Judith E. G. J. Wijnhoven

Light sources inside photonic crystals

We have measured the optical fluorescence spectra of dye incorporated in high-quality photonic crystals made from colloids. The spectra reveal a stopgap that is due to Bragg reflection with strikingly reduced attenuation compared with plane-wave transmission. The modified attenuation is independent of the position of the sources in the sample and is brought about by diffuse scattering from defects near the surface. In the presence of a photonic bandgap, the diffuse component would disappear. Thus we have found a simple, unambiguous probe for the presence of photonic bandgaps.

Hollow-fibre flow field-flow fractionation of polystyrene sulphonates

Abstract Hollow-fibre flow field-flow fractionation has been used for the characterization and separation of sodium polystyrene sulphonates in aqueous solutions. The elution behaviour of the polymers was found to follow theoretical predictions with a fair accuracy. Diffusion coefficients of standards of different molecular mass could be calculated from elution data. A method was developed to determine the polydispersity of narrow standards by measuring peak widths under different flow regimes. Flow programming has been used to fractionate samples with a wide molecular mass range.

Influence of injected mass and ionic strength on retention of water-soluble polymers and proteins in hollow-fibre flow field-flow fractionation

The retention behaviour of water-soluble polymers and proteins in hollow-fibre flow field-flow fractionation was studied. For a charged polymer such as polystyrene sulphonate (PSS), a low ionic strength of the carrier solution can completely disturb the retention mechanism by overloading, resulting in early-eluting, deformed peaks. Overloading was observed at polymer concentrations far below the so-called semidilute region. Under conditions where overloading effects could be observed, no evidence was found for a threshold value of the amount injected below which overloading did not occur. For proteins the influence of the ionic strength on overloading is less than for PSS, although there is an effect when the pH of the eluent is not at the isoelectric point of the protein. In contrast, non-charged pullulan is not effected by the ionic strength of the eluent and much larger amounts can be injected before significant overloading occurs.