Intensity issues for standard blank during the first calibration

Hi everyone : We use an ICPMS 7800 and, before and after the first standard calibration, the signal intensity vary a lot.

The intensity for most of the elements tracked decrease strangely, even if we take care of cleaning the autosampler probe with washes before and after the calibration.

  • Matrix of Standards : 2.7% v/v HNO3
  • Matrix of wash ("Rince pre-std", "Diluent Blank") : 2% HNO3, 0.5% HCl

As you can see on the screenshot below, the intensity for the same solution can vary from single to double. On the same note, the intensity of the wash before calibration decrease slowly for most elements tracked, even with 10 washes. As a result, all the blanks following are negative until the next recalibration.

 (knowing that we leave the ICPMS warm up 20 minutes at its start, before doing the performance report, a tune of the instrument, adding the internal standard line and 10 minutes of wash again, so the ICPMS is turned on for approximately an hour before the "Rince Pre-Std".  We wash it as well at the end of each analysis with a dedicated wash solution, according to Agilent advises.)

do you have any idea what could happen here? Thanks!

Parents
  • Does anyone in your lab use a dandruff shampoo? The most common shampoos have zinc pyrithione as the active ingredient. Do a swab sample of your lab where standards and blanks are prepared to see if you have Zn contamination.

  • Good to know! I asked around but nobody used dandruff shampoo for a while in the labs where the solutions were prepared
    The swab test is an excellent idea, thank you very much.

  • You're quite welcome. 

    Another possibility - we had an unidentifiable Zn contamination that we traced to the cardboard cap liner in some of our sample (digestate) bottles. If you use this type of bottle, the cap has a cardboard liner that has a shiny surface. That shiny surface is mineral-derived, and when we digested a bunch of bottle caps, we found  a strong Zn signature (plus a few other ions such as Mg, Al, Ca, etc.). So take a look at your containers, raw sample, digestate, and standards, and see if any of them have this cap liner. 

Reply
  • You're quite welcome. 

    Another possibility - we had an unidentifiable Zn contamination that we traced to the cardboard cap liner in some of our sample (digestate) bottles. If you use this type of bottle, the cap has a cardboard liner that has a shiny surface. That shiny surface is mineral-derived, and when we digested a bunch of bottle caps, we found  a strong Zn signature (plus a few other ions such as Mg, Al, Ca, etc.). So take a look at your containers, raw sample, digestate, and standards, and see if any of them have this cap liner. 

Children
  • Thank you again, it's a really valuable advice.
    Actually we use plastic Falcon tubes and, so far, it seems to not contaminate a lot the samples. But we had the very same issue you describe with the syringes used to filter the samples (rubber+silicone of the plunger high in Zn+ other several elements). 

    Since, we take great care of washing everything with TG nitric acid (even the autosampler parts) and reducing the number of containers/pipettes used to store samples and solutions, so we are short on assumptions! Hope the swab test will bring out the source(s) of contamination.

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