2 Introduction to AES Atomization Emission Sources Advantages of plasma Flame – still used for metal atomsElectric Spark and ArcDirect current PlasmasMicrowave Induced PlasmaInductively Coupled Plasma – the most important techniqueAdvantages of plasmaSimultaneous multi-element Analysis – saves sample amountSome non-metal determination (Cl, Br, I, and S)Concentration range of several decades (105 – 106)Disadvantages of plasmavery complex Spectra - hundreds to thousands of linesHigh resolution and expensive optical componentsExpensive instruments, highly trained personnel required
3 10A Plasam Source AES Plasma Three main types an electrically conducting gaseous mixture containing significant concentrations of cations and electrons.Three main typesInductively Coupled Plasma (ICP)Direct Current Plasma (DCP)Microwave Induced Plasma (MIP)
4 ICP Inductively Coupled Plasma (ICP) Plasma generated in a device called a TorchTorch up to 1" diameterAr cools outer tube, defines plasma shapeRapid tangential flow of argon cools outer quartz and centers plasmaRate of Argon Consumption L/MinRadio frequency (RF) generator 27 or 41 MHz up to 2 kWTelsa coil produces initiation sparkIons and e- interact with magnetic field and begin to flow in a circular motion.Resistance to movement (collisions of e- and cations with ambient gas) leads to ohmic heating.Sample introduction is analogous to atomic absorption.
6 Nebulizer convert solution to fine spray or aerosol Ultrasonic nebulizeruses ultrasound waves to "boil" solution flowing across discPneumatic nebulizeruses high pressure gas to entrain solution
7 Electro-thermal vaporizer ETV electric current rapidly heats crucible containing samplesample carried to atomizer by gas (Ar, He)only for introduction, not atomization
8 Plasma structure Brilliant white core Flame-like tail Ar continuum and linesFlame-like tailup to 2 cmTransparent regionwhere measurements are made (no continuum)
9 Plasma characteristics Hotter than flame (10,000 K) - more complete atomization/ excitationAtomized in "inert" atmosphereIonization interference small due to high density of e-Sample atoms reside in plasma for ~2 msec andPlasma chemically inert, little oxide formationTemperature profile quite stable and uniform.
10 DC plasma First reported in 1920s DC current (10-15 A) flows between C anodes and W cathodePlasma core at 10,000 K, viewing region at ~5,000 KSimpler, less Ar than ICP - less expensiveLess sensitive than ICPShould replace the carbon anodes in several hours
11 Atomic Emission Spectrometer May be >1,000 visible lines (<1 Å) on continuumNeedhigher resolution (<0.1 Å)higher throughputlow stray lightwide dynamic range (>1,000,000)precise and accurate wavelength calibration/intensitiesstabilitycomputer controlledThree instrument types:sequential (scanning and slew-scanning)Multichannel - Measure intensities of a large number of elements (50-60) simultaneouslyFourier transform FT-AES
13 Sequential vs. multichannel Sequential instrumentPMT moved behind aperture plate,or grating + prism moved to focus new l on exit slitPre-configured exit slits to detect up to 20 lines, slew scancharacteristicsCheaperSlowerMultichannel instrumentPolychromators (not monochromator) - multiple PMT'sArray-based systemcharge-injection device/charge coupled deviceExpensive ( > $80,000)Faster
15 Sequential monochromator Slew-scan spectrometerseven with many lines, much spectrum contains no informationrapidly scanned (slewed) across blank regions (between atomic emission lines)From 165 nm to 800 nm in 20 msecslowly scanned across lines0.01 to nm incrementcomputer control/pre-selected lines to scan
16 Slew scan spectrometer Two slew-scan gratingsTwo PMTs for VIS and UVMost use holographic grating
17 Scanning echelle spectrometer PMT is moved to monitor signal from slotted aperture.About 300 photo-etched slits1 second for moving one slitCan be used as multi channel spectrometerMostly with DC plasma source
18 AES instrument types Three instrument types: sequential (scanning and slew-scanning)Multichannel - Measure intensities of a large number of elements (50-60) simultaneouslyFourier transform FT-AES
20 Applications of AES AES relatively insensitive small excited state population at moderate temperatureAAS still used more than AESless expensive/less complex instrumentationlower operating costsgreater precisionIn practice ~60 elements detectable10 ppb range most metalsLi, K, Rb, Cs strongest lines in IRLarge # of lines, increase chance of overlap
22 ICP/OES INTERFERENCES Spectral interferences:caused by background emission from continuous or recombination phenomena,stray light from the line emission of high concentration elements,overlap of a spectral line from another element,or unresolved overlap of molecular band spectra.CorrectionsBackground emission and stray light compensated for by subtracting background emission determined by measurements adjacent to the analyte wavelength peak.Correction factors can be applied if interference is well characterizedInter-element corrections will vary for the same emission line among instruments because of differences in resolution, as determined by the grating, the entrance and exit slit widths, and by the order of dispersion.
23 Physical interferences of ICP causeeffects associated with the sample nebulization and transport processes.Changes in viscosity and surface tension can cause significant inaccuracies,especially in samples containing high dissolved solidsor high acid concentrations.Salt buildup at the tip of the nebulizer, affecting aerosol flow rate and nebulization.Reductionby diluting the sampleor by using a peristaltic pump,by using an internal standardor by using a high solids nebulizer.
24 Interferences of ICP Chemical interferences: include molecular compound formation, ionization effects, and solute vaporization effects.Normally, these effects are not significant with the ICP technique.Chemical interferences are highly dependent on matrix type and the specific analyte element.
25 Memory interferences: When analytes in a previous sample contribute to the signals measured in a new sample.Memory effects can resultfrom sample deposition on the uptake tubing to the nebulizerfrom the build up of sample material in the plasma torch and spray chamber.The site where these effects occur is dependent on the element and can be minimizedby flushing the system with a rinse blank between samples.High salt concentrations can cause analyte signal suppressions and confuse interference tests.
28 10B. Arc and Spark AES Arc and Spark Excitation Sources: Limited to semi-quantitative/qualitative analysis (arc flicker)Usually performed on solidsLargely displaced by plasma-AESElectric current flowing between two C electrodes
29 Carbon electrodesSample pressed into electrode or mixed with Cu powder and pressed - Briquetting (pelleting)Cyanogen bands (CN) nm occur with C electrodes in air -He, Ar atmosphereArc/spark unstableeach line measured >20 sneeds multichannel detection
31 spectrograph Beginning 1930s photographic film Cheap Long integration timesDifficult to develop/analyzeNon-linearity of line "darkness“Gamma functionPlate calibration
32 Multichannel photoelectric spectrometer multichannel PMT instrumentsfor rapid determinations (<20 lines) but not versatileFor routine analysis of solidsmetals, alloys, ores, rocks, soilsportable instrumentsMultichannel charge transfer devicesRecently on the marketOrignally developed for plasma sources