We have a few sizes of John Guest 3/8" QC X 1/2" Male Adapters.  This threaded male to male adapter is 3/8" to 1/2" uses push-fit technology for easy installation and simple disconnecting without the use of tools.  It has a high grade chemical resistance and is made of food grade quality that can be used for pure water and reverse osmosis applications.  These fittings are for water usage only, these are not suitable for compressed air or vacuum applications.