Comments
Precision is poor when the sample dispensed from the syringe is between 50% and 200% of the loop volume. When partial-filling, and the volume dispensed exceeds 50%, some sample is lost out vent tube #6, and the amount lost varies. When complete-filling, and the volume dispensed is < 200%, much sample remains in the loop, and the amount is variable, especially when the volume loaded and the rate of loading are inconsistent. As more sample is loaded, the amount in the loop approaches a constant. See Appendix F for a full discussion of what happens inside the injector.

There are many causes for non-reproducible results other than injector malfunction:
Flow rate changes cause non-reproducible retention times and peak areas (area is inversely proportional to flow rate). Peak height is also influenced, but only slightly. Mobile phase composition changes cause non-reproducible retention times and peak heights. Peak areas are relatively unaffected.

Temperature changes cause non-reproducible retention times, peak areas, and peak heights, but the magnitude is usually small and varies with compounds and mobile phases. Rheodyne Technical Note 5 discusses accuracy and precision in LC injection. All Rheodyne literature is available at www.rheodyne.com. See comments on next page.

Symptom #7
Peak area and height of all peaks are inconsistent from one injection to another:

  • If you require more than one syringe full to load the sample loop, see Cause A.
  • If you are loading a volume of sample 50% to 200% of the loop, see Cause B.
  • If you are returning to LOAD before ten loop volumes have passed, see Cause C.
  • If none of these:
    Overfill the loop (see Fig. 24). Stay in LOAD with syringe in place and plunger depressed.
  • If vent leaks continuously, see Cause D.
  • If vent does not leak, see Cause E.

Fig. 24. Overfill the loop with at least two to three loop volumes.

Cause A
The syringe is too small. Removing it in LOAD pulls sample from the loop.

Solution
Use a syringe with an appropriate volume.

Cause B
You are in the problem zone. See Fig. 25, comments, and Appendix F.

Solution
Load the loop with < 50% or > 200% of its volume and be consistent with the rate of loading and the volume loaded.

 

 
 

Fig. 25. The problem zone produces poor precision.

 

Cause C
Not all of the sample is swept from the loop.

Solution
Remain longer in INJECT. See Appendix E.

 

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