Presentation on theme: "Atomic Emission Spectroscopy Yongsik Lee May 14, 2004."— Presentation transcript:
Atomic Emission Spectroscopy Yongsik Lee May 14, 2004
Introduction to AES ► ► Atomization Emission Sources Flame – still used for metal atoms Electric Spark and Arc Direct current Plasmas Microwave Induced Plasma Inductively Coupled Plasma – the most important technique ► ► Advantages of plasma Simultaneous multi-element Analysis – saves sample amount Some non-metal determination (Cl, Br, I, and S) Concentration range of several decades (10 5 – 10 6 ) ► ► Disadvantages of plasma very complex Spectra - hundreds to thousands of lines High resolution and expensive optical components Expensive instruments, highly trained personnel required
10A Plasam Source AES ► ► Plasma an electrically conducting gaseous mixture containing significant concentrations of cations and electrons. ► ► Three main types Inductively Coupled Plasma (ICP) Direct Current Plasma (DCP) Microwave Induced Plasma (MIP)
ICP ► ► Inductively Coupled Plasma (ICP) Plasma generated in a device called a Torch Torch up to 1" diameter Ar cools outer tube, defines plasma shape Rapid tangential flow of argon cools outer quartz and centers plasma Rate of Argon Consumption L/Min Radio frequency (RF) generator 27 or 41 MHz up to 2 kW Telsa coil produces initiation spark ► ► Ions 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.
Nebulizer ► ► convert solution to fine spray or aerosol ► ► Ultrasonic nebulizer uses ultrasound waves to "boil" solution flowing across disc ► ► Pneumatic nebulizer uses high pressure gas to entrain solution
Electro-thermal vaporizer ETV ► ► Electrothermal vaporizer (ETV) electric current rapidly heats crucible containing sample sample carried to atomizer by gas (Ar, He) only for introduction, not atomization
Plasma structure ► Brilliant white core Ar continuum and lines ► Flame-like tail up to 2 cm ► Transparent region where measurements are made (no continuum)
Plasma Plasma characteristics ► ► Hotter than flame (10,000 K) - more complete atomization/ excitation ► ► Atomized in "inert" atmosphere ► ► Ionization interference small due to high density of e- ► ► Sample atoms reside in plasma for ~2 msec and ► ► Plasma chemically inert, little oxide formation ► ► Temperature profile quite stable and uniform.
DC plasma ► ► First reported in 1920s ► ► DC current (10-15 A) flows between C anodes and W cathode ► ► Plasma core at 10,000 K, viewing region at ~5,000 K ► ► Simpler, less Ar than ICP - less expensive ► ► Less sensitive than ICP ► ► Should replace the carbon anodes in several hours
Atomic Emission Spectrometer ► ► May be >1,000 visible lines (<1 Å) on continuum ► ► Need higher resolution (<0.1 Å) higher throughput low stray light wide dynamic range (>1,000,000) precise and accurate wavelength calibration/intensities stability computer controlled ► ► Three instrument types: sequential (scanning and slew-scanning) Multichannel - Measure intensities of a large number of elements (50-60) simultaneously Fourier transform FT-AES
Desirable properties of an AE spectrometer
Sequential vs. multichannel ► ► Sequential instrument PMT moved behind aperture plate, or grating + prism moved to focus new l on exit slit Pre-configured exit slits to detect up to 20 lines, slew scan ► ► characteristics Cheaper Slower ► ► Multichannel instrument Polychromators (not monochromator) - multiple PMT's Array-based system ► ► charge-injection device/charge coupled device ► ► characteristics Expensive ( > $80,000) Faster
Sequential vs. multichannel
Sequential monochromator ► ► Slew-scan spectrometers even with many lines, much spectrum contains no information rapidly scanned (slewed) across blank regions (between atomic emission lines) ► ► From 165 nm to 800 nm in 20 msec slowly scanned across lines ► ► 0.01 to nm increment computer control/pre-selected lines to scan
Slew scan spectrometer ► Two slew- scan gratings ► Two PMTs for VIS and UV ► ► Most use holographic grating
Scanning echelle spectrometer ► ► PMT is moved to monitor signal from slotted aperture. About 300 photo-etched slits 1 second for moving one slit ► ► Can be used as multi channel spectrometer ► ► Mostly with DC plasma source
AES instrument types ► ► Three instrument types: sequential (scanning and slew-scanning) Multichannel - Measure intensities of a large number of elements (50-60) simultaneously Fourier transform FT-AES
Multichannel polychromator AES Rowland circle Quantitative det. 20 more elements Within 5 minutes In 10 minutes
Applications of AES ► ► AES relatively insensitive small excited state population at moderate temperature ► ► AAS still used more than AES less expensive/less complex instrumentation lower operating costs greater precision ► ► In practice ~60 elements detectable 10 ppb range most metals Li, K, Rb, Cs strongest lines in IR Large # of lines, increase chance of overlap
Detection power of ICP-AES
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. ► ► Corrections Background 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 characterized Inter-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.
Physical interferences of ICP ► ► cause effects associated with the sample nebulization and transport processes. Changes in viscosity and surface tension can cause significant inaccuracies, ► ► especially in samples containing high dissolved solids ► ► or high acid concentrations. Salt buildup at the tip of the nebulizer, affecting aerosol flow rate and nebulization. ► ► Reduction by diluting the sample or by using a peristaltic pump, by using an internal standard or by using a high solids nebulizer.
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.
Memory interferences: ► ► When analytes in a previous sample contribute to the signals measured in a new sample. ► ► Memory effects can result from sample deposition on the uptake tubing to the nebulizer from 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 minimized by flushing the system with a rinse blank between samples. ► ► High salt concentrations can cause analyte signal suppressions and confuse interference tests.
Typical Calibration ICP curves
Calibration curves of ICP-AES
10B. Arc and Spark AES ► ► Arc and Spark Excitation Sources: Limited to semi-quantitative/qualitative analysis (arc flicker) Usually performed on solids Largely displaced by plasma-AES ► ► Electric current flowing between two C electrodes
Carbon electrodes ► ► Sample pressed into electrode or mixed with Cu powder and pressed - Briquetting (pelleting) ► ► Cyanogen bands (CN) nm occur with C electrodes in air -He, Ar atmosphere ► ► Arc/spark unstable each line measured >20 s needs multichannel detection
Arc and Spark spectrograph
spectrograph ► ► Beginning 1930s ► ► photographic film Cheap Long integration times Difficult to develop/analyze Non-linearity of line "darkness“ ► ► Gamma function ► ► Plate calibration
Multichannel photoelectric spectrometer ► ► multichannel PMT instruments for rapid determinations (<20 lines) but not versatile For routine analysis of solids ► ► metals, alloys, ores, rocks, soils portable instruments ► Multichannel charge transfer devices Recently on the market Orignally developed for plasma sources