Principles and Practical Aspects of Preparative Liquid Chromatography

Degassing unit
Damper
Damper
Mixer
Channel A
Channel B
Pump
Autosampler
Figure 3.2 Schematic of a solvent delivery system for
high-pressure mixing of gradients, showing the two solvent
channels for generation of binary elution gradients.
3.2 Sample introduction
3.2.1 Flow-through-needle
design of autosampler
In preparative LC the number of samples and the amount of sample to
be injected varies significantly. As a consequence we need to consider
manual injection techniques as well as automatic liquid samplers – known
as autosamplers – and injection pumps as possible techniques for transfer
of the sample to the flow path. Autosamplers are available with two
different design approaches; flow-through-needle and fixed-loop.
The flow-through-needle design easily handles a variety of injection
volumes and there is usually no loss of sample when filling the sample
loop to the maximum. However, a disadvantage of this design is the larger
dwell volume resulting from the size of the sample loop and metering
device. This is a particular drawback when switching between analytical
and preparative injection modes. Nevertheless, flow-through-needle
technology facilitates easier workflows and eliminates sample losses
caused by improper injection steps. Figure 3.3 shows how the eluent
flows through the needle and onto the column.
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