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大气环境颗粒物中的铬(Cr)主要以三价铬(Cr(Ⅲ))和六价铬(Cr(Ⅵ)) 2种价态存在。Cr(Ⅲ)是人体进行糖类和脂类等代谢活动必不可少的微量物质;Cr(Ⅵ)则是剧毒物质,具有致癌性。1990年,美国的《清洁空气法》将Cr(Ⅵ)化合物列入188种有害空气污染物,美国环境保护局也将其列入18种核心污染物[1]。我国的《环境空气质量标准》规定其年平均浓度限值为25 pg·m−3[2]。近年来,相关学者围绕Cr这种环境颗粒物中重要的有害过渡金属[3-5],展开了Cr的污染特征和来源解析、潜在生物危害和健康风险评估以及在环境中可能存在的化学作用等[6-9]方面的研究。
大气颗粒物产生化学过程往往只持续较短时间(大约几个小时)。切实模拟和研究其产生、传输、消耗的动态过程,需要在该时间精度内进行分析测定[10-11]。在高时空分辨率背景下,研究大气化学过程中过渡金属的污染特征,可为探索此类气溶胶的理化特性、并控制其潜在毒性提供重要见解。建立灵敏而可靠的测量方法需要解决3个问题:1)如何提高测量的灵敏度(达到更低的检测限);2)如何降低测量过程中的潜在干扰;3)如何在处理和测量过程中降低样品中待测物质的损失。目前,多数研究者倾向于使用原子吸收光谱技术测定样品中的总Cr,包括火焰原子吸收光谱法(FAAS)、石墨炉原子吸收光谱法、电热原子吸收光谱法(ET-AAS)等[12-13]。但是,此类方法的样品前处理较为繁琐,所需仪器设备造价昂贵。紫外/可见光分光光度法(UV/Vis)灵敏度很高,通过适当的显色试剂进行络合反应可在高吸收率条件下测量水溶性金属,常被用于溶液中金属浓度的高精度测量[14-15]。以此为基础,可通过配备长光液体波导毛细管来实现UV/Vis定量测定环境样品中的水溶性金属[16-17],结合测量系统的开发,还能有序测量环境颗粒物样品中的金属浓度[18-19]。
结合采样和前处理操作,设计并评估了一种基于分光光度法连续测量大气环境细颗粒物(PM2.5)中铬浓度的系统,以便于精确了解大气颗粒物中铬的污染水平及其来源,提供一种测量环境大气中水溶性痕量金属的思路,以期为开发实际监测技术奠定基础。
基于分光光度法测量PM2.5中铬浓度方法的设计与评估
Development and evaluation of a method for measurement of chromium in fine particulate matter based on spectrophotometry
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摘要: 设计并评估了一种基于分光光度法以实现连续测量环境细颗粒物(PM2.5)中重金属铬(Cr)的测量系统。该系统工作流程为:用颗粒物采样器进行样品采集,通过酸化等前处理过程制成样品溶液,加入二苯碳酰二肼(DPC)进行络合反应;在长光程流通池(LWCC)中,用分光光度法对生成的有色铬络合物(Cr-苯肼羟基偶氮苯)进行吸光度测定;比对测定结果与标准样品标定结果,得出样品中铬的实际浓度。实验室测试结果和外场采样观测的结果均表明,系统具有较好的采样性能和测量精度。多次校准系统获得的线性响应R2均大于0.99。以空白样品3倍的标准偏差确定的系统检测限(LOD)为0.13 ng·m−3。由相同样品通过该系统和电感耦合等离子体质谱(ICP-MS)分别测定得到的数据具有良好的一致性。用该系统对南京信息工程大学大气观测站采集的大气颗粒物中总Cr浓度进行测定,得到其在测定期间的变化趋势与PM2.5基本一致。该系统是一种准确、稳定、便捷的PM2.5中铬元素的分析监测技术。
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关键词:
- 细颗粒物(PM2.5) /
- 铬 /
- 分光光度法 /
- 石英膜采样 /
- 过渡金属元素
Abstract: A new system is developed and evaluated for the measurement of chromium (Cr) content in ambient fine particulate matter (PM2.5). In this measurement system, an ambient sample is collected by quartz filters, extracted by acidified solutions, and reacted with 1, 5-diphenylcarbazide (DPC) to form a chelate, which is measured by spectrophotometry using a long-optical liquid waveguide capillary (LWCC) cell and comparing with a standard Cr sample solution. Both laboratory and field experiments demonstrated that this new technique was easy to deploy and had high accuracy. Duplicated calibrations were performed which showed consistant results with R2 > 0.99. The system detection limit (LOD) is 0.13 ng·m−3, which is determined by three times the standard deviation of Cr levels of a blank sample. Our new method showed good agreement with the-well-established inductively coupled plasma mass spectrometry (ICP-MS). The total chromium concentration was measured by the new system at the atmospheric observation station of Nanjing University of Information Science and Technology (NUIST), the trend of which agrees with that of PM2.5. Results demonstrate that the new system is an accurate, stable, convenient and promising technique for the measurement and analysis of ambient particulate chromium contents. -
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