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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农产品产出的地球化学条件成为近年来农业地学研究的一个重要领域。土壤类型、地球化学环境和气候条件等是决定名特优农产品分布及其品质的主要因素。板栗作为名特优农产品是河南信阳六大支柱产业之一,种植资源丰富,栽培历史悠久[1],但由于长期管理粗放,施肥不当,造成土壤质量下降,影响了板栗品质的提高,减少了产区果农的经济收入。为了提高板栗的经济效益,许多学者从各自的专业出发,对板栗良种的选择、栽培技术、病虫害防治以及采后保鲜进行了大量的研究[2-6],已有研究表明在排除种子、气候、栽培管理技术等因素外,名特优农产品的特殊品质受当地特定的生态地质因素控制,表现出了与地质环境之间显著的相关性[7- 9]。
本文在已有工作基础上,通过采集信阳地区的土壤和板栗样品进行测试分析,在实验数据的基础上采用相关分析的方法对土壤地球化学背景和板栗品质进行研究,力求从生态环境角度研究板栗的产量和品质与地理、地质、土壤地球化学之间的关系,揭示影响作物生长的土壤地质条件,为合理发展板栗的种植基地和指导板栗的施肥管理提供理论基础。
1. 样品采集与分析方法
1.1 土壤样品采集
根据板栗种植区的土壤类型等因素,在信阳县浉河港乡分4个采样区共采集了30件土壤样品(本文的数据为样点几何平均值)和对应的板栗样品,土壤采样深度为0~20 cm的表层土。4个采样区的土壤类型都为黄棕壤土类。样区1、样区2、样区3为淡岩黄砂石土土属,成土母质为花岗岩类风化的残积-坡积物;样区4分布的岩石地层为泥盆系南湾组,母岩为砂页岩类,为黄岗土土属。
1.2 土壤样品分析方法
分析项目及方法[10]如下。①全量氮、氧化钾、氧化钙、硼:硝酸-盐酸消化,X射线荧光光谱仪测定(型号ZSX 100,日本理学公司);②全量砷:原子荧光光度计测定(型号XDY-I,上海三科仪器有限公司);③汞:硝酸-高氯酸消解,原子荧光光谱仪测定(型号XGY-1011A,廊坊开元高技术开发公司公司);④有效铁、有效锰、有效铜、有效锌:DTPA浸提,原子吸收分光光度计测定(型号GGX-9,北京海光仪器公司);⑤有效磷、速效钾:硝酸-高氯酸消化,IRIS Intrepid电感耦合等离子体发射光谱仪测定(型号GGX-9,美国Thermo公司);⑥氨态氮:紫外分光光度仪测定(型号U-2001,上海元析仪器有限公司);⑦pH值:pH计法测定;⑧容重:环刀法测定;⑨有机质:K2Cr2O7容量法-外加热法测定;⑩阳离子交换量(CEC):乙酸钠火焰光度计测定。
采集的板栗样品送至实验室测定其中的N、P、K、Ca、Mg、B、Cu、Fe、Mn、Se、Zn 等11种元素和蛋白质、还原糖、淀粉、蔗糖等有机物质。其中无机元素分析测试方法同土壤样品;粗蛋白质采用开氏法(国家标准方法)测定,还原糖用铜还原直接滴定法,淀粉用蒽酮法测定,蔗糖用盐酸水解法测定。
2. 板栗主要成分分析和营养成分综合评价
2.1 板栗主要成分的含量水平
土壤的生态地质环境条件影响着土壤中矿质元素的赋存形态,并进而影响到元素在土壤中的迁移分布和在植株体内的吸收和富集[11]。由于气候条件、土壤条件及管理水平的差异,信阳板栗果实中的淀粉、蔗糖、粗蛋白质、还原糖及N、P、K等一些无机元素也各不相同。信阳板栗的主要成分含量见表 1。
有机物质的含量范围:蔗糖含量在35.4~43.9 mg/g之间,平均值为41.1 mg/g;淀粉含量在230~290 mg/g,平均值为254.4 mg/g;粗蛋白质含量在34.4~38.8 mg/g,平均值为36.5 mg/g;还原糖在9.5~12.1 mg/g,平均值为10.6 mg/g。
各无机元素的含量范围:N为11.0~15.2 mg/g,P为0.786~0.884 mg/g,K为0.140~0.144 mg/g,Ca为0.137~0.180 mg/g,Mg为0.53~0.62 mg/g,B为6.18~11.20 mg/kg,Fe为8.17~11.35 mg/kg,Mn为29.26~50.71 mg/kg,Cu为3.40~4.60 mg/kg,Zn为6.10~6.20 mg/kg,Se为0.006~0.012 mg/kg。有毒有害元素As含量为0.042 mg/kg,Hg含量为0.003 mg/kg,这两种元素的含量都大大低于食品中重金属限量卫生标准(As < 0.5 mg/kg,Hg < 0.01 mg/kg)。
2.2 板栗营养成分的综合评价
根据板栗果实的主要营养成分对板栗品质的影响大小,综合专家的意见,参考1989年中国农业部苹果、梨果实品质评定的权重标准和有关资料[12-13],给板栗果实的各主要营养成分赋予权重值,见表 1。
表 1 板栗果实中主要成分含量及其权重分配Table 1. The main ingredients and weighted distribution of chestnut fruit成分 w/(mg·g-1) 权重分配值 样区1 样区2 样区3 样区4 淀粉 252 249 290 230 0.25 蔗糖 42.2 43.3 43.9 35.4 0.30 粗蛋白质 34.5 38.8 34.4 38.5 0.20 还原糖 9.5 10.8 10.0 12.1 0.05 N 14.2 13.4 11.0 15.2 0.005 P 0.787 0.807 0.786 0.884 0.025 K 0.140 0.142 0.144 0.141 0.004 Ca 0.151 0.137 0.172 0.180 0.03 Mg 0.61 0.62 0.53 0.57 0.004 B 8.10 10.20 11.20 6.18 0.05 Fe 8.17 11.35 8.82 10.26 0.025 Mn 29.26 34.58 50.71 28.54 0.04 Cu 4.6 3.4 3.5 3.4 0.003 Zn 6.1 6.2 6.1 6.1 0.004 Se 0.009 0.008 0.006 0.012 0.01 如果以板栗的品质指数(QI值)表示板栗综合品质水平,QI值就可以衡量板栗样品间的品质水平差异,并可以进行不同产地、不同时间板栗品质水平比较。首先对表 1中的数据进行正规化处理,得出数值
,然后根据公式 ,求出QI值。其中xi为板栗果实中营养成分的原始数值,xmin为板栗果实营养成分数值中的最小值,xmax为板栗果实营养成分数值中的最大值,Ci是对xi进行正规化处理后的数值,Ri为对应测定项目在板栗品质中的重要性(权重)。为了使QI值明确化(4个QI值之和为1),并对QI值进行归一化处理(表 2)。可以看出不同产区内的板栗品质之间的差异,其优劣顺序为:样区3≈样区2>样区1>样区4。
表 2 信阳板栗果实营养品质指数Table 2. The nutritional quality index of xinyang chestnut fruit品质指数 样区1 样区2 样区3 样区4 QI值 0.382 0.681 0.683 0.325 对QI值归一化 0.185 0.328 0.330 0.157 3. 板栗品质与土壤理化性质的关系
3.1 土壤的理化特征
3.1.1 土壤全量元素分析
土壤中元素全量分析结果列于表 3。从表 3可以看出,土壤中某些元素在采样区的差异性很大,如CaO、MgO、As、B、Co、Cu、Mo等元素。与河南A层土壤平均值[14]相比较,样区土壤中全N、B、Se略低于中国A层土壤(由表土层组成)背景值,土壤中CaO除样区1外,其他三个样区都高于土壤背景值。采样区土壤中的As、Cd、Cu、Cr、Hg、Pb等重金属元素虽然有的略高于全国土壤背景值,但都低于绿色食品土壤环境质量所规定的标准[15]。因此信阳地区具备板栗生长的土壤条件,而且符合绿色食品生产的标准。
表 3 土壤中元素全量Table 3. The total contents of elements in the soil成分 w/(mg·g-1) 中国A层土壤
背景值[16]绿色食品产地
土壤环境
质量标准[17]样区1 样区2 样区3 样区4 N 0.869 0.913 0.887 1.306 1.0~2.0 - P2O5 0.657 1.117 1.637 2.144 1.00~1.50 - K2O 10.1 23.9 24.6 30.0 20.0 - CaO 3.8 17.3 16.2 11.7 9.9 - Na2O 6.1 11.0 22.5 25.8 - - Fe2O3 35.2 48.5 62.5 63.9 55.7 - MgO 5.8 22.8 19.1 14.8 10.5 - SiO2 851.6 703.1 630.4 636.3 - - 成分 w/(mg·kg-1) 中国A层土壤
背景值[16]绿色食品产地
土壤环境
质量标准[17]样区1 样区2 样区3 样区4 As 5.50 2.50 3.75 2.25 9.20 < 20~25 B 12.40 44.10 7.00 4.80 39.00 - Cd 0.15 0.12 0.12 0.13 0.07 < 0.3 Co 8.07 11.40 15.90 12.40 12.00 - Cu 17.10 26.80 34.10 18.20 20.00 < 50 Cr 41.70 67.70 63.40 19.70 54.00 < 120 Hg 0.029 0.023 0.021 0.027 0.04 < 0.25 Mn 245.00 643.00 718.00 756.00 540.00 - Mo 7.45 0.28 2.34 2.95 1.20 - Pb 45.80 21.60 20.20 25.30 24.00 < 50 Sb 0.84 0.70 0.44 0.50 - - Se 0.31 0.15 0.29 0.18 0.22 - Zn 98.40 73.60 123.10 89.00 68 - 注:“-”表示没有资料显示相关的数值。 3.1.2 土壤有效态元素分析
土壤元素的地球化学行为在很大程度上受其赋存状态的影响[18-20],一般来说土壤中有效态元素含量的高低,取决于土壤中微量元素含量的高低,并直接影响植物的生物有效性[21]。研究区土壤有效态元素分析和主要理化指标见表 4。采样区土壤中有效Fe、有效Mn、有效Zn的含量都大大高于相应的河南A层土壤元素平均值;有效Cu的含量样区1、4低于平均值,样区2、3高于平均值。采样区有机质含量在17.98~19.87 mg/g,高于平均值,样区4的容重高于其他3个样区,pH值能够满足板栗生长的酸性土壤条件。阳离子交换量在8.79~13.18 cmol/kg,土壤保肥性较好。总的来说,信阳板栗种植区土壤条件能够较好地满足板栗生长的要求。
表 4 土壤有效态元素分析和主要理化指标Table 4. The available elements and main physicochemical indexes of soil成分 w/(mg·g-1) 河南A层土壤
元素平均值样区1 样区2 样区3 样区4 CaO 3.8 17.3 16.2 11.7 - 全N 0.869 0.913 0.887 1.306 - 有机质 18.02 18.04 19.87 17.98 - 成分 w/(μg·g-1) 河南A层土壤
元素平均值样区1 样区2 样区3 样区4 氨态N 82.4 101.0 96.15 82.4 - 有效P 2.93 32.30 45.10 88.50 - 速效K 82.7 63.1 112 102.9 - 有效Mn 66.27 100.9 111.74 93.49 17.8 有效Fe 76.20 68.13 68.22 62.46 21.2 有效Cu 0.76 1.36 1.88 0.84 1.33 有效Zn 3.20 3.91 3.94 2.99 0.5 有效B 0.29 0.24 0.34 0.25 - pH值 5.00 4.90 4.99 4.82 - 容重(g/cm) 1.24 1.20 1.17 1.40 - 阳离子交换量
(cmol/㎏)8.79 10.86 13.18 12.41 - 注:“-”表示没有资料显示相关的数值。 3.2 板栗品质与土壤理化性质的相关关系
通过SPSS软件对表 2中归一化后的4个QI值和表 4中对应4个样区土壤主要理化指标进行相关性分析,得出板栗果实品质和土壤中主要理化指标的相关关系,见表 5。
3.2.1 板栗品质与土壤容重、有机质、阳离子交换量的关系
土壤容重是表明土壤好坏的一个重要基本数据。如表 5相关系数数据所示,本区板栗品质与土壤容重呈显著负相关(r=-0.835),说明土壤容重小有利于板栗品质的提高,因此在板栗种植区应对土壤进行深耕,使土壤疏松多孔,增加土壤的透气性,避免因土壤容重增大引起土壤紧实板硬、缺少团粒结构,造成板栗品质的下降。
表 5 板栗品质与土壤理化性质的相关系数Table 5. The correlation coefficient of chestnut quality-soil physiochemical index土壤理化
指标与归一化的QI值
相关系数容重① -0.835 有机质 0.598 阳离子交换量 0.329 pH值 0.349 有效Zn① 0.998 有效Cu 0.899 有效Mn 0.712 有效B 0.874 有效Fe 0.006 全N 0.145 氨态N① 0.969 速效P -0.234 速效K -0.117 CaO 0.775 注:①表示在5%水平上显著相关。 土壤有机质含有各种营养元素,是土壤微生物生命活动的能源。对土壤水、气、热各种肥力因素起着重要的调节作用,对土壤结构、耕作性也有重要的影响。采样区有机质总体比较丰富,与板栗品质呈正相关(r=0.598),表明有机质在提高板栗产量和质量方面都起着重要的作用。
阳离子交换量与板栗品质呈正相关(r=0.329),这说明土壤胶体吸附阳离子数量越多,土壤保水保肥的能力越强,越有利于板栗品质的提高。
3.2.2 板栗品质与土壤pH值的关系
板栗适宜的土壤pH值为4~7,最适值为5~6,超过7.5时生长不良[22]。本区pH值范围在4.82~5.00之间(表 4),较好地满足了板栗树生长的酸性条件,如表 5所示,土壤pH值与板栗品质呈正相关(r=0.349)。根据孙鲁平等[12]研究,pH值与板栗品质的相关系数达到-0.62,是土壤各性质中与板栗品质关系最为密切的一个因素。本区研究的结果是土壤pH值与板栗品质的相关系数为正,是因为本区pH < 5.0,为强酸性,而pH值最适宜范围5~6为强酸~酸性,表明土壤pH值越接近5~6的最适宜范围,板栗品质就会越好,这与前人的研究结果是一致的。
3.2.3 板栗品质与土壤微量元素的关系
如表 5所示,土壤中的有效态微量元素Zn、Cu、Mn、B与板栗的品质呈正相关,相关系数为rZn=0.998,rCu=0.899,rMn=0.712,r=0.874,相关性较大,其中有效Zn与板栗品质呈极显著正相关,而有效Fe和板栗品质的相关性不大(r=0.006)。因此在进行板种植时,要在有效态Zn、Cu、Mn、B元素含量高的土壤上规划板栗园区;或者对土壤结构进行改良,增加土壤中有效态Zn、Cu、Mn、B元素的含量。
B是影响板栗产量和品质的重要元素之一,当土壤中的有效硼含量低于0.5 μg/g时,就会形成有蓬无实的空苞[23]。本地区全B含量在4.8~44.1 μg/g之间,而有效B(0.24~0.34 μg/g)低于临界值(0.5 μg/g),这说明信阳板栗种植区板栗空苞的主要原因与本区土壤缺B有很大的关系,造成了板栗的产量下降并且影响了板栗的品质。在调查中也发现板栗园的大面积空苞现象,这和本地区低B现象相一致。
3.2.4 板栗品质与土壤全氮、氨态氮、有效磷、速效钾的关系
土壤中N、P、K元素是农产品生长必要的也是最重要的元素,直接决定着板栗的生长情况,对板栗的产量和品质有重要的影响,这三种元素对作物产量的提高已经毋庸置疑。通过全氮、氨态氮、有效磷、速效钾与板栗品质的相关性研究,如表 5所示,相关系数为r全氮=0.145,r氨态氮=0.969,r有效磷=-0.234,r速效钾=-0.177。因此增加氮含量,特别是氨态氮可以提高板栗的产量和品质;有效磷与板栗品质呈负相关,因此有效磷含量的增加不利于板栗品质的提高,其机理尚不清楚,由于磷的增产效益较高[24],施肥时应注意增施磷肥引起品质下降的问题。而速效钾与板栗品质的相关性不大,对板栗品质的提高作用不大,施钾肥可能主要表现在增产方面。
3.2.5 板栗品质与土壤钙含量的关系
一般认为板栗是忌钙植物,不能生长在含钙的土壤上。因为碳酸钙含量超过一定水平后土壤呈中性或碱性反应,而板栗是喜酸性植物[25-27],钙含量高的土壤质地也较为黏重,对板栗的产量和品质也有不利的影响。本研究中板栗品质与CaO呈正相关(r=0.775),其原因可能是以前的研究侧重于提高板栗的产量,而本研究主要侧重CaO与板栗品质的相关关系。
4. 结语
越来越多的研究表明,名特优农产品优良品质的形成均有着其特定的农业地质背景。本文正是从土壤地球化学的角度,利用相关分析的方法对板栗品质和土壤理化指标的关系进行了分析研究,查明了板栗品质提高的土壤地质背景,为信阳地区因地制宜、合理规划板栗产业布局和合理施肥提供了理论基础。板栗品质与土壤有机质、阳离子交换量、pH值、全氮、氨态氮、CaO、有效锌、有效铜、有效锰、有效硼呈正相关关系,增加全氮、氨态氮、有效硼、有效锰、有效铜、有效锌的含量可以提高板栗品质。而板栗品质与土壤容重、速效磷、速效钾呈负相关关系,因此在施用磷肥和钾肥增加速效磷、速效钾,提高板栗产量的同时,应注意板栗品质下降的问题。
值得说明的是,本研究中板栗品质与CaO呈正相关关系,这与板栗是忌钙植物的观点相矛盾,其中的机理有待进一步研究。
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