In conclusion, the ion transmission devices between the source and mass analyzers influence the ion population's internal energy and in-source fragmentation. Contemporary instruments increasingly use RF-based ion guides including traveling wave ion guides and ion funnels. When used at high gas pressure, these ion guides can cool down the ion effective temperature close to the gas temperature ), even if higher internal temperatures were reached in the source region where the ions were produced. A quantitative example is given by Carpenter et al., who deduced that benzylpyridinium ions reached ~650 K in the source, then were thermalized at 300 K in an RF hexapole . Note that traveling wave ion guides , differential mobility analyzers and trapped ion mobility interfaces ) can also substantially heat the ions, but this subject will not be covered here.