土壤重金属快速检测技术研究进展

毛雪飞，刘霁欣，钱永忠

土壤重金属快速检测技术研究进展

毛雪飞，刘霁欣，钱永忠

（中国农业科学院农业质量标准与检测技术研究所/农业农村部农产品质量安全重点实验室，北京 100081）

近年来，随着我国工农业的高速发展，尤其是无节制的矿藏开采、“三废”排放、汽车尾气以及农业化学投入品的滥用，重金属污染已成为我国当前最严重的环境污染问题之一，因此土壤重金属监测工作显得尤为重要。但是，目前的土壤重金属检测标准方法仍以实验室确证性分析为主，无法用于土壤重金属的现场、快速分析，从而难以从源头上及时、有效地对土壤重金属污染进行监测和预防，开发重金属快速检测设备和技术势在必行。从土壤样品的基质特点来看，固体进样分析是最可行的技术方案，主要包括电热蒸发（ETV）原子光谱、X射线荧光光谱（XRF）、激光烧蚀（LA）、激光诱导击穿光谱（LIBS）、X射线吸收光谱（XAS）、中子活化（INAA）等。上述固体进样分析技术均无需样品消解处理，高效、快捷，但是部分技术的检出能力和稳定性尚难以满足土壤质量标准的全部要求，如XRF、LA、LIBS，还有部分技术难以实现现场化，如LA、XAS、INAA等。因此，基于电热蒸发（ETV）固体进样的原子光谱分析技术在分析灵敏度、稳定性和小型化方面具有特殊的优势。ETV是利用电加热将样品中的待测元素以气溶胶的形式导入原子化器或激发源的技术，可实现土壤中常见重金属元素的快速、高效导入，技术简单、通用性强，适用于原子吸收、原子荧光、原子发射、无机质谱等多种检测系统。ETV常采用碳、金属、石英等材料，如石墨管、多孔碳管、钨丝、铼丝、石英管，其中利用高熔点金属的电磁感应电热蒸发技术具有无冷点、升/降温速度快、易于小型化的优势。但是，土壤样品基质复杂，基体干扰一直是困扰ETV技术应用的核心瓶颈问题。新型的气相富集（GPE）、介质阻挡放电（DBD）、基体改进及背景校正等技术，有望实现土壤基体干扰的有效消除。特别是GPE技术，在特异性捕获消除基体干扰的同时，还可以通过预富集提高仪器的分析灵敏度。通过上述技术的集成与创新，可以有效解决固体进样的分析灵敏度和基体干扰问题，这将为土壤重金属速测技术的研发提供新的思路，从而为土壤环境监测与治理工作提供有效的技术支撑。

土壤；重金属；快速检测；电热蒸发；固体进样；基体干扰

0 引言

近年来，随着我国工农业的高速发展，尤其是无节制的矿藏开采、“三废”排放、汽车尾气以及农业化学投入品的滥用，重金属污染已成为我国当前最严重的环境污染问题之一。2014年4月，环境保护部与国土资源部联合发布的《全国土壤污染状况调查公报》显示：全国耕地土壤点位污染超标率达19.4%，主要污染物为Cd、Ni、Cu、As、Hg、Pb等重金属，会直接或间接影响到食品安全和人体健康。为了及时掌握、预警土壤中重金属的污染情况，2016年国务院启动了《土壤污染防治行动计划》（土十条），提出了土壤环境质量监测点位对所有县（市、区）全覆盖的要求，在“土壤详查”等工作中每年花费大量人力、物力和财力开展重金属污染监测。目前，土壤重金属检测的标准方法仍以实验室确证性分析为主，如原子吸收光谱法（atomic absorption spectrometry，AAS）[1]、原子荧光光谱法（atomic fluorescence spectrometry，AFS）[2]、电感耦合等离子体发射光谱法（induced coupled plasma optical emission spectrometry，ICP-OES）[3]、电感耦合等离子体质谱法（induced coupled plasma mass spectrometry，ICP-MS）[4]、波长色散型X射线荧光光谱法（wavelength dispersion X-ray fluorescence，WDXRF）[5]等。但是，上述方法需要复杂、耗时的样品制备以及消解或提取处理，或较大的仪器尺寸和复杂精密的硬件配置，无法用于土壤的现场、快速分析，从而难以从源头上及时、有效地对土壤重金属污染进行监测和预防。因此，本文对当前重金属快速检测仪器与方法，及其对土壤样品来说最核心的技术瓶颈——“基体干扰”问题进行综述，以期为土壤环境监测与治理研究提供技术参考。

1 土壤重金属的固体进样分析技术概述

与现有的液体进样原子光谱方法相比，固体进样分析是最有可能实现土壤重金属现场快速检测的技术，其无需消解处理，具有较高的样品导入效率，可有效提高分析灵敏度、缩短分析时间、避免痕量元素的损失，同时减少有害化学试剂的使用，更加环保和安全。实际上，早期的原子光谱仪器大多基于固体样品的直接分析，如钾盐、钠盐火焰发射谱线的观测。现代意义上的固体进样原子光谱仪器，可以追溯到1957年L’VOV[6]利用石墨坩埚电热蒸发固体NaCl，采用AAS测定。这是现代石墨炉AAS（graphite furnace atomic absorption spectrometry，GF-AAS）的雏形，后经Massmann改进为管式石墨炉，并由L’VOV设计增加了原子化平台以改善空间温度分布。

目前，可实现固体进样元素分析的技术，主要包括电热蒸发（electrothermal vaporization，ETV）[7]、XRF[8]、激光烧蚀（laser ablation，LA）[9]、激光诱导击穿光谱（laser induced breakthrough spectrometry，LIBS）[10]、X射线光电子能谱（X-ray photoelectron spectroscopy，XPS）[11]、X射线吸收光谱（X-ray absorption spectroscopy，XAS）[12]、中子活化（instrumental neutron activation analysis，INAA）[13]等。其中，电热蒸发（electrothermal vaporization，ETV）所需的材质来源广，易改装和小型化，通用性强，是研究和应用最多的固体进样分析技术之一，将在下文予以专门阐述。

1.1 X射线荧光光谱法

XRF是指原子在X射线或粒子的激发下发射X射线荧光（0.01—10 nm），检测器测量荧光的波长和强度从而实现元素的定性和定量分析。除了XRF，X射线光谱还有X射线衍射（X-ray diffraction，XRD）、XPS等。XRF是一种性能优异的多元素分析手段，分析速度快，其中能量色散型XRF（energy dispersive X-ray fluorescence spectrometry，EDXRF）结构简单、功率较小，易于小型化，可用于现场快速检测，已在地矿、钢铁、材料等领域获得广泛应用[5, 8, 14-16]。但是，EDXRF的能量分辨率较差，对背景干扰较为敏感，常采用基体匹配作为标准曲线策略，同时对光源、滤光片、靶材、算法等加以优化和改进，以提高分析灵敏度和消除基体干扰[14,16]。当前的便携式XRF对于土壤中Cu、Zn、As、Pb、Ni、Cr等元素的检出能力可以达到土壤质量标准的要求，仍难以满足土壤中亚mg·kg-1级Cd和Hg元素的精准测定需求。

1.2 激光样品导入技术

LA[17]和LIBS[18]是利用高功率脉冲激光聚焦到固体样品表面，使样品等离子化或蒸发后以气溶胶形式传输进入检测器，简单、快速，空间分辨率高，可用于元素的微区分析。其中，LA须与ICP-MS联用，无法现场使用；LIBS构造简单，易于小型化、现场化。但是，由于激光激发样品的绝对量过小，其分析灵敏度多在mg·kg-1以上级，同时受限于样品表面特性对激光的吸收和激发效能差异，分析稳定性仍是限制LA和LIBS进一步发展应用的主要因素。目前，LA和LIBS主要用于常/微量元素的定性或半定量分析，还很难用于痕量重金属的快速检测。

1.3 等离子体进样技术

分析仪器中常用的等离子体包括ICP、微波等离子体（MWP）等“高温”等离子体，以及介质阻挡放电（dielectric barrier discharge，DBD）、辉光放电、尖端放电等低温等离子体（LTP），其蕴含丰富的能量可以激发或刻蚀固体样品[19-22]。如DUAN等[19]研发了一种MWP-AES的直接固体进样装置，通过等离子体与固体样品直接作用，将样品加热、使其元素原子化并连续激发，成功地测定了地质样品中Cu、Pb、Cr和Co等元素的含量。内炬蒸发（ITV）技术是利用ICP的“高温”等离子体作用将样品中的元素蒸发和激发，再导入发射光谱或质谱检测器，实现了样品激发与导入的“无缝”衔接，极大地提高了传输效率[23]。目前，ITV-ICP技术已用于部分生物和水样品中Cr、V、Sr、Pb、Cd、Mn、Mg、Ca、Be、Zn、Ti、Ba等元素的分析[24-25]。但是，上述基于“高温”等离子体技术的固体进样仪器装置依然存在功耗高、体积大的问题，难以用于现场快速分析。

此外，LTP也具有导入固体样品中元素的能力，如XING等[20]将DBD-LTP探针与ICP-MS联用，剥蚀薄层材料中的待测元素并转化成气溶胶，再导入检测器；XING等[21]还利用该技术对岩石样品中元素进行二维成像。相对于LA和LIBS，DBD-LTP探针结构简单、成本低廉，也是一种可行的微区分析技术[22]。但是，DBD-LTP的激发能力还不够强，刻蚀的均匀性和分析的稳定性还需进一步提高。

1.4 其他固体进样技术

此外，XAS、INAA等仪器[12-13]可多元素分析，灵敏度高，其中XAS还可给出元素形态的特征信息。但是，XAS作为同步辐射大科学装置，而INAA是一种放射性分析技术，两者都无法用于土壤样品的现场快速检测。

2 电热蒸发固体进样技术

ETV是利用电加热将样品中的待测元素以干燥气溶胶的形式导入原子化器或激发源的技术，有时ETV自身也是原子化器或激发源。ETV技术简单，常采用碳、金属、石英等材料；样品导入快、进样效率高，土壤中常见的重金属元素都可以通过ETV实现导入[26-30]；同时，适用于原子吸收、原子荧光、原子发射、无机质谱（ICP-MS）等多种检测系统，通用性非常强。

2.1 碳材料ETV技术

碳材料ETV装置最早源于L’VOV石墨坩埚原子化器，多制成管、舟、杯状等作为进样器[31-34]。其中，石墨炉原子吸收（graphite furnace atomic absorption spectrometry，GF-AAS）的石墨管是最常用的ETV装置，在ETV-AAS上使用时既是蒸发器也是原子化器，但与ICP-MS/OES联用时仅作为蒸发器，完成样品的干燥、灰化和蒸发过程[35]。还有研究将W[36]、Ir[37]、Ta[33]等高熔点金属热解或电镀在进样器和石墨管表面，可有效改善管内温度分布、提高碳材料寿命。目前，基于碳材料的ETV技术已成功用于土壤、地矿、食品等样品[34, 28-40]中Cd、Cu、Zn、Hg、Ni、Mn、Pb、As、Cr等数十种元素的分析。

但是，常用的石墨管ETV耗电高、散热慢，需要复杂的冷却和电源系统，难以小型化，如德国Jena公司生产的基于高聚焦短弧氙灯光源、中阶梯光栅高分辨率分光系统和电荷耦合器件图像传感器（charge coupled device，CCD）检测器的GF-AAS[41]。由于石墨炉的样品承载量有限，必须将土壤样品处理成干燥、均质的粉末才能置入GF进行蒸发，一般在毫克级，这大大增加了样品制备和分析过程的难度。LIU等[42]对多孔碳材料进行了改良，稳定性与石墨管接近，但蒸发功耗仅需0.3—0.5 kW，远低于后者，加上多孔碳自身的空隙大，因此散热快，无需额外冷却系统，对土壤中Cd的方法检出限（limit of detection，LOD）可以达到μg·kg-1以下。同时，多孔碳样品舟的样品承载量可以达到100 mg，这些都为碳材料ETV装置小型化及应用提供了思路。

2.2 金属材料ETV技术

高熔点金属也是良好的ETV材料，如W、Mo、Pt、Ta、Re等，可制成丝、舟、管、片等形状[37, 43]。金属钨具有良好的导电导热性能和延展性，熔点高、化学惰性，并且成本低、易获取，是当前常用的ETV金属材料。其中钨丝（tungsten coil，TC）应用最为广泛，可与AAS、AFS、AES和ICP等串联[44-45]，易于仪器小型化和便携化，且不像GF那样易与样品成分产生难蒸发的碳化物，从而影响测定重现性。如HOU等[44]首次将TC-ETV直接插入Ar-H2火焰石英管原子化器中，形成了一种新型、小型化、紧凑的AFS进样装置，对Cd，Pb，Au和Ag等元素的绝对LOD分别可以达到0.02、0.6、2.5、4.4 pg；HOU等[45]比较了TC与不同的检测器串联的检测技术，其中，TC-ETV-AAS中Cd的LOD为10 pg，而TC-ETV-AFS中As、Se、Cr、Sb和Pb的LOD分别为950、320、1400、330和160 fg（TC-ETV-AAS、TC-ETV-AFS的检出能力在pg级，甚至更低）；当与ICP串联，其LOD与传统ICP检测器接近。为了防止金属材料的氧化，使用TC时常在氩气中补充氢气作为还原性保护气氛，并有提供自由基源、促进原子化的作用。

TC-ETV用于土壤样品导入的主要难点在于样品承载和基体干扰。由于TC无支撑结构，无法承载固体样品，因此一般多将土壤样品制成悬浮液再导入TC，依靠表面张力承载，但最大也仅能加载10—20 µL。若将高熔点金属制成舟或杯状，虽然可以承载固体样品[46-47]，如OKAMOTO等[46]利用钨舟承载样品并电热蒸发，ICP-MS做为检测器测定固体生物样品中Cd的LOD为0.84 ng·g-1。但是对于直接电阻加热方式，加热体质量或尺寸过大会导致过高的功率需求，以及升温、导热等性能的限制，这就失去了TC那样的特殊优势。而且，部分研究中仍选择将样品消解后再加热蒸发消解液进行元素测定[47]，依然难以实现现场快速检测。

实际上，不管是碳材料还是高熔点金属材料ETV，一般多采用电阻式加热，必须在加热体两端引入电极，所以电极的接触好坏对加热的影响极大，而且易于损坏；另一方面，为保证电极的良好接触，往往会在电极附近形成一个低温区域，这个区域也容易造成元素的残留，影响测定的稳定性。本课题组研制的电磁感应电热蒸发器（inductive electrothermal vaporization，IETV），通过强制电流趋肤增加表面电阻，突破了常规铁磁性金属材料居里点1 200℃以上失磁无法继续加热的瓶颈，实现了非接触ETV进样。初步实验结果表明，IETV装置升温速率＞300 ℃·s-1、最高温度＞2 000 ℃，实现了样品中Cd、Hg、As、Pb等元素的全部导入。

2.3 石英材料ETV技术

石英管（quartz tube，QT）是原子光谱中常用的原子化器，也可用于ETV，但受自身材料局限，耐受温度最高仅有1 000 ℃左右，因此主要适用于Hg、Cd、Pb等中低温元素。如基于催化热解原理的测汞仪，一般采用QT-ETV来实现样品中Hg的快速导入[48-50]，可用AAS或AFS检测，是目前最为成功的基于ETV技术的商品化仪器，如有研究分别利用ETV-AFS与ETV-AAS检测土壤中的Hg，其LOD分别为0.08、0.5 μg·kg-1[48-49]。针对测Hg时的基体干扰，主要采用金汞齐或塞曼扣背景的技术予以消除。此外，LIU和MAO等[7]研制了一体化石英管，前段QT-ETV作为进样系统，后段QT作为气相富集（gas phase enrichment，GPE）装置用于预富集Pb，实现了食品样品中Pb的直接固体进样分析，方法LOD达到2—3 pg[7,51]。

3 基体干扰及消除技术

土壤样品基质成分复杂，除了大量的矿物质，还富含微生物、腐殖质等有机组分，直接电热蒸发会导入大量的基体，如土壤样品中共存元素的化学干扰、复杂基质的背景干扰、微颗粒物的物理干扰、分子吸收干扰、特定元素的电离干扰等，可能对仪器的各个系统带来不利影响，因此基体干扰一直是限制ETV技术广泛应用的瓶颈。为了实现固体进样元素分析的准确测定，研究者尝试多种技术手段来消除基体干扰，如GPE[52-54]、基体改进剂[43,55-57]、标准加入法[30]、基体匹配[58]、塞曼背景校正[59]、程序升温[7, 52-54,60]以及化学计量学校正[61]等。

3.1 气相富集技术

GPE主要以气相分析物为对象，如ETV[51-54]、化学蒸气发生（chemical vapor generation，CVG）[62-63]都是较好的气相导入技术，所以GPE装置大多直接耦合到原子光谱仪器中，效率高、通用性强。早期的原子光谱仪器由于分析灵敏度不高，为了提高元素检出能力，常采用气球法[64]、冷凝法[65]、溶液/固体吸附[66-68]等方法对气相分析物进行预富集。这是GPE技术最初的用途，当时的GPE装置结构复杂、操作繁琐。近年来，随着材料、等离子体等技术的发展，与GPE密切相关的材料表面改性、快速升降温、微等离子体放电均可简单实现，这为GPE的发展提供了新的思路。

GPE在仪器系统中的主要作用有两点：一是通过预富集提高分析灵敏度；二是通过与基体干扰物的分离作用降低或消除基体干扰。常见的GPE捕获材料主要有石英、碳、金属材料等，如石英管[62-63,69-71]、石墨炉[72-75]、金丝[53,76-77]、钨丝[42,52-54,78-80]、镀贵金属的钨丝[81-82]等；捕获/释放方式主要有冷/热处理[65,83]、合金捕获[53,60]、放电捕获/释放[62-63,71]等。对于GPE的样品导入，HG[62-63]是研究最多的方式，但只适用于液相分析物，或部分元素的固体悬浮液/泥浆法[84]进样分析。若将GPE技术用于土壤样品ETV导入时的基体干扰消除，则可充分发挥两种技术的优势，有望实现土壤重金属的直接进样分析。

3.1.1 ETV与GPE联用 目前，GPE与ETV联用的研究还相对较少，只有Hg、Cd、Pb、As等元素可以分别通过金阱（捕Hg）[53]、钨阱（捕Cd）[42,85]和石英阱（捕Pb、As）[7,86]实现气相富集，其中最成功的测汞仪和测镉仪已实现商品化，仪器检出限可达pg—ng级别，可用于土壤的固体进样快速检测。但是，目前尚无研究将金、钨、石英等捕获材料有效耦合，实现多种目标重金属元素的同时捕获，这也是ETV-GPE技术的难点之一。对于土壤的基体干扰，由于Hg的蒸发温度较低，不易蒸出大量干扰基体，因此催化热解-金汞齐原理的测汞仪可以完全消除测Hg时的干扰，实现溶液标准曲线的直接定值。但是，蒸发温度越高，随蒸发携带的基体干扰则更为复杂，由于尚未摸清具体干扰的来源和作用机制，目前还很难完全消除土壤中Cd、Pb、As等元素的基体干扰，仍需使用标准加入法或基体匹配法予以校正。

3.1.2 介质阻挡放电GPE技术 DBD亦称无声放电，是一种常温常压下非平衡态交流放电技术，也是一种产生低温等离子的有效方式，常用作原子光谱的原子化器、发射光谱激发源和化学蒸气发生源[87-89]。KRATZER等[90]最早报道了DBD作为原子化器时Bi的残留问题，但未能实现捕获和释放过程的精确控制。LIU等[62-63]利用同轴型双介质层DBD装置，构建了氢化物发生（hydride generation，HG）DBD-AFS系统，实现了O2和H2气氛切换下DBD 对As捕获/释放的精确控制，As的LOD可以达到1.0 ng·L-1，分析灵敏度提高一个数量级。与石英、石墨炉、金属等GPE技术相比，DBD能够实现常温下对待测元素的捕获和释放，无需加热过程，释放快，并可避免因温度反复变化而出现的捕获材料退化问题。

此外，由于DBD中包含紫外辐射以及大量自由基、离子、激发态原子、分子碎片等化学性质异常活跃物质，对有机物具有非常好的降解效果[89,91-93]，具有消除ETV导入的有机干扰物的能力。本课题组已经研发了相关的干扰消除装置，并申请了国家发明专利。总体来看，DBD装置价格低廉、结构简单、尺寸小巧，通用性强，HG和ETV[89]均可实现样品导入，是一种简单、低耗、易控的捕获/释放元素手段和有机干扰消除技术，有潜力在ETV-GPE中得到广泛应用。

3.2 基体改进剂技术

又称化学改进剂，是改善基体干扰最常用的方法，如钯等铂系金属固体颗粒[44]或盐[94]、磷酸盐[55]、铵盐[95]、镁盐[96]、Triton X-100[97]、四甲基氢氧化铵[98]、NaF[99]、PTFE[100]、8-羟基喹啉[101]以及混合改进剂[102]等，通过与干扰元素或待测元素结合以降低或提高蒸发温度而与基体分离，还有改善原子化环境、助熔、避免难熔物产生等作用。目前，上述基体改进剂在液体进样系统的GF-AAS上使用最多。然而，由于改进剂需要与样品中待测元素或干扰物质充分反应才能发挥作用，因此与固体的土壤粉末样品很难充分接触。有研究[103]采用悬浮液技术制备土壤样品，再与基体改进剂混合使用，但是土壤的悬浮液制备难度较大，同时干扰消除效果也非常有限。其实，原先在催化剂合成领域广泛使用的“自发单层分散”（spontaneous monolayer dispersion，SMD）理论[104]，恰好具备在ETV过程中实现的可能，也就是说样品中的待测元素可以通过“载体”表面的自发分散，而与基体干扰物分离，从而有可能消除ETV的化学干扰。

此外，最早由SHUTTLER提出的持久性化学改进剂也是常用的基体改进技术[105]，主要使用高熔点金属Ir、Pd、Pt、Rh、Ru等，生成难熔化合物的Hf、Mo、Nb、Re、Ta、Ti、V、W、Zr等，以及生成共价碳化物的B、Si等[100]，或者上述改进剂的混合使用。持久性化学改进剂主要采用涂覆、镀层等方式用于原子化器、电热蒸发器，可反复使用，并延长加热装置寿命。但为了减少持久性化学改进剂的损失，必须严格控制加热的温度。由于直接ETV导入的固体土壤样品基体过于复杂，在持久性化学改进剂的实际使用过程中，不仅基体干扰消除作用效果不明显，而且会严重缩短其使用寿命。

3.3 标准加入与基体匹配法

基体匹配或标准加入法做标线也是一种可行的基体干扰校正方法[106]，若有条件获得基体标物，其固体便携的特点也较为适于现场操作，可以提前准备好标曲所需的基体样品，在现场直接上机使用。如OLESZCZUK等[106]在研究中利用固体标准物质校准检测咖啡样品中的Cu，LOD为0.06 μg·g-1。但是，与基体相近或待测元素浓度已知的标准物质不易获得，需要提前筛选大量的典型样品基体，构建样品库进行基体分类与校正，并且现场需要预判样品适用的基体类型，大大增加了分析难度。其实，基体匹配的方法只能校正与浓度无关的干扰，终究只是一种有限的补救措施。

3.4 其他基体干扰消除技术

其他一些技术也可以实现部分元素或特定条件下的基体干扰消除，如塞曼背景校正、程序升温、连续光源与高分辨率分光系统的背景校正等。德国Jena公司在固体进样GF-AAS中将高聚焦短弧氙灯光源、中阶梯光栅高分辨率分光系统和CCD检测器技术联用发挥到了极致，通过背景校正技术实现了毫克级样品的直接进样与基体干扰消除[107-108]，但对于复杂基质的土壤样品仍显得捉襟见肘。另外，塞曼扣背景是AAS常用的背景校正技术，在ETV进样方面应用最为成功的是Lumex公司生产的基于塞曼背景校正AAS技术的测汞仪，对于基体较为简单的土壤样品或者进样量较少的情况下，可以实现溶液标准曲线直接定值。LIU等[53]利用Hg和Cd元素的蒸发温度差，实现了样品中两种元素的顺序蒸发，并利用金阱、钨阱分别捕获Hg和Cd，LOD分别为0.07、0.05 μg·kg-1，这也是利用程序升温技术消除基体干扰的一种尝试。

4 土壤重金属快速检测技术发展的思考

当前，土壤重金属的现场快速检测已经列入国家重大科研需求。要实现土壤重金属现场、快速、准确测定的目标，无论是样品消解还是提取处理都无法满足实际的检测需求，因此无需消解处理的固体进样元素分析技术已成为最可行的技术方案。在诸多的固体进样元素分析技术中，XRF、LIBS和ETV均具有自身的优劣势，其中XRF有可能解决土壤重金属mg·kg-1级或亚mg·kg-1级的检测需求，也是当前重金属快速检测仪器市场表现较为突出的技术。但是，若要进一步提升固体进样技术的分析灵敏度，特别是XRF难以解决的Hg和Cd等痕量重金属的检测问题，ETV技术则具有特殊的优势。ETV作为一种简单、高效、通用性强的进样方式，在样品导入过程中几乎不会损失，因此具有较高的样品导入效率；同时，适用于吸收、荧光、发射等多种原子光谱检测器以及无机质谱，可以达到pg级的绝对检出能力，因此是一种极具潜力的固体样品导入技术。但是，由于固体样品复杂基质的直接导入，不可避免地带来不同程度的基体干扰，从而影响测试的灵敏度、准确度和精密度。特别对ETV技术，其蒸发出来的大量基体气溶胶可能对仪器的各个系统带来不利影响，因此基体干扰一直是限制其广泛应用的瓶颈。总体来说，当前急需系统性地研究ETV过程中土壤基体干扰的来源及作用机制，以期为消除干扰提出理论支撑。同时，通过GPE、DBD[109]、基体改进、背景校正等技术的创新与集成，有望实现土壤基体干扰的有效消除，构建电热固体进样的重金属速测仪器设备。这也是固体进样元素分析技术体系中非常重要的一个环节，从而最终为实现土壤重金属的现场、快速、准确测定提供可靠的技术手段。

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Technical Review of Fast Detection of Heavy Metals in Soil

MAO XueFei, LIU JiXin, QIAN YongZhong

(Institute of Quality Standard and Testing Technology for Agri-Products, Chinese Academy of Agricultural Sciences / Key Laboratory of Agri-Food Safety and Quality, Ministry of Agriculture and Rural Affairs, Beijing 100081)

Recently, with the high-speed development of industry and agriculture in China, the contamination of heavy metal has become a severe environmental problem caused by immoderate mining operation, "three wastes" emissions, vehicle exhaust, and misuse of agricultural chemical inputs. So, it is very important to monitor the contamination of heavy metals in soil. However, the national and industrial standards of detecting heavy metals in soil mainly focus on the traditional analytical approaches employed in laboratory at present. So, it is still difficult to achieve the on-site and fast detection of heavy metals in soil, which gives rise to such difficulty of monitoring and preventing the source contamination effectively and timely. In view of the matrix of soil sample, solid sampling analysis should be feasible to the fast detection of heavy metals, including electrothermal vaporization (ETV), X-ray fluorescence spectrometry (XRF), laser ablation (LA), laser induced breakthrough spectrometry (LIBS), X-ray photoelectron spectroscopy (XPS), and instrumental neutron activation analysis (INAA). The solid sampling techniques do not require digestion treatment and is thereby fast and efficient. However, among them, the detection limit and stability of XRF, LA, and LIBS cannot satisfy the all demands in standards of soil quality; on the other hand, it is too difficult to reach the miniaturization and on-site testing for LA, XAS, and INAA. By comparison, ETV is a kind of solid sampling tool with excellent advantages such as high analytical sensitivity, favorable stability, and being easy to be miniaturized, using electric heating to introduce analysesaerosol from the sample into the atomizer or exciter for measurement. ETV is able to introduce heavy metals fast and efficiently, which is versatile to atomic absorption spectrometry (AAS), atomic fluorescence spectrometry (AFS), atomic emission spectrometry (AES), and induced coupled plasma mass spectrometry (ICP-MS). As usual, many materials such as carbon, metals, and quartz can be utilized for ETV, which are frequently processed into graphite furnace, porous carbon tube, tungsten coil, rhenium coil, quartz tube and so on. Among various ETV approaches, electromagnetic induction ETV is characterized with no cold zone, fast heating or cooling and miniaturization. Considering the complicated soil matrices, however, ETV has been always confronted with the bottleneck problem, namely matrix interference. Through integrating these advanced techniques including gas phase enrichment (GPE), dielectric barrier discharge, matrix modifier, background correction and so on, the matrix interference will be eliminated completely for the detection of heavy metals in soil when solid sampling by using ETV atomic spectrometers. Especially for GPE, it can realize both two aims at one time: eliminating matrix interference and improving analytical sensitivity. This review is about to bring some valuable suggestions for innovating the fast detection of heavy metals in soil, to play parts in the environmental monitoring and protection in the future.

soil; heavy metals; fast detection; electrothermal vaporization; solid sampling; matrix interference

2019-04-28；

2019-08-22

国家重点研发计划（2017YFD0801203、2017YFF0108203）、国家自然科学基金面上项目（31571924）、中国农业科学院基本科研业务费专项（Y2019XK05）

毛雪飞，E-mail：mxf08@163.com & maoxuefei@caas.cn。通信作者钱永忠，Tel：010-82106298；E-mail：qyzcaas@163.com。通信作者刘霁欣，Tel：010-82106540；E-mail：ljx2117@gmail.com

（责任编辑 李云霞）