焚烧秸秆对大气环境具有很大的危害, 燃烧过程中重金属元素会随着烟尘飘散到空气中, 严重污染大气环境, 探测随秸秆燃烧烟尘扩散到空气中的重金属元素具有重要意义。 采用激光诱导击穿光谱(LIBS)技术对秸秆燃烧烟尘进行原位在线探测, 对其中的重金属元素进行在线分析。 实验仪器由Nd:YAG单脉冲激光器(波长1 064 nm, 激光能量为290 mJ·pulse^-1, 重复频率10 Hz), Avantes光谱仪(AvasSpec-ULS2048-4Channel-usb2.0, 光谱检测范围200~890 nm, 分辨率0.13 nm)、 反射镜、 聚焦透镜(焦距为150 mm)、 时序发生器组成, 光谱仪的延迟时间设定为6 μs。 激光器发射出的激光经反射镜与聚焦透镜聚集到烟尘中, 并产生高温等离子体, 经光谱仪采集信号获得LIBS光谱。 实验样品为华东地区成熟水稻茎叶。 首先对空气进行LIBS探测, 可以从光谱图中观察到N, O, H_α和H_β等元素, 同一实验条件下再对烟尘进行LIBS探测并得到光谱图, 观察到秸秆燃烧产生的烟尘中含有C, Mg, Ca, Mn, Na和K等元素, 其中Mn为重金属元素, 验证了LIBS探测秸秆燃烧烟尘中重金属元素的可行性。 在烟尘光谱图中同时观察到CN分子(自由基)谱线的存在, 秸秆燃烧过程中产生的CO₂分子在激光的辐射下与空气中的N₂发生反应生成CN分子(自由基), 在同一条件下对激光焦点进行人工吹气, 得到的光谱图中同样探测到了CN分子谱线。 利用LIFEBASE软件拟合烟尘中的CN分子谱线, 获得拟合数据, 同时得到CN分子的振动温度为8 000 K, 转动温度为7 700 K。 制备含铅的秸秆样品, 将相同质量的两份样品分别浸泡在不同浓度的乙酸铅溶液中, 并进行烘干处理。 对两种样品进行LIBS探测并得到光谱图, 通过对比含铅秸秆样品与原始秸秆样品, 含铅秸秆样品的光谱图中多出波长为357.261, 363.898, 368.370, 373.945和405.747 nm的五条谱线, 与NIST数据库对比发现五条谱线都为铅元素的光谱线。 以CaⅡ(393.329 nm)谱线为参考线对含铅样品的光谱图进行归一化处理, 观察到含铅浓度高的样品光谱图中铅的特征峰值强度比含铅浓度低的样品光谱图中铅的特征峰值强度高, 验证了激光诱导击穿光谱技术对秸秆燃烧烟尘中重金属元素的半定量分析的可行性。

The burning of straw has great harm to the atmospheric environment. During the combustion process, heavy metal elements will float into the air along with the smoke, causing a certain degree of pollution to the atmospheric environment. Therefore, it is of great significance to detect the heavy metal elements that spread into the air with the smoke of straw combustion. Laser Induced breakdown Spectroscopy (LIBS) is used to detect the straw smoke, and to analyze the heavy metal. The experimental setup consists of Nd:YAG single pulse laser (wavelength: 1 064 nm, laser energy: 290 mJ·pulse^-1, frequency: 10 Hz), Avantes spectrometer (AvasSpec-ULS2048-4Channel-usb2.0, spectral range: 200~890 nm, resolution: 0.13 nm), reflector, focusing lens (focal length: 150 mm), pulse delay generator. The delay time of spectrometer was set at 6 μs. The laser beam is focused on the smoke by the lens, and generates high-temperature plasma. The LIBS spectrum signal is obtained by the spectrometer. The samples were mature rice stems and leaves in east China. For the LIBS detection of air, N, O, H_α, and H_β elements can be observed in the spectra. Under the same experimental conditions, the smoke of the burning straw was detected by LIBS and the spectrum demonstrates that the smoke consists of C, Mg, Ca, Mn, Na, K and other elements. The observation of the heavy metal element Mn verifies the feasibility of the LIBS detection of heavy metal elements in smoke of burning straw. At the same time, the molecular spectrum of CN radical was observed in the smoke spectrum. CO₂ molecules generated in the combustion process of straw could react with N₂ in the air under the irradiation of laser to form CN molecules. Then the detection of the breathing of human by LIBS was carried out under the same condition, and CN emission bands were also observed in the obtained spectra. What’s more, LIFEBASE software is used to fit the emission bands of the CN molecule. Moreover, the result shows that the vibration temperature and rotation temperature of CN molecules are 8 000 and 7 700 K, respectively. Straw samples containing Pb were prepared by using lead acetate solution. Two samples of the same quality were soaked in different concentrations of Pb solution and dried. Online in situ LIBS detection was carried out on the two samples, and the spectra were obtained. By comparing the straw samples containing Pb with the original straw samples, five spectral lines with wavelengths of 357.261, 363.898, 368.370, 373.945 and 405.747 nm were found in the spectrum of the straw samples containing Pb. Compared with the NIST database, it is found that the five spectral lines are all characteristic spectral lines of Pb. Taking CaⅡ(393.329 nm)as a reference line in the signal normalization process, the intensity of Pb line in the sample with the high concentration of Pb is significantly higher than that in the sample with the lower concentration of Pb. It verifies that the online in situ LIBS detection is promising to apply to the semi-quantitative analysis of heavy metals of straw combustion and soot.