3 - Detection methods

Summary

The CW method

Frequency and time domains

We have seen in Chapter 1 that the essential features of a resonance phenomenon may be investigated by looking at the system's response to a small sinusoidal disturbance of varying frequency. This is called the continuous wave (CW) method and is the technique traditionally used in most branches of spectroscopy. Alternatively, as we also saw in Chapter 1, we may look at the time response to a transient excitation, this being called the pulse method in NMR.

Results of the two methods have been shown to be equivalent, one response being the Fourier transform of the other. However, if we look at the history of the subject we find that studying the frequency response (CW) was the method used almost exclusively for the first ten years of NMR, i.e. 1946–56. In Andrew's book (Andrew, 1955) for instance 27 pages are devoted to CW experimental methods while pulse methods are treated in a page and a half. Erwin Hahn is usually thought of as the father of pulsed NMR due to his important paper in 1950. Other important contributors include Carr and Purcell (1954) and Torrey (1952).

Since Andrew's book was written, pulsed NMR has to a large extent eclipsed CW NMR. There are two main reasons for this. Firstly, it is much easier to measure the relaxation times T₁ and T₂ using pulse methods. In fact the long T₂ values found in some liquids can only be measured by the pulse technique because magnet inhomogeneity masks the weaker effects of spin–spin coupling. This point will be taken up again in the following chapter.