Principle of Feed Injection of the G4767A SFC Multisampler

In classic flow-through injection (HPLC mode), the sample is drawn into the loop while the valve is in bypass position. To inject the sample, the valve switches from the bypass to the main pass position. The pump flow is redirected and by that carries the sample towards the column. During injection, the mobile phase does not dilute the sample.

In Feed Injection, the sample is drawn into the loop and precompressed, while the valve is in the bypass position. Instead of switching to the main pass position (as in HPLC mode), the valve switches to feed position. The movement of the metering device piston pushes the precompressed sample into the pump flow (mobile phase stream). In this process, the sample and the mobile phase are mixed in the valve unit. By that, the mobile phase automatically dilutes the sample.

As the pump flow does not carry the sample, but it is fed into it by the metering device piston, there are more parameters that can be optimized in Feed Injection compared to flow-through injection. The feed speed is the speed with which the sample is pushed into the pump flow during Feed Injection. This parameter affects the degree of sample dilution by the mobile phase (pump flow). A high feed speed means a low degree of sample dilution. As the sample is less diluted, the solvent in which the sample is initially existent, is injected alongside the sample onto the column. Depending on the properties of this solvent, poor peak shapes may result due to solvent effects (e.g. overlapping peaks). A lower feed speed results in a higher degree of sample dilution, as proportionally more mobile phase is mixed with the same amount of sample during injection. When the mobile phase dilutes the sample more, the solvent effect is weaker. A weaker solvent effect improves peak shapes and results in narrower and more defined peaks.

Feed Injection parameters also affect recovery and carry over. To achieve the goal of good recovery and low carry over, the sample must be completely flushed out to the column. Low recovery leads to high carry over, which may result in carry over peaks after the sample peak.

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