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Chemistry 311: Instrumentation Analysis Topic 2: Atomic Spectroscopy

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Topic 2: Atomic Spectroscopy

Atomic line widths: Narrow line widths reduce the possibility of spectral overlap and thus interferences. The band width at half height is used to indicate width. This is also sometimes called the effective line width Δλ½.

Line broadening arises from 4 sources; Uncertainty Effect: Broadening due to the uncertainty principal relating to the uncertainty in state lifetimes Doppler effect: Broadening due to the Doppler effect (see below) Pressure effects: Broadening due to collisions of the emitted or absorbing species causing small changes in the energy level of the ground state. Electric and magnetic effects: Broadening due to the Zeeman effect or in other words the splitting of states due to presence of electric (or mag. fields)

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Topic 2: Atomic Spectroscopy

Results from the Doppler shift phenomenon common for sound waves.

The magnitude of the Doppler shift increases with the velocity that the emitting species is traveling to or from the detector (observer). No shift is observed by species moving perpendicular to the detector.

Atomic velocity is distributed over a range described by a Maxwell-Boltzman distribution. The average velocity increases as the square root of the absolute temperature. In flames, Doppler effect broadens lines two orders of magnitude greater than natural line widths.

Introduction to Atomic Absorption Spectrometry: Doppler effect:

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Temperature Effects on Atomic Spectra: Temperature has a significant effect on the population of excited vs ground atomic species. The distribution is described by the Boltzman equation;

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Topic 2: Atomic Spectroscopy Temperature Effects on Atomic Spectra: Emission spectra are very dependant on the temperature of the atomizer. However, even in very hot flames only a small fraction of atoms are in excited states, Absorption and fluorescence are much less temperature dependant. High temperature increases atomization of the sample, while also enhancing Doppler effect and increasing ionization. Although, adsorption involves more states, it also involves measuring a small difference (A = log P0 - log P), thus trade off

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Complications and Sources of Continuum Spectra: Molecular Species produce broad absorption bands due to the close “spacing” of vibration and rotation states associated with excited states and ground states.

These broad continuum spectra can interfere (overlap) with a number of discrete atomic transitions.

Formation of molecular species can also reduce the effective gaseous atomic concentration of analytes, also resulting in reduced analyte signal.

The adverse effects from characteristic molecular species can be reduced by selecting appropriate characteristic signals, altering flame chemistry and in some cases by background subtraction.

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Topic 2: Atomic Spectroscopy Sample Introduction Techniques in Atomic spectroscopy: • The goal of the sample introduction technique is to transfer a reproducible and representative sample into the atomizer. • This step limits the accuracy, precision and the detection limits of the technique. • This is strongly dependant on the physical and chemical state of the analyte and sample matrix. For solid samples, sample introduction is a major problem. For liquids and gases, this is relatively simple. Liquids are converted to a fine mist or aerosol and then introduced or vaporized.

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Topic 2: Atomic Spectroscopy Pneumatic Nebulizers:

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Topic 2: Atomic Spectroscopy Other Atomic Absorption Introduction Techniques: •  Ultrasonic Nebulizers: Similar aerosol effect to pneumatic, however, quartz crystal used to produce a dense, homogenous mist. •  Electrothermal Vaporizers (Liquid and Solid): Sample is vaporized by rapid resistive heating of a graphite or tantalum material. Material is entrained in a flow of inert gas. A transient peak is observed. Peak Height or Peak area used to quantify. •  Hydride Generation Techniques: Volatile Hydrides are generated by a chemical reaction (see below). Higher transfer efficiency of specific analyte types; arsenic, antimony, tin, selenium, bismuth and lead. Results in an increase in sensitivity of 10 to100.

•  Direct Sample Insertion (Solid): Sample is physical placed into atomizer by a probe or similar and transient signal is produced.

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Other Atomic Absorption Introduction Techniques:

Arc and Spark Ablation (Solid): Electrical discharges involving the surface of a sample can lead to the ablation of surface material to form a plume of particulate and vapors which is then swept into atomizer by inert gas.

Laser Ablation: Material ablated from surface by an intense laser beam. Versatile because the material can be of almost any type.

Glow discharge Techniques: A glow discharge takes place in a low-pressure atmosphere (1 to 10 torr) of argon gas between a pair of electrodes maintained at a dc potential of 250 to 1000 V. The argon gas to break down into positively charged argon ions and electrons. The electric field accelerates the argon ions to the cathode surface that contains the sample. Neutral sample atoms are sputtered from surface.

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Topic 2: Atomic Spectroscopy Sample Atomization Techniques:

•  Flame Atomization: The sample is nebulized into a gaseous oxidant flow, mixed with a fuel and then transported into a flame region for atomization.

•  Electrothermal Atomization: A small aliquot of sample (µL) is injected into a graphite furnace where it is atomized via resistively heating to ~3000°C

•  Glow Discharge Atomization: Small amounts of solid conductive samples are sputtered from a surface acting as an electrical cathode. An electrically accelerated stream of ionized argon is provides the energy for the sputtering and atomization process.

•  Hydride Atomization: Volatile metal hydrides are produced from a chemical reaction and then atomized by heating in a quartz tube.

•  Cold-Vapor Atomization: Mercury metal has sufficient volatility to be atomized by vaporization and thus the atomic absorption (@253.7 nm) can be obtained cold. Can produce low ppb detection limits.

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Topic 2: Atomic Spectroscopy

Solution Nebulization DesolvationSpray Aerosol

Molecules Atoms AtomicIonsDissociation Ionization

ExcitedMolecules

ExcitedAtoms

ExcitedAtomicIons

TemperatureDependantPopulationexcitation

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Topic 2: Atomic Spectroscopy Flame Atomization Techniques: The temperature and chemistry of the flame will have a significant impact on the relative populations of molecules, atoms, ions and there electronically excited states. This will subsequently have a significant impact on the analytical signal.

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Topic 2: Atomic Spectroscopy Flame Structure: Primary combustion zone: In hydrocarbon, blue luminescence zone (C2, CH etc.). Thermal equilibrium not yet reached. Interzonal region: Common zone to use for spectroscopy. rich in free atoms and the hottest part of the flame Secondary combustion zone: Atoms and other reagents converted to more stable species, such as oxides.

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Topic 2: Atomic Spectroscopy Electrothermal Atomizers (Graphite Furnace): A small aliquot (µL) of sample (or entire sample) is introduced into a graphite furnace that is heated electrically to 2000°C to 3000°C.

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Topic 2: Atomic Spectroscopy Electrothermal Atomizers (Graphite Furnace):

Advantages: Sample size 0.5 µL to 10 µL, with detection limits of 10-10 to 10-13 g of analyte.

Disadvantages: Reproducibility (RSD) 5% to 10% whereas in flame 1% or better. Methods are slow, dynamic range is low.

Graphite Furnace is normally only used when detection limits are an issue.

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Topic 2: Atomic Spectroscopy Interferences in Atomic Absorption:

Spectral interferences: These result from the absorption or emission of radiation by an interfering species that overlaps or cannot be resolved from the absorption of the analyte species.

Chemical Interferences: These result from various chemical processes that are occurring during atomization that alter the absorption characteristics of the analyte.

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Topic 2: Atomic Spectroscopy

Spectral interferences: Interferences from overlapping emission lines usually not a problem, very narrow from hollow cathode and well characterized.

Combustion by-products other than analyte and particulate most significant

Background correction can correct for fuel and oxidant interferences

Sample matrix interferences more challenging; •  Oxides and hydroxides; may need to change flame chemistry •  Organic material species → incomplete combustion products scattering •  Most dependent on flame chemistry, temperature ∴ vary to min. prob. •  Add excess to standards for “radiation buffer effect”

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Topic 2: Atomic Spectroscopy Spectral Interferences in Atomic Absorption: Historically, spectral interference in graphite furnace most severe, very specialized corrections applied to minimize problem.

Two Line Correction: An additional spectral line from the source, close in frequency to the analyte wavelength can be employed. (special case & rare)

Continuum Source Correction: Signal from a continuous (deuterium lamp) is alternately passed through the analyte zone. Most common.

Zeeman Effect Correction: In a strong magnetic field, the magnetic field generated by the spinning electron alters the “energy” or wavelength of transitions. For Singlet transitions, 3 lines -σ, π, +σ result. π lines absorb radiation polarized parallel to magnetic field,σ perpendicular to field

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Topic 2: Atomic Spectroscopy

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Topic 2: Atomic Spectroscopy Chemical Interferences in Atomic Absorption: Chemical interferences are much more common than spectral ones. Reduce by selecting appropriate conditions. Treat flame as a stable solvent and apply equilibrium concepts.

Low Volatility Compounds: Anion presence leads to non-volatile ionic species. ie., calcium absorbance drops off linearly with sulfate or phosphate contamination. The presence of other metals can also reduce signal possibly due to complex, heat stable metal oxides being formed.

Reduce or eliminate by higher temperature Add releasing agents to preferably bind with interference. Add protective agents to form stable volatile complexes such as EDTA

Dissociation Equilibria: In a flame complex equilibrium are established such as;

M + O MO M + 2OH M(OH)2

alkaline-earth oxides are stable, lead to broad molecular bands Other reactions also reduce such as: Na + Cl NaCl

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Topic 2: Atomic Spectroscopy Chemical Interferences in Atomic Absorption: Ionization Equilibria: In low temperature flames, such as those produced with oxygen as a oxidant, ionization is low and the loss of analyte by the reaction following reaction is low. Not the case for hotter flames.

The equilibrium constant for this reaction is;

As the equilibrium constant implies that introduction of an easily ionized element such as Potassium or Cesium will lead to the production of additional e- by the reaction below.

B B+ + e- (2)

The [M+] can then be significantly reduced as increasing the [e-] will drive reaction 1 back toward reagents.

On the other hand for easily ionized atoms such as the alkali metals, using a hotter flame may reduce the analyte signal due to ionization but increase the signal by eliminating hydroxide and oxide formation. Therefore complex

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Atomic Absorption Techniques: US EPA method 7000A: 4.8 Glassware - All glassware, polypropylene, or Teflon containers, including sample bottles, flasks and pipets, should be washed in the following sequence: detergent, tap water, 1:1 nitric acid, tap water, 1:1 hydrochloric acid, tap water, and reagent water.

(Read and understand specifications for reagents)

5.7 Calibration standards - ... preparation of standards which produce an absorbance of 0.0 to 0.7. Calibration standards are prepared by diluting the stock metal solutions at the time of analysis. ... calibration standards should be prepared fresh each time a batch of samples is analyzed. Prepare a blank and at least three calibration standards in graduated amounts in the appropriate range of the linear part of the curve. The calibration standards should be prepared using the same type of acid or combination of acids and at the same concentration as will result in the samples following processing. ...Calibration curves are always required.

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Atomic Absorption Techniques: US EPA method 7000A:

7.2 Direct aspiration (flame) procedure: •  In general, after choosing the proper lamp for the analysis, allow the lamp to warm up for a minimum of 15 minutes •  Align the instrument, position the monochromator at the correct wavelength, select the proper monochromator slit width, and adjust the current according to the manufacturer's recommendation. Subsequently, •  Light the flame and regulate the flow of fuel and oxidant. Adjust the burner and nebulizer flow rate for maximum percent absorption and stability. •  Run a series of standards of the element under analysis. •  Construct a calibration curve by plotting the concentrations of the standards against absorbances. •  Aspirate the samples and determine the concentrations either directly or from the calibration curve. •  Standards must be run each time a sample or series of samples is run.

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Topic 2: Atomic Spectroscopy 8.0 QUALITY CONTROL 8.2 A calibration curve must be prepared each day with a minimum of a calibration blank and three standards. After calibration, the calibration curve must be verified by use of at least a calibration blank and a calibration check standard (made from a reference material or other independent standard material) at or near the mid-range. The calibration reference standard must be measured within 10 % of it's true value for the curve to be valid.

8.3 If more than 10 samples per day are analyzed, the working standard curve must be verified by measuring satisfactorily a mid-range standard or reference standard after every 10 samples. This sample value must be within 20% of the true value, or the previous ten samples reanalyzed.

8.6.1 Dilution test - For each analytical batch select one typical sample for serial dilution to determine whether interferences are present. The concentration of the analyte should be at least 25 times the estimated detection limit. Determine the apparent concentration in the undiluted sample. Dilute the sample by a minimum of five fold and reanalyze. Agreement within 10% between the concentration for the undiluted sample and five times the concentration for the diluted sample indicates the absence of interferences, and such samples may be analyzed without using the method of standard additions.

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Atomic Emission Spectroscopy - Atomization Emission Sources:

Flame Electric Spark and Arc Direct current Plasmas Microwave Induced Plasma Inductively Coupled Plasma

Advantages: Simultaneous multi-element Analysis Some non-metal determination (Cl, Br, I, and S) Concentration range of several decades

Disadvantages: Spectra very complex hundreds to thousands of lines High resolution and expensive optical components Expensive instruments, highly trained personnel required

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Topic 2: Atomic Spectroscopy Plasma Sources: A Plasma is an electrically conducting gaseous mixture containing significant concentrations of cations and electrons. There are three main types;

• Inductively Coupled Plasma (ICP) • Direct Current Plasma (DCP) • Microwave Induced Plasma (MIP)

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Inductively Coupled Plasma (ICP):

•  Plasma generated in a device called a Torch •  Rate of Argon Consumption 5 - 20 L/Min •  Radio frequency (RF) generator 27 or 41 MHz •  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. Rapid tangential flow of argon cools outer quartz and centers plasma. Sample introduction is analogous to atomic absorption.

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Topic 2: Atomic Spectroscopy

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Topic 2: Atomic Spectroscopy

Inductively Coupled Plasma (ICP): •  Sample atoms reside in plasma for ~2 msec and reach temperatures of 4000 to 8000 K. •  Ionization interference small due to high density of e- •  Plasma chemically inert, little oxide formation •  Temperature profile quite stable and uniform.

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Topic 2: Atomic Spectroscopy

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Topic 2: Atomic Spectroscopy Inductively Coupled Plasma (ICP): Sequential: Step between atomic emission lines quickly (sec/line) Simultaneous Multi-channel: Measure intensities of a large number of elements (50-60) simultaneously Sequential: Most use holographic grating Some 2 sets of slits, one visible, one UV range

Sequential Skew-scan: Scan fast between wavelengths and slow near wavelength

Scanning Echelle Spectrometer: Photomultiplier is moved to monitor signal from slotted aperture.

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Topic 2: Atomic Spectroscopy The next slide is coming

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Topic 2: Atomic Spectroscopy 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. •  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: 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. •  Reduced by diluting the sample or by using a peristaltic pump, by using an internal standard or by using a high solids nebulizer. •  Salt buildup at the tip of the nebulizer, affecting aerosol flow rate and nebulization.

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Topic 2: Atomic Spectroscopy ICP/OES INTERFERENCES (cont.):

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: result 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 and 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.