DPI Implementation A31 Hardware

A schematic of the hardware employed to conduct DPI measurements is shown in Fig. 15. The output of a coherent light source, plane polarised (helium-neon laser, A = 632.8 nm) is focussed (using abeam expander) on to the end face of a sensor chip (as described in Fig. 13). Between the light


Fig. 15. Schematic diagram of a dual polarisation interferometer. DSP Digital signal proces-


Fig. 15. Schematic diagram of a dual polarisation interferometer. DSP Digital signal proces-

source and the waveguide structure there is a ferroelectric liquid crystal half-wave plate (polariser switch) to select the polarisation of light with which to excite the waveguide structure. This is controlled by the DSP, typically switching at 50 Hz. The high tolerance to input-coupling conditions of the waveguide structure is important here as refractive displacement of the beam occurs during the process of activating the polariser switch. However, displacement of the fringe image is observed during the switching process. The (diffracted) light output from the waveguide structure is allowed to fall on a camera (1024 X 1024 pixels). The image containing the interference fringe image is captured by the DSP. A fast Fourier transform algorithm is used to calculate any change in fringe position (phase) since the last measurement was made, and this is reported to an attendant PC. The polariser switch is set to admit the alternate polarisation and the process is repeated. These steps are repeated throughout the course of an experiment.

In addition to the optical train, there is also a fluidic train. The waveguide structure is brought into close proximity with a fluidic manifold that provides two flow-through cells, each of which is 2 ^l in volume. One side of each flow cell comprises the active waveguide surface; it is this surface upon which proteins are immobilised and their behaviour and structure studied. The provision of the second flow cell enables biological control experiments to be conducted simultaneously. The fluidic set-up is such that standard high performance liquid chromatography equipment maybe used (e.g. pumps, de-gassers and injection valves), which are familiar to most experimental practitioners.

As the instrument is measuring refractive indices to a very high accuracy, it is necessary to control the temperature very closely. An autonomous

Fig. 16. DPI. A bench-top instrument for the study of protein structure in real time (Cross et al. 2004)

temperature control unit using two-stage temperature control ensures that the temperature in the region of the waveguides is maintained at ±1 mK.

Once the data have been collected by the PC they are saved to disc and displayed in real time for the user to see. The data may also be resolved using Maxwell's equations to display the data in terms of thickness and refractive index. This information may be further manipulated to provide additional information such as mass per unit area and the volume fraction of the layer that is occupied. A dual-polarisation interferometer is shown in Fig. 16.

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