This Information Applies To: Agilent Single Needle Autosamplers
Background
Sometimes the user may observe liquid flowing from Port four on the autosampler injection valve. During an injection sequence, it is normal to have liquid from port four. This port is used to expel the solvent when the Analytical plunger is reset to the default position or home position.
This article will provide some insights on why there may be liquid from port 4 during the injection. If liquid is observed during any other time that would indicate that there is an issue with the sampler.
Figure 1 identifies the various components and the flow path in an autosampler.
Figure 1. Autosampler flow path components
1. Injection Valve, 2.Analytical Head, 3. Sample loop, 4. Needle, 5. Needle seat capillary
The autosampler has a six-port injection valve labeled 1 through 6. Only four ports can be connected at any given time and therefore the valve will be either in the Mainpass or Bypass position.
In Mainpass, the connected ports are 1>2 and 5>6. Flow comes in from the pump at port 1 and always leaves the valve to the column at port 6.
In Bypass, the connected ports are 1>6 and 5>4.
Note: Port 3 is not ever used and is typically plugged.
The flow enters the autosampler from the pump via port 1 on the injection valve. It then leaves via port 2 to the Analytical Head to the Sample Loop>Needle>Needle Seat and returns to the injection valve via port 5. It leaves via port 6 to the column.
The following figures show the various stages and port connections during an injection sequence.
Pre-injection
Figure 2 shows the pre-injection stage. The blue color indicates the flow path through the sampler. Notice the position of the Analytical plunger (red arrow) and that port 4 is not connected to 5 (red ellipse). This is the normal mode when the Autosampler is sitting idle and is Mainpass.
Prepare to inject
The valve switches to bypass mode and notice that port 4 is connected to 5 (Figure 3). The sample loading pathway (ports 2 to 5 is now at atmospheric pressure). The plunger “homes” (returning to default position) expelling liquid via port 4. The volume of liquid displaced is equivalent to the previous injection volume.
Sample load
The needle is now in the vial and the plunger draws the requested injection volume in the sample loop (Figure 4).
Inject and run
The valve switches once again to Mainpass and the sample is moved along by the mobile phase out of the sample loop and into the column (Figure 5).
So when the analytical plunger goes to "home" position during the prepare to inject step, liquid will be forced out of port 4. This example is one instance where it is okay to see liquid from that port.
There is another case where liquid can be expelled from port 4 and that is during what is called a multidraw injection sequence. The multidraw technique is used when making large sample injection volumes. The autosampler will draw and load injection volumes into the multidraw capillary. After the sample load step, the needle will be placed in the needle seat and the Analytical plunger will “home” the plunger expelling the drawn volume into the multidraw capillary. The sample load process repeats until the required volume is drawn, then the INJECT AND RUN step will execute.
Other than the above two instances, there should never be any liquid seen from port four. If any is observed, it would be an indication of what is called crossport leak and Agilent service support should be consulted.
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