冷原子荧光光谱中数字基线估计方法的研究

The Study of Digital Baseline Estimation in CVAFS

水体中汞的存在形态有单质汞、无机汞和有机汞三种。其中,甲基汞是主要的有机汞形态,毒性远高于单质汞和无机汞。测量水体中的甲基汞的方法有很多,冷原子荧光光谱法是测量水体中甲基汞的推荐方法。冷原子荧光光谱法是原子发射光谱法和原子吸收光谱法综合发展而来的元素分析方法。经过多年的发展与完善,是元素分析最常用的分析技术之一,具有灵敏度高、检出限低等特点,被广泛应用于环境科学、生命科学、地质等领域。由于检测仪器的背景噪声和色谱柱分离效果的影响,冷原子荧光光谱出现基线漂移和信号拖尾等干扰因素,严重影响冷原子荧光光谱数据的峰面积计算和痕量甲基汞的定量分析。其中,基线漂移是最主要干扰因素。目前,改进模拟器件参数和数字基线估计是解决基线漂移的的两种重要手段。在改进模拟器件参数方面,有激发光源使用空心阴极汞灯、闭环控制的热阴极低压汞灯等,但存在实验设备复杂、成本高昂等缺陷;在数字基线估计方面,有最小二乘法、差值拟合法等,但存在基线估计不稳,含量计算不准等缺点。基于此,提出了一种基于小波变换的数字基线估计方法。首先,分析甲基汞的冷原子荧光光谱微观信号和基线漂移现象,建立冷原子荧光光谱信号和基线漂移数理模型;其次,根据冷原子荧光光谱信号模型的特点,以小波变换为研究基础,研究合适的母小波模型,将母小波模型与基线漂移模型进行卷积,卷积结果恒为零,理论上证明了基线漂移现象经过小波变换后会被消除;再次,以100 pg标样甲基汞为例,实验验证了小波变换能够有效地消除基线漂移的干扰和信号拖尾的影响;最后,在仪器相对标准差(RSD)为1.29%~3.40%的条件下,对0,10,20,50,100,500以及1000 pg的标样甲基汞溶液进行5次重复实验,分别建立小波变换前后峰面积平均值校准曲线,校准曲线的相关系数(R)由小波变换前的0.994提高到小波变换后的0.997。实验结果表明,该方法能够有效地消除测量仪器基线漂移和信号拖尾的影响,提升了系统测量准确性。

There are three forms of mercury in water:elemental mercury,inorganic mercury and organic mercury.Methylmercury is the main organic mercury form,and is much higher than that of elemental mercury and inorganic mercury.Cold vapor atomic fluorescence spectrometry(CVAFS)is the recommended method for measuring methylmercury in the water.CVAFS is an element analysis method developed from atomic emission and absorption spectrometry.After years of development and improvement,it has become one of the most commonly used technologies for element analysis.It is widely used in environmental protection,life science,geology and other fields with the characteristics of high sensitivity and low detection limit.However,affected by the background noise of the excitation light source,electronic components of the detection instrument and the separation effect of the chromatographic column,the signal of CVAFS will haveproblems such as baseline drift and signal tailing,which will seriously influence the peak area calculation of the CVAFS’s data and the quantitative analysis of trace methylmercury.Baseline drift is the most critical problem.At present,improving analogue device parameters and digital baseline estimation are two important ways to solve baseline drift.In terms of improving the parameters of the analogue device,there are hollow cathode mercury lamps and closed-loop controlled hot cathode low-pressure mercury lamps with disadvantages such as complex experimental equipment and high cost.The digital baseline estimation includesthe least square method,difference fitting method and so on,as all of them have weaknesses like unstable baseline estimation and inaccurate content calculation.Thus,a digital baseline estimation method based on wavelet transform was proposed.Firstly,by analyzing the microscopic signal of CVAFS and baseline drift of methylmercury,the mathematical model of the signal of CVAFS and baseline drift wasestablished.Secondly,according to the characteristics of the signal of the CVAFS model and wavelet transform,an appropriate mother wavelet model was established.The mother wavelet model was convoluted with the baseline drift model,and the convolution result was always zero.Theoretically,it indicated that the baseline drift wouldbe eliminated after wavelet transform.Thirdly,taking 100 pg standard sample methylmercury as an example,the experiments verified that wavelet transformation could eliminate baseline drift and solve the problem of signal tailing.Finally,under the condition that the relative standard deviation(RSD)of the instrument is 1.29%~3.40%,the experiments were repeated for 5 times for standard methylmercury solutions of 0,10,20,50,100,500 and 1000 pg,and the calibration curves of the average peak area before and after wavelet transform were established respectively.The calibration curve’s correlation coefficient(R~2)is increased from 0.994 to 0.997 after the wavelet transform.The experimental results showed that this method could effectively eliminate the influence of baseline drift and signal tailing and improve the system’s measurement accuracy.