Explain about Low pressure gradient and High Pressure
gradient systems in HPLC?
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Answer Posted / tariq
HPLC gradient elution systems can be categorized according to their configuration as high-pressure gradient elution systems or low-pressure gradient elution systems.
IN A LOW-PRESSURE GRADIENT ELUTION SYSTEM
(1) Driving the Solenoid (Mobile Phase Mixing Valve)
In a low-pressure gradient elution system, a solenoid valve switches in synchronization with the pump intake stroke to mix multiple mobile phases. The small-capacity plunger pump requires the solenoid valve to switch very frequently and is disadvantageous to making accurate concentration settings.
(2) Effects Due to Mobile Phase Compressibility
No mobile phase intake occurs in the hatched area as the pressure increases, this area gets larger and the concentration error increases.
(3) Correcting Errors Due to Solenoid Valve Response Delay
A time delay occurs from the time of signal input until the solenoid valve operation is complete.
(4) Mechanical Errors
Mechanical errors in the plunger drive mechanism (cams, etc.) affect the concentration accuracy
(5) Accuracy
The main difference between the systems is the ability of the high pressure one to generate more accurate and reproducible gradients at flow rates and within narrow ranges of mobile phase variations.
(6) Tests
In a low-pressure gradient elution system you can’t perform the test of high resolutions. It is used to perform simple protein test with low resolutions
(7) Technology
It is an old technology formerly used
(8) Outgassing
Outgassing of dissolved gases can cause more problems in low-pressure mixing so degassing of mobile phase is essential
(9) Maintenance
The low pressure gradient system required more maintenance.
(10) Ternary Or Quaternary Gradients
The low pressure system will let you run ternary gradients or quaternary if you wish, and let you quickly select up to 4 different solvents. However, you a binary high pressure system with a switch valve that will let you use or select up to four solvents.
(11) Dwell Volume
Dwell volume is the physical volume of liquid between the point of mixing and the column. On a high pressure mixing system, the mixing occurs after the pumps, and usually there is only a small dwell volume. On a low pressure mixing system, the mixing occurs before the pump, and the dwell volume includes the volume in the mixing chamber and the low pressure tubing leading to the pump, in addition to the tubing after the pump. This volume is usually much larger than with high pressure mixing, and can be 1 mL or more.
(12) Gradient Range
The optimum gradient range for the low pressure pump is between 5% to 95% organic. The binary pump shows optimum performance from 0% to 100% organic, which is advantageous for compounds eluting at very low or at very high organic percentage.
(13) Mixing Efficiency
Another advantage for the binary pump is the high mixing efficiency, which is necessary when TFA is used as a modifier.
In the end
HP mixing systems will always outperform LP mixing system under all conditions. Hence two Pumps are most suitable for high performance applications of a gradient system requiring high pressures with pulseless and accurate flow.
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