Amazon has packs of 100 10ml plastic test tubes with screw-on caps for 11.99. It looks like these would be good for keeping powders in, especially for general carry-on or sharing samples with friends. My box just came today. I've seen the type with push in caps before, but never trusted them. These look pretty secure. http://www.amazon.com/gp/product/B005Z4R9HI
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