IDEX Troubleshooting Guide
space
space
Sample injection solutions for Rheodyne®

space space space space space

Troubleshooting Table of Contents | Previous Page | Next Page | Close Window

 
Leaks

space


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 nonreproducible results other than injector malfunction: Flow rate changes cause nonreproducible 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 nonreproducible retention times and peak heights. Peak areas are relatively unaffected.

Temperature changes cause nonreproducible 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 in the Technical Support section.

The contribution to nonreproducibility made by the noninjector parts of the system can be determined by making a series of injections with the loop completely full of sample each time an injection is made. Under these conditions, the injector will transfer sample to the column with a volumetric precision of about 0.1%. The amount by which the observed precision of areas and heights is larger is the amount due to the noninjector parts of the system.

To make such a "completely full" injection: Load the loop with ten loop volumes to displace all air and mobile phase. Use the same rate of loading each time. Keep the syringe in the port while turning to INJECT (standard procedure). Inject immediately after loading to ensure that a leaking rotor seal, should it exist, does not have time to displace sample from the loop. During loading, observe that no more than 10% of the sample flows back out the needle port. None should flow out if the needle seal is functioning properly, but this test allows for even a malfunctioning seal. The 10% limit means that at least nine loop volumes will pass through the loop — enough to produce high precision.

Rheodyne RheBuild™ Kits are available for all injectors. Kits contain only genuine Rheodyne parts together with tools and instructions.

 
space

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 Figure 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.

Overfilling Sample Injection Loop

Figure 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 Figure 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.

Poor Precision

Figure 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.

Cause D
A cross-port scratch on the rotor seal allows mobile phase to leak into the loop while in the LOAD position, displacing sample (see Figure 26). To confirm: Load the loop with five loop volumes of sample and inject immediately. This makes an accurate injection of a complete loop volume. Observe the peaks. Load the loop again, but delay turning to INJECT, allowing time for any leakage into the loop to displace sample. Inject and observe the peaks. Smaller peaks indicate leakage. The longer the delay, the smaller the peaks.

Solution
Replace the rotor seal as in Symptom 3. Examine the ceramic surface of the stator face assembly and replace the assembly if it is chipped, cracked, or if any of the six holes are blocked (see Comments).

Rotor Seal in LOAD Position with Scratches

Figure 26. View of the rotor seal with the injector in the LOAD position. A scratch between ports 3 and 4 can displace sample from the loop.

Cause E
The problem may not be the injector. Check to see if the needle seal is working. When it is loose, not all of the sample dispensed from the syringe enters the loop (see Figure 27). You can discover if the seal is leaking by using the procedures described in Symptom 1. See also Symptom 9 and Comments.

Solution

Re-form the seal if it is leaking. Make sure that you are using the proper syringe needle.

Bad Needle Seal in Sample Injection

Figure 27. With a bad needle seal (A) sample leaks around the needle seal and into the needle port. With a good needle seal (B) there is no leakage.

Back to Top

Troubleshooting Table of Contents | Previous Page | Next Page | Close Window