A heterocyclic drug with 2 nitrogen atoms in the rings is analysed on Waters Alliance 2690 (stainless sample loop). A Supelco Discovery Amide C16 is used. The mobile phase is 25 mM phosphate/10 mM dimethyloctylamine pH 7.4 (at pH 3 the drug has no fluorescence). The problem is that a peak always appears at the RT of the drug when a relatively concentrated sample is injected, even after extensive wash with mobile phase, after injecting mobile phase 15 times and/or needle wash. It is suggested that the drug stick to the stainless sample loop and a PEEK loop be used. Has anyone seen this problem before? Any suggestions will be greatly appreciated.
MAny thanks.

I have never seen anything stick to a stainless steel loop, while I have seen stuff stick to PEEK frits or the rotor seal in a Rheodyne injector. On the other hand, none of this should apply to the standard Alliance injector. How do you do the needle wash in this system?

What is the sample dissolved in? Are your sample and mobile phase compatible? Is there any possibility that precipitation is occurring in the sample loop?

It is known, that 316ss (or metal in general) has caused sample components of very polar nature to 'stick' - see certain pesticides; this can also happen in other parts of the sample injection area like the valve seal; what about metal frits in the column ?
Have you thought about residual components from previous injections contaminating the injector or parts around the injection point ? What about a service of the injector ? Injectors must be serviced dependent on the usage . No car would run without an oil check once in a while.

As Waters suggested, I switched to Waters 717 autosampler (previously Alliance 2690). The problem was solved.

Thanks a lot for your imput.