The Hiden HPR-60 molecular beam mass spectrometer is a compact skimmer inlet MS for the analysis of atmospheric plasma and reactive gas phase intermediates. Radicals are sampled via a multistage differentially pumped skimmer inlet and transferred to the MS ion source with minimal interaction with other species and without wall collisions. Customizable inlets allow connection to many different reactor systems, including atmospheric plasmas. The skimmer system, combined with a triple filter precision mass spectrometer, offers a sampling system with ultra fast response and high accuracy.