Calcium detection using XRF

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 $\ce{Ca}$?

• 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. – Stian Yttervik May 9 '17 at 16:43