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Atomic Emission Spectroscopy

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1 Atomic Emission Spectroscopy
Yongsik Lee May 14, 2004

2 Introduction to AES Atomization Emission Sources Advantages of plasma
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 (105 – 106) Disadvantages of plasma very complex Spectra - hundreds to thousands of lines High resolution and expensive optical components Expensive 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 types Inductively Coupled Plasma (ICP) Direct Current Plasma (DCP) Microwave Induced Plasma (MIP)

4 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.

5 Sample introduction Nebulizer Electrothermal vaporizer
Table 8-2 methods of sample introducton

6 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

7 Electro-thermal vaporizer ETV
electric current rapidly heats crucible containing sample sample carried to atomizer by gas (Ar, He) only for introduction, not atomization

8 Plasma structure Brilliant white core Flame-like tail
Ar continuum and lines Flame-like tail up to 2 cm Transparent region where measurements are made (no continuum)

9 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.

10 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

11 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

12 Desirable properties of an AE spectrometer

13 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 Expensive ( > $80,000) Faster

14 Sequential vs. multichannel

15 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

16 Slew scan spectrometer
Two slew-scan gratings Two PMTs for VIS and UV Most use holographic grating

17 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

18 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

19 Multichannel polychromator AES
Rowland circle Quantitative det. 20 more elements Within 5 minutes In 10 minutes

20 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

21 Detection power of ICP-AES

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.

23 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.

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 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.

26 Typical Calibration ICP curves

27 Calibration curves of ICP-AES

28 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

29 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

30 Arc and Spark spectrograph

31 spectrograph Beginning 1930s photographic film Cheap
Long integration times Difficult to develop/analyze Non-linearity of line "darkness“ Gamma function Plate calibration

32 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

33 Homework 10-1, 10-2, 10-5, 10-7

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