Source of Zn contamination

Zinc and contamination in general is a tricky problem to nail down since it is literally everywhere. The best approach when troubleshooting contamination is to take a systematic approach.

Vials can be the largest source of contamination and it can vary from vial to vial, so I always recommend doing a prewash step for all vessels and vials involved in the prep work. Are you transferring the digested solution to another vial for analysis?

Reagents are another source of contamination since we use concentrated acids for digestions, so we always have to rule those out. Gloves, kimwipes, parafilm - those all have trace amounts of zinc as well and sometimes we don't realize we are adding it just by handling samples. Pipette tips can also have trace levels so some customers doing low level analysis will prewash their tips.

For the stir bars I would soak them my diluent matrix and test the leachate for each one and see how much it varies if you've already 100% ruled out the vials and determined they are not the cause. Perhaps the stir bars are not being cleaned well enough between uses.

Zinc can also come from ceiling tiles and such so sometimes it's a matter of keeping solutions well covered and doing work in a hood to minimize exposure to air until you are ready to run. 

Chelsea gave a great response and I agree about the source possibly being the autosampler vials/tubes- I would just add the following:

It's unlikely that the Zn contamination is from the stir bars. We tend to see the following elements from stir bar contamination – Al, Cu, Cr, Fe, Ni. This is because the most common type of stir bar that CEM uses/sells are PTFE coated AlNiCo stirring bars. If you aren't seeing Al, Cu, Cr, Fe or Ni also, then it's not likely coming from the stir bars. I would also note that stir bars should be considered as consumables and should be replaced from time to time. 

Hi Chelsea, thank you for your comprehensive reply!

I encounter this contamination for years already. Sometimes more, and sometimes less pronounced. So over he years I have tested a lot of things. I started soaking the polypropylene sample vials in diluted nitric acid and rinsing with UPW. Quartz vessels are always cleaned with nitric acid. Nitric acid is trace level grade.  I also started,as you suggested, to clean the disposable pipette tips before use. But you are right, Zn is everywhere, and it's a problem for my analysis. Because I always digest 500 mg sample and diluted this to 50 ml with UPW the dilution factor is 100. So even a slight contamination is multipied 100 times .

Hi Tina, thanx for your reply!

me suspecting the stirring bars was a last resort for me . I did know that these were so-called AlNiCo magnets. But over the years I really don't know what to check anymore. Maybe the source is to be found, like Chelsea suggested, in indoor environment like ceiling, particles from AC. If that is the case maybe 'low-level' Zn analysis is something I simply can not do.

Yes the dilution factor will always make any contamination issue appear so much worse. I once had a small lead contamination issue that came from diluent and even thought it was only a few ppb, when used for samples the dilution factor resulted in hundreds of ppb for the results. Such a headache! If you suspect it might be in the environment you can actually test this hypothesis, Make a blank solution with your diluent. Set up a run to take place over the course of a couple hours and run that solution over those few hours. If you see the Zn levels creep up slowly you know it’s likely somewhere in the air.

Good suggestion Chelsea! I will try measuring a blank solution over a longer period in time. However if it is really environment related.......