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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田口方法是日本著名的质量工程学家田口玄一博士于20世纪70年代创立的质量工程技术[1-2],它强调“产品的质量首先是设计出来的,其次才是制造检验出来的”,提出了信噪比(S/N)的概念来评价测量系统的测量特性[3-4]。我国一些企业引入日本质量管理中的田口方法,如机械行业、兵工企业探索全面质量管理与田口方法的结合,收效显著[5-7]。田口方法的基本思想是用正交表安排实验方案,以误差因素模拟造成产品质量波动的各种干扰;以S/N作为衡量产品质量稳定性的指标,通过对各种实验方案的统计分析、找出抗干扰能力最强、调整性最好、性能最稳定、最可靠的设计方案;并以质量最小为原则,合理确定参数的容差,以达到成本最低、质量最优的综合效果。在实验室方法研究过程中,误差波动为非期望部分,即有害部分,计算对分析有用部分与有害部分值的比值,即为S/N值。S/N值越大,说明测量系统抗干扰能力越强,灵敏度越高,线性越好,测量系统更稳健、更可靠[8-9]。
田口方法在地质样品痕量金/超痕量金分析中的应用
目前测定地质样品痕量金、超痕量金常用的三种方法有:活性炭吸附富集-发射光谱法、泡塑吸附富集-石墨炉原子吸收光谱法(GFAAS)、活性炭吸附富集-电感耦合等离子体质谱法(ICP-MS)。其中活性炭吸附富集-发射光谱法使用国产设备,成本低,流程稍长;泡塑吸附富集-石墨炉原子吸收光谱法,操作简便,使用进口石墨管,成本高;活性炭吸附富集-电感耦合等离子体质谱法,检出限低,对环境条件要求较高,成本稍高。由于矿石矿物的组成往往非常复杂,存在大量的其他金属、非金属等基体成分,三种分析方法的检出限为0.1~0.3 ng/g。
当一个地质实验室有多种分析方法可供选择时,应优先选择更为稳健的实验体系和方法进行地质样品分析。以金的测定为例,由于地质样品中金的含量较低,一些样品存在“粒金效应”,分析流程长,因此合理选择地质样品中痕量金的测定方法具有重要的现实意义。同时,对实验室检测数据进行测量不确定度分析时,除了使用目前相关标准推荐的方法进行评定外,也可以使用田口分析方法进行不确定度分析[10,11]。特别是在一些仪器设备无法溯源的、自制的、寻找不确定度分量有困难的情况下,使用田口分析方法估计整个测量系统不确定度是切实可行的。利用田口分析方法还可以对影响测试结果的多个因素(如温度、湿度、压力、时间等)进行多水平设定,采用正交试验方法进行多因素分析,获知对测试结果影响较大的因素,在实验中加以严格控制,以使实验室检测结果更加准确可靠。甚至可以对于同一实验室的不同范围进行不确定度评定,例如同一检测人员与多个检测人员的检测数据综合分析、同一台设备与多台设备的检测数据分析、同一批次化学试剂与不同批次化学试剂的检测数据分析、不同样品处理条件的检验数据分析等。还可以用田口分析方法对检测设备进行期间核查,确定检测设备的计量性能是否发生变化,为是否需要进一步调试提供理论依据。
基于田口方法的思想和地质分析的需求,本文将该法应用于地质实验室,选择三种不同原理的分析测试方法(活性炭吸附富集-发射光谱法、泡塑吸附富集-石墨炉原子吸收光谱法、活性炭吸附富集-电感耦合等离子体质谱法),对三个金标准物质的金含量进行多次测定,对其三种分析方法的测试数据分别计算特性值(S/N),通过比较三者的S/N值和相对标准不确定度,确定更稳健的测试体系,为地质样品中痕量超、痕量金的测试提供优选方案,以此提高测试结果的准确性,减少测试结果的不确定度。
实验部分
仪器和主要试剂
X-series Ⅱ型电感耦合等离子体质谱仪(美国Thermo公司)。
SOLAARM-6型原子吸收光谱仪(美国ThermoFisher公司)。
WP-1型一米光栅摄谱仪(北京第二光学仪器厂)。
金标准储备溶液:ρ(Au)=1 g/L(国家标准物质研究中心提供的国家一级标准物质)。
金标准溶液:ρ(Au)=100 μg/L,由金标准储备溶液逐级稀释而成。
硫脲溶液(12 g/L):称取12 g硫脲溶于1 L水中,此溶液现用现配。
抗坏血酸溶液(20 g/L):称取20 g抗坏血酸溶于1 L水中,此溶液现用现配。
王水溶液:75 mL 盐酸与25 mL硝酸混合,摇匀,用时配制。
盐酸、硝酸均为分析纯。
实验用水为超纯水,实验所用试剂均为分析纯。
实验材料
活性炭吸附剂的制备:将1000 g市售活性炭倒入塑料桶中,加20 g/L氟化氢铵溶液500 mL浸泡7天,中间搅拌数次,过滤,用盐酸和超纯水洗净氟离子,烘干备用。
泡塑的准备:将市售聚醚型聚胺酯泡塑剪去边皮后,剪成约0.3 g一块,用10%盐酸浸泡30 min后用水漂洗,挤干后放入塑料瓶中保存备用。
金含量的分析方法
活性炭吸附富集-发射光谱法(A法)
称取0.074 mm过筛的样品10 g于瓷舟中,在高温炉内经650℃焙烧2~3 h后,取出放冷,转入250 mL烧杯中,以20 mL水润湿,加入40~60 mL王水,盖上表面皿摇匀。
于电热板上低温(约70℃)溶解约1 h,再升温,摇动数次,待浓缩至原体积的一半时取下,趁热以水冲洗表面皿与杯壁至60 mL(为防止矿渣龟裂,可加入适量纸浆),摇匀,将溶液连同残渣倒入已铺好滤纸并加有1%盐酸纸浆的20 mL布氏漏斗内(漏斗底部与活性炭吸附柱连接)进行抽气过滤,用热的4%稀王水洗烧杯2次,洗布氏漏斗中的矿渣5~6次(每次10 mL),迅速取下布氏漏斗,用热的2%氟化氢铵溶液洗柱5次(每次10 mL),用热的5%盐酸(分析纯盐酸,用蒸馏水配制)洗柱5次,最后用10 mL热蒸馏水洗柱1次,停止抽气。
从柱中取出活性炭纸浆饼,放入10 mL瓷坩埚中,移入马弗炉,于650~700℃灰化1~2 h,取出放冷,灰分约为0.25 mg。灰分超重时,可引起背景加深,使测定结果偏高。可用王水溶解再次吸附灰化,此灰样以纯碳粉补至2.5 mg,加缓冲剂5 mg,用玻璃棒搅匀,全部装入一个电极中,进行光谱测定。
金标准溶液的配制:金标准系列为5 mg炭粉中含金 0.01、0.02、0.04、0.1、0.3、1、3、10、30 μg。以纯炭粉为基准物质各配制1 g。准确分取金的标准溶液,其中金含量各为2、4、8、20、60、200、600、2000、6000 μg,放入9个烧杯中,以5%王水稀释至100 mL,每个标准溶液各用装有活性炭的吸附柱先后吸附2次,使全部富集在柱中。灰化,以活性炭灰分补至100 mg,再以纯碳粉补至1 g,研磨均匀。此标准50 mg与缓冲剂100 mg混匀,可使用10次。
泡塑吸附富集-石墨炉原子吸收光谱法(B法)
称取10 g样品于25 mL瓷坩埚中,置于650℃的高温炉中灼烧1.5 h,取出冷却后倒入200 mL三角烧瓶中,加少量水润湿样品,加入新配制的50%王水50 mL、1 mL Fe(Ⅲ)溶液,加盖置于电热板上加热2~3 h,溶液体积蒸至约10 mL时取下,用水稀释至约100 mL,放入一块泡塑,排去气泡,将三角烧瓶置于振荡器上振荡1 h,取出泡塑,用水洗净,挤干,放入预先加入5 mL硫脲溶液的25 mL比色管中,排去气泡,于100℃沸水浴中保持20 min,趁热取出泡塑,溶液冷却,以下按标准曲线的绘制步骤上机测定。
标准曲线的绘制:称取100 μg/L金标准溶液0.00、0.25、0.50、1.00、2.00、4.00 mL于10 mL比色管中,用硫脲溶液稀释至10 mL刻度,摇匀后于100℃沸水浴中保持20 min,取下冷却,即配制成0.0、2.5、5.0、10.0、20.0、40.0 μg/L的金标准系列。按照制定的仪器工作条件上机测定。
活性炭吸附富集-电感耦合等离子体质谱法(C法)
称取10 ~20 g样品于50 mL瓷坩埚中,置于600~650℃的高温炉中灼烧1~2 h取出,冷却后将样品转移至250 mL烧杯中,用少许水润湿,加入50 mL王水(现用现配),加表面皿置于180℃电炉板上加热溶解1 h,取下盖,蒸至体积约20 mL,取下,用活性炭动态吸附柱进行减压抽滤,用2%王水洗烧杯和漏斗3~4次。取下布氏漏斗,先后用0.5%氟化氢铵热溶液、2%盐酸和温水洗吸附柱各4~5次。取下活性炭纸饼,放入15 mL瓷坩埚中,先在低温然后升至650℃高温炉中灰化40 min,灰化产物用2 mL 50%的沸王水溶解后转移至10 mL比色管中,用水稀释至刻度,摇匀。按设定的仪器工作条件,采取在线加入内标方式进行测定。
标准曲线的绘制:移取ρ(Au)=100 μg/L的标准溶液0.00、0.50、1.50、2.50、5.00、10.00 mL于50 mL容量瓶中,加2.5 mL王水,稀释至刻度,摇匀,得到ρ(Au)=0、1.0、5.0、10.0、30.0、50.0、100.0、300.0 ng/mL的标准溶液。标准系列按照样品处理步骤进行。
数据处理
静态测量的特性值S/N
在测量工程学中,测量特性值的S/N分为静态测量的特性值S/N及动态测量的特性值S/N。在痕量金标准物质定量分析时,通常用静态测量的特性值,即被测量为固定值。
被测量的测量结果为y,记:
$ \mu = E\left( y \right),\sigma = \sqrt {V\left( y \right)} $
(1) 式中,μ、σ分别表示测量结果y的期望值和标准差。令:
$ \gamma = \frac{\sigma }{\mu },\widehat \eta = \frac{1}{{{\gamma ^2}}} = \frac{{{\mu ^2}}}{{{\sigma ^2}}} $
(2) 为测量特性值 y 的信噪比,信噪比(S/N)越大,说明测量结果的相对标准偏差γ(或相对标准不确定度)越小,测量特性值 y 的灵敏度 μ2 越大,测量特性值越稳健,即测量结果越可靠[12,13]。
数理统计方法
对同一被测量进行n次独立重复测量,设测量值为y1、y2、y3、...、yn。
根据数理统计理论:
$ \overline y = \frac{1}{n}\sum\limits_{i = 1}^n {{y_i}} $
(3) $ {\widehat \sigma ^2} = {V_e} = \frac{1}{{n - 1}}\sum\limits_{i = 1}^n {{{\left( {{y_i} - \overline y } \right)}^2}} $
(4) $ {\widehat \mu ^2} = {\left( {\overline y } \right)^2} - \frac{{{V_e}}}{n} = \frac{1}{n}\left( {{S_m} - {V_e}} \right) $
(5) 其中: $ {S_m} = \frac{1}{n}{\left( {\sum\limits_{i = 1}^n {{y_i}} } \right)^2} $
(6) 则: $ \widehat \eta = \frac{{{S_m} - {V_e}}}{{n{V_e}}} $
(7) 田口博士参照通信理论的方法对
$ \widehat \eta $ 取常用对数并扩大10倍,换算为以分贝(dB)为单位的信噪比,即:$ \eta = 10\lg \frac{{{S_m} - {V_e}}}{{n{V_e}}} = 10\lg \widehat \eta $
(8) $ \gamma = \frac{1}{{\sqrt {\widehat \mu } }} $
(9) 数据统计和分析方法评价
金标准物质测试数据统计分析结果
分别采用上述三种分析方法,对三个痕量金标准物质GAu-2a、GAu-9a、GAu-10a进行了15次测试,测试结果分别见表 1、表 2、表 3。
表 1 GAu-2a样品测试结果Table 1. Analytical results of Au in GAu-2a sample测量次数 w(Au)/(ng·g-1) A法 B法 C法 1 1.01 0.71 0.87 2 0.74 0.83 0.86 3 1.25 0.59 0.88 4 0.80 0.82 0.89 5 0.83 0.89 0.73 6 0.83 0.94 0.70 7 0.87 1.04 0.70 8 0.81 0.71 0.90 9 0.86 0.89 0.77 10 0.83 1.27 0.72 11 0.83 1.07 0.78 12 0.82 0.58 0.81 13 0.88 0.88 0.68 14 0.83 0.80 0.75 15 0.85 0.73 0.79 表 2 GAu-9a样品测试结果Table 2. Analytical results of Au in GAu-9a sample测量次数 w(Au)/(ng·g-1) A法 B法 C法 1 1.59 1.65 1.72 2 1.43 1.38 1.73 3 0.92 1.41 1.60 4 1.04 1.38 1.88 5 1.55 1.61 1.47 6 1.51 1.46 1.31 7 1.57 1.60 1.69 8 1.61 2.00 1.54 9 1.65 1.42 1.72 10 1.48 1.54 1.65 11 1.59 1.54 1.41 12 1.54 1.36 1.69 13 1.28 1.17 1.50 14 1.42 2.08 1.61 15 1.44 2.18 1.64 表 3 GAu-10a样品测试结果Table 3. Analytical results of Au in GAu-10a sample测量次数 w(Au)/(ng·g-1) A法 B法 C法 1 5.22 4.98 5.91 2 5.44 5.31 5.78 3 5.27 5.69 5.51 4 5.30 5.13 4.68 5 5.11 4.77 4.85 6 5.47 5.09 5.23 7 5.18 4.99 4.76 8 5.14 4.76 5.00 9 5.11 5.54 5.99 10 5.02 5.38 5.54 11 5.03 4.62 4.96 12 5.05 5.60 4.86 13 5.11 6.71 5.96 14 7.37 5.69 6.13 15 5.09 4.66 4.78 根据表 1~表 3的分析数据,按照公式(1)~(9)计算得到了测量三个标准物质的痕量金三种分析方法的特性值S/N和相对标准不确定度,统计结果见表 4。
表 4 痕量金三种分析方法的测量数据处理结果Table 4. Data processing results of trace gold using three analytical methods金标准物质 分析方法 Sm Ve $ \widehat \eta $ η/dB γ 极限值 GAu-2a A法 11.336 0.014 52.700 17.220 0.138 11.20 B法 10.838 0.034 21.410 13.310 0.216 7.29 C法 9.330 0.006 107.740 20.324 0.096 14.30 GAu-9a A法 31.162 0.044 46.850 16.710 0.146 10.69 B法 37.699 0.083 30.250 14.810 0.182 8.79 C法 38.914 0.021 123.680 20.920 0.090 14.90 GAu-10a A法 425.729 0.339 83.580 19.220 0.109 13.20 B法 415.224 0.296 93.540 19.710 0.103 13.69 C法 426.027 0.275 103.350 20.140 0.098 14.12 以测量特性值S/N作为地质实验室测试方法的评价指标,其优点有:①反映了输入特性(被测量真值)与输出特性(读数值)之间的线性关系,线性关系越显著,Ve越小,S/N值越大。②度量了输出特性y的稳健性,稳健性越好,则Ve越小,S/N值越大。③度量了测试方法灵敏度μ2大小,灵敏度越高,S/N值越大。④以分贝值表示的S/N值不仅计算方便,
而且经过对数变换后更接近于正态分布,可进行方差分析。
对于GAu-2a样品,Au含量的标准值为0.85 ng/g,ηC>ηA>ηB,γC<γA<γB,说明电感耦合等离子体质谱法的信噪比最大和相对标准不确定度最小,发射光谱法次之,石墨炉原子吸收光谱法稍差。
对于GAu-9a样品,Au含量的标准值为1.6 ng/g,ηC>ηA>ηB,γC<γA<γB,说明电感耦合等离子体质谱法的信噪比最大和相对标准不确定度最小,发射光谱法次之,石墨炉原子吸收光谱法稍差。
对于 GAu-10a样品,Au含量的标准值为5.1 ng/g,ηC>ηB>ηA,γC<γB<γA,说明电感耦合等离子体质谱法的信噪比最大和相对标准不确定度最小, 其他两种方法的信噪比和相对标准不确定度基本一致。
三种分析方法的总体评价
通过三种分析方法体系的测定结果比较来看,信噪比和相对标准不确定度相差不大。相对而言,电感耦合等离子体质谱法的数据分散性较小,测试稳定性更好,测试结果更为可靠。从表 4可以看出,不同含量的三个金标准样品的Au含量越低,测试结果的信噪比越低,稳定性越差。对于Au含量在1 ng/g以上(痕量金)的样品,三种方法测试结果的信噪比基本一致,根据实验室的实际情况,选择这三种方法中的任一种均可满足要求;对于Au含量在1 ng/g以下(超痕量金)的样品,由于泡塑吸附富集-石墨炉原子吸收光谱法测试结果的信噪比较低,建议采用活性炭吸附富集-电感耦合等离子体质谱法。
结语
地质实验室可根据测试任务类型、含量高低、时间、成本、质量要求等因素,选择合理的痕量、超痕量金的分析测试方法。
本文将田口测量质量评价理论应用于地质样品中痕量、超痕量金测试方法的选择过程,建立了多种测试方法的优选方案。通过比较分析结果的信噪比(S/N)和相对标准不确定度,结果表明,对于活性炭吸附富集-发射光谱、泡塑吸附富集-石墨炉原子吸收光谱、活性炭吸附富集-电感耦合等离子体质谱三种痕量、超痕量金测试方法,活性炭吸附富集-电感耦合等离子体质谱法总体评价稍优,石墨炉原子吸收光谱法与发射光谱法基本一致。对于地质样品中痕量金(含量大于1 ng/g)的测试,三种分析测试方法均能够满足要求;对于超痕量金(含量小于1 ng/g)的测试,活性炭吸附富集-电感耦合等离子体质谱法是较好的测试方案。
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