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AAS AES AFS

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Description about several method of Spectroscopy

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Page 1: AAS AES AFS

•Materials Characterization 2

Atomic Spectroscopy

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Atomic Spectroscopy

•Atomic Absorption Spectroscopy (AAS)

•Atomic Emission Spectroscopy / Flame Emission Photometer (AES/FEP)

•Inductively Coupled Plasma Spectroscopy (ICP)

•Atomic Fluorescence Spectroscopy (AFS)

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Atomic Spectroscopy

• Sample Introduction– Flame

– Furnace

– ICP

• Sources for Atomic Absorption– Hollow Cathode Lamps

• Sources for Atomic Emission– Flames

– Plasmas

• Wavelength Separators

• Calibration and other considerations

• Comparison of Techniques

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Atomic Spectroscopy

• Atomic spectroscopy is the sub-division of spectroscopy that investigates the line spectra that are observed upon interaction of electromagnetic radiation with free atoms.

• As free atoms can absorb as well as emit electromagnetic radiation of the same energy (wavelength),

• Atomic spectra can be observed as absorption spectra, emission spectra and fluorescence spectra, a combination of the former two.

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Atomic spectra

• As the spectra are closely related to the structure and the properties of atoms, from which they are absorbed or emitted, they are specific for the atom under consideration and can be used for its detection.

• When different atomic spectra are considered it becomes obvious that the arrangement of the spectral lines depends on the position of the atom in the periodic table.

• Elements with only one valence electron (e.g. alkaline metals) exhibit spectra with relatively few lines;

• elements with a greater number of valence electrons (e.g. transition metals) exhibit spectra with a significantly larger number of lines.

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The arrows indicate transitions, which the electrons can undergo. Downward arrows indicate emission; upward arrows indicate absorption, i.e. excitation.

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Atomic Spectra

Absorption and emission spectra of sodium (schematic)

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Atomic vs Molecules Spectra

•Atomic spectra consist of pure electronic transitions and appear as Sharp, Narrow lines, width ~0.01 nm.–because there is little interaction with anything in

the gas

–because atoms are simple compared to molecules

–narrow lines means easy to distinguish from noise

–Doppler and pressure broadening increase line widths, but they are still narrow for our purposes

•Molecules spectra may be spread out over 100 nm

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Problem in atomic spectra

Atomic spectra can move away towards and away from detector because 2 broadening phenomena :

• Doppler broadening– Result Doppler shift 100x wider than normal (10-5nm)

– Not much can do about it except recognize that is occurs

• Pressure broadening / collision broadening– Spectra collide with other spectra because some energy

exchange

– More effect if temperature arise

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Spectral interference

• AA or AE lines interferencefrom another element if Δλ< 0.01 nm (rare)

• Background interference froma.scattering of the source beamb.broadband absorption by molecular

combustion products

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Atomic Absorption Spectrometer

Source Wavelength Selector

Sample

DetectorSignal Processor Readout

PPo

Chopper

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Atomic-Absorption Spectroscopy

•Atomic absorption measures the absorption, by free atoms in a sample solution, of characteristic radiation generated by a lamp.

•Used to determine elemental concentrations in solutions down to the ppb level.

•Solid samples must be dissolved before analysis. This is typically done either by fusion or acid digestion techniques.

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Atomic-Absorption Spectroscopy

•AA Spectroscopy requires atoms to be in their gas phase.

•As a result, the sample must be vaporized or dissolved at high temperatures.

•Sample solutions are aspirated, nebulized (converted into a fine mist) and carried into a flame.

•The flame vaporizes the mist into a gas containing absorbing free atom species.

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The Sources

• Hollow cathode lamps (HCL)

• Superlamps (S-HCL)

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Hollow Cathode Lamps (HCL)

• An HCL usually consists of a glass tube containing a cathode, an anode, and a buffer gas (usually a noble gas).

• The cathode is made from the material that is to be determined.

• A large voltage across the anode and cathode will cause the buffer gas to ionize, creating a plasma. The buffer gas ions will then be accelerated into the cathode, sputtering off atoms from the cathode.

• Multi elements lamps are available

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Superlamps

• Superlamps are recommended for the determination of elements, which have their main analytical lines in the far UV range, such as arsenic and selenium.

• Superlamps, in contrast to regular HCL, are equipped with an additional heating device, so that they need a socket with four instead of two electric cables.

• The heating device is supplied by the so-called boost current, which intensifies the electric current to the cathode.

• This way a lot more sputtered metal atoms can be excited than in conventional HCL, and the risk that a cloud of metal atoms in the ground state is forming in front of the cathode is significantly reduced.

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The Atomizer

The following atomization techniques are nowadays used in AAS:

• Flame technique with Nebulizer

• Graphite furnace technique / Electrothermal

• Hydride and Cold Vapor techniques

• HydrEA technique (combination of Hydride and Graphite furnace technique)

Flame Graphite furnace Hydride technique

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Flame Technique

• The sample is transferred into liquid form, e.g. by dissolution.

• The nebulizer aspirates the solution and transfers it into a fine aerosol.

• Large droplets are separated in the mixing chamber, and the aerosol is mixed with the fuel gas and additional oxidant.

• The aerosol-fuel gas –oxidant mixture is ignited above the burner head

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Flame Technique

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Flames

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Flame absorbance profiles

•Mg: longer time > more Mg atomsinto 2o zone > oxidation

•Ag: not easy to be oxidizedlonger time > more Ag atoms

•Cr: forming stable oxideslonger time > less Cr atoms

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Flames correction

•what would happen if the flame were too hot?– elements would ionize or enter an excited state

– these excited species would not absorb the correct light

– signal would decrease

•what would happen if the flame were not hot enough ?– compounds may not completely atomize

– the compounds would not absorb the same light as the element

– signal would decrease

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Graphite Furnace / Electrothermal

• higher sensitivity than nebulizer/flame combo

• poor reproducibility

• sample is atomized in seconds

• high sensitivity (0.1 to 100 pg)

• low reproducibility (5% to 10% RSD)

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Graphite Furnace

• With this technique the sample to be investigated may be liquid or solid, and is introduced directly into a graphite tube.

• A controlled voltage is applied at the ends of the graphite tube, which is heated rapidly to high temperatures (up to 2600°C) due to its resistance.

• Using time-controlled stepwise heating of the graphite tube the sample solution is first dried, and then the matrix can be destroyed or removed, until finally the element of interest is atomized.

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Hydride and Cold Vapor techniques

• Use for Mercury and the elements that are forming volatile hydrides (e.g. As, Se, Sb, Te, Sn, Bi)

• The solution for measurement is mixed with sodium borohydride solution in a suitable apparatus. The generated hydrides are purged out of the solution using a carrier gas flow.

• Analyte can frequently be separated completely from the matrix.

• Atomization may be carried out in a heated quartz tube placed in the beam of the spectrometer.

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The optical system

The optical components that are required for an AA spectrometer may be combined into two major group:

• The monochromator, which has the duty of dispersing the incoming radiation spectrally, and to prevent that any radiation, except for the analytical line, reaches the detector.

• Lenses and mirrors which focus the radiation of the HCL, first in the atomization zone (flame, graphite tube, quartz tube), then on the entrance slit of the monochromator, and finally on the detector.

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The optical system

For Removal of Atomic Emission :

• Place a chopper before the flame

• The signal from the source is modulated by the chopper

• Thus, a AC signal is produced on top of a DC signal that originates from emission in the flame

The chopper is a device which impedes the signal from the hollow-cathode into two beams. One of the beams is called the analytical beam and the other is called reference beam.

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Monochromator

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The detector

• The detection of radiation in conventional AA spectrometers is typically accomplished by a photomultiplier tube (PMT).

• A PMT is an electronic tube that is capable of converting a photon current into an electrical signal and of amplifying this signal.

• A PMT consists of a photo cathode and a secondary electron multiplier.

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Quantitative analysis in AAS

• Basis for the quantitative evaluation of absorption spectra is the law of LAMBERT and BEER, which says that the absorbance A is proportional to the thickness d of the absorbing layer and the concentration c of the absorbing matter.

I0 = Incident radiation intensityId = Transmitted radiation intensityελ = Molar absorbance coefficientc = Molar concentrationl = Thickness of the absorbing layer

IdI0

d

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Quantitative analysis in AAS

• The Lambert-Beer Law can be strictly applied only for monochromatic radiation and ideally diluted solutions.

• In non-ideal solutions ελ is no longer independent of the concentration, as the properties of the absorbing matter might be influenced by interactions between molecules, such as dissociation or complex formation.

• Transferring this generally valid formula to AAS, we arrive at:

kλ = Absorption coefficientN0 = Number of atoms in the absorption volume l = Length of the absorbing layer

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Limit of detection

• The limit of detection (LOD) is a measure for the analyte content or mass, above which the presence of the analyte in a solution for measurement can be detected with a certain statistical probability compared to a blank test solution.

• The LOD is defined (IUPAC) as the analyte concentration or mass that gives a signal corresponding to three times the baseline noise of the blank test solution (3s-criterion).

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Limit of detection

Unit in ng/mL

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Limit of quantification

• At the limit of quantification (LOQ) is the lowest analyte concentration or mass that can be determined quantitatively with a certain precision.

• In the concentration or mass range between the LOD and the LOQ the analyte may be detected, but no quantitative evaluation is permissible.

• The LOQ corresponds roughly a value three times the LOD or ten times the standard deviation of the blank test solution.

• LOD and LOQ have to be determined separately for each sample type (matrix) and must not only be determined for calibration solutions.

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Interferences in AAS

•Spectral interference:due to spectral overlapping.

•Chemical interferencealteration of AA signals due to chemical

processes

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Chemical interference

• Formation of low volatility compounds– anions (in flame or furnace) + cations (in sample)

→ low volatility compound → reduces atomization

• Dissociation equilibria– formation and dissociation of metal oxides of the analyte.

e.g. MO ↔ M + O; M(OH)2 ↔ M + 2OH

• Ionization equilibria– formation of metal ions in the flame> AA decreases

– ionization of metal increases with increasing T

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AAS Problems

• Temperature level and consistency are key

• Alignment the source light is importance

• Analyte can be lost by volatilization prior the to absorbance of light

• Refractory species and excessive amount of complexes can be form in flame if temperature relatively low

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What is Atomic Emission

• Rapid relaxation of excited species is accompanied by emitting of ultraviolet and visible light at discrete wavelengths (line spectra), which are useful for quantitative and qualitative analysis.

• The excitation source (stimulus) to promote chemical species up to excited states can be flame , plasma, electric arc and spark.

• In atomic emission spectroscopy, we measure the emission of light from atoms. The intensity is proportional to the concentration of atoms in the particular excited state

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Typical steps in AES:

1.Place the sample in solution.2.Form an aerosol (nebulization).3.Evaporate the solvent from the aerosol droplets.4.Decompose the resulting particles into atoms.5.Thermally excite a small fraction of the atoms.6.Select a narrow wavelength range of the

spontaneously emitted radiation to be measured.7.Measure the intensity in this wavelength range, and

analyze the data

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Application of atomic emission spectroscopy (AES)

• Sample preparation (similar to the atomic absorption spectroscopy)

• Element determination: almost all metallic elements can be measured by AES; A vacuum spectrometer is necessary for the determination of boron, phosphorus, nitrogen, sulfur and carbon, because the emission lines of these elements lie below 180 nm where the components of air absorbs.

• Line selection: the strongest line may not be the best if it is not very separate from the lines of other existing elements;

• Calibration curves: plot of the output current or voltage of a transducer vs. the concentration of analytes; at very high concentration of analytes, log-log plots are employed;

• Interferences: chemical and matrix interferences are significantly low for plasma sources;

• Detection limit: comparable or better than other atomic spectral methods.

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Atomic Emission Spectrometer & ICP

Source Wavelength Selector

Sample

DetectorSignal Processor Readout

P

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Flame Emission Spectrometer

Flame photometry, more properly called flame atomic emission spectrometry, is the traditional method for determining the concentration of electrolytes in fluids.

It is a fast, simple, and sensitive analytical method that measures metals, such as Na+ and K+, that are easily excited at flame temperature.

The solution must be aspirated or sprayed in to a flame in order to be quantified.

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Flame Emission Spectrometer

• Emission flame photometry relies on the principle that the alkali metal salt absorbs energy from a non-luminous flame in order to reach its excited state.

• The electron is highly unstable in this position; thus, almost immediately after, falls back to its original state while emitting energy in the form of light.

Na+ Na+* (absorbed energy from flame)Na+* Na+ + hv (589 nm)

• The light is measured as a wavelength, specific to the atom being observed. The intensity of the emission is proportional to the concentration of the element in solution.

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Flame Emission Application

•used mostly for alkali metals > easily excited even at low

temperatures

•Na, K

•need internal standard (Cs usually) to correct for variations flame

•Advantages- cheap

•Disadvantage- not high enough temperature to extend to many other elements

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Flame Emission Application

•flame emission not common because for most single-element determinations, absorption methods proved as good or better results in terms of–accuracy,

–convenience,

–and detection limits

•for multielements analysis, plasma sources are far superior to flames in most regards

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Stimulus Source for AE: Plasma

• A plasma is an electrical conducting gaseous mixture containing a significant concentration of cations and electrons. The concentrations of the two are such that the net charge approaches zero.

• Argon gas, one of the most widely used plasma species, provides temperature as high as 10,000 K.

• Three types of plasma considered for atomic emission spectroscopy:1.Inductively coupled plasma (ICP);2.Direct current plasma (DCP);3.Microwave induced plasma (MIP).

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Inductively coupled plasma (ICP)

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Inductively coupled plasma (ICP)

• In the ICP, the plasma is created in argon.• A few argon ions are formed, and these

ions absorb energy from the radio-frequency field. They are accelerated and moved in the closed annular path.

• Collision between higher energy argon ions and free atoms generates more argon ions --- thus forms the plasma.

• The temperature of the plasma is ~10,000 K (5500oC).

• The sample is introduced in the central tube, and argon cooling gas is supplied to keep the tubes from melting.

• The RF coil is water cooled.• In the region 10 to 30 mm above the

core, the plasma is optically transparent. The region 15 – 20 mm is typically for spectral observation.

• The residence time for sample in the plasma is short, ~ 2 ms.

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Sample introduction in ICP

1.Nebulizers --- the most widely used method for sample introduction. Good for solution samples. Argon flow rate is about 0.3 –

1.5 mL/min.

2.Electrothermal vaporization --- good for both liquid and solid samples.

3.Ablation devices --- good for solid samples.

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Advantages and Disadvantages of ICP

• Advantages :ability to identify and quantify all elements with the exception of

Argon; the ICP is suitable for all concentrations from ultratrace levels to

major components; detection limits are generally low for most elements with a

typical range of 1 - 100 g / L. multielemental analysis can be accomplished, and quite rapidly.

• Disadvantages Although in theory, all elements except Argon can be determined

using and ICP, certain unstable elements require special facilities for handling the radioactive fume of the plasma.

ICP has difficulty handling halogens--special optics for the transmission of the very short wavelengths become necessary.

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Direct current plasma (DCP)

•The direct current plasma is created by the electronic release of the two electrodes. The samples are placed on an electrode. In the technique solid samples are placed near the discharge to encourage the emission of the sample by the converted gas atoms.

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Direct current plasma (DCP)

•In the DCP, the plasma is also created in argon.

•The temperature at the arc core is ~10,000 K.

•DCP uses less argon, and the auxiliary power supply is simpler and less expensive.

•The graphite electrode must be replaced every few hours, while in ICP little maintenance is required.

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Direct current plasma (DCP)

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Arc and Spark / Electrothermal ES

• involves use of electrical discharge to give high temperature environment

• Mostly used for solids, but also can for liquids or gas phase samples

• types of discharge used:DC arc: high sensitivity, poor precisionDC spark: intermediate sensitivity and precisionAC spark: low sensitivity, high precision

• Because of difficulty in reproducing the arc/spark conditions, all elements of interest are measured simultaneously by use of appropriate detection scheme.

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Arc and Spark / Electrothermal ES

• For solid/metal, samples act as electrode

• For liquid, samples are placed in a high purity graphite cup & heated by DC/AC discharge to desolvate, vaporize, atomize, and excite

• Arc temperatures of 4000-10000 K

• Much more efficient than flame methods, more elements are excited and emit

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Arc/Spark Emission Spectrometer

Arc created by electrodes separated by a few mm, with an applied current of 1-30 A

Concave grating disperse frequencies, photographic film records spectra

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Atomic Fluorescence Spectroscopy

• Techniques based on the absorption of light by free atoms in the gas phase followed by atomic fluorescence, typically in the UV and Visible region of the electromagnetic spectrum

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Atomic Fluorescence Spectroscopy

•optical emission from gas-phase atoms that have been excited to higher energy levels–Enhancement of sensitivity over AA

–Examine electronic structure of atoms

• Light source–Hollow Cathode Lamp

– Laser

•Detection–Similar to AA

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Atomic Fluorescence Spectroscopy

• This technique incorporates aspects of both atomic absorption and atomic emission

• Fluorescence is a luminescence that is mostly found as an optical phenomenon in cold bodies, in which the molecular absorption of a photon triggers the emission of a photon with a longer (less energetic) wavelength.

• The intensity of this "fluorescence" increases with increasing atom concentration, providing the basis for quantitative determination.

• The source lamp for atomic fluorescence is mounted at an angle to the rest of the optical system, so that the light detector sees only the fluorescence in the flame and not the light from the lamp itself.

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Basic Instruments of AFS

Source

Sample

Wavelength Selector Detector Signal Processor Readout

PPo

Chopper

90o

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Fluorescence Sources

• Hollow Cathode Lamp (HCL) and Electrodeless Discharge Lamp (EDL) sources are not of sufficient intensity for meaningful analyses

• Lasers produce high intensities and narrow bandwidths ideal for fluorescence measurements

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Applications of AFS

• Due to the success in the development of AA and AE methods, applications in AF are limited and currently there is no commercially available instrumentation to perform atomic fluorescence analysis.

• But now AFS available on market specially to determined Hg on sample

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Atomic Spectroscopy Comparison of Instruments

Instrument Cost Speed Sensitivity

Flame-AA Low (~$10-15K)

Slow Moderate(~0.01 ppm)

GF-AA Moderate(~$40K)

Slowest Very Good

Sequential ICP-AES

Moderate Medium Moderate

Simultaneous ICP-AES

High Fast Good

ICP-MS Highest (~$200K)

Fast Excellent

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LOD Comparison of Atomic Spectroscopy

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Emission vs Absorption

Flame Emission More Sensitive

Sensitivity About the Same

Flame Absorption More Sensitive

Al, Ba, Ca, Eu, Ga, Ho, In, K, La, Li, Lu, Na, Nd, Pr,Rb, Re, Ru, Sm, Sr, Tb, Tl, Tm, W, Yb

Cr, Cu, Dy, Er, Gd, Ge, Mn, Mo, Nb, Pd, Rh, Sc, Ta, Ti, V, Y, Zr

Ag, As, Au, B, Be, Bi, Cd, Co, Fe, Hg, Ir, Mg, Ni, Pb, Pt, Sb, Se, Si, Sn, Te, Zn