Extra Column Dispersion - Chromatographic Apparatus for Trace Analysis > Page 60

The apparatus consists essentially of two six-port valves and a small low volume LC pump. The first valve controls sample access to a pre-column, 5 cm long, 1 mm I.D. packed with reverse phase having a large particle diameter (100-120 mm) to permit passage of any small diameter particulate matter (e.g. blood corpuscles) that might be present in the sample. The sample pump is supplied by two-way tap from either of two reservoirs containing water or normal saline. The sample pump displaces the contents of the open loop through the packed loop and allows it to b washed with either water or normal saline.

A syringe is used to fill the open loop which may be a water sample, blood, tissue dispersion etc.. If the sample is not of biological origin the samples displaced it into the packed tube by water from the reservoir. If the sample is blood, or, a cell suspension the sample is displace by saline to ensure that no protein is precipitated in the packed tube. In general, a volume equivalent to twice that of the open loop is pumped through the packed tube to waste. If saline is used to wash the packed tube free of particulate matter then the tube is washed free of saline with water. The water wash is also important as it not only washes the packed tube free of saline, it also washes lines from the solvent selection valve through the sampling system free of saline. After the water wash, the packed loop now contains the sample for separation, focused as a sharp band at the front of the packing in contact with pure water. The second valve is now rotated and the packed loop placed in line with the solvent supplied from the gradient system and the sample eluted in the usual manner.