments.210... 

Spectroscopy covers a very wide area which has been widened further since the mid-1960s by the development of lasers and such techniques as photoelectron spectroscopy and other closely related spectroscopies. The importance of spectroscopy in the physical and chemical processes going on in planets, stars, comets and the interstellar medium has continued to grow as a result of the use of satellites and the building of radiotelescopes for the microwave and millimetre wave regions. 

Besides the continuous improvements of FTIR-VCD instruments described above, some exciting new developments related to VCD measurements have been reported in recent years. These include the developments of matrix isolation FTIR-VCD instruments and of laser based real time VCD spectrometers. These new developments are associated with brand new applications and research directions, such as combining the matrix isolation technique with VCD spectroscopy to probe conformationally flexible chiral molecules and H-bonded chiral molecular complexes, and using femtosecond laser VCD instruments to record time resolved VCD spectra for monitoring fast chemical reactions or folding and unfolding events of peptides and proteins in solution. These will be discussed in more detail in Sects. 4.5 and 4.6. 

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