Sample rigidity

FRED (www.eyesopen.com/ Exhaustive sampling rigid protein, Chemscore, PLP, Single molecule docking ... 

We have endeavoured to section styrene-swollen block copolymers, but whilst the monomer makes the sample rigid and easy to cut at -100°C, attempts to preserve the specimens in tlie TEM have been hampered by the lack of a cold stage. Subsequent IPN products have significantly enlarged styrene-rich, unstained domains, consistent with 70% styrene uptake. Thus in the case of Solprene 416 cut normal to cylinders, domain diameters increase from about 10 nm to 30 nm (Figure Id) consistent with Yeo s (16) estimates. 

Fig. 1. Effect of A- and -tensor anisotropy on the line shape on the ESR spectrum of a typical nitroxide (a) Immobilized and perfectly oriented nitroxides taken at the indicated orientations of the external magnetic field with respect to the nitroxide axis system. ib) Immobilized nitroxides in a polycrystalline sample (rigid limit spectrum), (c) Fast-motional spectrum in a solvent of very low viscosity (a and g are isotropic hyperfine splitting and isotropic g-factor, respectively).

The situation is more complex for rigid media (solids and glasses) and more complex fluids that is, for most materials. These materials have finite yield strengths, support shears and may be anisotropic. As samples, they usually do not relax to hydrostatic equilibrium during an experiment, even when surrounded by a hydrostatic pressure medium. For these materials, P should be replaced by a stress tensor, <3-j, and the appropriate thermodynamic equations are more complex. 

The take-home lesson is that the vast majority of high-pressure studies are on solids or other rigid media and are not done under hydrostatic conditions. The stresses and stress-related properties may vary throughout the sample. Unless the probes are very local and focus on a small region of the sample, measurements are averages over a range of, often uncharacterized, conditions. 

Other artifacts that have been mentioned arise from the sensitivity of STM to local electronic structure, and the sensitivity of SFM to the rigidity of the sample s surface. Regions of variable conductivity will be convolved with topographic features in STM, and soft surfaces can deform under the pressure of the SFM tip. The latter can be addressed by operating SFM in the attractive mode, at some sacrifice in the lateral resolution. A limitation of both techniques is their inability to distinguish among atomic species, except in a limited number of circumstances with STM microscopy. 

Usually, in AFM the position of the tip is fixed and the sample is raster-scanned. After manual course approach with fine-thread screws, motion of the sample is performed with a piezo translator made of piezo ceramics like e. g. lead zirconate tita-nate (PZT), which can be either a piezo tripod or a single tube scanner. Single tube scanners are more difficult to calibrate, but they can be built more rigid and are thus less sensitive towards vibrational perturbations. 

---