Agilent Technologies 1260 Infinity II User Manual page 18

1
Introduction
Overview of the Hydraulic Path
What is Solvent Compressibility Compensation?
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 II 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.
18 InfinityLab LC Series 1260 Infinity II Binary Pump User Manual