ZnS:Mn electroluminescent (EL) device active layers were prepared using the RF magnetron sputtering method employing a powder target. The active layer was prepared under specified sputtering conditions, and in situ analysis was carried out by quadrupole mass spectroscopy (QMS) and optical emission spectroscopy (OES). The effects of gas impurities as a function of the sputtering environment on the active layer quality, and EL device characteristics were examined. Gas impurities in the sputtering environment were observed by QMS. Gases of mass numbers 18 (H2O), 28 (CO/N-2), and 32 (O-2) a.m.u. had unfavorable effects on the preparation of the active layer, which demonstrates the importance of reducing the concentration of these gases in the preparation of high quality active layers for EL devices. In the optical emission spectra in sputtering plasma, when the Ar (750.3 nm) peak intensity is normalized as unity, the inverse of emission intensity at 472.2 and 636.2 nm from Zn could be utilized as a quality factor of the sputtering environment.