Ultraviolet-visible spectroscopy instrumentation

In addition to IR and NMR spectroscopy there are many other instrumental techniques that are useful to the organic chemist. Two of these, ultraviolet-visible spectroscopy and mass spectrometry, are discussed in this chapter. Ultraviolet-visible spectroscopy is presented first. The use of this technique to obtain information about the conjugated part of a molecule is described. Then mass spectrometry is discussed. This technique provides the molecular mass and formula for a compound. In addition, the use of the mass spectrum to provide structural information about the compound under investigation is presented. 

Many analytical laboratories are equipped with an infrared spectrometer, be it an older-style dispersive machine or a more modern Fourier-transform instrument. The results obtained from this particular technique are typically used in conjunction with the information gained from a variety of other analytical methods, such as nuclear magnetic resonance spectroscopy, mass spectrometry, ultraviolet-visible spectroscopy, or chromatography, in order to obtain information abbut a wide range of samples. 

D6t6rminQtion of UlolBCUlsr Structuro is one of the central themes of organic chemistry. For this purpose, chemists today rely almost exclusively on instrumental methods, four of which we discuss in this text. We begin in this chapter with infrared (IR) spectroscopy. Then in Chapters 13 and 14, we introduce nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS), respectively. A brief introduction to ultraviolet-visible spectroscopy is contained in Chapter 20 as part of our discussion of conjugated systems. 

Sometimes more than one technique is required to deduce the structure of a compound. You will find several problems in this book that require you to use two or three techniques at the same time. In this chapter, we will look at three instrumental techniques mass spectrometry, infrared spectroscopy, and ultraviolet/visible spectroscopy. 

Chapter 14 introduced three instrumental techniques that are used to determine the structures of organic compounds mass spectrometry, IR spectroscopy, and ultraviolet/ visible spectroscopy. Now we will look at a fourth technique nuclear magnetic resonance (NMR) spectroscopy. NMR spectroscopy helps to identify (he carbon-hydrogen framework of an organic compound. 

The phenomenon of fluorescence has been synonymous with ultraviolet (UV) and visible spectroscopy rather than near-infrared (near-IR) spectroscopy from the beginning of the subject. This fact is evidenced in definitive texts which also provide useful background information for this volume (see, e.g., Refs. 1-6). Consequently, our understanding of the many molecular phenomena which can be studied with fluorescence techniques, e.g., excimer formation, energy transfer, diffusion, and rotation, is based on measurements made in the UV/visible. Historically, this emphasis was undoubtedly due to the spectral response of the eye and the availability of suitable sources and detectors for the UV/visible in contrast to the lack of equivalent instrumentation for the IR. Nevertheless, there are a few notable exceptions to the prevalence of UV/visible techniques in fluorescence such as the near-IR study of chlorophyll(7) and singlet oxygen,<8) which have been ongoing for some years. 

The application of ultraviolet and visible spectroscopy to the identification and measurement of carbenium ions derived from aromatic and dienic monomer has already been discussed (see Sect. II-G-2). The use of this technique to monitor stable carbenium salts is also well known. We have finally stressed in a preceding section that the fate of certain anions could be followed spectrophotometrically during a cationic polymerisation. The limits of detection allowed by the values of the extinction coefficients of all these species and by the sensitivity of present-day instruments is 10 to 10 M. 

Analysis is an integral part of research, clinical, and industrial laboratory methodology. The determination of the components of a substance or the sample in question can be qualitative, quantitative, or both. Techniques that are available to the analyst for such determinations are abundant. In absorption spectroscopy, the molecular absorption properties of the analyte are measured with laboratory instruments that function as detectors. Those that provide absorbance readings over the ultraviolet-visible (UV-vis) light spectrum are commonly used in high-performance liquid chromatography (HPLC). The above method is sufficiently sensitive for quantitative analysis and it has a broader application than other modes of detection. 

Because it offers an integrated treatment of nuclear magnetic resonance (NMR), infrared (IR), and ultraviolet-visible (UV-VIS) spectroscopy, and mass spectrometry (MS), Chapter 13 is the longest in the text. It is also the chapter that received the most attention in this edition. All of the sections dealing with NMR were extensively rewritten, all of the NMR spectra were newly recorded on a high-field instrument, and all of the text figures were produced directly from the electronic data files. 

Today, a number of different instrumental techniques are used to identify organic compounds. These techniques can be performed quickly on small amounts of a compound and can provide much more information about the compound s structure than simple chemical tests can provide. We have already discussed one such technique ultraviolet/visible (UVA/is) spectroscopy, which provides information about organic compounds with conjugated double bonds. In this chapter, we will look at two more instrumental techniques mass spectrometry and infrared (IR) spectroscopy. Mass spectrometry allows us to determine the molecular mass and the molecular formula of a compound, as well as certain structural features of the compound. Infrared spectroscopy allows us to determine the kinds of functional groups a compound has. In the next chapter, we will look at nuclear magnetic resonance (NMR) spectroscopy, which provides information about the carbon-hydrogen framework of a compound. Of these instrumental techniques, mass spectrometry is the only one that does not involve electromagnetic radiation. Thus, it is called spectrometry, whereas the others are called spectroscopy. 

Traditional instrumental techniques, such as nuclear magnetic NMR, mass spectrometry infrared (IR) spectroscopy, ultraviolet-visible spectrophotometry, and gas and liquid chromatography and size-exclusion chromatography, are used extensively for purity assessment and molecular structure and molecular weight measurements of monomers and polymers [61]. 

Unlike IR spectroscopy where nowadays FT instrumentation is solely used, in Raman spectroscopy both conventional dispersive and FT techniques have their applications, the choice being governed by several factors. The two techniques differ significantly in several performance criteria, and neither one is best for all applications. Contemporary dispersive Raman spectrometers are often equipped with silicon-based charge coupled device (CCD) multichannel detector systems, and laser sources with operating wavelength in the ultraviolet, visible or near-infrared region are employed. In FT Raman spectroscopy, the excitation is provided exclusively by near-infrared lasers (1064 nm or 780 nm). 

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