Valve Switching Method
(low pressure mixing)
This method uses a single pump and two switching valves, or
one switching three-way valve to produce the gradient.
The valves alternate between components A and B as the mixture is
pumped. Nonlinearity occurs in the gradient due to the fact that pumps
pulse and have more suction during one part of their cycle. As the pump
suction stroke goes in and out of phase with either component, the pump
will pull too much or too little of a given component into the mixture.
The blips in the gradient curve can be reduced significantly by
including a large mixing volume in the system, but this reduces the
response in step gradients and greatly increases dilution of sample and
other buffers.
Also, the valves cannot switch instantaneously. It takes a certain
amount of time to switch. This limits the minimum time it can dwell on
either component A or B, and thus the minimum % gradient that can be
formed.
| Advantages |
Disadvantages |
| Reduced cost due to one
pump
Can form multi-step gradients
Can perform step gradient |
Limited gradient range
(typically 10-90%
Low pressure mixing can cause out-gassing, cavitations, and air
entrainment
Phasing between pump pulsation and valve switching causes
localized inaccuracies in the gradient |
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