We are developing a method detecting @254nm, and noticed that there is a contamination peak from the HPLC vial caps: e.g., when injecting mobile Phase without caps, there is no such peak. But if capped and shook a while, then the peak appears. Also the peak grows with the time of contacting with the vial caps. Mobile Phase-H2O/MeOH(1:1). We will evaluate different caps, but I am wondering somebody would share their experience. The UV max @ ca.260, little absorption before 225 and after 270nm.

Any idea what it could be, or any special caps recommended? Any comments/help will be greatly appreciated. Thanks in advance.

You might put your caps in methanol or acetone overnight and then dry them. This will hopefully remove the extractibles from the caps.

We see very often extractibles show up, especially with fluorescence detection and GC. Some of them are probably plastizers.

Thanks Jimmy. Our method is for stability test, sometime we have to run long sequence, for early vials, the extractable is not a big issue, but for later vials, we noticed more/more this extractible(peak getting higher). Although we managed to make this peak not an interference with other peaks, we would love not to see it, so I am wondering any suggestion which suppliers may have better products to this problem. Thanks.

Noname,
There used be several suppliers of different type of cap inserts, e.g., teflon lined, butyl rubber, etc. Is that not the case now?
Jim

There have been no problems with teflon lined (completely teflon covered on side facing sample!!) caps here, such that we donīt even check anymore. Now if you used such teflon caps it sure would be instructive to know what liquids you had in your vials. Maybe we should be more careful? Did you wash the vial with septum prior to use?
In GC with an autosampler we had extensive extraction of unknowns by hexane when we used vial septa which were rubber/silicon edged with teflon lining in the center (for better seal, it did not visibly expose the vialīs content to the rubber...), or after the teflon was punctured once by the needle.

Thanks for all the suggestion. Currently we only have one type of cap(kind of cheap one), silicone type. We use H2O/MeOH mixture as sample solvent, But pure MeOH/ACN also see the same peak. We don't wash vials/caps either, just use as supplied.

Anyway, we will order some other types like teflon-lined, and will test to see if still interfering. Thanks a lot.