最近完成的1.1毫米和870微米银河系巡天发现了大量大质量尘埃核，而我们发现可以通过尘埃核上CN、HCN、HNC三条谱线的积分强度比变化找出化学丰度受到周围环境影响的源。我们选择200个已探测到N2H+,HCO+和NH3的大质量尘埃核，其中包括宁静核（starless core）、中间核(intermediate core)、活跃核(active core)和红色核(red)四个阶段。通过CN、HCN和HNC的成图观测排除丰度受到影响的源，研究积分强度比HCN/HNC、(CN+HCN)/HNC、N2H+/HCO、N2H+/HNC的变化是否可以作为化学时钟，以及大质量恒星形成早期阶段的化学特性和化学模型。利用丰富的多波段巡天数据和水脉泽观测数据研究样本的物理特性。本项研究有望找到示踪大质量恒星形成的化学时钟，在大质量恒星形成早期阶段物理化学特性与演化序列的研究中获得重要成果。

The galaxy surveys at 1.1mm and 870um have been finished recently，they could find all dense dust cores in the interstellar medium. These dust cores provide us a good sample to find all stages of high-mass star formation. Our observation study toward 30 high-mass star formation regions suggest that we could identify those sources affected by the environment by studying the integrated intensity ratios among CN(1-0),HCN(1-0) and HNC(1-0).After deleting these sources， we found that the intensity ratios CN/HNC, (CN+HCN)/HNC increase with the evolutionary stages. Therefore, We plan to select one large sample of dust cores associated with all early stages of high-mass star formation, and observe CN(1-0)\HCN(1-0) and HNC(1-0) toward them. we will identify the sources affected by the environment and drop them. We will study the integrated intensity raio variation of HCN/HNC、(CN+HCN)/HNC、N2H+/HCO、N2H+/HNC and their chemical model. We will also study the physical conditions of these dust cores based on muti-wavelength archival data and H2O maser observation.This study may find chemical clock for the early stages of high-mass star formation, and get good progresses in understanding high-mass star formation and its evolution of early stages.