A mixture of precursor and carrier gases were used to create colder molecular ions than could be achieved without a buffer. For jet sources, normal carrier gases are argon or helium. An apparatus was constructed which enabled two gases to be mixed in a third cylinder, which could be fully evacuated (via an independent rotary pump), to allow the use of spontaneously flammable gases. A 0-5 bar pressure gauge monitored the filling and nozzle pressures (see Figure 5.1). Gas flow into the mixing cylinder could be carefully controlled through the use of a VGMD7R leak valve. Typical carrier gas to precursor gas ratios were 5:1, 10:1 and 20:1, and a typical nozzle (stagnation) pressure was 3 bar.