Chemistry Reference
In-Depth Information
3.4.4 S
olvent
m
AnAgement
LC pumps are sometimes categorized according to the way solvents are blended
(Figure 3.1). Low-pressure designs use a single pump to deliver mobile phases
generated by an upstream proportioning valve. High-pressure systems use two or
more pumps to proportion solvents downstream at high pressure. As illustrated in
Figure 3.9, the most significant difference between high-pressure and low-pressure
systems is in the system volume. While low-pressure systems usually exhibit less
compositional ripple in the chromatographic baseline, high-pressure systems usually
have lower volumes. Therefore, if speed or high throughput is desired, high-pressure
systems are usually preferred. However, low-pressure systems can usually accommo-
date a larger number of different mobile phase solvents, and software-conigurable
solvent-select valves are also frequently used on method development systems to
expand capability. Regardless of the type of system used, it is important to remem-
ber that a proper determination of the volume is important for any system used in
method development. Problems related to method transfer can often be traced to
differences in system, dwell, or gradient delay volumes, as no two systems will have
exactly the same volume. The volume difference is particularly significant when
transferring methods between low-pressure and high-pressure systems. In addition,
problems may also result from how the volumes are calculated [19]. Accurate volume
determinations for high-pressure systems can be made using a step gradient method
because the mobile phase is generated post-pump. For accurate low-pressure system
volume determinations, a linear gradient must be used, to take into account the pre-
pump volume from the solvent proportioning valve. It is for this reason that many
method developers now recommend programming in a short isocratic step at the
Multi-Pump High-Pressure System
Smaller system volume = Dwell volume
Pump 1
Mixer
Detector
Injector
Column
Pump 2
Single-Pump Low-Pressure System
Larger system volume = Dwell volume
B
Solvent
Delivery
A
C
Injector
Detector
Column
D
Proportioning
valve
FIgure 3.9
Schematic diagrams of a low-pressure mixing system using a single pump
and a four-position solvent proportioning valve (bottom), and a high-pressure mixing system
using multiple pumps (top).
