Tips On Using Injectors
The Model 7125, 7725, and other front-loading models can use both the partial-filling method and the complete-filling method of loading the loop. These are discussed in Appendix F. Following is a summary of procedures.

1) Steps to inject a sample:
a. With the handle in LOAD, insert the syringe into the needle port until it stops.
b. Dispense the sample; turn the handle rapidly to INJECT.
c. Remove the syringe.
d. Wait until you are ready to run the next sample, then return to LOAD.

2) Flush in the INJECT position, not in LOAD. After turning to INJECT, it is OK to remove the syringe, but keep the handle in this position so the loop is continually flushed with mobile phase.

Manual flushing of the needle port after every injection, to prevent cross contamination, is rarely necessary. A patented direct-connection port design connects the tip of the syringe needle directly to the end of the sample loop; there is no connecting passage that traps sample, which can enter the loop when the next sample is loaded.

It is good practice to flush the needle port after every ten or twenty injections. This keeps it filled with liquid, which baths the syringe needle and dilutes any sample that contaminates this region during movement of the syringe into or out of the port. It also keeps the needle port and vent tube #5 filled with solvent, preventing air from inadvertently entering the loop. To flush, use from 0.1 to 1 mL of mobile phase. Do it while still in the INJECT position, so flow goes directly out vent tube #5 and bypasses the loop which has already been flushed by the pump. Appendix E has more information.

3) Flush using the Needle Port Cleaner, not a needle.
Use the Needle Port Cleaner (a small Teflon® part without a needle attached), Rheodyne part number 7125-054, attached to a luer tip syringe. This flushes the entire length of the port. A fully inserted needle flushes none of it.

4) Do not load a sample volume equal to the loop volume.
You will lose up to 20% of sample out the vent tube, producing poor accuracy and precision. Load < 50% of the loop volume (partial-filling) or > 200% (complete-filling).

5) A 20 µL sample loop does not contain 20 µL. The size designations of loops are nominal. Actual volumes differ because of the tolerance of the tubing bore. Tubes are cut to length and not volumetrically calibrated. Accuracy of large loops (2 mL) is about 5%, intermediate loops (20 µL) 10%, and small loops (5 µL) 30%. Although complete-filling provides the best precision (reproducibility), use partial-filling if you must know with accuracy the actual volume injected.

6) Keep vent tubes and needle port at the same level. Adjust the end of the vent tubes to the same height as the needle port, so liquid does not siphon out. Siphoning sucks air into the loop. Liquid left inside the needle port will bathe the syringe needle as it enters, preventing traces of sample from depositing on the wall of the port.

7) Use the proper syringe needle.
The needle should be #22 gauge 0.7 mm (5 cm, 2 in) OD, 5.1 cm (2 in) long, with a 90° point (square end) and no electrotaper. Model 3725 requires a #16 gauge needle (1.6 mm - 1.7 mm OD) of the same length.

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