Page 340 - Tandem Techniques

Page 324

eluent, and the mixture of column eluent and silver dispersion, must be kept as short as possible to
minimize peak dispersion.

Figure 8.26
The Liquid Chromatography/Raman
(LC/RA) Tandem Instrument

The preparation of the silver dispersion must be carried out extremely carefully by the citrate reduction
process [25] and is heated, prior to mixing with the column eluent, by means of a coiled heat exchanger.
The temperature is adjusted so that the mixture entering the Raman cell is ca 50-70°C. This was found
essential to achieve the necessary sensitivity. The connection between the mixing T and the Raman cell
consists of 12 cm of knotted tube, which helps ensure that the silver dispersion and the column eluent
are completely mixed before entering the flow cell. The flow cell consists of a glass capillary, 100 mm
long and 1 mm I.D. fitted into an aluminum holder.

The laser is the Coherent Innova 100 Kr and the 476 nm line is used at 40 mW power. The excitation
+
beam is set at 45° to the normal of the surface and the Raman scattered light is collected in a back-
scattering geometry. The scattered light is focused onto a Spex Triplemate spectrometer, where it is
dispersed onto a diode array, which is operated at -40°C to achieve maximum sensitivity. The solutes
used for testing the system were adenine, guanine, xanthine and hypoxanthine. An example of