Principle of Feed Injection of the G7137B Hybrid Multisampler, and the G7167C Hybrid Multisampler

The introduction of the specially designed 3-position/6-port Injection Valve in Hybrid Multisamplers enables the implementation of two injection modes in one sampler module. The availability of classical flow-through injection and new Feed Injection in one LC System extends the capability of handling the broader variety of the samples with different, sometimes challenging, properties.

In classic flow-through injection, the sample is drawn into the sample loop while the injection valve is in bypass position. To inject the sample, the injection valve switches from the bypass to the main pass position. The pump flow is redirected and by that starts carrying the sample from the sample loop towards the column. During injection, the mobile phase does not dilute the sample, pushing it forward as a solvent plug. The flow-through injection mode introduces some extra delay volume to the flow path between pump and column mainly in a size equal to sample loop and metering device, extending the analysis time. This effect can be reduced using Automatic Delay Volume Reduction method parameters, switching the injection valve to bypass once the sample is completely moved out of the sample loop.

In Feed Injection, the sample is drawn into the sample loop, while the injection valve is in the bypass position. Instead of switching to the main pass position (as in flow-through injection mode), the injection valve switches to feed position. The movement of the metering device piston pushes the sample into the pump flow (mobile phase stream). In this process, the sample and the mobile phase are mixed in the junction point created by the injection valve, and the mobile phase dilutes the sample. The introduction of the sample in Feed Injection mode allows to avoid extra delay volume appearance in the flow path and analysis time extension.

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. 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 carryover. To achieve the goal of good recovery and low carryover, the sample must be completely flushed out to the column. Low recovery leads to high carryover, which may result in carryover peaks after the sample peak.

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