A rotary dosing device for use in analytical instrumentation quickly transfers a sequence of precise molar quantities of gas from a primary stream into a secondary stream. The device has a rotating chamber with dosing ports that cycle through three states: fill, equilibrate, and transfer. The device cycles in an overlapping manner such that as one dose volume fills with gas from the primary stream, another equilibrates at a known pressure and temperature, and another transfers its contents to the secondary stream. The device initiates its operation so that the first transfer in a sequence is a properly filled and equilibrated dose from the primary stream. Rather than cycling a single dose volume through the three states multiple times, the overlapping operation of the rotary doser enables multiple precise molar quantities of gas to be transferred in one-third the time.