Understanding HPLC Pumps (Part I): Common Misconceptions
Table of contents
Introduction
In recent months, we received many inquiries from users for advice about HPLC pumps. Since our Welch columns are used by users who work with instruments from different manufacturers, this article will not limit to any specific brand when discussing HPLC pump principles and maintenance.
(Note: all images in this article belong to their respective owners. These images are only used for explanation purpose and we do not hold their copyrights)
First Question: How to Understand “The Number of Pump Channels?”
“Is your HPLC pump single, binary, or quaternary?”
To answer this question, it needs to be clarified that the number of channels (single, binary, quaternary, etc.) is the number of independently controlled solvent channels, excluding the wash solvents for the pump or autosampler. It depends only on the independently controlled solvent lines, not on the number of pump heads or modules.
Example 1: Why does this look like two pumps but is called a single pump?
This Shimadzu LC-10AT is a single pump. Although it appears to have two pump heads, both are driven synchronously by a single motor. The main and auxiliary heads work together in a serial configuration to deliver the mobile phase, as shown in the schematic below.
Example 2: One module, two pump heads — how many channels?
This one is a quaternary low-pressure mixing pump.
Example 3: Also one module with two heads —
But this one is a binary high-pressure pump with a built-in damper.
Common Misunderstandings About HPLC Pumps
The misunderstanding of the number of channels is a common misconception that easily arise when users don’t fully understand the differences of pumps between brands, series, or models, along with several other misconceptions. We'll fully discuss them in this chapter.
Myth 1: The number of pump heads equals the number of channels
As seen in Figure 1 above, even though the LC-10AT has two heads, it’s still a single-channel pump, because there is only one solvent channel.
Some single pumps also integrate main and secondary chambers with two plungers, as shown in the figure below.
Myth 2: The number of modules equals the number of channels
Older Shimadzu models like LC-10AT or LC-20AT are commonly used by our clients and most of them feature high-pressure binary mixing. This caused some users to falsely assume binary systems have exactly two modules.
However, both Figure 2 (quaternary) and Figure 3 (binary) pumps are single-module designs.
Agilent’s 1260 Infinity III, for another instance, also offers both binary and quaternary options in one module.
Myth 3: High-pressure systems can withstand higher pressure than low-pressure systems
For example, the Shimadzu LC-20AT can be configured as either a quaternary low-pressure (one LC-20AT pump & four low-pressure proportioning valves) or a binary high-pressure (two LC-20AT pumps & mixer) system — yet the maximum operating pressure is identical.
“High pressure” and “low pressure” refer to whether solvent mixing occurs at high or low pressure, not to the instrument’s pressure limit.
Myth 4: Binary high-pressure systems are the same as UPLC
Some assume that since binary systems mix under high pressure, they are equal to UPLC systems. This is false.
In fact, as described above, high- and low-pressure mixing only describe solvent mixing positions, not system pressure ratings.
| Characteristics | HPLC | UPLC | Advantages |
|---|---|---|---|
| Working Pressure | ≤ 400 bar | 600–1500 bar | Enables smaller particles and faster flow |
| Particle Size | 3–5 µm | 1.7–2 µm | Higher efficiency and sharper peaks |
| Analysis Speed | Slower | 3–5× faster | Time-saving, higher throughput |
| Sensitivity | Lower | Higher | Detects trace analytes |
| Solvent Consumption | Higher | Up to 80% less | Lower cost, greener operation |
| System Volume | Larger | Smaller (in dead volume) | Less peak dispersion, higher resolution |
Table 1. Comparison between HPLC and UPLC
Myth 5: Parallel pumps are always better than serial pumps
The table below self-explains it.
| Comparison Aspect | Dual-Plunger Serial Pump | Dual-Plunger Parallel Pump |
|---|---|---|
| Pressure Pulsation | Moderate | Very low |
| Flow Precision | Moderate (RSD ≈ 0.1–0.5%). Instantaneous flow may fluctuate. | Very high (RSD < 0.1%). Stable instantaneous flow, ideal for gradient elution or trace analysis |
| System Dead Volume | Small. Dead volume mainly comes from liquid cylinder and check valve. Ideal for fast separations (e.g. UPLC) | Larger by 5–15%, may affect fast separations (e.g. peak broadening). Mixer configuration required. |
| Maintenance Cost | Lower; simpler structure, less consumables (valves/seals), lower maintenance frequency | Higher; doubled amount of valves/seals, regular mixer cleaning required |
Table 2. Comparison between parallel and serial pumps
Test Yourself: Can you identify the number of channels in each of the four flow diagrams below?
+ Answers (click to expand):
- Single pump
- Quaternary low-pressure pump
- Binary high-pressure pump
- Quaternary high-pressure pump
Continue to Part II:
Understanding HPLC Pumps: Practical Maintenance
