Understanding Pressures within an HPLC System

Answer

This Information Applies To: Agilent Analytical Scale LC Systems 

Issue

To troubleshoot high backpressure within an HPLC system, it’s necessary to understand the expected pressure throughout an operating system. The total pressure for an LC is measured by a pressure sensor in the pump. In the hypothetical LC schematic in Figure 1, pressure sensors are placed at various points to give an understanding of what each hypothetical sensor would read.

Background

Assume that the total backpressure on the system is 350 bar. 

Figure 1. Locations in a typical LC flow path and the expected pressure readings with a total backpressure of 350 bar
 For the description and expected pressure at the numbered locations, see Table 1.

Some useful considerations from the pressure readings.

1. In an ideal HPLC, the bulk of the backpressure should be from the Column/Guard column.
2. Capillary and other components will add pressure, but this pressure increase should be minimal.
3. The sensor at position 4 indicates a useful feature in troubleshooting a high pressure clog in the Sampler. If there was an obstruction somewhere in the needle/sample loop or needle seat contributing 150-bar additional backpressure, the pressure will be 177 instead of 327, when switching from mainpass [normal position] to bypass.  
4. You can readily tell what each capillary or component is contributing to the overall backpressure. For example, the Guard column is contributing 15 bar.
5. A good troubleshooting tip is to always know the expected backpressure for each application so you can readily tell when there is high backpressure. Having a column use logbook recording the daily operating backpressure is a good practice.

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