原子吸收光譜法
早在1859年，Kirchhoff成功的解釋了太陽光譜中暗線產生的原因，並應用於太陽外圍大氣組成的分析。但其作為一種分析方法卻是從1955年澳大利亞物理學家A.Walsh發表了「原子吸收光譜在化學分析中的應用」的論文以後才開始的。
As early as 1859, Kirchhoff successfully explained the dark lines in thesolar spectrum and applied them to the analysis of the composition of the sun'souter atmosphere.However, its use as an analytical method was not introduceduntil 1955, when the Australian physicist A.Walsh published his paper"Application of Atomic Absorption Spectroscopy in Chemical Analysis".
關鍵詞 原子吸收光譜 原子化 原子吸收分光光度計1 原子吸收光譜法概述
原子吸收光譜法是根據基態原子對特徵波長光的吸收，測定試樣中待測元素含量的分析方法，簡稱原子吸收分析法。
Atomic absorption spectrometry is an analytical method to determine thecontent of the element to be measured in the sample according to the absorptionof the ground-state atom to the characteristic wavelength light, abbreviated asAAS.
2 原子吸收光譜法的基本原理
2.1基本概念
共振吸收線：當電子吸收一定能量從基態躍遷到能量最低的激發態時所產生的吸收譜線，簡稱共振線。
Resonance absorption line: The absorption line generated when an electron absorbs a certain amount of energy from the ground state to the lowest energ yexcited state.
共振發射線：當電子從第一激發態躍遷回基態時，發射出同樣頻率的光輻射所對應的譜線稱之為共振發射線，簡稱共振線。
Resonant emission line: when the electron transitions from the first excited state to the ground state, the spectral line corres ponding to the emission of optical radiation of the same frequency is called the reson anceemission line, referred to as the resonance line.
2.2原子吸收譜線的輪廓
理論上講，原子吸收光譜應該是線狀譜。但實際上並不是絕對單色的幾何線，而是具有一定寬度的譜線。以吸收係數Kv為縱坐標，頻率ν為橫坐標作圖。原子吸收線的輪廓以吸收線的中心頻率ν0或半寬度△ν1/2來表徵。
The oretically, atomic absorption spectra should be linear.But in fact,it is not an absolute monochromatic geometric line, but a spectral line with acertain width.Take the absorption coefficient KV as the ordinate andthe frequency ν as the abscissa to plot.The outline of the atomic absorption line is characterized by the central frequency ν0or half width △ν1/2of the absorption line.
圖2.1 原子吸收譜線
譜線變寬的影響因素是多種的，一般由兩方面因素決定。一為原子本身的性質決定了譜線的自然寬度；二是由於外界因素的影響。自然變寬△νN：不受外界因素的影響，譜線本身固有的寬度。與原子發生能級躍遷時激發態原子平均壽命有關，壽命越長，譜線寬度越窄。都卜勒變寬△νD：Doppler變寬是在測量條件下影響最大的因素，其是由於原子熱運動引起的，又稱之為熱變寬。當處於熱力學平衡態時，可用圖2.2表示。壓力變寬：其是由於產生吸收的原子與蒸汽中原子或分子相互碰撞而引起譜線的變寬，可分為兩類：Lorentz變寬和Holtzmork變寬，具體概念不再介紹。
The influencing factors of spectral line broadening are various,generally determined by two factors.First, the nature of the atom it selfdetermines the natural width of the spectral line.Second, due to the influence of external factors.Natural broadening △ νn: The intrinsic width of the line itself, which is not affected by external factors.It is related to the average life of the excited state atom when the energy level transition occurs. The longer the life, the narrower the spectral line width.Doppler broadening △νD: Doppler broadening is the most influential factor under the measurement conditions, which is caused by the thermal movement of atoms, alsoknown as thermal broadening.When in thermodynamic equilibrium state, Equation2.2 can be expressed as follows。Pressure broadening: it is caused by the collision between atomsproducing absorption and atoms or molecules in steam, which can be divided into two categories: Lorentz broadening and Holtzmork broadening. The specificconcept will not be introduced.
圖2.2
2.3原子蒸氣中基態和激發態原子的分配
原子吸收光譜是以測定基態原子對同種原子特徵輻射的吸收為依據的。在進行分析時，首先要使樣品中待測元素化合物狀態轉變為基態原子，此過程稱之為原子化過程。在原子化過程中，多數原子處於基態，少部分吸收較高的能量轉變為激發態，這兩種原子的數目在一定溫度下符合Boltzmann分布定律，即圖2.3。
Atomic absorption spectroscopy is based on the measurement of the absorption of the same atomic characteristic radiation by the ground-stateatoms.During the analysis, the state of the element to be measured in the sample should first be transformed into the ground state atoms, which is called the atomization process.In the atomization process, most atoms are in the ground state, while a few absorb higher energy and transform into excitedstates. The number of these two kinds of atoms conforms to Boltzmann distribution law at a certain temperature, as shown in Fig. 2.3。
圖2.3
2.4原子吸收值與待測原子濃度的定量關係
原子吸收包括積分吸收和峰值吸收。其中積分吸收是指原子蒸氣層中的基態原子吸收共振線的全部能量，公式如圖2.4，其是原子吸收光譜法重要理論依據。
圖2.4
Atomic absorption includes integral absorption and peak absorption. Ofwhich the integral absorption refers to the total energy of the ground stateatomic absorption resonance lines in the atomic vapor layer, and the formula isshown in Figure 2.4, which is an important theoretical basis for atomicabsorption spectroscopy.
峰值吸收是指基態原子蒸氣對入射光中心頻率線的吸收。在一定條件下，基態原子蒸氣的峰值吸收與試液中待測元素的濃度成正比，可以通過峰值吸收的測量進行定量分析。為測定峰值吸收係數K0，需用銳線光源代替連續光源，即用與吸收線中心頻率相同，半寬度比吸收線更窄的發射線做光源。根據Beer-Lambert's Law可以推出：當銳線光源強度及其他實驗條件一定時，基態原子蒸氣的吸光度與試液中待測元素的濃度及光程的乘積成正比。
Peak absorption refers to the absorption of the center frequency line ofthe incident light by the ground state atomic vapor.Under certain conditions,the peak absorption of ground state atomic vapor is proportional to theconcentration of the element to be measured in the test solution, which can bequantitatively analyzed by measuring the peak absorption. In order to determinethe peak absorption coefficient K0, the continuous light source needs to bereplaced by a sharp line light source, that is, an emission line with the samefrequency as the center of the absorption line and a narrower half width thanthe absorption line is used as the light source.According to Beer-Lambert'sLaw, when the intensity of the sharp light source and other experimental conditionsare constant, the absorbance of the ground state atomic vapor is proportionalto the concen tration of the element to be measured in the test solution and the product of the optical path.
3 原子吸收分光光度計
3.1概述
原子吸收光譜分析用的儀器稱為原子吸收分光光度計或原子吸收光譜儀。原子分光光度計主要由光源、原子化系統、單色器、檢測系統四部分組成。原子吸收分光光度計主要有兩種類型：單光束分度計和雙光束分度計。其中單光束分度計結構簡單，但會因為光源不穩定引起基線漂移。（圖3.1）雙光束分度計經光源發出的光被斬光器分成兩束光，一束測量光，一束參比光。連束光交替進入單色器進行檢測。（圖3.2）
圖3.1 單光束分度計
圖3.2 雙光束分度計
The instrument used for atomic absorption spectroscopy analysis is calledatomic absorption spectrophotometer or atomic absorption spectrometer.Atomicspectrophotometer is mainly composed of light source, atomic system,monochromator, detection system four parts.There are two main types of atomicabsorption spectrophotometers: single-beam graders and double-beam graders.Thesingle-beam indexer has a simple structure, but it will cause baseline driftdue to the instability of the light source.The light emitted by the lightsource is divided by the chopper into two beams, one measuring beam and onereference beam.Continuous beam of light alternately into the monochromator fordetection.(figure 3.2)
3.2光源
光源作用是發射出待測元素的特徵譜線。為保證峰值吸收的測量，要求光源必須能發射出比吸收線寬度更窄，強度大、背景低，使用壽命長的線光譜。常見的銳線光源有空心陰極燈、無極放電燈、蒸氣放電燈。
The function of the light source is to emit characteristic spectral lines of the element to be measured.In order to ensure the measurement of peak absorption, the light source must be able to emit a line spectrum narrower thanthe absorption line width, with high intensity, low back ground and long servicelife.Common sharp line light sources include hollow cathode lamp, electrolessdischarge lamp and vapor discharge lamp.
3.2.1空心陰極燈
空心陰極燈又稱為是元素燈。其是由一個在鎢棒上鑲鈦絲或鉭片的陽極和一個由發射所需特徵譜線的金屬或合金製成的空心陰極組成（圖3.3）。
在兩極之間施加300-500V電壓時，陰極燈開始輝光放電，電子從空心陰極射向陽極，並與周圍惰性氣體碰撞使之電離。氣體陽離子在電場力作用下撞擊陰極內壁，使陰極表面上的自由原子濺射出來，濺射出的金屬原子再與電子、正離子碰撞而被激發，當激發態原子返回基態時，輻射出特徵頻率的銳線光譜。
The hollow cathode lamp is also known as the elemental lamp.It consistsof an anode mounted on a tungsten rod with a titanium wire or tantalum plateand a hollow cathode made of a metal or alloy with the characteristic spectrallines needed for emission.(figure 3.3)
When a voltage of 300-500V is applied between the two poles, the cathodelamp begins to glow discharge. Electrons shoot from the hollow cathode to theanode and collide with the surrounding inert gas to ionize it.The gas cationimpacts the inner wall of the cathode under the action of electric field force,so that the free atoms on the surface of the cathode are sputtered out. Thesputtered metal atoms collide with electrons and positive ions and are excited.When the excited state atoms return to the ground state, the sharp linespectrum of characteristic frequency is radiated.
圖3.3 空心陰極燈
3.3原子化系統
將試樣中待測元素變成氣態基態原子的過程稱為試樣的「原子化」。完成「原子化」的設備稱為原子化器或原子化系統。其是原子吸收分光光度計的一個關鍵裝置，對靈敏度和精確度有很大影響，是分析誤差最大的來源。
The process of changing the element to gaseous ground state atoms in the sample is called "atomization" of the sample.The device that performs the "atomization" is called an atomizer or an atomization system.It is a key device of atomic absorption spectrophotometer, which has a greatinfluence on the sensitivity and accuracy, and is the source of the biggest analysis error.
3.3.1火焰原子化法
火焰原子化包括兩個步驟，首先將試樣溶液變成細小霧滴，然後霧滴接受火焰供給的能量形成基態原子。即霧化階段和原子化階段。火焰原子化器包括霧化器、預混合室和燃燒器等。（圖3.4）其操作簡便，重現性好，對大多數元素有較高的靈敏度，應用廣泛，但火焰原子化法原子化效率低，一般不能直接分析固體樣品。
Flame atomization involves two steps: first, the sample solution isturned into small droplets, and then the droplets receive the energy from the flame to form the ground state atoms.Atomization stage and atomizations tage.The flame atomizer includes atomizer, premixing chamber and burner,etc.(figure 3.4)It has the advantages ofsimple operation, good reproducibility, high sensitivity to most elements andwide application. However, the atomization efficiency of flame atomization is low, so it is generally unable to directly analyze solid samples.
圖3.4 火焰原子化器
3.3.2石墨爐原子化器
管式石墨爐原子化法直接採用程序升溫和直接進樣的方式對試樣進行原子化，過程包括乾燥、灰化、原子化、淨化四個階段。（圖3.5）石墨爐原子化法原子化效率遠大於火焰原子化法，其絕對檢出限可達10-14g，絕對靈敏度高，無論是固體樣品還是液體樣品均可直接進樣，樣品用量小，液體樣品1~100μL，固體樣品20~40μg。
The tube graphite furnace atomization method directly uses programmedtemperature rise and direct sample injection to atomize the sample. The processincludes four stages: drying, ashes, atomization and purification.(Fig. 3.5)The atomization efficiency of graphite furnace atomization method is muchhigher than that of flame atomization method, with the absolute detection limit of 10-14g and high absolute sensitivity. Both solid samples and liquid samples canbe directly imported, and the sample dosage is small, 1~100μL for liquidsamples and 20~40μg for solid samples.
圖3.5 石墨爐原子化器
還有其他類型的原子化方法，如電加熱原子化、化學原子化、等離子原子化、陰極濺射原子化等，不再一一贅述。
There are other types of atomization methods, such as electric heating atomization, chemical atomization, plasma atomization, cathodic sputteringatomization, etc., which will not be described in detail.
3.4單色器和檢測系統
單色器由入射狹縫、出射狹縫和色散元件組成。其是將待測元素的吸收線與鄰近譜線分開並阻止其他譜線進入檢測器。使檢測系統只接受共振吸收線。
檢測系統由光電轉換器、信號放大器和顯示記錄器等組成。其結構及原理不再一一贅述。
The monochromator consists of an inlet slit, an outlet slit and a dispersion element.It separates the absorption lines of the element to bemeasured from the adjacent spectral lines and prevents other spectral lines from entering the detector.Make the detection system accept only resonanceabsorption lines.
The detection system consists of photoelectric converter, signa lamplifier and display recorder.Its structure and principle will not be described one by one.
參考文獻
[1]武漢大學主編.分析化學（下冊）.第5版.北京：高等教育出版社，2006
[2]葉憲曾，張新祥編著.儀器分析教程.第2版.北京:北京大學出版社，2007