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1、Chapter 6 Atomic Absorption Spectrometry （原子吸收光谱） AAS,原子吸收分光光度计（PerkinElmer),Basic principle of atomic absorption spectrometric analysis Structure of atomic absorption spectrophotometer Interference and elimination Quantitative analytical methods Sensitivity, characteristic concentration and detecte
2、d limit brief introduction of atomic fluorescence spectrometry (AFS),掌握原子吸收光谱分析法的基本原理，了解原子吸收光谱仪的构造，重点掌握原子吸收分光光度定量分析方法。,Outlines,1802年Wollaston发现太阳光谱的暗线； 1859年Kirchhoff和 Bunson解释了暗线产生的原因；,暗线是由于大气层中的钠原子对太阳光选择性吸收的结果,1.Instroduction of AAS,1.1 History,Hollow Cathode Lamps 1955年Walsh发表了一篇论文“Application o
3、f atomic absorption spectrometry to analytical chemistry”,解决了原子吸收光谱的光源问题，50年代末 PE 和 Varian公司推出了原子吸收商品仪器。,1.2 Characters of AAS Little interference and/or matrix effects Accurate and precise analysis Application of most elements Simple operation and low-cost,1.3 Principle of AAS,Atomic vapor absorb t
4、he radiation emitted by the special source, the absorption is proportion to the number of the atoms Terms: Resonance-Absorption (共振吸收）,表征吸收线轮廓（峰）的参数： 中心频率O（峰值频率） ： 最大吸收系数对应的频率或波长; 中心波长: （nm）， 半宽度：O,2. Line profile(轮廓) of atom absorption,一束不同频率强度为I0的平行光通过厚度为l的原子蒸气，一部分光被吸收，透过光的强度Iv服从吸收定律,Line broadeni
5、ng caused by P387 (1) Natural width 自然宽度 10-4 ,(2) Doppler broadening if an atom emits radiation while moving toward the detector, the waves will be compressed, and the wavelength will be shorter if an atom emits radiation while moving away from the detector, the waves will stretched out, and the wa
6、velength will be longer related effect for absorption,Chapter 8,11,(3) Pressure broadening （压力变宽） Collision among atoms causing the changing of energy Lorentz broadening：atoms of different elements Holtmark broadening：collision in atoms of one element Lorentz变宽与Doppler变宽有相同的数量级，也可达10-3nm。,(4) 自吸变宽 (
7、5) electric and magnetic field effects,3. Measuring of atomic absorption,(1)Integral Absorption 积分吸收 Integral absorption is proportional to N0,Difficulties in measuring of the integral absorption,Resolution Power of integrator for measuring the absorption Sensitivity of the detector for measuring th
8、e absorption,若用一般光源照射时，吸收光的强度变化仅为0.5%。灵敏度极差,(2)Peak absorption 峰值吸收 1955, Walsh: Peak absorption is proportional to the number of atoms in flame,Narrow-line light source (锐线光源) （1）光源的发射线与吸收线的0一致。 （2）发射线的1/2小于吸收线的 1/2。,When we carry out absorption measurement using a narrow line source, absorbance is
9、 expressed as,Where I0 I are respectively intensities of incident light and passing light through the sample on the frequency region Dne, they are given by respectively,Considering I = I0n e-KnL , we get,When incident light of frequency range is so narrow, that is to say, when we use narrow-line sou
10、rce, absorption coefficient Kn can be regarded as a constant equal to peak absorption coefficient K0,So,Where K0 depends on line width.,Under normal operation condition for AAS, line profile is mainly determined by Doppler broadening, hence,It is obvious that when a narrow-line source is employed as
11、 that of atomic absorption, the absorbance is directly proportional to atomic number of the sought-for element at the ground state in atomic vapor.,大多数化合物均已离解，且绝大多数以基态原子状态存在，激发态原子不足0.1%, 因此，可用N0代表吸收辐射的原子总数。,This is the base for quantitative analysis of AAS,4. Instrumentation of AAS (原子吸收分光光度计),4.1 G
12、eneral introduction Single-beam design(单光束型) Double-beam design(双光束型) Compensate for a possible drift of the primary radiation source Function of chopper(斩光器）,a. Single-beam b. Double-beams,Schematic figure of AAS instrument (原子吸收分光光度计仪器结构图),光源的功能是发射被测元素的特征共振辐射。,4.2 Radiation sources,(1)Hollow catho
13、de lamp (HCL)空心阴极灯,Cathode made of analyte element (metal or salt) Emits same wavelengths that analyte absorbs,Principle,Ionized gas generated in lamp Ne(g) + e- Ne+(g) + 2 e- Ne+ accelerates from anode (+) to cathode (-) Ne+ strikes cathode and vaporizes some of the surface metal Some of the vapori
14、zed metal atoms are promoted to the excited state and emits light,(2),4.3 Atomization system,Structure Nebulizer (雾化器) and burner (燃烧器) Total consumption burner and premix burner (全耗型和预混合型),(1) Flame atomization,火焰原子化,Characteristic of flame,Velocity of burning Composition and Temperature of flame F
15、uel to oxidant ratio(燃助比) Stoichiometric flame(化学计量火焰)：high temperature，stable，little background and interference Fuel-rich flame(富燃火焰)：reductive (还原性)，(有利于氧化物离解)，如Mo Fuel-poor flame (贫燃火焰) Oxidative, high temperature, for Ag, Cu,flame,Comparison of commonly-used flames,Spectrum of the flame,(2) Ele
16、ctrothermal atomizer (电热原子化器）,Structure of Graphite Furnace (GF) atomizer石墨炉原子化器,A basic graphite furnace atomizer is comprised of the following components: graphite tube electrical contacts enclosed water cooled housing inert purge gas controls,GF atomizer process: drying step, ashing step and atom
17、ization step,干燥：去除溶剂，防样品溅射； 灰化：使基体和有机物尽量挥发除去； 原子化：待测物化合物分解为基态原子，此时停止通 Ar，延长原子停留时间，提高灵敏度； 净化：样品测定完成，高温去残渣，净化石墨管。,(3) Low-temperature atomization (低温原子化方式),Cold-vapor atomizatio：determination of Hg Hydride atomization (氢化物原子化法)：Determination of As, Sb etc.,4.4 Optical system,Bandpass (带宽）W： W=DS10-3 S
18、：slit width，D：reciprocal linear dispersion of grating Selection of slit,4.5 Detector,Photomultiplier 光电倍增管,光阴极,5 Interferences in AAS P409,5.1 Spectral interferences,(1) From light source谱线干扰 P417,Cause,Eliminating method,Overlap of lines 吸收线重叠的干扰：指试样中共存元素吸收线与待测元素共振线的重叠干扰。,Non-absorption line in the
19、 bandpass 发射线的邻近线的干扰：指空心阴极灯的元素、杂质或载气 元素的发射线与待测元素共振线的重叠干扰。,Decrease slit and light current, or changing analytical line,Changing analytical line or separating interfering element 选择其它吸收线,(2) Background interferencefrom atomizer背景干扰,(a) Cause： Flame absorption and scatter(散射） Absorbing：Components（CO,C
20、H,CN,OH） and Molecule； Scatter: Particles of salt 固体微粒对共振线的散射造成的假吸收，当吸收线在短波且基体浓度很高时，光散射干扰较严重。,(2)光谱背景的校正,用邻近非共振线校正背景 P418 Absorption of non-resonance line near the analytical line,用分析线测量原子吸收与背景吸收的总吸光度，在分析线 邻近选一条非共振线，此时测出的是背景吸收，两次测量值之 差即为校正背景后的吸光度。这种校正方法准确度较差，只适 用于分析线附近背景分布比较均匀的情况。,用连续光源校正背景 P418,用锐线
21、光源测定分析线的原子吸收和背景吸收的总吸光度, 再用氘灯(紫外区)或碘钨灯(可见区)在同一波长测定背景吸收， 计算两次测定吸光度之差，即为校正背景后的吸光度。由于空 心阴极灯与氘灯两种光源放电性质不同，能量分布不同，会导 致背景校正不足或过度。,(b) Background-Correction method,Two light sources: narrow-line and continum Narrow line: atom absorption plus background Continum source: Background absorption Subtracting of t
22、he two absorptions,Shortcoming （190-360 nm)，two sources,Zeeman background correction P420,Based on Zeeman effect(塞曼效应): Spectral line splitting in magnetic field 外加磁场垂直于光的传播方向时，基态原子吸收线分裂成不同组分的偏振成分 成分，平行于磁场振动，波长不变 + 与-成分，垂直于磁场振动，波长位移,5.2 Physical interference (物理干扰、基体效应）P411,Causes Physical character
23、s: 试样与标准样品物理性质有差别产生的干扰，如基体的黏度，表面张力等。 Eliminating method Preparing standard solution with same matrix of the sample, or standard addition method,5.3 Chemical interference (化学干扰)P410,Causes Chemical reaction between element being analyzed and other compound affects the atomization process Eliminating
24、methods Adding release agent(释放剂) that reacts with interferents more rapidly than the analyte does (Sr2+ or La3+ can be added to prevent analyte from reacting with PO43- in sample) Adding protecting agent(保护剂) EDTA or 8-hydroxyquinone Chelates to metal, shielding it from reaction but the resulting c
25、omplex remains volatile 饱和剂：加入足够的干扰元素，使干扰趋于稳定。 用N2OC2H2火焰测钛时，在试样和标准溶液中加入300mgL-1以上的铝盐，使铝对钛的干扰趋于稳定。,5.4 Ionization interference (电离干扰) P412,Cause Ionization of atoms at high temperature, which decrease atoms of ground level Elimination method Adding eliminating-ionization agent采用低温火焰或在试液中加入过量的更易电离的化合
26、物(消电离剂)，能够有效地抑制待测元素的电离。在火焰温度下，消电离剂首先电离，产生大量的电子，抑制了被测元素的电离。,常用的消电离剂：CsCl、KCl、NaCl等,M = M+ + e- (M = analyte atom) This is bad because the monochrometer wavelength is set for that of neutral atom absorption/emission Add ionization suppressor - an easily ionizable salt (usually CsCl or KCl) that floods
27、 the sample with electrons CsCl Cs + Cl Cs Cs+e- (much smaller ionization potential) Causes the M = M+ + e- equilibrium to shift,Ionization Potential Table,6 Analytical methods,6.1 Parameters of analysis (1) Analytical line,每种元素都有几条可供选择使用的吸收线。一般选待测元素的共振线作为分析线，可以得到最好的灵敏度。在测量高含量元素时，也可选次灵敏线。,光谱通带的选择以排除
28、光谱干扰和具有一定透光强度为原则。无邻近干扰线（如测碱及碱土金属）时，选较大的通带，以提高信噪比和测量精密度，降低检出限；反之（如测过渡及稀土金属），宜选较小通带，以提高仪器的分辨率，改善线性范围，提高灵敏度。,(2) Slit width 单色器光谱通带的选择（调节狭缝宽度）,(3)Lamp current 空心阴极灯电流的选择,在保证有稳定和足够的辐射光通量的情况下，尽量选较低的灯电流。实际工作中，通过绘制吸光度-灯电流曲线选择最佳灯电流。一般空心阴极灯上标有允许使用的最大工作电流，一般为1-6mA，需要预热10-30min。,依据不同试样元素选择不同火焰类型。,(5) Height 观测
29、高度,调节观测高度（燃烧器高度），可使元素通过自由原子浓度最大的火焰区，灵敏度高，观测稳定性好。,(4) Atomization conditions,6.2 Analytical methods,Calibration curve： A: 0.1-0.5 Causes of the bend of calibration： Lower atomization efficiency at high concentration 当含量增高时，压力变宽因素使辐射线中心频率与吸收线中心频率不重叠造成等,Standard curve method 配制一系列浓度不同的标准溶液，在相同测定条件下，测定标
30、准系列溶液和待测试样溶液的吸光度，绘制A-c标准曲线，由待测溶液的吸光度值在标准曲线上得到其含量。,(2)Standard addition method(标准加入法) 当试样组成复杂，待测元素含量很低时，应采用标准加入法进行定量分析。,Eliminating matrix interference,直线外推法,Conclusion,1、共振发射线和共振吸收线的定义； 2、什么叫做锐线光源，需要满足的条件； 3、谱线变宽的原因； 4、基态原子数与原子化温度的关系：波尔兹曼分布定律； 5、测量原子吸收的基本关系式：A=Kc 6、原子吸收分光光度计的主要部件及类型； 7、火焰和非火焰原子化的过程；
31、 8、原子化的方法有火焰原子化法和无火焰原子化法。 9、原子吸收的干扰分为几类，如何抑制或消除； 10、化学干扰是原子吸收分析中主要干扰。如何抑制或消除； 11、背景干扰如何校正； 12、什么是灵敏度和检测限，如何表示； 13、简述定量分析的标准加入法； 14、测定条件如何选择。,Brief Introduction of atomic fluorescence spectrometry （AFS),Principle: Excited by radiation and eradiation Highly sensitive and well selective,Fluorescence Spectrometer,Source,Wavelength Selector,Sample,Detector,Signal Processor Readout,P,Po,Chopper,90o,