## 3 Sample Environments
### Light  
The use of visible or UV light to modify samples on-line has found some applications in chemistry and in biological systems. An example is the use of UV light in a solution with a UV photo-destructible anionic sodium 4-hexylphenylazosulfonate (C6 PAS) surfactant in combination with an inert surfactant hexaethylene glycol monododecyl ether (C12E6). Upon exposure to the UV light from a high-pressure Hg lamp, the C6 PAS fell apart and released the content of its micelles to react with the inert surfactant. Hg lamps produce a high light intensity so care should be taken in controlling sample temperature [107]. Similar experiments on reversible phase separation systems using narrow bandwidth light sources have also been reported. Here two different UV wave lengths (350 and 450 nm respectively) were used to initiate phase separation and to reverse it [108]. Photorheological studies, where the rheological material properties are influenced by the exposure to UV light, have also been reported, but it should be noted that the UV irradiation was not performed on-line with the SANS experiments [109]. Visible light produced by a 'white' halogen lamp was used in SANS experiments to gain insight into the gelation process of the conjugated optoelectronic polymer poly(3 hexylithiophene-2,5-diyl) (P3 HT). Here the effect of the exposure to light was found to retard the growth of microstructures [110,111]. Although the authors mention that they monitored the light intensity, no information about differences in growth of microstructures as function of dose rates was provided. The conformation of biological molecules involved in the photosynthesis process as a function of exposure to light is a research subject that is well-suited for study by SANS. Not only because of the advantages that contrast variation via deuteration can bring, but also due to the fact that one does not have to worry about the interactions of photoelectrons, which invariably are created in X-ray-based experiments. Here, the issue is not so much recognizable radiation damage, but instead, the less recognizable effects of the direct interaction of X-ray photoelectrons possibly causing conformational changes [112]. Using live cells that were, both on- and off-line, exposed to 'cool white LED light' with an intensity of \(\approx 20\) umol of photons m\({}^{-2}\) s\({}^{-1}\), the structural organization of membrane systems in cyanobacteria were investigated [113]. An illustrative example of the results is shown in Figure 12. It should also be mentioned that quasi-elastic neutron scattering experiments were performed under similar illumination conditions. Figure 12: SANS data from the membranes of live cyanobacteria cells in both light and dark conditions. WT, CB, CK, and PAL indicate data from different cyanobacteria mutants. The panels marked (**A**,**B**) show data obtained under light and dark conditions, respectively. There are subtle differences apparent between the two illumination conditions. The numbers indicate the different peaks visible in the spectrum. For a full explanation, one is referred to the original manuscript. Figure adapted and provided by the authors [113]. Similar experiments, using SANS and Quasi Elastic Neutron Scattering (QENS), were used to determine the volumetric changes of the light sensitive biological pigment rhodopsin, although the experimental details with respect to the exact illumination conditions are somewhat vague in the manuscript [114]. Microwave radiation can be used for sample heating, but an alternative use is to influence chemical reactions, although the exact way that microwaves influence chemical reactions is not entirely clear. It can change the rate constants, but the reaction pathway may also change. Preliminary neutron scattering experiments with an on-line microwave generator were performed, but these experiments were inconclusive [115] and have not received much follow-up.