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In HPLC, there are two methods for compound
elution: isocratic elution with a single mobile phase and gradient
elution with a concentration gradient of multiple mobile phases.
Gradient elution involves varying the proportions of several
different mobile phases to adsorb and desorb solutes on the
stationary phase. The delivery pump is responsible for handling the
different mobile phases. There are two types of gradient delivery
systems used in gradient elution: "high-pressure gradient systems"
and "low-pressure gradient systems." The terms "high pressure" and
"low pressure" do not refer to whether the analysis is conducted at
ultra-high pressure like in UHPLC.
A "high-pressure gradient system," also known as a "binary pump
system," uses two pumps for mixing. The mobile phases are mixed
after the pumps, i.e., under "high pressure," hence the term
"high-pressure gradient."
A "low-pressure gradient system" is a mixing system that uses a
single pump (1-pump gradient system). In the case of mixing four
liquids, it is also known as a "quaternary pump system." The mobile
phases are mixed before the pump, i.e., under "atmospheric pressure
(low pressure)," hence the term "low-pressure gradient."
In the early days of HPLC, pumps were expensive, so a method was
adopted to enable gradient elution with a single pump. This involved
placing a "switching valve" before the pump to switch between mobile
phases, which were then mixed and delivered by a single pump.
Although modern systems can handle four different mobile phases and
seem convenient, "low-pressure gradient systems" have significant
drawbacks, mainly involving the "switching valve" and the "mixer"
after the pump.
Switching valves can be either solenoid valves or switching
valves, and the amount of mobile phase flowing through can vary
depending on the speed of the switching operation. Since simply
switching between mobile phases does not result in mixing, a "mixer"
with a larger capacity than the switched mobile phase volume is
required after the pump to achieve uniform mixing. The capacity of
the mixer affects the "delay volume" of the gradient, meaning the
start time of the gradient can change.
In low-pressure gradient systems using a single pump, the
capacity and switching speed of the switching valve and mixer can
vary between gradient devices, making it difficult to obtain the
same chromatogram and inter-laboratory reproducibility unless the
exact same model is used.
On the other hand, high-pressure gradient systems usually use two
pumps to vary the flow rates of two mobile phases. With modern
high-precision pumps, the same gradient can be achieved even with
different manufacturers. The mixer capacity for mixing the mobile
phases is small, and sometimes a T-union is sufficient, leading to
improved gradient precision and inter-laboratory reproducibility.
For this reason, binary pump systems are common in expensive LC-MS
systems.
Inexpensive and seemingly convenient low-pressure gradient
systems can cause various problems, often blamed on the column. As a
column manufacturer, I strongly recommend the
adoption of high-pressure gradient systems.
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