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I have a mixture of metal oxides in powder form. Mainly Iron oxide, silicon dioxide, and calcium oxide. I know for certain calcium oxide is in the mixture, but my handheld XRF is not detecting it.

I am analyzing my sample through a very thin plastic wrap, polypropylene.

I have even taken a sample of pure lime, and my handheld XRF is not detecting any calcium. Just a bunch of others such as $\ce{Si, Al, P}$ and so on.

How might this be?

Is the plastic having an adverse effect?

My XRF is in 'general metals' mode; does it need calibrating to detect the $\ce{Ca}$?

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    $\begingroup$ This is assumption, so not making an official answer for it, BUT: Phosphorous and Ca share peaks so if the amount of Ca and P is within the same order of magnitude it could be you need a calibration or better equipment to handle that mixture. I know because at my place of work we are managing the same issue, only we are interested in the P content. There is a special set of calibrations and control samples to reduce the problem. $\endgroup$
    – Stian
    Commented May 9, 2017 at 16:43

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Ca cannot be detected in General Metal Mode. You have to change to "Mining Mode" In this mode, you can measure Ca properly.

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