CONTINUOUS WAVE CW NMR SPECTROSCOPY

An NMR spectrum is effectively a graph of the intensity of absorption of Rf radiation (y-axis) against the frequency of the Rf radiation (x-axis). Since frequency and magnetic field strength are linked by the Larmor equation, the x-axis could also be calibrated in units of magnetic field strength. In a CW NMR spectrometer, the x-axis of the output device (usually a pen plotter) is coupled to the frequency sweep so that the response of the sample is displayed as the frequency of the Rf transmitter varies. 

NMR spectroscopy is a quantitative technique and NMR spectra are usually recorded with an integral which indicates the relative areas of the absorption peaks in the spectrum. The area of a peak is proportional to the number of protons which give rise to the signal. In most NMR spectrometers, the integral is represented as a horizontal line plotted over the spectrum. Whenever a peak is encountered, the vertical displacement of the integral line is proportional to the area of the peak. 

NMR spectroscopy is an excellent tool for the analysis of mixtures - if a sample contains more than one compound then the areas of the signals belonging to each species in the NMR spectrum will reflect the relative concentrations of the species in the mixture. 

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In the conventional NMR experiment, a radio-frequency field is applied continuously to a sample in a magnetic field. The radio-frequency power must be kept low to avoid saturation. An NMR spectrum is obtained by sweeping the rf field through the range of Larmor frequencies of the observed nucleus. The nuclear induction current (Section 1.8.1) is amplified and recorded as a function of frequency. This method, which yields the frequency domain spectrum f(ai), is known as the steady-state absorption or continuous wave (CW) NMR spectroscopy [1-3]. 

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