Contamination zirconium versus sapphire pistons

[Tony]

Hi All,
We recently bought a new Thermo Surveyor HPLC system to replace an aging Thermo TSP system. However, transferring a method from the TSP system to the new Surveyor system resulted in the appearance of a new system peak that coelutes with a key analyte. The mobile phases contain acetonitrile, water, acetic acid, and triethylamine at pH 10; separation is by C18 and detection is by fluorescence.

By substituting various Surveyor system components with the TSP counterparts in a process of elimination, the source of the contaminating peak was narrowed down to between the solvent proportioning valve and the pumps, inclusive. The mobile phase, solvent tubing, degasser, injection valve, tubing from the sample vial to the injector valve, and column were all eliminated as possible contamination sources. As the contaminant elutes as a distinct peak, it must be entering the flowpath upstream of the column exit and being resolved on column; therefore plumbing downstream of the column and the detector were not considered as contamination sources.

This leaves the proportioning valve and pump chambers as the only possible sources of contamination. The proportioning valve materials, check valves, and pump seals are made of the same material as on the TSP system. However, in the Surveyor, the pistons are made of zirconium ceramic; someone suggested to me these might be the cause of the problem - the pistons in the old TSP system are sapphire. I can get replacement sapphire pistons for the Surveyor, but are zirconium pistons known to be problematic for some analyses??

thanks
Tony

[tom jupille]

A couple of questions (to confirm assumptions)
- gradient separation?
- the interfering peak shows up even with a "dummy" gradient (no injection)?
- the interfering peak is "well retained" (i.e., somewhere in the middle of the gradient)?
- reversed-phase column?

I'm not aware of any specific problems with zirconia at pH 10. Aside from the possibility of contamination during assembly, I find it hard to visualize anything hydrophobic enough to be significantly retained by reversed-phase coming off a ceramic piston.

I'd be more inclined to suspect a polymer component (seals, transfer lines, etc.). Even though they are made of the same material in both systems, it's possible that you have a "bad" or contaminated component.

Any chance you can "borrow" (or get access to) another Surveyor system and try running your gradient on that (if nothing else, put the arm on your Thermo sales rep)? That should tell you whether the problem is related to the Surveyor materials or is specific to your particular system.

[Mark Tracy]

Yes, but not in the way you describe. For ordinary HPLC mobile phases, ZrO2 is quite stable. Only when you go to extremes of pH do any problems appear. (Dionex recently went back to sapphire because we pump sodium hydroxide all the time.) I agree with Tom, you have a contaminated component. No factory can be expected to run high-sensitivity fluorescence tests as part of their QC.

By the way, pH 10 is high enough to worry about the rotor seal of the injection valve. Is it made from Vespel? If so, you are near the edge of its pH range, and it can leach stuff.

[Tony]

Tom and Mark,
Thanks for your replies. To answer Tom's queries; yes, a gradient is running, it is reversed-phase, the peak shows up mid-run and appears when there is no injection, even when the injection valve is taken out of the pump-to-column flowpath.

Mark: I suspect the Surveyor rotor seal is Vespel; this was also the default on the TSP system. Experience revealed the mobile phase severely degraded the Vespel seal after a couple of months use. This manifested itself as broadened analyte peaks because of a significant increase in dead volume as the rotor seal channels opened up. However, this degradation was not accompanied by the appearance of any contaminant peak.

I do think there is some contaminated polymer somewhere. It can only be the pump seals; I plan to replace these and try substitutng some sapphire pistons at the same time.

thanks
Tony

[Mark Tracy]

Once I had a problem with pump seals leaching after exposure to pH 10 ammonia buffer. It only showed up on MS. I never completely resolved the issue. It is worth checking into.