This Information Applies To: Agilent Binary and Quaternary LC Pumps
Issue
When performing a gradient analysis, it’s important to know what component determines composition if it becomes necessary to troubleshoot chromatographic inconsistencies. Those inconsistencies can manifest themselves as peak retention time shifts (late or early peaks) and merged or missing peaks. Troubleshooting strategy depends on the pump type being used. This article will describe how binary and quaternary pumps create gradient runs.
Resolution
Binary Pump
Figure 1 shows the front of a typical binary pump.
Figure 1. Binary Pump
1. Degasser 2. Solvent Selection Valve, 3. Pressure Sensor, 4. Mixer, 5. Damper, 6. Purge Valve, 7. Pump B, 8. Pump A, 9. Inlet Check Valve, 10. Outlet Check Valve
Figure 2 shows the flow schematic within the binary pump.
Figure 2. Binary Pump Schematic
1. Seal Wash Solvent, 2.Seal Wash Pump, 3. Pump A, 4. Solvent Selection Valve, 5. Solvents, 6. Purge Valve, 7. Pump B, 8. To Sampler, 9. Pressure Sensor/Damper, 10. Inlet Check Valve, 11. Outlet Check Valve, 12. Mixing Chamber, 13. Mixer
Binary pumps have two pump heads and are referred to as Pump A or Channel A and Pump B or Channel B. If there is an SSV (solvent selection valve), then the user can choose either A1 or A2 for Pump A and B1 or B2 for Pump B. The user needs to select either A1 or A2 to be mixed with either B1 or B2. If there is no SSV, then there will be only one solvent line for Pump A and one for Pump B. Some pumps may have a built-in degasser, as this example shows, while others may have an external degasser.
By convention, Pump A will flow with aqueous solvent while Pump B will be for organic solvent. So, if a user sets a flow at 1 mL/min and wishes to have a 50:50 composition for the mobile phase, both Pump A and Pump B will be delivering solvent at 0.5 mL/min. If the user wants 100% organic, then Pump B will be flowing at 1.0 mL/min while Pump A is idle and vice versa. In this way, the user can program any composition change during the gradient run with a timetable.
Notice that Pumps A and B determine both the Flow Rate as well as the Composition Accuracy.
The solvents in a binary pump are mixed after the pump heads and is sometimes referred to as high pressure mixing. An advantage is that if there is outgassing, such as mixing water and methanol, it will be suppressed since it is under system pressure.
The simplest way to troubleshoot in case of retention time shifts is to flow 100% off Pump A and measure the flow accuracy in a volumetric flask. Repeat with Pump B. If the Flow Rates are accurate, then the Composition Accuracy will be accurate as well. The SSV doesn’t determine the flow accuracy so there is no need to test both channels off each pump.
Quaternary Pump
Figure 3 shows the front of a typical quaternary pump.
Figure 3. Quaternary Pump
1. Degasser, 2. Multichannel Gradient Valve, 3. Pump Head, 4. Inlet Check Valve, 5. Purge Valve, 6. Outlet Check Valve
Figure 4 shows the flow schematic within the pump.
Figure 4: Quaternary Pump
1. Solvents 2. Degasser, 3. Seal Wash Solvent, 4. Seal Wash Pump, 5. Multichannel Gradient Valve, 6. Pump Head, 7. Purge Valve, 8. Pressure Sensor/Damper, 9. Outlet Check Valve, 10. Inlet Check Valve
Quaternary pumps have a single pump head. They also have a device called Multichannel Gradient Valve (MCGV) where the user can select up to four different solvents. It is the device that will allow the user to select two or more channels to mix using solenoid valves.
So, if a user sets the flow at 1 mL/min and wishes to have a 50:50 composition from channels A and B, the pump head will flow at 1 mL/min while the MCGV will cyclically alternate between channels A solenoid valve 50% of the time and channel B solenoid valve 50% of the time. If the user wants 100% from channel A, then solenoid valve A will be fully open while the others are closed.
Notice then, that for a quaternary pump, the Composition Accuracy is determined by the MCGV while the Flow Rate is determined by the pump head.
When the solenoid valves open, slugs of solvent will move along to the pump head. In theory, there isn't a barrier between the solvent slugs as they move to the pump head, so one can say that mixing begins when the solvents leave the MCGV. For this reason, quaternary pumps are referred to as low pressure mixing as the MCGV operates at atmospheric pressure. True mixing, however, happens in the pump head where the movement of the pump piston will create turbulence and thoroughly mixes the solvents.
To troubleshoot the pump, set a flow through use a single channel and measure the flow accuracy using a volumetric flask. If the Flow Rate is accurate, then the issue may be due to the MCGV not mixing properly. To determine if that is true, then the MCGV Compositional Accuracy needs to be tested. See the article Quick Way to Manually Determine Gradient Composition Accuracy.
Therefore, for Binary Pumps, if the Flow Rate is accurate, then the Composition Accuracy will also be accurate. For Quaternary Pumps, both the Flow Rate and Composition Accuracy must be accurate.
