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The drying step– to volatilize the sample solvent– use some degrees above the solventsboiling point (for water use 110 C) andabout the same time in seconds as theinjected volume in µl– check that the solvent only volatilizesand not boil, otherwise badreproducibility may occur78

Ashing of the sample– Maybe the most important step– Remove sample matrix without losing theanalyte– Highly dependent on both the analyte andsample matrix– Conditions for every new sample typeshould be optimized– Inorganic matrix more difficult, becausehigher temperatures are needed79

The atomization step– This step is to vaporize and atomize theanalyte– the vaporization pressure of the analytedominate the supply of analyte atomsinside the tube– The convection of expanding gas anddiffusion dominate the transport ofanalytes out of the tube– rapid heating is necessary for goodsensitivity80

The burnout step– it is used for taking away parts of thesample not completely volatilized duringatomization– temperatures in the range 2700-3000 C ismost often used– if vaporization of the sample is achieved,completely, the sensitivity will slowlydecrease and the imprecision increase Cooling step– its purpose is to cool down the furnacebefore next sample is added81

Steps in GF atomizationtemperature progamme3000Temp ( C)25002000Atomization15001000Ashing500Dryingtime (sec)008220406080

Interferences with graphite furnaceand its control In the early days this technique wasaccepted as a highly interferencetechnique Later, new instrumental and analyticalprocedures were developed whichavoided the conditions found to be thesource of interference problems83

Interferences in GFAAS The graphite furnace atomization technique is muchmore affected by interferences than the flame whichdepends on higher concentrations of non-analytes–physical change in surface properties of the graphite tubeinfluences the atomization rate–Spectral molecular absorption by molecules or atoms otherthan the analyte atoms light scattering–non spectral or chemical molecular binding between analyte and matrix formation of less volatile compound reactions with the graphite tube84

Interference removalSpectral interferences Emission interference Non-analyte absorption and scattering85

1. Emission interference Radiation emitted by the hot graphite tube orplatform that covers a wide range ofwaqvelengths (250-800 nm) Following elements will be obscured:– Zn 213.9 nm; might be ok– Cr 357.9 nm– Ca 422.7 nm– Ba 553.7 nm mostly affected This effect is eliminated by removing thegraphite tube wall or platform from the viewof the detector. This is achieved by ine of the86following arrangements:

Setting a narrow monochromator slit Furnace alignment Cleaning graphite tube and windows toprevent light scattering Atomization temperature should not behigher than required87

2. Non-analyte absorption and scattering It is considered as a background absorption It is the most severe spectral interferenceproblem It occurs as a non specific attenuation of light atthe analyte wavelength due to matrixcomponents in the sample It leads to having a broad band covering 10100’s nm due to molecular absorption caused by“undissociated sample matrix” components inthe light path at atomizationRemedy? Matrix modification Furnace control procedures 88Background correction as with flame techniques

Matrix modification Matrix should be more volatile than the anayte so asto be removed before atomization1. Formation of easily volatile matrix components, e.g.,Using NH4NO3 or ammonium acetate to modify NaClmatrixNaCl NH4NO3NaNO3 NH4Clnonvolatile matrixMatrixmodifierMore volatile compounds The more volatile components can be driven off atlower pyrolysis temperature89

2. Converting the analyte element into a lessvolatile (if it is too volatile) component usinga modifier e.g., Ni (NO3)2 for Sedetermination. Se is a highly volatile component With the modifier Se form nickel selenide.Thus, Se can be heated up to 900 oC withoutloss. Consequently the matrix would beremoved without affecting Se.90

3. Varying the sample volume Can be done if concentration allows Reduce the sample size by injecting smallersamples, thus the mass of background producingmatrix components will be reduced.4. Spectral background correction A continuous background correction is not veryefficient specially for complex matrices. Matrixmodification is a must. Zeeman background correction is very efficient forcomplex matrices91

Chemical Interference It occurs when other components of the samplematrix inhibit the formation of free analyte atomsM o XoFree analyte atoms92MXDifferent matrix component

Interference Removal1. Ashing with the optimal temperature–matrix modification (mentioned above) Delay of anayte release to furnace allows time for aconstant furnace temperature thermal stabilization of analyte (to hold the analyteon the graphite surface to a higher temperature)and vaporization of interference during the ashingstep– Thus, volatility of interfering matrix compoundis increased On the other hand volatility of interferingcompound is decreased and selective vaporizationduring the atomization steptakes place93

2. Use of pyrolytically coated Tubes and Platforms Ordinary graphite has a porous surface thus it formswith the analyte a carbide that is nonvolatie The tubes are more or less always fabricated frompyrolytic carbon or covered with a pyrolytic carbon layer.It’s a more inert form of the graphite. It is more densesurface and less absorption Tubes could sometimes be coated with tungsten ortantalum for prolonged lifetime or other specialapplications The platform technique is used to vaporize the analyteinto a more uniform and higher temperature than therelease from the wall. The analyte vaporizes after thetubewall and gas phase reach a steady state94

L'vov Platform – Thin graphite plate inserted intotube with flat trough– L'vov barely contacts sides of tube– Sample is heated more uniformly by air– Platform delays the appearance of analyte untilthe furnace has reached atomization temp95

Functions of the platform It resists tndency for sample to saok into surfacebefore or during drying It tolerates high acid concnetration It gives bigger signal and less background– It is heated gradually by radiation from walls– When atomization step is reached the temperature ofthe sample and furnace environment will be similar– No chance for sudden cooling and recombinationforming molecular species inhibiting atomizationI96

Disadvantages of Graphite Furnace Prone to interferences, poor precision Must take Step to control Precision:1. Autosamplers vs. Manual Injection - more precise2. Duplicate analyses for all samples (repeat if pooragreement)3. Standard addition method - making standards up insample4. Background correction for Molecular Absorption)1. D22. Zeeman Background Correction3. Matrix Modifiers to controlatomization97

INDUCTIVELY COUPLEDPLASMA 98Main components of the instrumentPerformance of ICPICP-mass spectrometryinterferences

Introduction The device which produces the ICP is commonlyreferred to as the ICP torch. It consists of two to fourArgon flows depending on the manufacturer: Nebulizer gas (inner Argon flow), at about1 L/min, carries the analyte aerosol Sheath gas for producing a laminar flow toimprove low excitation energy elements eg groupI & II elements Auxiliary gas (if present), lifts the plasma abovethe injector tube, used when measuring organics Plasma gas, at about 12-16 L/min, sets theplasma conditions, e.g., excitation temperature99

Sample Introduction An ICP-AES instrument consists of a sampledelivery system, an IC plasma to generate the signal,one or more optical spectrometers to measure thesignal, and a computer for controlling the analysis. The most common sample delivery system consistsof a peristaltic pump and capillary tube to deliver aconstant flow of analyte liquid into a nebulizer.100

Inductively Coupled Plasma Torch101

How does plasma generate? RF induction coil powered by RF generatorwill generate a fluctuating magnetic fieldaround the coil Ionization of Ar is initiated by a spark Ar and e- will interact with the magnetic fieldthus they will be forced to move in annularpaths Omic heating is the result of their resistanceto the forced movement in limited area102

Inductively Coupled PlasmaEmission Source103

Inductively Coupled PlasmaEmission Source and Supply104

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An ICP torch can be used to atomize andionize materials for quantitative trace andultratrace elemental analysis The compounds in the samples are brokendown into their elemental form and somefraction of each elemental species issimultaneously ionized The monocationic ( 1) form is preferredfor the assay, and conditions are set toenhance production of that form The torch is normally operated at atemperature of 3000K107

Schematic diagram of a typical ICP-OES instrument108

Schematic diagram showing the differentregions in the IC Plasma109

Processes that take place in the plasma Aerosol vapor is transported to the plasma Vapor desolvates Atomization occurs within the plasma Atoms get excited to atomic and ionic states Rich spectra produced because of presence of bothatomic and ionic lines Because of their different excitation energies,different emission lines will have maximum intensitiesat different vertical positions in the plasma110

Typical detection limits in ICP-OES111

112

Typical detection limits in ICP-OES113

SamplingLiquids Form an aerosol ( 5 µ droplet size) of the liquidsample Eliminate larger droplets The use of liquids facilitates automatic sampling:Gases May use gasses directly or indirectly, e.g., formhydridesSolids May use slurries (if 95% of particles 5 µ) May use spark ablation or laser ablation to generatea small metal vapor114

Viewing position The plasma generated in an ICP can be viewedby the spectrometer, side-on or end-on. These viewing positions are called radial andaxial viewing, respectively. Each has advantages and disadvantages, soeach tends to be used for different applications:radial viewing for normal analysis and complexmaterials, axial viewing for low detection limitsin simpler materials.115

116

Advantages of Radial View Should collect signal from the entire normalanalytical zone– no torch adjustment for different elements– fewer matrix effects and interferences, especially inorganics– less stray light– better detection limits in difficult matrices, especiallyin alkalis and organics– can run any matrix– less maintenance of the torch– torches last longer especially with dissolved salts– lower consumption of argon117

118Advantages of Axial View: More intensity Better detection limits in simple matrices

RF-Generator ICP's generally require an RF power of 1-2 kW maximum outputto maintain the plasma. High efficiency required - especially for organics (not less than1.5 Kw)Frequency Fixed versus Free Running– Fixed: Crystal controlled, eg at 40 MHz– Free running: floats eg at 40 MHz 2 MHz 40 MHz versus 27 MHz– Because of the skin-effect in RF plasmas, higherfrequency gives a thinner plasma with a widerdynamic range (less self-absorption), with lowerbackgrounds and fewer interferences.119

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90% element coverage 0.1 to 10 ppb sensitivity for mostelements RSD 2% to 4%121

Typical laser ablation/ICP-MS mass spectrum of a rock sample122

Comparison of Detection LimitsBlack ICP/MS Cross-Hatch ICP/AES123Gray Furnance AA

Comparison of ICP/AES & ICP/MS for the analysis ofCerium solutionsICP/AES (100 ppm Ce)Background ionsoriginated fromthe Ar, O, H andN elementsICP/MS (10 ppm Ce) Simple to interpret, especiallyfor rare elements Less interference fromatmospheric contaminants ormolecular bandsNitric acid is commonly used as acidification agent rather than hydrochloric acid, sulfuric acid andphosphoric acid in sample preparation. Why?124

Limitations for ICP/MS Quantitation Formation of monocationic ions, doubly-charged ionsand stable metal oxides ions in the torch depends onthe identity of the element and several experimentalconditions, such as the plasma r.f. power and thenebulizer gas flow rate. Spectroscopic interference:1. Isobaric Interference- isotopes of different elements (or isobaricinterference) e.g. 87Rb and 87Sr ; 40Ar and40Ca ; 58Ni and 58Fe 125

– Polyatomic Ion Interference interactions between species in the plasmaand species in the matrix or atmosphere, e.g.molecular ions from the acids of digestione.g. ClO and 51V Which element(s) does not have isobaricinterference caused by the presence of otherelements?126

Matrix effect:(a)Limited tolerance for salt concentration( 1% is acceptable but 0.1% is preferred!)– The deposition of salts leads to adecrease in the aperture diameter, so thesensitivity worsens and the signalgradually decreases as a function of time(b)Organic solvents tend to quench the plasmatorch127

Approaches to reduce the occurrence ofpolyatomic interferences Adjust the plasma conditions to minimize the formation ofpolyatomic speciese.g. use 5-10% N2 in Ar, the axial temperature of the plasmaincreases by 1000 Kand the electron density increases by30%. A marked reduction of MO ions Minimizing the amount of water vapor introduced into theplasma in order to limit the signal intensity of oxygencontaining speciese.g. reduce the spray chamber temperature(?) Use He instead of Ar as plasma gas to avoid interference frompolyatomic ions containing argon Use electrothermal volatilization for sample introduction 128Use high resolution ICP/MS

Approaches to correct for or overcomematrix effect Dilution Matrix matching Use of internal standards– Additionof a known and preferably an equalconcentration of an internal standard to all samples (&standards). The analytical signal is taken as the ratio ofthe signals for the analyte elements in the sample (&standards) and the signal for the internal standard Standard addition– A known amount of the element(s) of interest is added tothe sample and hence suffers the same matrix effect asthe analyte129

1. Atomic Emission Signal Intensity of emission KN f K'N a K''C N f number of free atoms in flame N a number of absorbing atoms in flame C concentration of analyte in the sample K, K and K'' depend upon: Rate of aspiration (nebulizer) Efficiency of aspiration (evaporation efficiency) - Flow rate of solution