A catalyst for oxidizing ethylene and producing ethylene oxide, said catalyst being prepared by pretreating a porous carrier with a solution containing a lithium compound and a cesium compound, thereafter, impregnating a solution containing a silver compound and a cesium compound in the pretreated porous carrier and then heat-treating the impregnated porous carrier. The use of the catalyst of this invention provides high selectivity in the production of ethylene oxide by the vapor contact oxidation of ethylene.

A multipole ion guide which begins in one pumping stage and extends continuously into one or more subsequent pumping stages has been incorporated into an atmospheric pressure ion source mass spectrometer system. Ions delivered into vacuum from an Electrospray, Atmospheric Pressure Chemical Ionization or Inductively Coupled Plasma ion source are guided and focused into a mass analyzer with high efficiency using the multipole ion guide. The background pressure over a portion of the multipole ion guide length is high enough to cause kinetic energy cooling of ions traversing the ion guide length due to ion collisions with neutral background gas molecules. This ion kinetic energy cooling lowers energy spread of ions traversing the multipole ion guide length. The multipole ion guide DC offset potential can be used to adjust the mean ion energy and the ion guide a.sub.n and q.sub.n values can be set to reduce or expand the range of ion mass to charge which will be transmitted through the ion guide. These features of multipole ion guides and multiple pumping stage multipole ion guides are used to improve performance and lower the cost of Atmospheric Pressure Ion source mass spectrometer instruments.