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12 ch4icpoes theory 12 ch4icpoes theory Document Transcript

  • Chapter 4 Inductively Coupled Plasma – Optical Emission Spectroscopy 4.1 INTRODUCTIONInductively coupled plasma – optical emission spectroscopy (ICP-OES) hasbeen acclaimed as one of the most versatile analytical tools for quantitativemultielement analysis77. It is a very powerful analytical tool that allows for thedetection of low concentration levels.The ICP source produces a plasma, which is a stream of high-energy ionisedgas by inductively coupling an inert gas such as argon with a high-frequencyfield. When a sample is injected through the centre of the plasma atemperature of 10 000 K allows for the desolvation, dissociation, atomisationand excitation of the elements in the sample. This results in emission of lightof unique frequencies for the given elements.This light is proportional to the concentration of the elements in the sampleand is measured by an emission spectrometer. The spectrometer is capableof separating the unique frequencies into discrete wavelengths andquantifying the results.Some advantages of the ICP – OES due to its high temperature are: • 5 – 7 orders of linearity-wide linear dynamic range • Increase in detection limits • No chemical interferences • Minimum interelement effects • Excellent accuracy and precision 54
  • ICP – OES 4.2 INSTRUMENTATION77ICP – OES is an elemental analysis technique that utilises spectra emitted byfree atoms or ions generated within a source known as an ICP. The set-up ofthe ICP-OES is shown below in Figure 4.1.Figure 4.1: Instrumental setup of an ICP – OES73 4.2.1 SourceThe source produces emission spectra characteristic of the elements presentin a sample. The ideal characteristics of an excitation source are: • to excite lines of all the elements of interest • to provide reproducible excitation conditions from sample to sample • to provide sufficient line intensity to achieve the required detection limits • to provide a low spectral background • to provide uniform and reproducible sample vaporisation and efficient atomisation 55
  • ICP – OESSome of the available excitation sources for AES are: • Flame • Electrical discharges e.g. DC arc, high – voltage spark • Microwave – induced plasma • ICP • Direct current plasmas a. Inductively Coupled PlasmaA plasma is a very hot gas, usually argon, in which a significant proportion ofthe atoms or molecules is ionised. The plasma is surrounded by a time –varying magnetic field, which causes it to be inductively coupled. This meansthat current flows are induced in the ionised medium, which cause resistiveheating of the plasma gas and enable it to be self–sustained. This occurs asthe hot plasma ionises some of the incoming non–ionised gas and theprocess is continuously repeated.A plasma is produced in the following manner:A water–cooled induction coil connected to a radio frequency generatorinduces a strong high–frequency magnetic field in a relatively small volumethat is flushed with argon (Figure 4.2). The argon is subsequently exposed toa high–voltage Tesla discharge that ignites the plasma by creating seedelectrons and ions. The electrons accelerate and encounter resistancethrough collisions with argon atoms, which results in the generation of a largeamount of heat and an avalanche of ions. A vortex flow of argon is used tocool the inside walls of the torch which are exposed to temperatures of up to10 000 K. It also serves to centre and stabilise the plasma. The hightemperature and long residence time of the sample results in completevaporisation of the sample solvent and total breakdown of the analyte into freeatoms, which are available for excitation. This process occurs in a chemicallyinert environment. 56 View slide
  • ICP – OESFigure 4.2: Inductively Coupled Plasma73 4.2.2 OpticsA spectrometer is used to isolate light from the various wavelengthscorresponding to emission lines of the different analyte elements and todifferentiate this light from the plasma background emission. A spectrometercontains a grating, a fine slit and an imaging system in order to improveresolution. Better resolution is needed so that adjacent line overlap does notoccur.There are two main types of spectrometers, which are used with the ICP.Firstly, there is a monochromator, which has only one secondary slit and canthus only measure one wavelength at a time. Multielemental analysis is donesequentially when using a monochromator. The second type of spectrometeris the polychromator, which has a secondary, slit permanently fixed at each ofthe selected analysis lines. If each of the slits has its own photomultiplier tube 57 View slide
  • ICP – OESthe analytes in a sample can be analysed simultaneously. This spectrometeris much faster but it is rigid as it can only measure at fixed wavelengths. 4.3 ICP PROCESS77The sample is introduced into the plasma as an aqueous aerosol afternebulisation. As the aerosol moves up through the plasma a variety ofprocesses occur. These processes are summarised in Figure 4.3 and arediscussed below.Figure 4.3: Processes, which occur in the plasma72Firstly, the aerosol droplets are desolvated to produce a solid salt particle.These particles are then vaporised to produce gas-phase molecular species.The molecular species subsequently undergo atomisation and with sufficient 58
  • ICP – OESenergy ionisation. Atoms and ions are also capable of combining with otherfree atoms to form “stable” molecular species.If an atom is to emit radiation, according to the law of conservation of energy,the atom must first absorb a corresponding amount of energy provided by anexternal source such as the plasma. Thus further energy transfer to the atomsis needed to raise their electrons to an excited state. The main excitationprocesses for the analyte are: • Electron impact excitation: e- + M M* + e- • Ion-electron radiative recombination: M+ + e M* + hvAnalyte line emission will occur when the excited state returns to a lowerenergy state with the release of a photon of radiation. 4.4 SAMPLE INTRODUCTION77-79ICP – OES is a versatile instrument and can be operated with a variety ofdifferent devices for the introduction of liquid, gaseous and solid samples. Asummary of the sample introduction devices available for ICP – OES is shownin Figure 4.4 and some of these techniques will be discussed below. Sample Introduction Liquids Solids Gases Pneumatic Ultrasonic Laser Ablation Arc or Spark Hydride Gas Nebuliser Nebuliser Ablation Generation Chromatography Electrothermal Direct Electrothermal Vaporisation Introduction VaporisationFigure 4.4: Summary of available sample introduction systems for ICP – OES 59
  • ICP – OES 4.4.1 Liquid Sample IntroductionThe introduction of liquid samples is mostly carried out by nebulisation. Thusintroduction of a liquid occurs as an aerosol into the excitation source.Nebulisation techniques are simple, reliable and relatively inexpensive. Thedisadvantage of nebulisation is that it is slow; interferences occur and up to99.5 % of the introduced sample goes to waste, which leads to high detectionlimits. A selection of nebulisers are discussed below: a. Pneumatic NebulisersSeveral types of pneumatic nebulisers are used for ICP–OES. These includethe concentric nebuliser, the cross–flow nebuliser and the Babingtonnebuliser.Samples are fed into the nebuliser by a pump or by aspiration, which resultsfrom the Venturi–effect. The sample solution is then split into droplets underthe influence of a high–speed gas flow that also serves as the aerosoltransport gas. Only a fraction of the introduced sample is nebulised, the restflows into the waste. The aerosol consists of droplets of various sizes thus it isblown into a spray chamber where the larger droplets are removed. i. Concentric NebuliserIn a concentric nebuliser, the sample solution is fed through a capillary, whichis surrounded by a second capillary through which a nebuliser gas flows(Figure 4.5). 60
  • ICP – OESFigure 4.5: Concentric nebuliser74Concentric nebulisers are self–aspirating and can be operated without aperistaltic pump. However, best analytical precision is obtained when thesample is pumped at a rate just below the free sample uptake rate of thenebuliser. Concentric nebulisers have high operation stability but they arefound to suffer from blockages. ii. Cross – Flow NebuliserIn cross–flow nebulisation the analyte solution is fed through a verticallymounted capillary and is nebulised by the force of a gas flowing horizontallyover the tip of the sample capillary (Figure 4.6).Figure 4.6: Cross–flow nebuliser74Most cross–flow nebulisers require force-feeding of the sample solution with aperistaltic pump. These nebulisers are also stable and are found to be lessprone to blockages. iii. Babington NebuliserIn a Babington nebuliser a liquid film is nebulised by blowing it against a wallto produce an aerosol. The benefit of this nebuliser is that the sample doesnot move through a thin capillary thus blockages are less likely to occur. 61
  • ICP – OESA modified version of the Babington nebuliser known as a V–groove nebuliserhas been developed for ICP–OES. In this type of nebuliser the solution flowsdown a V–shaped groove and is subsequently blown onto an impacter by agas flowing from a hole below the sample hole (Figure 4.7).Figure 4.7: V–groove nebuliser74 b. Ultrasonic Nebuliser80Ultrasonic nebulisers operate by directing a sample solution over the surfaceof a transducer plate. A piezoelectric membrane usually acts as thetransducer plate and vibrates at high energy due to the application of RFenergy at 1 MHz. The sample solution is shattered into fine droplets when itmoves over the rapidly vibrating transducer. The droplets are swept along in agas stream and are desolvated with a heater/condenser before beingtransported to the ICP (Figure 4.8). 62
  • ICP – OESFigure 4.8: Ultrasonic Nebuliser77Ultrasonic nebulisers have excellent detection power but their instability,especially with high salt content solutions, and the need for desolvation aredisadvantages. c. Electrothermal VaporisationIn electrothermal vaporisation a small volume of sample (solid or liquid) isplaced on a conductor e.g. carbon rod or tantalum filament. The conductor isheated resistively to produce a vapour, which is carried to the ICP by aninjector gas. An appropriate heating sequence enables the separation of thesolvent, matrix and analyte. 4.4.2 Gaseous Sample IntroductionGaseous samples can be introduced directly into the ICP without anycomplications. a. Hydride GenerationThe analytical sensitivities of elements such as As, Sb, Se and Bi arerelatively poor with the use of conventional sample introduction systems. Thusdeterminations at low concentration levels are difficult. This problem can beovercome with the use of hydride generation. In this technique the analyte istransferred into the vapour phase as a hydride. Sample preparation, acidityand the amount of reagent used in this process differ from element toelement. Thus hydride generation cannot be used if the multielementcapability of the ICP is to be employed. 4.4.3 Solid Sample IntroductionSolid sample introduction has not been researched as thoroughly as theintroduction of liquids. Some techniques have difficulties with respect tocalibration, sample conditioning and analytical performance. However, direct 63
  • ICP – OESinsertion, arc or spark sampling devices, electrothermal vaporisation and laserablation have been successfully applied to solid sample introduction. a. Laser Ablation81In the laser ablation technique, energy in the form of a focused laser beam isdirected onto the sample and causes material to be vaporised and sputteredfrom the surface. The vapour and particulates released from the surface aretransported in an argon stream to the plasma. Laser ablation is a relativelyunique technique as the focusing characteristics of lasers permit sampling ofvery small areas thus localised in situ microanalysis and spatially resolvedstudies can be done. b. Arc and Spark Ablation81In arc and spark ablation interaction of the discharge with a sample surfaceproduces vapour and particulate matter, which is transported to the ICP in agas stream. Two types of discharge exist namely arcs and sparks. Thedischarges operate in an oxygen free environment. For successful ablation,the sample must be conductive and can be in any physical form as long as areproducible discharge is formed between the sample and electrode. c. Direct Sample InsertionDirect sample insertion is where the sample is inserted on a probe directly intothe ICP. Rapid heating of the sample results in vaporisation of analyte speciesinto the ICP. This sample introduction process is extremely efficient hencehigh sensitivity is achieved. The solid is usually ground into a powder orsegmented into pieces before being placed in a probe. The probes are madefrom materials such as graphite, tantalum or tungsten. Data is collected astime–dependant intensities because differential volatilisation occurs. 64
  • ICP – OES 4.5 INTERFERENCES72Of the various analytical techniques that exist not one can be claimed to beinterference free. Thus an instrument is selected on the particularrequirements that must be met for that particular analysis. Some of thecommon interferences of ICP–OES will be discussed below. 4.5.1 Matrix InterferenceThe efficiency of the sample introduction system used is affected by thesurface tension, viscosity and dissolved solid content of the samples. Thesedifferences among samples and standard solutions can cause differences inthe nebuliser uptake rate and in the efficiency of sample transport to theplasma. This causes fluctuations in the results of analyses. For ICP analysisthe total dissolved solid content of samples should be kept to a maximum of0.5 % for the best results. Samples with higher total dissolved solid contentstend to clog the nebuliser thus frequent cleaning is needed. Matrixinterferences can be avoided by matrix matching or by using the internalstandard or standard addition methods. 4.5.2 Chemical and Physical InterferencesDue to the high operating temperature of the argon plasma (± 10 000 K) theICP is relatively free from chemical interferences. This temperature is highenough to cause dissociation of most chemical bonds and compounds to theatomic state. The plasma is also essentially oxygen free.Physical interferences occur due to sample consumption, alteration of sampletransport and droplet formation processes. These processes do not have asignificant effect in ICP, as the consumption rate is relatively small. 65
  • ICP – OESA peristaltic pump controls the sample flow rate in ICP, which makes theuptake rate independent of the viscosity of the sample thus minimisingchemical, and matrix effects. 4.5.3 Ionisation InterferencesIonisation interferences result from donation of electrons by concomitantspecies in the sample, which alter the atom or ion concentration of the speciesbeing determined. The electron-rich nature of the ionised argon gas tends tobuffer the ionising effect of the high-temperature environment. The degree ofionisation, which occurs in ICP, is constant within the overall reproducibility ofICP measurements. 4.5.4 Spectral or Background InterferencesBackground shifts occur due to light emission by the excitation source at theanalyte wavelength. Spectral interferences occur when an element in asample has an emission line close to that of the analyte wavelength. Threetypes of spectral interferences occur namely stray light, partial overlap ofnearby or wing–broadened spectral lines or direct overlap of unresolvedspectral lines.The interferences may arise from emission of the unwanted light from sourcessuch as the matrix, solvent, air and gases.Spectral interferences can be minimised by correct wavelength selection,observation height selection, background correction and interfering elementcorrection. 66