In modern analytical instrument systems, especially in liquid chromatography (HPLC), gas chromatography (GC), ion chromatography (IC), and continuous flow analysis systems, the injection valve is one of the core components for achieving precise sample introduction. It not only determines the repeatability and accuracy of the sample entering the analysis system, but also directly affects the reliability of the analysis results and the degree of automation of the system. As a gateway connecting samples and detectors, it plays an irreplaceable role in fields such as scientific research, environmental monitoring, pharmaceuticals, food safety, and clinical testing.

1、 Basic structure and working principle

The core structure of the injection valve includes a valve body, rotor, stator, handle or drive motor, and multiple flow path interfaces. There are usually two main working states: "Load" and "Inject".

In the "loading" state, the mobile phase (carrier liquid) is directly passed through the chromatographic column, while the sample is injected into the quantitative loop (Sample Loop) through a syringe. The quantitative loop is a stainless steel tube with a known volume, which determines the volume of each injection (such as 10 μ L, 20 μ L, 100 μ L, etc.), ensuring high precision and repeatability of the injection volume.

When switching to the "injection" state, the valve core rotates 60 degrees (six way valve), and the flow direction of the mobile phase is redirected to flow through the quantitative loop, flushing the sample into the chromatographic column and starting the separation process. The entire switching process is fast, reliable, and has a small dead volume, minimizing sample diffusion and peak broadening to ensure good chromatographic resolution.

2、 Types and Automation Development

According to the operation mode, it can be divided into two types: manual injection valve and automatic injection valve (Auto sampler).

The manual injection valve is operated by the operator to switch states by rotating the handle. It has a simple structure, low cost, and easy maintenance, and is widely used in conventional laboratories. Its representative brands, such as the 7725i and 7725T models from Rheodyne (now part of Agilent) in the United States, are renowned for their durability and high sealing performance.

With the increasing demand for analysis throughput and automation, automatic injection valves have become the standard configuration of instruments. The automatic injection system integrates a robotic arm, injection needle, cleaning station, and electronic control module, which can complete the entire process of sample grasping, quantitative ring filling, valve position switching, injection, and cleaning according to preset programs, achieving unmanned and high-throughput analysis. This type of system is particularly suitable for pharmaceutical QC, biopharmaceuticals, and large-scale environmental testing scenarios.

3、 Key performance indicators and selection points:

-Injection accuracy and repeatability: usually expressed as relative standard deviation (RSD), high-quality RSD can be less than 0.5%;

-Pressure resistance: It needs to be matched with the operating pressure of the system. HPLC usually has a pressure resistance of over 6000 psi;

-Dead volume: The smaller the better to reduce peak diffusion;

-Chemical compatibility: The valve body and sealing materials (such as PEEK, ruby, ceramics) need to withstand various solvents, acids, bases, and buffer salts;

-Service life: High quality can withstand hundreds of thousands of cycles of operation.

When selecting, it is necessary to comprehensively consider the analysis method, sample properties, mobile phase type, and system pressure to ensure that it matches the entire analysis system.

4、 Maintenance and common problems:

-Rinse the quantitative ring and flow path with an appropriate solvent after use;

-Prevent particulate matter from entering the system and prevent scratching the sealing surface;

-Regularly replace the rotor sealing ring;

-Avoid excessive force to rotate the handle.