Comments
To flush the needle port: Use about 1 mL of mobile phase. Use a Needle Port Cleaner (Rheodyne Part Number 7125-054) and only when the handle is in INJECT. This puts fluid in the entire length of the needle port — guide, tube, and needle seal. The cleaner does this because it seals against the front of the guide. A fully inserted needle flushes none of these parts. If you have no cleaner, use a luer tip syringe. See the illustration in Fig. 18.

In routine operation, remove the syringe after turning to INJECT, but keep the handle in this position so the loop is continuously flushed with mobile phase. To keep the port filled with fluid, flush every ten or twenty injections.

Air in the loop prior to loading is usually pushed out through vent tube #6 when using the complete-filling method. But, when using partial-filling, not enough sample enters to displace the air; Symptom #10 is therefore more likely to occur when using this method.

The decrease in pressure is instantaneous because the air compresses immediately. It takes the pump awhile to replace the loss of volume and to return to normal pressure. Large loops can produce a large loss of pressure which takes a longer time to return to normal.

Symptom #10
System pressure decreases instantly when the injector is switched from LOAD to INJECT, and then slowly returns to a normal level:

• If there is only a small and momentary drop in system pressure, see Cause A.
• If there is a large and momentary drop in system pressure, see Cause B.

Cause A
Air is present in the needle port, specifically the needle seal (see Fig. 32). When the syringe is inserted, the needle pushes a small air bubble into the loop. Upon injection, the system pressure collapses the bubble, causing pressure to drop momentarily.

Solution
Remove the air from the needle port by flushing (see comments). Keep the needle port filled with fluid by occasional flushing.

Fig. 32. Air (yellow) present in the needle port is pushed by the syringe during loading into the sample loop.

Cause B
The vent tube(s) is not level with the needle port. This allows air to enter the loop via siphoning, when in the LOAD position (see Fig. 33). Upon injection, the system pressure collapses the bubble, causing pressure to drop momentarily.

Solution
Adjust the vent tube(s) so the outlet is at the same horizontal level as the needle port (see Symptom #3).

Fig. 33. Air (yellow) siphons into loop via vent tube #6 when it is higher than the needle port.

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