Thursday, January 6, 2011
Sample Slice Selection in NMR Spectroscopy
In MRI, field gradients are used routinely for slice selection while imaging a sample. Gradients are also finding applications in high resolution NMR spectroscopy employing fast data collection techniques using parallel data acquisition for multiple slices of the same sample. A good example of this can be found in Carlos' BLOG (and the references therein). The slice selection is accomplished by turning on a linear field gradient across the sample while applying an excitation pulse. While the gradient is on, the frequencies of each of the NMR resonances is spread out according to length of the sample and the strength of the field gradient across the sample. A particular individual slice of the sample can be selected by either modifying the strength of the gradient while using a semi-selective pulse of a given excitation frequency or by modifying the offset frequency of the semi-selective pulse for a constant gradient strength. Both the gradient strength and the excitation profile of the pulse determine the thickness of the slice selected. When the gradient is turned off and the receiver turned on, the FID representing only the spectrum of the selected slice of the sample is collected. An educational example is shown in the figure below. A sample of toluene and H2O/D2O was prepared. As these two liquids are immiscible, the sample is layered with the less dense toluene on top and the more dense water on the bottom. The bottom trace in the figure shows a conventional 1H NMR spectrum. Since the pulse used to collect the spectrum was a hard 90° pulse with a wide excitation profile, one can see both the toluene and the water in the spectrum. For the middle and upper traces, a field gradient of 48 G/cm was turned on while a 100 µs 90° Gaussian excitation pulse was applied. The only difference between the middle and upper traces is the offset frequency used in the excitation pulse. The middle trace represents the spectrum of a slice of the water in the bottom layer of the sample and the upper trace represents the spectrum of a slice of the toluene in the top layer of the sample.