Different Ionization Modes in Gas Chromatography-Mass Spectrometric Determination of Organochlorine Pesticides and Polychlorinated Biphenyls in Food
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摘要: 对食品中有机氯农药和多氯联苯的气相色谱-质谱联用(GC-MS)分析方法中三种离子化方式，电子轰击电离(EI)、正化学电离(PCI)和负化学电离(NCI)进行了总结和比较。PCI-MS/MS方法和EI-MS/MS方法都有很高的选择性和较高的灵敏度；PCI方法在分析含硝基、羰基等基团的化合物时有明显优势，EI则在分析狄氏剂、异狄氏剂、硫丹及其代谢物时比PCI表现稍好，而NCI-MS的灵敏度最高，但抗干扰能力稍弱，且不适合分析滴滴涕类和多氯联苯类化合物。在食品安全分析中，三种质谱方法的准确性好，精密度高，检测限较低，都能够满足食品中农残检测的要求，在日常检测工作中可互为补充和替代。同时指出，GC在有机氯化合物分析中仍表现出明显的优越性；常规的GC-MS尤其在EI电离模式下，易受到基质干扰而使谱图变得复杂；新型离子化方式包括高选择性化学电离技术的应用，将是食品安全中GC-MS联用分析的发展方向之一。Abstract: Different modes of electron impact (EI), positive chemical ionization (PCI) and negative chemical ionization (NCI) in Gas Chromatography-Mass Spectrometry (GC-MS) determination of organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) in food are summarized and compared in this paper. Both PCI-MS/MS and EI-MS/MS have high selectivity and sensitivity. PCI-MS/MS has obvious advantages in the analysis of compounds containing nitro or carbonyl groups, while EI-MS/MS has slightly better performance when dealing with dieldrin, endrin, endosulfan and its metabolites. NCI-MS has the highest sensitivity, but weaker anti-interference ability, and is not suitable for the analysis of DDTs and PCBs. In summary, these three mass spectrometry methods, which are complementary and alternative to the routine analysis, can satisfy the requirements of pesticide residue analysis in food by providing good accuracy, good precision and low detection limits.Also it points out that Gas Chromatography showed a superiority in organochlorine compound analysis, but for traditional GC-MS, especially in the EI ionization mode, the spectrum is more complicated because of matrix interference. New ionization modes including the highly selective chemical ionization, will be one of future developments and trends of Gas Chromatography applications in the field of food safety analysis.
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电感耦合等离子体质谱(ICP-MS)具有灵敏度高、干扰少、多元素同时测定、线性范围大、检出限低的特点，适合地质样品中微量级多元素同时测定。对于微量元素的测定通常采用氢氟酸、硝酸在封闭溶样罐中高温、高压分解，该方法能有效分解岩石、矿物中的难溶矿物[1-2]，由于ICP-MS仪器的高灵敏度，大部分微量元素的检出限可达到化探样品分析要求。Ag有两个同位素，107Ag(51.84%)和109Ag(48.16%)，它们分别受到91Zr16O和93Nb16O氧化物离子的强烈干扰，由于化探样品中Zr和Nb含量大大高于Ag，即使用Zr和Nb的单标准氧化物产率进行校正，其结果误差仍然较大，因为其干扰信号强度已经超过了样品中Ag本身的强度。虽然使用膜去溶装置可以大大降低氧化物离子干扰，实现Ag的准确测定[3]；但该装置价格高，拥有膜去溶装置的实验室较少。目前对于化探样品Ag的测定，国内大部分实验室仍然采用传统的发射光谱法[4-9]，该方法费时、费力、结果不稳定。因此，迫切需要对化探样品Ag的测定方法进行改进。
本文应用P507萃淋树脂将用于ICP-MS测定常规微量元素的溶液进行简单的分离，干扰元素Zr和Nb可被有效除去，而Ag和内标元素Rh可被有效回收，实现了化探样品中低含量Ag的快速测定。
1. 实验部分
1.1 仪器及工作条件
Bruker Aurora M90电感耦合等离子体质谱仪(美国布鲁克·道尔顿公司)。在每次测试前，为了降低仪器本底，样品锥和截取锥都需仔细清洗。在5％硝酸溶液中Ag的仪器背景值通常都小于50 cps，使用普通灵敏度模式，仪器灵敏度通常调整为>400000 cps/1 ng/mL 115In，相对标准偏差(RSD)通常小于3％。本实验采用雾化器自吸进样，仪器工作参数见表 1。
1.2 材料与主要试剂
P507萃淋树脂：粒径80 ~120目(北京瑞乐康分离科技有限公司)。
表 1 仪器工作参数Table 1. nstrumental operating parameters of ICP-MSI工作参数 设定值 射频功率 1400 W 反射功率 < 2 W 等离子体气 15 L/min 辅助气 0.90 L/min 护鞘气 0.25 L/min 雾化气 0.95 L/min 扫描次数 5 测定次数 5 每个质量通道数 1 测定方式 Peak Hopping 停留时间 10 ms 样品锥孔径 1 mm 截取锥孔径 0.4 mm 雾化室温度 3℃ 交换柱：采用5 mL塑料移液枪头作为交换柱，底部垫自制聚四氟乙烯棉；称取0.45 g的P507萃淋树脂于烧杯中，加入约5 mL水，转移至交换柱中，待水流尽时，在上部垫一层自制聚四氟乙烯棉；用10 mL的3 mol/L硝酸淋洗，最后用5 mL的5%硝酸平衡交换柱，待用。
封闭溶样器：自制不锈钢-聚四氟乙烯封闭溶样装置，体积10 mL[1]。
多元素混合标准储备溶液：100 μg/mL (Accu-Standard Inc，USA)。
硝酸：通过石英亚沸蒸馏提纯。
氢氟酸：采用聚四氟乙烯对口瓶亚沸蒸馏提纯，实验用水用Millipore纯化装置制备，电阻率18 MΩ·cm。
1.3 实验步骤
准确称取0.0500 g样品于带不锈钢外套的聚四氟乙烯密封溶样装置中，加入1 mL氢氟酸和1 mL硝酸，加盖密封，在烘箱中于185℃加热12 h，取出冷却后在电热板上低温蒸干。最后加入2 mL硝酸、1 mL 500 ng/mL的Rh内标溶液、3 mL水，重新盖上盖密封，放入烘箱中于135℃加热3 h溶解残渣。冷却后取0.4 mL溶液于15 mL离心管中，用5%硝酸稀释至6 mL。该溶液可用于ICP-MS测定常规微量元素。
待微量元素测定完成后，将剩余溶液倒入交换柱中，直至加满交换柱，其余溶液弃去，并立即用水清洗离心管，用原离心管承接，该溶液即可用于以Rh为内标Ag的测定。
2. 结果与讨论
2.1 Ag与Zr和Nb的分离
P507是酸性磷类萃取剂，又名2-乙基己基膦酸单2-乙基己基酯，常用于稀土元素分离以及稀土元素的相互分离[10-13]，在Sm-Nd同位素测定中也常用P507或P204萃淋树脂实现Sm与Nd的相互分离[14]。该树脂的另一个特点是对Ti、Nb、Ta、Zr、Hf、W、Sn和Mo等元素的四价离子强烈吸附，即使用高浓度的盐酸或硝酸也很难将其洗脱下来，只有用氢氟酸才能将这些元素有效洗脱，该类树脂也可用于Lu-Hf同位素分离[15-16]。本研究利用该树脂这一特性，在约1.2 mol/L的硝酸介质中成功地实现了Ag和内标元素Rh与干扰元素Zr和Nb的有效分离。
取200 ng混合标准溶液于15 mL离心管中，用1.2 mol/L硝酸稀释至5 mL，将此溶液过柱，15 mL离心管承接，用4 mL的5%硝酸分两次清洗离心管及交换柱，在承接溶液的离心管中加入100 ng的Rh 内标溶液，最后稀释至10 mL，ICP-MS测定。各元素的回收率见表 2。由表 2可以看出，98%以上的Zr和Nb被P507树脂吸附，而95%以上的Ag和Rh通过交换柱，说明P507萃淋树脂能有效地将Ag和Rh与Zr和Nb分离。
表 2 各元素在P507萃淋树脂上的回收率Table 2. The recovery of elements for P507 levextrel resin元素 回收率/% Zr 0.93 Nb 1.44 Mo 2.83 Sn 0.43 Hf 0.58 Ta 0.42 W 3.32 Cd 103.0 Ag 95.8 Rh 97.5 2.2 方法检出限
按样品前处理同样程序处理5份流程空白，测定结果见表 3。其绝对浓度值3倍标准偏差除以称样量，即为方法的检出限，计算Ag的检出限为0.005 μg/g，低于化探样品分析的检出限要求(0.02 μg/g，见DZ/T 0130.5—2006)。
表 3 方法的空白值Table 3. Blank level of the method空白 m(Ag)/μg 空白1 0.0004 空白2 0.0002 空白3 0.0003 空白4 0.0003 空白 m(Ag)/μg 空白5 0.0004 平均值 0.0003 标准偏差 0.000075 2.3 树脂的再生
交换柱使用后立即用水洗柱一次，然后用3 mol/L硝酸5 mL洗柱1次，再用水洗柱两次，最后用1.2 mol/L硝酸5 mL平衡交换柱，待用。树脂在使用一段时间后，其吸附的Ti、Zr、Nb等元素可能达到饱和，这时树脂吸附效率会降低。一般在使用5~10次后，用2 mol/L氢氟酸5 mL将这些元素洗脱下来，树脂即可继续使用。如果发现用氢氟酸洗脱后交换柱的效率仍然很低，说明P507萃取剂已流失，这时需要更换新树脂。
3. 标准物质分析
按上述分析流程对岩石及土壤系列国家一级标准物质进行分析，本方法的测定结果与标准值基本一致(见表 4)，完全能够满足化探样品分析要求。
表 4 标准物质测定结果Table 4. Analytical results of Ag in reference materials标准物质
编号w(Ag)/(μg·g-1) 标准值 本法测量值 GBW 07103 0.033±0.010 0.026±0.008 GBW 07104 0.071±0.014 0.065±0.010 GBW 07105 0.040±0.012 0.051±0.009 GBW 07106 0.062±0.010 0.055±0.007 GBW 07302 0.066±0.015 0.065±0.008 GBW 07305 0.36±0.04 0.35±0.02 GBW 07306 0.36±0.04 0.31±0.05 GBW 07307 1.05±0.09 1.12±0.08 GBW 07311 3.2±0.5 3.02±0.32 GBW 07312 1.15±0.16 0.99±0.11 4. 结语
利用ICP-MS仪器的碰撞池技术可以消除氧化物离子干扰，但仪器灵敏度会降低，碰撞气体有可能带入新的干扰，Ag也需要进行单独测定；膜去溶装置可去除气溶胶中的大部分水分，降低氧化物离子干扰，提高仪器灵敏度，实现Ag与其他常规微量元素同时测定，但进样时间可能延长，设备也较贵。而本文应用P507萃淋树脂对ICP-MS用于测定常规微量元素的溶液进行简单分离，就可实现化探样品中待测元素Ag和内标元素Rh与干扰元素Zr、Nb的有效分离，Ag的检出限达到0.005 μg/g，低于化探分析要求(0.02 μg/g)。
相比于其他方法，本方法省略了称样及分解等样品前处理步骤；且由于在样品处理过程中加入了内标元素，因此最后的溶液不需要准确定容，待测元素与内标元素都具有很高的回收率，过柱分离的溶液只需3～4 mL即可，节省了时间，提高了分析效率。不足在之处在于：虽然本方法相对于传统的发射光谱法更简单、快速，但Ag也需要进行分离并单独测定。在本方法拓展应用方面，由于高含量W和Mo样品中W可能以单矿物形式存在，需要进行碱熔才能保证分解完全，利用P507萃淋树脂的这一特性，有可能实现W和Mo与大量基体元素和干扰元素的分离富集。
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