Spectrometers nuclear magnetic resonance

The most common nuclei examined by NMR are and 13C, as these are the NMR sensitive nuclei of the most abundant elements in organic materials. H represents over 99% of all hydrogen atoms, while 13C is only just over 1% of all carbon atoms further, H is much more sensitive than 1 C on an equal nuclei basis. Until fairly recently, instruments did not have sufficient sensitivity for routine 13C NMR, and H was the only practical technique. Most of the time it is solutions that are characterized by NMR, although 13C NMR is possible for some solids, but at substantially lower resolution than for solutions. 

In general, the resonant frequencies can be used to determine molecular structures. H resonances are fairly specific for the types of carbon they are attached to, and to a lesser extent to the adjacent carbons. These resonances may be split into multiples, as hydrogen nuclei can couple to other nearby hydrogen nuclei. The magnitude of the splittings, and the multiplicity, can be used to better determine the chemical structure in the vicinity of a given hydrogen. When all of the 

13C resonances can be used to directly determine the skeleton of an organic molecule. The resonance lines are narrow and the chemical shift range (in ppm) is much larger than for H resonances. Furthermore, the shift is dependent on the structure of the molecule for up to three bonds in all directions from the site of interest. Therefore, each shift becomes quite specific, and the structure can be easily assigned, frequently without any ambiguity, even for complex molecules. 

Very commonly, however, the sample of interest is not a pure compound, but is a complex mixture such as a coal liquid. As a result, a specific structure determination for each molecular type is not practical, although it is possible to determine an average chemical structure. Features which may be determined include the hydrogen distribution between saturate, benzylic, olefinic, and aromatic sites. The carbon distribution is usually split into saturate, heterosubstituted saturate, aromatic + olefinic, carboxyl, and carbonyl types. More details are possible, but depend greatly on the nature of the sample, and what information is desired. 

The NMR experiment can be conducted in a temperature range from liquid nitrogen (-209°C) to + 150°C. This gives the experimenter the ability to slow down rapid molecular motions to observable rates or to speed up very slow or viscous motions to measurable rates. 

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FIGURE 13 5 Diagram of a nuclear magnetic resonance spectrometer (Reprinted with permis Sion from S H Pine J B Hendrickson D J Cram and G S Hammond Organic Chemistry 4th ed McGraw Hill New York 1980 p 136)... 

The basic instrumentation used for spectrometric measurements has already been described in Chapter 7 (p. 277). The natures of sources, monochromators, detectors, and sample cells required for molecular absorption techniques are summarized in Table 9.1. The principal difference between instrumentation for atomic emission and molecular absorption spectrometry is in the need for a separate source of radiation for the latter. In the infrared, visible and ultraviolet regions, white sources are used, i.e. the energy or frequency range of the source covers most or all of the relevant portion of the spectrum. In contrast, nuclear magnetic resonance spectrometers employ a narrow waveband radio-frequency transmitter, a tuned detector and no monochromator. 

P.A. Barnard, C. Gerlovich, and R. Moore, The validation of an on-line nuclear magnetic resonance spectrometer for analysis of naphthas and diesels, presented at the ISA Analytical Division Meeting, Calgary, Alberta, Canada, 2003. 

The C-NMR spectrum ofindinavir sulfate, shown in Figure 13, was obtained using a Bruker Instruments model AMX-400 nuclear magnetic resonance spectrometer operating at a frequency of 100.55 MHz as an approximate 4.16 % w/v solution in deuterium oxide. The 67.4 ppm resonance of dioxane was used as an external reference standard. Peak assignments are found in Table 8, and make use of the numbered structural formula given previously [11]. 

Pearson, R.M. and Adams, J.Q. 1990. Automatic use of small nuclear magnetic resonance spectrometers for quality control measurements. In NMR Applications in Biopolymers (J.W. Finley, S.J. Schmidt, and A.E. Serianni, eds.) pp. 499-509. Plenum Press, New York. 

Pulsed nuclear magnetic resonance spectrometer (NMR), >20 MHz, with maximum dead time plus pulse width of 10 p.sec (e.g., Bruker Canada Minispec Oxford Instruments Analytical QP20+)... 

R. Kaptein, J. Chem. Soc. D., 732 (1971). The rules apply to reactions carried out in the strong magnetic field of the nuclear magnetic resonance spectrometer. 

Another method (ASTM D-4808) covers the determination of the hydrogen content of petroleum products, including vacuum residua, using a continuous-wave, low-resolution nuclear magnetic resonance spectrometer. Again, sample solubility is a criterion that will not apply to coal but will apply to coal extracts. More recent work has shown that proton magnetic resonance can be applied to solid samples and has opened a new era in coal analysis by this technique (de la Rosa et al., 1993 Jurkiewicz et al 1993). 

In contrast is the hugely successful American firm of Beckman Instruments, which constructed and marketed pH meters from 1935 and the DU Spectrophotometer from 1941. Papers of a biographical nature based on interviews with Arnold Beckman have been published.104,105 The development of nuclear magnetic resonance spectrometers by the firm of Varian is considered in another paper, with emphasis on the introduction of the Varian A-60, the first commercial instrument intended for the broadly trained chemist as opposed to the custom-built tools for the research specialist.106... 

Drury, D.D., Dale, B.E., Gillies, R.J. (1988). Oxygen transfer properties of a bioreactor for use within a nuclear magnetic resonance spectrometer. Biotechnol. Bioeng. 32, 966-974. 

Auxiliary Instruments. Auxiliary instruments can be used on the fly as special detectors, or analytes can be trapped and taken to other instruments. Instruments that have been used with chromatography include the mass spectrometer (MS), the infrared spectrometer (IR), the nuclear magnetic resonance spectrometer (NMR), the polarograph, the fluorescence spectrophotometer, and the Raman spectrometer, among others. The two most popular ones are MS and IR, and they will be discussed in more detail in Chapter 11. In the beginning of this chapter we noted the utility iof GC/MS and LC/MS. 

Simplified block diagram of a nuclear magnetic resonance spectrometer. 

Although the determination of HA or HB selectivity is relatively straightforward the techniques for isolation of pyridine nucleotides from the reaction mixtures are tedious and time consuming. Two more recent techniques use either proton magnetic resonance or electron impact and field desorption mass spectrometry. The technique of Kaplan and colleagues requires a 220 MHz nuclear magnetic resonance spectrometer interfaced with a Fourier transform system [104], It allows the elimination of extensive purification of the pyridine nucleotide, is able to monitor the precise oxidoreduction site at position 4, can be used with crude extracts, and can be scaled down to /nmole quantities of coenzyme. The method can distinguish between [4-2H]NAD+ (no resonance at 8.95 8) and NAD+ (resonance at 8.95—which is preferred) or between [4A-2H]NADH (resonance at 2.67 8, 75 4B = 3.8 Hz) and [4B-2H]NADH (resonance at 2.77 8, J5 4A = 3.1 Hz). 

A nuclear magnetic resonance spectrometer comprises essentially a magnet and a frequency generator. Around these two elements arc fitted a receiver, a calculator and generally digital recording systems. 

The low cost of many fine infrared spectrophotometers has contributed to their availability to most pesticide residue analysts, as contrasted to mass and nuclear magnetic resonance spectrometers. The sensitivity inherent in infrared measurements for identification purposes is only exceeded by mass spectrometry. This sensitivity has been reahzed by the development of suitable and practicable microtechniques, and some of these have been available for at least 10 years. 

Fig. 16. Block diagram of the crossed coil nuclear magnetic resonance spectrometer due to Bloch,...

Copolymer compositions were determined by a high resolution nuclear magnetic resonance spectrometer (180 HMz). Copolymers of methyl methacrylate and styrene were dissolved in deuterated chloroform for the analysis. Deuterated pyridine was the solvent for the methyl methacrylate - methacrylic acid copolymers. Elemental analysis was also used in copolymer composition analysis to complement the NMR data. 

All phases of analytical development are ideally supported by chemical separation techniques such as HPLC, TLC, GC, SFC, and CE. HPLC continues to be the primary method of analysis throughout the pharmaceutical development process. Although HPLC is limited in its ability to separate more than 15-20 components in a single analysis, and variations in columns and instrumentation manufacturer to manufacturer complicate transfer of methods, HPLC can readily be implemented to meet ICH requirements for method performance. For early-phase methods, HPLC can be coupled dynamically to mass and nuclear magnetic resonance spectrometers to facilitate the identification of unknown impurities. In later phases, HPLC can be implemented in a fully automated format as a high-throughput method for release and stability testing. 

MEMS Example Force-Detected Nuclear Magnetic Resonance Spectrometer... 

George T, Madsen L, Tang W, Chang-Chien A, Leskowitz G, Weitekamp D (2001) MEMS-based force-detected nuclear magnetic resonance spectrometer for in situ planetary exploration. In IEEE, pp 273-278... 

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