The solvent from the bottle in the solvent cabinet enters the pump through an active inlet valve. Each side of the binary pump comprises two substantially identical pump units. Both pump units comprise a ball-screw drive and a pump head with two sapphire pistons for reciprocating movement.
A servo-controlled variable reluctance motor drives the two ball-screw drives in opposite directions. The gears for the ball-screw drives have different circumferences (ratio 2:1) allowing the first piston to move at double the speed of the second piston. The solvent enters the pump heads close to the bottom limit and leaves it at its top. The outer diameter of the piston is smaller than the inner diameter of the pump-head chamber allowing the solvent to fill the gap in between. The first piston has a stroke volume in the range of 20 µL to 100 µL depending on the flow rate. The microprocessor controls all flow rates in a range of 1 µL/min to 5 mL/min. The inlet of the first pumping unit is connected to the active inlet valve which is processor-controlled opened or closed allowing solvent to be drawn into the first pump unit.
The outlet of the first pump chamber is connected by a 500 µL absorber capillary to the second pump chamber. The outlets of the second chambers of both pump channels joined via a small mixing chamber. A coiled restriction capillary connects the mixing chamber via a pressure pulse damper, a mixer and a pressure sensor to the purge valve assembly. The outlet of the purge valve assembly is then connected to the attached chromatographic system.
When turned on, the pump runs through an initialization procedure to determine the upper dead center of the first piston of both pump channels. The first piston moves slowly upwards to the mechanical stop of the pump head and from there it moves back a predetermined path length. The controller stores this piston position in memory. After this initialization the pump starts operation with the set parameters for the two pump channels.
The active inlet valve is opened and the down moving piston draws solvent into the first pump head. At the same time the second piston is moving upwards delivering into the system. After a controller defined stroke length (depending on the flow rate) the drive motors are stopped and the active inlet valve is closed. The motor direction is reversed and moves the first piston up until it reaches the stored upper limit and at the same time moving the second piston downwards.
Then the sequence starts again moving the pistons up and down between the two limits. During the delivery stroke of the first piston the solvent in the pump head is pressed through the outlet valve into the second pumping unit. The second piston draws in half of the volume displaced by the first piston and the remaining half volume is directly delivered into the system. During the drawing stroke of the first piston, the second piston delivers the drawn volume into the system.
For pump specifications, see Performance specifications of the 1260 Infinity III Binary Pump (G7112B).
The flow path of the pump consists of pump chambers, sapphire pistons, polymer seals, stainless steel tubing of different dimension, pressure sensor, and so forth. All of these parts deform when pressurized. The sum of this deformation is called pump elasticity.
Let us look at a practical example: Piston 1 draws solvent at ambient pressure. The movement direction is reversed and the piston 1 now compresses the solvent until the operating pressure of the HPLC system is reached. The outlet valve opens, and solvent is pumped by piston 1 into pump chamber 2. Due to two factors, the solvent volume that is delivered into the system at high pressure is smaller than it is supposed to be:
The solvent is compressible
The pump has a certain elasticity which causes its internal volume to increase with pressure.
In order to compensate for these two influences, their contributions must be known. An elasticity calibration allows separating pump properties from solvent properties and therefore allows transferring solvent properties, which have been obtained from one pump to another pump with different elasticity.
Elasticity calibration is done with a solvent, which properties (compressibility, thermal expansion) are well-known and documented: pure water. When pumping water and using its property data for controlling the pump, any deviations from the theoretical pressure profile during solvent recompression are caused by the elasticity of the pump.
The Pump Elasticity Calibration calculates correction factors to compensate for the individual elasticity of the pump that is being calibrated. The elasticity is different for every pump and may change with the replacement of parts in the flow path, e.g. pump seals.
All binary pumps are elasticity calibrated at the factory and require recalibration only after preventive maintenance or major repairs to the flow path. Replacement of capillaries or PTFE frits are not considered as a major repair.
CAUTION
Incorrect pump elasticity calibration.
Solvent compressibility calibrations acquired with a miscalibrated pump will work, but they are not transferable to other pumps. A correct pump elasticity calibration is an essential prerequisite for successful solvent compressibility calibrations.
Although the compressibility of liquids is orders of magnitude lower than the compressibility of gases, without correction a noticeable volume error would be seen if typical chromatographic solvents are compressed to operating pressures as high as 600 bar. In addition, the compressibility depends on pressure, temperature and the amount of dissolved gas. In order to minimize the influence of the latter, the use of a vacuum degasser is mandatory for a high flow and composition precision. Unfortunately, the influence of the temperature on compressibility is non-linear and cannot be calculated.
The Agilent 1260 Infinity III Binary Pump features a multi point compressibility calibration. The compressibility of a solvent is determined at different pressures from 0 – 600 bar and stored in an XML file. This file can be distributed to other pumps because the solvent compressibility is independent from the pump.
The binary pump and ChemStation come with predetermined solvent compressibility data for the most common HPLC solvents like water, acetonitrile, methanol, etc. Users can calibrate their own solvent mixtures with the help of an easy to use calibration procedure in the Agilent Lab Advisor software.
Let us use the practical example from the last section once again to understand how compressibility compensation works:
Piston 1 draws solvent at ambient pressure. The movement direction is reversed and piston 1 now compresses the solvent until the operating pressure of the HPLC system is reached. The outlet valve opens, and solvent is pumped by piston 1 into pump chamber 2.
Without any compensation, the delivered volume at operating pressure would be too low. In addition, it would take a noticeable amount of time to recompress the solvent to operating pressure. During this time frame, no solvent would be delivered into the system and as a result a high pressure fluctuation (known as pressure ripple) would be observed.
When both solvent compressibility at the current operating pressure and pump elasticity are known, the pump can automatically correct for the missing volume by drawing the appropriate larger solvent volume at ambient pressure and speed up the piston during the recompression phase in the first pump chamber. As a result, the pump delivers the accurate volume with any (calibrated) solvent at any pressure at a greatly reduced pressure ripple. For applications that require lowest transition volume of the pump, damper and mixer can be bypassed.
For compatibility with older methods from G1312A Binary Pumps, the previous one-point compressibility compensation is available, too. However, since the compressibility is a non-linear function, one single compressibility value per solvent will only give good results at one particular pressure.
The smaller the solvent volume in the pump chamber is, the faster it can be recompressed to operating pressure. The binary pump allows to manually or automatically adjust the pump stroke volume of the first piston in the range of 20 – 100 µL. Due to the compression of the solvent volume in the first pump chamber, each piston stroke of the pump will generate a small pressure pulsation, influencing the flow ripple of the pump. The amplitude of the pressure pulsation mainly depends on the stroke volume and the compressibility compensation for the solvent in use. Small stroke volumes generate less pressure pulsation than larger stroke volumes at the same flow rate. In addition, the frequency of the pressure pulsation will be higher. This will decrease the influence of flow pulsations on retention times.
In gradient mode, a smaller stroke volume results in less flow ripple and reduces the composition ripple.
The binary pump uses a processor-controlled ball screw system for driving its pistons. The normal stroke volume is optimized for the selected flow rate. Small flow rates use a small stroke volume while higher flow rates use a higher stroke volume.
The stroke volume for the pump is by default set to AUTO mode. This means that the stroke is optimized for the flow rate in use. A change to larger stroke volumes is possible but not recommended.
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