Principles and Practical Aspects of Preparative Liquid Chromatography

3.2.2 Fixed-loop design
of autosampler
When working with a system where – in alternating mode –
analytical‐scale and preparative scale injection volumes are required, the
size of the sample loop of a flow-through-needle design of autosampler
will contribute significantly to the dwell volume. Systems with large dwell
volumes will not perform well when using analytical flow rates. For these
applications the fixed-loop design of autosampler with two sample loops
of different sizes can eliminate this dilemma. A switching valve is used
to create completely separate analytical and preparative flow paths.
Nevertheless, there are several aspects we need to consider to be able
to work successfully with this fixed-loop design of autosampler with
dual loops.
Figure 3.8 shows the dual-loop concept. In this setup, the upper sample
loop has a larger volume suitable for preparative work at higher flow
rates. In contrast, the lower injection loop has a smaller volume and used
for analytical scouting at lower flow rates. Separate needle seats facilitate
individual filling of the loops with sample. We describe the injection cycle
in more detail in later sections.
Pump
Upper sample loop
Autosampler
Needle seats
Waste
2
3
4
5
Column
1 10
9
6
7
8
Waste
Lower sample loop
Figure 3.8 Schematic of a dual-loop autosampler, showing the flow directed
through the lower, small-volume sample loop for analytical application work.
Because the loop is a part of the capillary system, the dwell volume
increases with increasing loop size. As a consequence, we recommend
keeping the dwell volume as small as possible, balancing it against the flow
rates that will be used for chromatographic separation. As a rule of thumb,
the ratio of total system void volume to applied flow rate should be equal to
or less than 2:1 to obtain reasonable chromatographic performance.
15