Masers are often detected from star formation regions and evolved stars. In addition to providing a good laboratory for study of maser processes, the intense maser emissions have been used as probes of the structures, dynamics and physical conditions of their surroundings. Among various galactic masers from different masing species and transitions, the 22 GHz H2O maser emission (the rotational transition between 616-523 levels) is most spectacular, with its high intensity, great variability and wide velocity range.
Since the first detection of the line (Cheung et al. 1969), much observational and theoretical work has been done, as documented by many excellent review papers (e.g. Reid & Moran 1981; Cohen 1989; Elitzur 1992; Anderson & Genzel 1993; Bowers 1993), and the present number of known galactic H2O maser sources has been increased to more than seven hundred in the northern sky. The most comprehensive catalog of H2O maser sources north of -30 deg was compiled by the Arcetri group (Comoretto et al. 1990 and Brand et al. 1993).
Since the 22 GHz system of the 13.7 m telescope of Purple Mountain Observatory was put into operation in 1990, systematic observations of H2O maser sources have been taken on the telescope. In addition to some monitoring programs, most of the observing time was spent on surveys towards CO outflows, compact HII regions, late-type stars and bright IRAS sources (e.g. Sun et al. 1995; Wu et al. 1995).
In several observing runs from August 1990 to January 1994, the total number of candidates for new water masers was about 360, with 110 objects detected. Among them 96 are new detections.