Please Help...Reproducability Problems

[KHall]

I am using a Dionex DX500 system and my injections are not reproducing. The area counts for all peaks are shifting from injection to injection, mostly getting progressively smaller. We have eliminated the column and matrix as causes. Any help would be much appreciated. Thanks!

[gstaepels]

Sounds to me that you may have a problem with your injector.
I use the following test procedure to measure the repeatability of the injector:
Replace the column by a piece of tubing
Run 95/5 water/acetonitrile at 1.00 ml/min (use a back pressure regulator if necessary) - UV at 265 nm
Inject 100 µl of 0.1% acetone in water.
Repeat the injection 12 times
Calculate the %RSD of the peak area's. %RSD should be less than 3%.
If necessary, clean the injector.
Succes

[juddc]

Check very carefully for leaks in the syringe and any valves or fittings and test the accuracy of the injector isolated from the rest of the system.

To check injector accuracy simply place an appropriate volume of DI water into a vial, weigh it carefully on an analytical balance, then inject it X-times at Y-volume and record the mass. The mass should equal (X)*(Y)*(Water density at ambient temperature) within a pretty narrow window which should be published by Dionex.

If everything checks out, then make sure your detector is operating properly and run an injector precision test under controlled conditions, which are usually available from the manufacturer. If you're using a UV detector, linearity and wavelength accuracy may be evaluated absent a pump and injector, if another type check with the mfr for OQ/PQ test protocols and specifications.

Best of luck!