土壤重金屬快速檢測技術研究進展

毛雪飛，劉霽欣，錢永忠

土壤重金屬快速檢測技術研究進展

毛雪飛，劉霽欣，錢永忠

（中國農業科學院農業質量標準與檢測技術研究所/農業農村部農產品質量安全重點實驗室，北京 100081）

近年來，隨著我國工農業的高速發展，尤其是無節制的礦藏開采、“三廢”排放、汽車尾氣以及農業化學投入品的濫用，重金屬污染已成為我國當前最嚴重的環境污染問題之一，因此土壤重金屬監測工作顯得尤為重要。但是，目前的土壤重金屬檢測標準方法仍以實驗室確證性分析為主，無法用于土壤重金屬的現場、快速分析，從而難以從源頭上及時、有效地對土壤重金屬污染進行監測和預防，開發重金屬快速檢測設備和技術勢在必行。從土壤樣品的基質特點來看，固體進樣分析是最可行的技術方案，主要包括電熱蒸發（ETV）原子光譜、X射線熒光光譜（XRF）、激光燒蝕（LA）、激光誘導擊穿光譜（LIBS）、X射線吸收光譜（XAS）、中子活化（INAA）等。上述固體進樣分析技術均無需樣品消解處理，高效、快捷，但是部分技術的檢出能力和穩定性尚難以滿足土壤質量標準的全部要求，如XRF、LA、LIBS，還有部分技術難以實現現場化，如LA、XAS、INAA等。因此，基于電熱蒸發（ETV）固體進樣的原子光譜分析技術在分析靈敏度、穩定性和小型化方面具有特殊的優勢。ETV是利用電加熱將樣品中的待測元素以氣溶膠的形式導入原子化器或激發源的技術，可實現土壤中常見重金屬元素的快速、高效導入，技術簡單、通用性強，適用于原子吸收、原子熒光、原子發射、無機質譜等多種檢測系統。ETV常采用碳、金屬、石英等材料，如石墨管、多孔碳管、鎢絲、錸絲、石英管，其中利用高熔點金屬的電磁感應電熱蒸發技術具有無冷點、升/降溫速度快、易于小型化的優勢。但是，土壤樣品基質復雜，基體干擾一直是困擾ETV技術應用的核心瓶頸問題。新型的氣相富集（GPE）、介質阻擋放電（DBD）、基體改進及背景校正等技術，有望實現土壤基體干擾的有效消除。特別是GPE技術，在特異性捕獲消除基體干擾的同時，還可以通過預富集提高儀器的分析靈敏度。通過上述技術的集成與創新，可以有效解決固體進樣的分析靈敏度和基體干擾問題，這將為土壤重金屬速測技術的研發提供新的思路，從而為土壤環境監測與治理工作提供有效的技術支撐。

土壤；重金屬；快速檢測；電熱蒸發；固體進樣；基體干擾

0 引言

近年來，隨著我國工農業的高速發展，尤其是無節制的礦藏開采、“三廢”排放、汽車尾氣以及農業化學投入品的濫用，重金屬污染已成為我國當前最嚴重的環境污染問題之一。2014年4月，環境保護部與國土資源部聯合發布的《全國土壤污染狀況調查公報》顯示：全國耕地土壤點位污染超標率達19.4%，主要污染物為Cd、Ni、Cu、As、Hg、Pb等重金屬，會直接或間接影響到食品安全和人體健康。為了及時掌握、預警土壤中重金屬的污染情況，2016年國務院啟動了《土壤污染防治行動計劃》（土十條），提出了土壤環境質量監測點位對所有縣（市、區）全覆蓋的要求，在“土壤詳查”等工作中每年花費大量人力、物力和財力開展重金屬污染監測。目前，土壤重金屬檢測的標準方法仍以實驗室確證性分析為主，如原子吸收光譜法（atomic absorption spectrometry，AAS）[1]、原子熒光光譜法（atomic fluorescence spectrometry，AFS）[2]、電感耦合等離子體發射光譜法（induced coupled plasma optical emission spectrometry，ICP-OES）[3]、電感耦合等離子體質譜法（induced coupled plasma mass spectrometry，ICP-MS）[4]、波長色散型X射線熒光光譜法（wavelength dispersion X-ray fluorescence，WDXRF）[5]等。但是，上述方法需要復雜、耗時的樣品制備以及消解或提取處理，或較大的儀器尺寸和復雜精密的硬件配置，無法用于土壤的現場、快速分析，從而難以從源頭上及時、有效地對土壤重金屬污染進行監測和預防。因此，本文對當前重金屬快速檢測儀器與方法，及其對土壤樣品來說最核心的技術瓶頸——“基體干擾”問題進行綜述，以期為土壤環境監測與治理研究提供技術參考。

1 土壤重金屬的固體進樣分析技術概述

與現有的液體進樣原子光譜方法相比，固體進樣分析是最有可能實現土壤重金屬現場快速檢測的技術，其無需消解處理，具有較高的樣品導入效率，可有效提高分析靈敏度、縮短分析時間、避免痕量元素的損失，同時減少有害化學試劑的使用，更加環保和安全。實際上，早期的原子光譜儀器大多基于固體樣品的直接分析，如鉀鹽、鈉鹽火焰發射譜線的觀測?，F代意義上的固體進樣原子光譜儀器，可以追溯到1957年L’VOV[6]利用石墨坩堝電熱蒸發固體NaCl，采用AAS測定。這是現代石墨爐AAS（graphite furnace atomic absorption spectrometry，GF-AAS）的雛形，后經Massmann改進為管式石墨爐，并由L’VOV設計增加了原子化平臺以改善空間溫度分布。

目前，可實現固體進樣元素分析的技術，主要包括電熱蒸發（electrothermal vaporization，ETV）[7]、XRF[8]、激光燒蝕（laser ablation，LA）[9]、激光誘導擊穿光譜（laser induced breakthrough spectrometry，LIBS）[10]、X射線光電子能譜（X-ray photoelectron spectroscopy，XPS）[11]、X射線吸收光譜（X-ray absorption spectroscopy，XAS）[12]、中子活化（instrumental neutron activation analysis，INAA）[13]等。其中，電熱蒸發（electrothermal vaporization，ETV）所需的材質來源廣，易改裝和小型化，通用性強，是研究和應用最多的固體進樣分析技術之一，將在下文予以專門闡述。

1.1 X射線熒光光譜法

XRF是指原子在X射線或粒子的激發下發射X射線熒光（0.01—10 nm），檢測器測量熒光的波長和強度從而實現元素的定性和定量分析。除了XRF，X射線光譜還有X射線衍射（X-ray diffraction，XRD）、XPS等。XRF是一種性能優異的多元素分析手段，分析速度快，其中能量色散型XRF（energy dispersive X-ray fluorescence spectrometry，EDXRF）結構簡單、功率較小，易于小型化，可用于現場快速檢測，已在地礦、鋼鐵、材料等領域獲得廣泛應用[5, 8, 14-16]。但是，EDXRF的能量分辨率較差，對背景干擾較為敏感，常采用基體匹配作為標準曲線策略，同時對光源、濾光片、靶材、算法等加以優化和改進，以提高分析靈敏度和消除基體干擾[14,16]。當前的便攜式XRF對于土壤中Cu、Zn、As、Pb、Ni、Cr等元素的檢出能力可以達到土壤質量標準的要求，仍難以滿足土壤中亞mg·kg-1級Cd和Hg元素的精準測定需求。

1.2 激光樣品導入技術

LA[17]和LIBS[18]是利用高功率脈沖激光聚焦到固體樣品表面，使樣品等離子化或蒸發后以氣溶膠形式傳輸進入檢測器，簡單、快速，空間分辨率高，可用于元素的微區分析。其中，LA須與ICP-MS聯用，無法現場使用；LIBS構造簡單，易于小型化、現場化。但是，由于激光激發樣品的絕對量過小，其分析靈敏度多在mg·kg-1以上級，同時受限于樣品表面特性對激光的吸收和激發效能差異，分析穩定性仍是限制LA和LIBS進一步發展應用的主要因素。目前，LA和LIBS主要用于常/微量元素的定性或半定量分析，還很難用于痕量重金屬的快速檢測。

1.3 等離子體進樣技術

分析儀器中常用的等離子體包括ICP、微波等離子體（MWP）等“高溫”等離子體，以及介質阻擋放電（dielectric barrier discharge，DBD）、輝光放電、尖端放電等低溫等離子體（LTP），其蘊含豐富的能量可以激發或刻蝕固體樣品[19-22]。如DUAN等[19]研發了一種MWP-AES的直接固體進樣裝置，通過等離子體與固體樣品直接作用，將樣品加熱、使其元素原子化并連續激發，成功地測定了地質樣品中Cu、Pb、Cr和Co等元素的含量。內炬蒸發（ITV）技術是利用ICP的“高溫”等離子體作用將樣品中的元素蒸發和激發，再導入發射光譜或質譜檢測器，實現了樣品激發與導入的“無縫”銜接，極大地提高了傳輸效率[23]。目前，ITV-ICP技術已用于部分生物和水樣品中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“高溫”等離子體技術的固體進樣儀器裝置依然存在功耗高、體積大的問題，難以用于現場快速分析。

此外，LTP也具有導入固體樣品中元素的能力，如XING等[20]將DBD-LTP探針與ICP-MS聯用，剝蝕薄層材料中的待測元素并轉化成氣溶膠，再導入檢測器；XING等[21]還利用該技術對巖石樣品中元素進行二維成像。相對于LA和LIBS，DBD-LTP探針結構簡單、成本低廉，也是一種可行的微區分析技術[22]。但是，DBD-LTP的激發能力還不夠強，刻蝕的均勻性和分析的穩定性還需進一步提高。

1.4 其他固體進樣技術

此外，XAS、INAA等儀器[12-13]可多元素分析，靈敏度高，其中XAS還可給出元素形態的特征信息。但是，XAS作為同步輻射大科學裝置，而INAA是一種放射性分析技術，兩者都無法用于土壤樣品的現場快速檢測。

2 電熱蒸發固體進樣技術

ETV是利用電加熱將樣品中的待測元素以干燥氣溶膠的形式導入原子化器或激發源的技術，有時ETV自身也是原子化器或激發源。ETV技術簡單，常采用碳、金屬、石英等材料；樣品導入快、進樣效率高，土壤中常見的重金屬元素都可以通過ETV實現導入[26-30]；同時，適用于原子吸收、原子熒光、原子發射、無機質譜（ICP-MS）等多種檢測系統，通用性非常強。

2.1 碳材料ETV技術

碳材料ETV裝置最早源于L’VOV石墨坩堝原子化器，多制成管、舟、杯狀等作為進樣器[31-34]。其中，石墨爐原子吸收（graphite furnace atomic absorption spectrometry，GF-AAS）的石墨管是最常用的ETV裝置，在ETV-AAS上使用時既是蒸發器也是原子化器，但與ICP-MS/OES聯用時僅作為蒸發器，完成樣品的干燥、灰化和蒸發過程[35]。還有研究將W[36]、Ir[37]、Ta[33]等高熔點金屬熱解或電鍍在進樣器和石墨管表面，可有效改善管內溫度分布、提高碳材料壽命。目前，基于碳材料的ETV技術已成功用于土壤、地礦、食品等樣品[34, 28-40]中Cd、Cu、Zn、Hg、Ni、Mn、Pb、As、Cr等數十種元素的分析。

但是，常用的石墨管ETV耗電高、散熱慢，需要復雜的冷卻和電源系統，難以小型化，如德國Jena公司生產的基于高聚焦短弧氙燈光源、中階梯光柵高分辨率分光系統和電荷耦合器件圖像傳感器（charge coupled device，CCD）檢測器的GF-AAS[41]。由于石墨爐的樣品承載量有限，必須將土壤樣品處理成干燥、均質的粉末才能置入GF進行蒸發，一般在毫克級，這大大增加了樣品制備和分析過程的難度。LIU等[42]對多孔碳材料進行了改良，穩定性與石墨管接近，但蒸發功耗僅需0.3—0.5 kW，遠低于后者，加上多孔碳自身的空隙大，因此散熱快，無需額外冷卻系統，對土壤中Cd的方法檢出限（limit of detection，LOD）可以達到μg·kg-1以下。同時，多孔碳樣品舟的樣品承載量可以達到100 mg，這些都為碳材料ETV裝置小型化及應用提供了思路。

2.2 金屬材料ETV技術

高熔點金屬也是良好的ETV材料，如W、Mo、Pt、Ta、Re等，可制成絲、舟、管、片等形狀[37, 43]。金屬鎢具有良好的導電導熱性能和延展性，熔點高、化學惰性，并且成本低、易獲取，是當前常用的ETV金屬材料。其中鎢絲（tungsten coil，TC）應用最為廣泛，可與AAS、AFS、AES和ICP等串聯[44-45]，易于儀器小型化和便攜化，且不像GF那樣易與樣品成分產生難蒸發的碳化物，從而影響測定重現性。如HOU等[44]首次將TC-ETV直接插入Ar-H2火焰石英管原子化器中，形成了一種新型、小型化、緊湊的AFS進樣裝置，對Cd，Pb，Au和Ag等元素的絕對LOD分別可以達到0.02、0.6、2.5、4.4 pg；HOU等[45]比較了TC與不同的檢測器串聯的檢測技術，其中，TC-ETV-AAS中Cd的LOD為10 pg，而TC-ETV-AFS中As、Se、Cr、Sb和Pb的LOD分別為950、320、1400、330和160 fg（TC-ETV-AAS、TC-ETV-AFS的檢出能力在pg級，甚至更低）；當與ICP串聯，其LOD與傳統ICP檢測器接近。為了防止金屬材料的氧化，使用TC時常在氬氣中補充氫氣作為還原性保護氣氛，并有提供自由基源、促進原子化的作用。

TC-ETV用于土壤樣品導入的主要難點在于樣品承載和基體干擾。由于TC無支撐結構，無法承載固體樣品，因此一般多將土壤樣品制成懸浮液再導入TC，依靠表面張力承載，但最大也僅能加載10—20 μL。若將高熔點金屬制成舟或杯狀，雖然可以承載固體樣品[46-47]，如OKAMOTO等[46]利用鎢舟承載樣品并電熱蒸發，ICP-MS做為檢測器測定固體生物樣品中Cd的LOD為0.84 ng·g-1。但是對于直接電阻加熱方式，加熱體質量或尺寸過大會導致過高的功率需求，以及升溫、導熱等性能的限制，這就失去了TC那樣的特殊優勢。而且，部分研究中仍選擇將樣品消解后再加熱蒸發消解液進行元素測定[47]，依然難以實現現場快速檢測。

實際上，不管是碳材料還是高熔點金屬材料ETV，一般多采用電阻式加熱，必須在加熱體兩端引入電極，所以電極的接觸好壞對加熱的影響極大，而且易于損壞；另一方面，為保證電極的良好接觸，往往會在電極附近形成一個低溫區域，這個區域也容易造成元素的殘留，影響測定的穩定性。本課題組研制的電磁感應電熱蒸發器（inductive electrothermal vaporization，IETV），通過強制電流趨膚增加表面電阻，突破了常規鐵磁性金屬材料居里點1 200℃以上失磁無法繼續加熱的瓶頸，實現了非接觸ETV進樣。初步實驗結果表明，IETV裝置升溫速率＞300 ℃·s-1、最高溫度＞2 000 ℃，實現了樣品中Cd、Hg、As、Pb等元素的全部導入。

2.3 石英材料ETV技術

石英管（quartz tube，QT）是原子光譜中常用的原子化器，也可用于ETV，但受自身材料局限，耐受溫度最高僅有1 000 ℃左右，因此主要適用于Hg、Cd、Pb等中低溫元素。如基于催化熱解原理的測汞儀，一般采用QT-ETV來實現樣品中Hg的快速導入[48-50]，可用AAS或AFS檢測，是目前最為成功的基于ETV技術的商品化儀器，如有研究分別利用ETV-AFS與ETV-AAS檢測土壤中的Hg，其LOD分別為0.08、0.5 μg·kg-1[48-49]。針對測Hg時的基體干擾，主要采用金汞齊或塞曼扣背景的技術予以消除。此外，LIU和MAO等[7]研制了一體化石英管，前段QT-ETV作為進樣系統，后段QT作為氣相富集（gas phase enrichment，GPE）裝置用于預富集Pb，實現了食品樣品中Pb的直接固體進樣分析，方法LOD達到2—3 pg[7,51]。

3 基體干擾及消除技術

土壤樣品基質成分復雜，除了大量的礦物質，還富含微生物、腐殖質等有機組分，直接電熱蒸發會導入大量的基體，如土壤樣品中共存元素的化學干擾、復雜基質的背景干擾、微顆粒物的物理干擾、分子吸收干擾、特定元素的電離干擾等，可能對儀器的各個系統帶來不利影響，因此基體干擾一直是限制ETV技術廣泛應用的瓶頸。為了實現固體進樣元素分析的準確測定，研究者嘗試多種技術手段來消除基體干擾，如GPE[52-54]、基體改進劑[43,55-57]、標準加入法[30]、基體匹配[58]、塞曼背景校正[59]、程序升溫[7, 52-54,60]以及化學計量學校正[61]等。

3.1 氣相富集技術

GPE主要以氣相分析物為對象，如ETV[51-54]、化學蒸氣發生（chemical vapor generation，CVG）[62-63]都是較好的氣相導入技術，所以GPE裝置大多直接耦合到原子光譜儀器中，效率高、通用性強。早期的原子光譜儀器由于分析靈敏度不高，為了提高元素檢出能力，常采用氣球法[64]、冷凝法[65]、溶液/固體吸附[66-68]等方法對氣相分析物進行預富集。這是GPE技術最初的用途，當時的GPE裝置結構復雜、操作繁瑣。近年來，隨著材料、等離子體等技術的發展，與GPE密切相關的材料表面改性、快速升降溫、微等離子體放電均可簡單實現，這為GPE的發展提供了新的思路。

GPE在儀器系統中的主要作用有兩點：一是通過預富集提高分析靈敏度；二是通過與基體干擾物的分離作用降低或消除基體干擾。常見的GPE捕獲材料主要有石英、碳、金屬材料等，如石英管[62-63,69-71]、石墨爐[72-75]、金絲[53,76-77]、鎢絲[42,52-54,78-80]、鍍貴金屬的鎢絲[81-82]等；捕獲/釋放方式主要有冷/熱處理[65,83]、合金捕獲[53,60]、放電捕獲/釋放[62-63,71]等。對于GPE的樣品導入，HG[62-63]是研究最多的方式，但只適用于液相分析物，或部分元素的固體懸浮液/泥漿法[84]進樣分析。若將GPE技術用于土壤樣品ETV導入時的基體干擾消除，則可充分發揮兩種技術的優勢，有望實現土壤重金屬的直接進樣分析。

3.1.1 ETV與GPE聯用 目前，GPE與ETV聯用的研究還相對較少，只有Hg、Cd、Pb、As等元素可以分別通過金阱（捕Hg）[53]、鎢阱（捕Cd）[42,85]和石英阱（捕Pb、As）[7,86]實現氣相富集，其中最成功的測汞儀和測鎘儀已實現商品化，儀器檢出限可達pg—ng級別，可用于土壤的固體進樣快速檢測。但是，目前尚無研究將金、鎢、石英等捕獲材料有效耦合，實現多種目標重金屬元素的同時捕獲，這也是ETV-GPE技術的難點之一。對于土壤的基體干擾，由于Hg的蒸發溫度較低，不易蒸出大量干擾基體，因此催化熱解-金汞齊原理的測汞儀可以完全消除測Hg時的干擾，實現溶液標準曲線的直接定值。但是，蒸發溫度越高，隨蒸發攜帶的基體干擾則更為復雜，由于尚未摸清具體干擾的來源和作用機制，目前還很難完全消除土壤中Cd、Pb、As等元素的基體干擾，仍需使用標準加入法或基體匹配法予以校正。

3.1.2 介質阻擋放電GPE技術 DBD亦稱無聲放電，是一種常溫常壓下非平衡態交流放電技術，也是一種產生低溫等離子的有效方式，常用作原子光譜的原子化器、發射光譜激發源和化學蒸氣發生源[87-89]。KRATZER等[90]最早報道了DBD作為原子化器時Bi的殘留問題，但未能實現捕獲和釋放過程的精確控制。LIU等[62-63]利用同軸型雙介質層DBD裝置，構建了氫化物發生（hydride generation，HG）DBD-AFS系統，實現了O2和H2氣氛切換下DBD 對As捕獲/釋放的精確控制，As的LOD可以達到1.0 ng·L-1，分析靈敏度提高一個數量級。與石英、石墨爐、金屬等GPE技術相比，DBD能夠實現常溫下對待測元素的捕獲和釋放，無需加熱過程，釋放快，并可避免因溫度反復變化而出現的捕獲材料退化問題。

此外，由于DBD中包含紫外輻射以及大量自由基、離子、激發態原子、分子碎片等化學性質異?；钴S物質，對有機物具有非常好的降解效果[89,91-93]，具有消除ETV導入的有機干擾物的能力。本課題組已經研發了相關的干擾消除裝置，并申請了國家發明專利?？傮w來看，DBD裝置價格低廉、結構簡單、尺寸小巧，通用性強，HG和ETV[89]均可實現樣品導入，是一種簡單、低耗、易控的捕獲/釋放元素手段和有機干擾消除技術，有潛力在ETV-GPE中得到廣泛應用。

3.2 基體改進劑技術

又稱化學改進劑，是改善基體干擾最常用的方法，如鈀等鉑系金屬固體顆粒[44]或鹽[94]、磷酸鹽[55]、銨鹽[95]、鎂鹽[96]、Triton X-100[97]、四甲基氫氧化銨[98]、NaF[99]、PTFE[100]、8-羥基喹啉[101]以及混合改進劑[102]等，通過與干擾元素或待測元素結合以降低或提高蒸發溫度而與基體分離，還有改善原子化環境、助熔、避免難熔物產生等作用。目前，上述基體改進劑在液體進樣系統的GF-AAS上使用最多。然而，由于改進劑需要與樣品中待測元素或干擾物質充分反應才能發揮作用，因此與固體的土壤粉末樣品很難充分接觸。有研究[103]采用懸浮液技術制備土壤樣品，再與基體改進劑混合使用，但是土壤的懸浮液制備難度較大，同時干擾消除效果也非常有限。其實，原先在催化劑合成領域廣泛使用的“自發單層分散”（spontaneous monolayer dispersion，SMD）理論[104]，恰好具備在ETV過程中實現的可能，也就是說樣品中的待測元素可以通過“載體”表面的自發分散，而與基體干擾物分離，從而有可能消除ETV的化學干擾。

此外，最早由SHUTTLER提出的持久性化學改進劑也是常用的基體改進技術[105]，主要使用高熔點金屬Ir、Pd、Pt、Rh、Ru等，生成難熔化合物的Hf、Mo、Nb、Re、Ta、Ti、V、W、Zr等，以及生成共價碳化物的B、Si等[100]，或者上述改進劑的混合使用。持久性化學改進劑主要采用涂覆、鍍層等方式用于原子化器、電熱蒸發器，可反復使用，并延長加熱裝置壽命。但為了減少持久性化學改進劑的損失，必須嚴格控制加熱的溫度。由于直接ETV導入的固體土壤樣品基體過于復雜，在持久性化學改進劑的實際使用過程中，不僅基體干擾消除作用效果不明顯，而且會嚴重縮短其使用壽命。

3.3 標準加入與基體匹配法

基體匹配或標準加入法做標線也是一種可行的基體干擾校正方法[106]，若有條件獲得基體標物，其固體便攜的特點也較為適于現場操作，可以提前準備好標曲所需的基體樣品，在現場直接上機使用。如OLESZCZUK等[106]在研究中利用固體標準物質校準檢測咖啡樣品中的Cu，LOD為0.06 μg·g-1。但是，與基體相近或待測元素濃度已知的標準物質不易獲得，需要提前篩選大量的典型樣品基體，構建樣品庫進行基體分類與校正，并且現場需要預判樣品適用的基體類型，大大增加了分析難度。其實，基體匹配的方法只能校正與濃度無關的干擾，終究只是一種有限的補救措施。

3.4 其他基體干擾消除技術

其他一些技術也可以實現部分元素或特定條件下的基體干擾消除，如塞曼背景校正、程序升溫、連續光源與高分辨率分光系統的背景校正等。德國Jena公司在固體進樣GF-AAS中將高聚焦短弧氙燈光源、中階梯光柵高分辨率分光系統和CCD檢測器技術聯用發揮到了極致，通過背景校正技術實現了毫克級樣品的直接進樣與基體干擾消除[107-108]，但對于復雜基質的土壤樣品仍顯得捉襟見肘。另外，塞曼扣背景是AAS常用的背景校正技術，在ETV進樣方面應用最為成功的是Lumex公司生產的基于塞曼背景校正AAS技術的測汞儀，對于基體較為簡單的土壤樣品或者進樣量較少的情況下，可以實現溶液標準曲線直接定值。LIU等[53]利用Hg和Cd元素的蒸發溫度差，實現了樣品中兩種元素的順序蒸發，并利用金阱、鎢阱分別捕獲Hg和Cd，LOD分別為0.07、0.05 μg·kg-1，這也是利用程序升溫技術消除基體干擾的一種嘗試。

4 土壤重金屬快速檢測技術發展的思考

當前，土壤重金屬的現場快速檢測已經列入國家重大科研需求。要實現土壤重金屬現場、快速、準確測定的目標，無論是樣品消解還是提取處理都無法滿足實際的檢測需求，因此無需消解處理的固體進樣元素分析技術已成為最可行的技術方案。在諸多的固體進樣元素分析技術中，XRF、LIBS和ETV均具有自身的優劣勢，其中XRF有可能解決土壤重金屬mg·kg-1級或亞mg·kg-1級的檢測需求，也是當前重金屬快速檢測儀器市場表現較為突出的技術。但是，若要進一步提升固體進樣技術的分析靈敏度，特別是XRF難以解決的Hg和Cd等痕量重金屬的檢測問題，ETV技術則具有特殊的優勢。ETV作為一種簡單、高效、通用性強的進樣方式，在樣品導入過程中幾乎不會損失，因此具有較高的樣品導入效率；同時，適用于吸收、熒光、發射等多種原子光譜檢測器以及無機質譜，可以達到pg級的絕對檢出能力，因此是一種極具潛力的固體樣品導入技術。但是，由于固體樣品復雜基質的直接導入，不可避免地帶來不同程度的基體干擾，從而影響測試的靈敏度、準確度和精密度。特別對ETV技術，其蒸發出來的大量基體氣溶膠可能對儀器的各個系統帶來不利影響，因此基體干擾一直是限制其廣泛應用的瓶頸?？傮w來說，當前急需系統性地研究ETV過程中土壤基體干擾的來源及作用機制，以期為消除干擾提出理論支撐。同時，通過GPE、DBD[109]、基體改進、背景校正等技術的創新與集成，有望實現土壤基體干擾的有效消除，構建電熱固體進樣的重金屬速測儀器設備。這也是固體進樣元素分析技術體系中非常重要的一個環節，從而最終為實現土壤重金屬的現場、快速、準確測定提供可靠的技術手段。

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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

國家重點研發計劃（2017YFD0801203、2017YFF0108203）、國家自然科學基金面上項目（31571924）、中國農業科學院基本科研業務費專項（Y2019XK05）

毛雪飛，E-mail：mxf08@163.com & maoxuefei@caas.cn。通信作者錢永忠，Tel：010-82106298；E-mail：qyzcaas@163.com。通信作者劉霽欣，Tel：010-82106540；E-mail：ljx2117@gmail.com

（責任編輯 李云霞）