is there a way to focus mining beam, to only take the material you need from a astroid?

There are mining beams that have better extraction rates for specific ores, which should increase the probability of that specific ore to get into your cargo bay. To selectively only mine one ore only works if you find pure roids, though. Workaround: jettison unwanted ores periodically.

http://www.vo-wiki.com/wiki/index.php?title=Equipment#Mining_Beams
Heres a list of the beams as you progress your mining license.

As toshiro said, the best way to ensure you get what you want is to find as pure a roid as possible.