The Nuclear Magnetic Resonance Spectrometer

By agreement, most workers report chemical shifts in delta (S) units, or parts per million (ppm), of the main spectrometer frequency. On this scale, the resonance of the protons in TMS comes at exactly 0.00 ppm (by definition). 

Note An old scale of chemical shift, no longer used, was called the tau (t) scale. On this scale, the resonance position of TMS was defined to be 10.00. To convert lvalues to rvalues, merely subtract them from 10. You will find rvalues in the older literature. 

The NMR spectrometer actually scans from high 8 values to low ones (as will be discussed in Section 3.6). Following is a typical chemical shift scale with the sequence of 8 values which would be found on a typical NMR spectrum chart. 

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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. 

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. 

Note that is unaffected by the transformation. In the nuclear magnetic resonance spectrometer of Fig, 6, the pick-up coil is oriented in such a way that M, is measured. If the lock-in amplifier is set to detect the siginal in-phase with the exciting rf-field ( cos (cot), cf, Eq. (13)), we obtain the "absorption mode signal (cf. Fig. 7) ... 

TTie Combipation of the liquid Chromatoeraph with the Nuclear Magnetic Resonance Spectrometer... 

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. 

The nuclear magnetic resonance spectrum of sodium valproate as shown in Figure 3 was obtained on a Varian Associates T-60 NMR Spectrometer in deuterium oxide containing tetramethylsilane as the internal standard. The spectral peak assignments (2) are presented in Table I. 

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]. 

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... 

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). 

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. 

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