Principles and Practical Aspects of Preparative Liquid Chromatography
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Pump<br />
Sampling loop<br />
Metering device<br />
Waste<br />
Needle seat<br />
4<br />
5<br />
6<br />
1<br />
3<br />
2<br />
Autosampler<br />
Column<br />
Figure 3.4 When the autosampler is in bypass mode, the eluent flow<br />
from the pump is diverted directly to the column, leaving the metering<br />
device <strong>and</strong> injection needle free to begin sampling.<br />
Switched out <strong>of</strong> the flow path, the injection needle is raised, a sample<br />
vessel such as a vial placed underneath, <strong>and</strong> the needle lowered into<br />
the sample. Withdrawing the plunger <strong>of</strong> the metering device pulls sample<br />
through the needle into the sampling loop, see Figure 3.5.<br />
Pump<br />
Sampling loop<br />
Sample vial<br />
Metering device<br />
Waste<br />
Needle seat<br />
4<br />
5<br />
6<br />
1<br />
3<br />
2<br />
Autosampler<br />
Column<br />
Figure 3.5 Drawing sample through the needle <strong>and</strong> into the sampling loop.<br />
When the required amount <strong>of</strong> sample has been transferred to the sampling<br />
loop, the needle is raised out the sample vessel, the vessel moved away,<br />
<strong>and</strong> the needle lowered onto the needle seat. The switching valve now<br />
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