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 $\ce{Ca}$?
Ca cannot be detected in General Metal Mode. You have to change to "Mining Mode"
In this mode, you can measure Ca properly.