An apparatus for mixing a vaporized precursor with a gas for producing silica particles is provided. The apparatus includes a mixer housing, a precursor delivery chamber having an outlet in communication with the mixer housing for delivering a vaporized precursor in the mixer housing, and an oxidizing gas delivery chamber having an outlet in communication with the mixer housing for delivering an oxidizing gas to be mixed with the vaporized precursor. The apparatus further includes a flashback member disposed within the mixer housing and between the outlet of the precursor delivery chamber and the outlet of the oxidizing gas delivery chamber. The flashback member is located at a minimum distance from the outlet of the oxidizing gas delivery chamber defined by Lminimum (cm) = 0.453U (Re)-0.5567, wherein U is the flow rate in cm/sec of precursor and Re is the flow Reynolds number. The flashback member may include a tapered surface on at least one side to reduce recirculation of vaporized gas.