CW instrument

Finally, it has been noted that some people think that they need TMS in their samples to enable them to lock. This is not the case On modem spectrometers, TMS is used for referencing only. There was a time when it was used for locking CW instruments (in an early form of spectrum averaging) but it is not used in that way for FT instruments now. 

In practice, using a CW instrument, the absorption of energy may be detected by subjecting the sample to radiation of varying frequency at a fixed value of the applied field or vice versa until the conditions required by equation (9.24) are met. At this point, the system is said to be in resonance, both upward and downward transitions occur, and a net absorption of energy is observed because of the small excess of nuclei in the lower level. 

Pulsed FT-NMR has facilitated the study of nuclei other than H where the sensitivity obtainable from a CW instrument is totally inadequate. In particular, 13C NMR, the sensitivity of which is nearly 10-4 less than that of the proton (Table 9.9), is now a well-established technique that yields information on the skeletal structure of complex molecules. The pulsed technique also enables proton spectra to be obtained from samples as small as a few micrograms. 

The biggest drawback of the CW instrument is that it takes several minutes to scan the entire spectrum (0-14 ppm) and is inherently of low sensitivity with limited sample or with less sensitive nuclei. Repeated scanning does little to improve the signal/noise ratio. Typically this type of instrument is used with neat or very concentrated samples. Other nuclei, such as 13C (approximately 6000 times less sensitive than 1H), are not usually analyzed by CW. 

The first NMR spectrometers developed were continuous-wave (CW) instruments and they are still in use for proton and fluorine NMR spectroscopy. In these instruments the irradiation frequency is fixed and the magnetic field strength of the magnet is slowly and continuously changed. When the correct magnetic field for the fixed frequency is reached for a proton in a particular chemical environment, then an absorption peak appears in the recorder of the instrument. The area of this absorption peak is a function of the number of protons in the sample that are in this particular chemical environment. 

NMR spectrometers of the late 1960s did not permit the detection of higher spin orders for sensitivity reasons, so no new name was coined for them the term used today is "higher multiplet effects". More importantly, with the cw instruments ubiquitous at that time a separation of different spin orders n was principally impossible. The advent of pulsed and Fourier transform spectrometers reduced that to a trivial task Because for a weakly coupled spin system the amplitude of the detectable signal is proportional to sim cos i , one simply has to acquire spectra with different flip angles i9 and form suitable linear combinations (e.g. one- and two-spin orders are separated by adding and subtracting two spectra acquired with = 45° and = 135°). ... 

We can now plot this peak as a chemical shift on a standard NMR spectrum chart (Fig. 3.16c).The peak for acetone appears at about 2.1 ppm. We have converted the time-domain signal to a frequency-domain signal, which is the standard format for a spectrum obtained by a CW instrument. 

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