9
In our second experiment we investigated the effect of connecting a dual channel Degasi vacuum degasser (Biotech AB, Sweden) on the inlet lines, between the solvent reservoir and pump. This is a set-up commonly used in many liquid chromatography applications to remove dissolved gases.
Figure 3 demonstrates that peristaltic pumps (even when using degassers) are prone to undissolved gas bubbles producing erroneous results from any connected liquid system without any indication of a problem with the pumping system.
Our fourth and fi nal experiment was designed to investigate the behaviour of a peristaltic pump if solvent in the feeding reservoir fl ask runs dry. This was simulated by temporarily pulling the inlet tube out of the fl ask while the peristaltic pump was running, and fl ow was monitored. Results of this experiment can be seen below in Figure 4.
As would be expected the measured fl ow rate dropped to zero quickly. The spikes of apparent fl ow shown are due to small droplets of residual solvent being pumped through the system.
Figure 2. Comparison of fl ow rate by dual cartridge peristaltic pump with vacuum degasser.
As is clearly shown in Figure 2, the measured fl ow results are now very similar for both channels, a completely different result from experiment 1.
The above demonstrates the value of using a degasser with a peristaltic pump feeding solvent to a liquid chromatograph or fl ow chemistry system. Undoubtedly the degasser has done its job, removing dissolved gases, thereby improving overall performance of the peristaltic pump.
In our third experiment we investigated the effect of an undissolved gas bubble on the fl ow rate of liquids delivered by a peristaltic pump.
To demonstrate the effect of undissolved gas we introduced a small gas bubble into the inlet line of one of the peristaltic pump channels while monitoring the fl ow rate (Figure 3).
Figure 4. Test of peristaltic pump empty reservoir.
Experiments 3 and 4 demonstrate that even a vacuum degasser is powerless when bubbles of undissolved gas travel are present in the solvent feed lines coming from your peristaltic pump.
To address the problem of undissolved gas bubbles in solvent feed to liquid chromatography and fl ow chemistry systems, Testa Analytical has developed the Solvent Line Monitor (Figure 5). The Solvent Line Monitor is a device designed to detect in real time undissolved gas bubbles within a length of translucent tubing.
Figure 3. Test of peristaltic pump air bubbles introduced.
The data above shows that the undissolved gas bubbles passed the vacuum degasser unperturbed. This caused the peristaltic pump to cavitate which is shown as periods of very high fl ow rate on the connected fl owmeter.
The reason for this effect is that an undissolved gas bubble will be compressed in the pump and once exiting the driven part of the tubing, it will expand thus accelerating the liquid creating a high fl ow rate period.
Figure 5: Solvent Line Monitor. (courtesy: Testa Analytical)
The Solvent Line Monitor is a small stand-alone device that provides real-time detection of undissolved gas bubbles in solvent fl ow lines. This unique device is designed to safeguard your laboratory against the above described problem of undissolved gas bubbles in your pumped liquid feed or running out of solvent in critical applications.
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