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基于LIBS的元素成像系统,需要保持较高空间分辨率的同时实现大面积复杂样本的快速元素检测,因此需要激光器、电动位移台、光谱仪以及光谱数据处理软件之间高速协同运作。基于LIBS的元素成像系统装置图如图 1所示。系统主要分为4个部分:激光光源、聚焦系统、光谱检测系统以及成像系统。激光光源提供的高能量的脉冲激光束,通过光路整形扩束后利用光阑进行截取,得到的圆形光斑经过聚焦系统后轰击样本表面,激光束与样本相互作用,样本表面发生快速的熔化和蒸发,材料在这个过程中被激发电离产生等离子体,等离子体冷却过程中发出特定元素的辐射光子,被光谱检测系统收集并检测,通过光谱数据处理软件分析特征波长以及对应的光谱强度,绘制元素分布图。成像系统则可以实现对样本表面、剥蚀位置和剥蚀状态进行实时成像观察。
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在LIBS元素成像系统中,激光光源是十分重要的部分。在等离子体产生的过程中,不同的激光具有不同的吸收特性;不同激光诱导产生的等离子体状态也不同。因此,等离子体的激发主要取决于脉冲激光的物理参量:波长、脉冲持续时间、脉冲能量、光束质量等。
目前,在LIBS元素成像技术中,应用比较广泛的是固体激光器、气体激光器以及准分子激光器。准分子激光是紫外波段光源,相比于红外激光,紫外激光作为光源,具有空间分辨率高、分馏效应小、屏蔽效应少等优点,因此,对生物组织的检测主要是利用紫外波段光源。此外,一些研究小组将飞秒激光光源应用于成像技术中,如ZORBA等人验证了使用频率加倍的Ti ∶sapphire激光器发射100个飞秒量级脉冲到样本表面上,达到亚微米空间分辨率的可能性[11]。飞秒激光的脉冲持续时间从几十飞秒到几百飞秒不等,由于这种非常窄的脉冲持续时间,能量沉积率非常高,导致与样本的相互作用与纳秒激光有很大的不同。表 1中是几种常见的激光光源及其参量。如表 1所示,利用非线性频率变换技术,可以将Nd∶ YAG激光器的基本波长(1064nm)转换为短波长(2次谐波532nm、3次谐波355nm和4次谐波266nm),拓宽了Nd∶ YAG激光器的应用领域。
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LIBS元素成像技术中,为了增加与样品相互作用处的辐照度,激光辐射通常通过光学系统聚焦到一个非常小的点上,该光学系统就称为聚焦系统。聚焦系统的特性对空间分辨率的提高至关重要,使用焦距为几个毫米、放大倍数大于5倍的物镜来聚焦光束,达到几个微米的空间分辨率[20]。此外,在聚焦系统中通入惰性气体,有利于提高光谱信号的稳定性,减少光路中激光能量的损耗,大多数实验中用到的惰性气体为氩气(Ar)、氦气(He)以及氩气和氦气的混合气体[21-22]。例如,美国QUARLES实验小组对聚焦光路以及样本台中通入氦气,使得地质样本中氟元素的检测限增加了几个数量级[23];法国DARWICHE等人研究了混合气体对信背比的影响,最后得出结论:在气压6000Pa下,氩气和氦气体积比为85∶ 15时,使得谱线信背比达到最大[24]。
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LIBS成像技术中另一个关键部分是光谱检测系统,由光谱仪和探测器组成。光谱仪的重要参量有:谱线范围、分辨率、灵敏度、采集速度,这些直接决定了光谱检测系统的性能。基于已发表的文献,使用较广泛的是Echelle型光谱仪和Czerny-Turner型光谱仪,Paschen-Runge型光谱仪使用较少。Echelle光谱仪具有光谱范围广的优点,特别适用于多元素探测(一般从紫外到近红外),但与其它光谱仪相比,Echelle光谱仪的入射狭缝比较窄(通常约为50μm),这会减少有效到达衍射光栅的光量并限制其灵敏度。此外,Echelle光谱仪需要读取整个电荷耦合器件图像来获得光谱,这导致了读出时间的增加,并将采集速率和运行速度降低到只有几赫兹。由于入口狭缝较大,Czerny-Turner光谱仪具有更高的灵敏度,当与电荷耦合器件结合使用时,采集速率也更快。然而,Czerny-Turner型光谱仪有一个主要的缺点:检测的光谱范围有限。有研究小组提出在测量过程中使用多个Czerny-Turner光谱仪[25-27],然而这一想法导致了系统成本的显著提高。
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LIBS成像系统为全光学系统,并且仪器相当简单,使其易于与其它兼容技术直接耦合。喇曼光谱技术与LIBS成像技术结合,两种基于激光的分析技术共享部分仪器,可以集合在一起研究样本表面的分子和元素,提供较全面的样品信息。HOESHE等人已经证明了这种结合在成像研究中的可行性,他们提出使用配备了双Echelle型光谱仪的双激光LIBS-Raman自动化微分析系统来表征铁矿石样品[28]。此外,LIBS元素成像技术与激光诱导荧光(laser-induced fluorescence, LIF)相结合,是一种增强光谱信号强度、减少背景干扰、提高信背比的很有效的方法。LI等人研究这种组合,通过分析在等离子体的不同位置激发荧光光谱,发现在等离子体的中心和外围,基质和目标元素的激发效率存在很大差异[29]。LIBS元素成像技术还与诸如激光剥蚀电感耦合等离子体质谱仪(laser ablation-inductively coupled plasma-mass spectrometry, LA-ICP-MS)这类不完全基于光学的技术相结合,RUSSO团队开发并研究了串联式LA-LIBS仪器,并将其应用于各种领域[30-31]。
基于LIBS的元素成像技术在古气候研究中的应用
Application of elemental imaging based on LIBS in paleoclimate research
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摘要: 古气候学是研究地球的过往气候的一门学科,可以预测未来气候变化,解决有关环境、资源等问题。基于激光诱导击穿光谱的元素成像技术可以快速、准确、原位分析复杂多样的大面积古气候样本,获得能够与气候建立联系的元素信息,在气候变化研究中展现出很好的应用前景。首先介绍了基于激光诱导击穿光谱的元素成像技术的基本原理,其次回顾了目前常用的成像系统的仪器配置,包括激光光源、聚焦系统和光谱探测系统等,最后介绍了国内外基于激光诱导击穿光谱元素成像技术分析古气候代理物的典型案例。该研究对基于激光诱导击穿光谱的元素成像技术在古气候研究中的应用有很好的指导作用。Abstract: Paleoclimatology is a discipline that studies the past climate of the earth, and its purpose is to predict the future climate change, to solve problems related to the environment, resources and so on. Based on laser-induced breakdown spectroscopy(LIBS), the complex and diverse large area paleoclimatology samples can be quickly, accurately and in situ analyzed by the element imaging technology, and the element information that can be linked with the climate can be obtained. LIBS thus has a good application prospect in climate change research. This paper first introduces the basic principles of element imaging technology based on LIBS. Secondly, it reviews the instrument configuration of the currently commonly used imaging system, including laser light source, focusing system and spectral detection system. Finally, typical cases of analyzing paleoclimate agents based on LIBS element imaging technology at home and abroad is introduced. Therefore, this paper has a good guiding role for the application of element imaging technology based on LIBS in paleoclimate research.
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