H/D Contrast Variation
Most of the elements in the periodic system have several stable isotopes that exhibit the same chemistry but have nuclei with different neutron content. The interaction of the nuclei with thermal neutrons can vary a lot among these isotopes. So does their scattering contribution. For example, details of alloy or liquid melt structures may be obtained by comparing the scattering intensities from samples containing different isotopes of one or several of the constituents. Of particular significance is the huge difference in scattering between hydrogen (H) and deuterium (D) with even a sign change in the interaction potential strength. Virtually all bio- and soft-matter molecules pertain to the realm of organic chemistry and contain substantial amounts of hydrogen. Labelling parts of larger molecules by H/D exchange or creating contrast in a mixture by combining D-molecules and H-molecules is used to focus the scattering signal origin to specific structures in the sample, the objects of interest can literally be stained and thus made visible.
In soft-matter research, this effect is an essential advantage of neutrons compared to other radiation probes, which enables otherwise unavailable insights into the structure, topology and dynamics.
Imagine a flexible necklace (a linear polymer) of connected molecules (beads) that can have many shapes, like one string of pearls in a bowl of many. To learn more about interactions and motions of this type of molecule, we would like to follow its tracks in space and possibly in time. Imagine now the bowl of identical strings in their closely entangled mixture. Even with a direct view of them it is virtually impossible to recognize the conformation of one selected single necklace. Staining one of the beads or just a few of the strings would of course be very helpful! For a polymer this means using a few H-chains mixed with a majority of D-chains. In that case neutrons “see” the difference and scattering intensity carries information on conformation and motions of the labelled chains.
At a resolution the size of molecular building blocks, the scattering properties of single atoms are averaged over the block, thus H/D content average can be varied or equalized (contrast matching). Molecules, polymers or bio-molecules may change from being “visible” to “invisible” by altering the ratio of normal and heavy water (H2O/D2O) in an aqueous solution or gel.