A microfluidic reaction device 10 includes a thermal buffer fluid passage [70] and a reactant passage [22] having mixing [40] and dwell time [50] sub-passages all defined within an extended body, the dwell-time sub-passage [50] having at least 1 ml volume, and the mixing sub-passage [40] being in the form of a unitary mixer not requiring precise splitting of flows to provide good mixing. The device is desirably formed in glass or glass-ceramic. The unitary mixer [40] is structured to generate secondary flows in the reactant fluid and is preferably closely thermally coupled to the buffer fluid passage [70] by sharing one or more common walls.