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Atomic Absorption and Atomic Fluorescence Spectrometry Section A By Matt Boyd, James Joseph, Jon Blizzard, Jackie Freebery, Hunter Bodle Atomization Techniques • AAS and AFS – Two techniques • Flame Atomization • Electrothermal Atomization Flame Atomization • Analyte is nebulized by flow of gaseous oxidants – Desolvations – Dissociation – Volitalized Types Flames • Figure 9-1 Fuel oxidant Temperature (Celsius) Maximum Burning (cm s-1) Natural Gas Air 1700-1900 39-43 Natural Gas Oxygen 2700-2800 370-390 Hydrogen Air 2000-2100 300-440 Hydrogen Oxygen 2550-2700 900-1400 Acetylene Air 2100-2400 158-266 Acetylene Oxygen 3050-3150 1100-2480 Acetylene Nitrous oxide 2600-2800 285 Flame Structure • Primary Combustion zone – Blue region, rarely used for spectroscopy • Interzonal region – Most widely used part • Secondary combustion zone – Products of inner core disperse Flame Atomizers • Uses – Atomic Absorption – Fluorescence – Emission Spectroscopy • Laminar-flow Burners are Commonly Used – Aerosol, oxidant, and fuel are burned in flame • Performance Characteristics – Most reproducible Electrothermal Atomizers • • • • Provides enhanced sensitivity Operates evaporating sample at low temps Ashing at higher temp Measures absorption and fluorescence – Used in the ICP Electrothermal Atomization • Occurs in open ended cylindrical graphite tube • Held between two contacts in water cooled housing • Two inert gas streams are provided Output Signal • Transducer output rises to a maximum • Rapid decay back to zero • Quantitative determinations – Peak height – Peak area Performance Characteristics • Advantages – Sensitivity – Relative precision • Disadvantages – Furnace methods – Analytical range Analysis of Solids with Electrothermal Atomizers • 1st- weigh grounded sample into a graphite boat and insert boat into furnace. • 2nd- prepare slurry of powdered sample by ultrasonic agitation in an aqueous solution. The slurry is then pipetted into furnace atomization. Specialized Atomization Techniques • Glow-Discharge Atomization • Hydride Atomization • Cold-Vapor Atomization Chapter Nine: Atomic Absorption and Atomic Fluorescence Spectrometry Section 9A: Sample Atomization Technique By Rachel Conroy Katie Payne Flame Atomization • Sample is nebulized by a flow of gaseous oxidant and fuel that carries it to a flame • Process – Desolvation – Volatilization – Dissociation – Ionization – Excitation to form spectra At each phase of atomization spectra can be obtained. Types of Flames • Oxidants – Air: 1700oC to 2400oC – Oxygen – Nitrous oxide • Burning velocity states when flame is stable – Too low: causes flashback – Too high: flame will blow off Table 9-1 Properties of Flames Maximum burning velocity. cm s -1 Fuel Oxidant Temperature oC Natural Gas Air 1700 – 1900 39 – 43 Natural Gas Oxygen 2700 – 2800 370 – 390 Hydrogen Air 2000 – 2100 300 – 440 Hydrogen Oxygen 2550 – 2700 900 – 1400 Acetylene Air 2100 – 2400 159 – 266 Acetylene Oxygen 3050 – 3150 1100 – 2480 Acetylene Nitrous oxide 2600 – 2800 285 Flame Structure • Primary Combustion Zone • Interzonal Area • Secondary Reaction Zone • Flame Profile Flame Atomizers Variables • Fuel and Oxidant Regulators – Double-diaphragm pressure regulators – Rotameter • Performance – Most reproducible – Low sensitivity Schematic of a laminar-flow burner, the typical atomizer used in AAS. Electrothermal Atomization • • • • Long residence time Measurements and vaporization Evaporated at a low temperature Ashed at a higher temperature Electrothermal Atomizers • • • • Graphite tube 2 inert gas streams provided Transverse configuration Pyrolytic carbon seal Shown is the cross-sectional view of a graphite furnace atomizer. The L’vov platform and its position in the graphite furnace. Other info • Output Signals – Measures peak height • Performance – Slow because of cooling cycles – Analytical range is narrow – High sensitivity • Analysis of Solids – Finely ground samples, slurry Specialized Atomization Techniques • Glow-discharge atomization • Hydride atomization • Cold-Vapor atomization Atomic Absorption Instrumentation 9-B Brian May Mandi Kauffman Tyler MacPherson Carolyn Inga Ginny Harrison Atomic Absorption Instrumentation • The AAS Consists of… – A radiation source – Sample Holder – Wavelength Selector – Detector – Signal Processor – Read Out Radiation Sources • Potentially highly specific because of narrow absorption lines. • These narrow lines also cause problems because a linear relationship between absorption and concentration requires narrow source bandwidth relative to the width of an absorption line, but even good monochromators have bandwidths significantly larger than the absorption lines. Problems Created • Non-linear calibration curves are inevitable when the AA is equipped with an ordinary spectrophotometer and continuum radiation source. • Small calibration curves are obtained because only a small amount of the radiation from the monochromator slit is absorbed by the sample, this gives poor sensitivity Solutions • The use of bandwidths narrower than the absorption lines. This is done by exciting the atoms with a lamp, filtering the light, and choosing appropriate operating conditions(source temperature and pressure). • This disadvantage to this method is that it require an additional source lamps for each element, or group of elements. • Hollow Cathode Lamps (9B-1)Sample • -Most common source for atomic absorption measurements • -Consists of a tungsten anode and a cylindrical cathode sealed in a glass tube which is filled with either argon or neon gas a pressure of 15torr • -Cathode is constructed from the metal whose spectrum is desired (or, if not constructed from the metal, it then serves to support a layer of that metal) • -When a potential of about 300V is applied across the electrodes, ionization occurs of the inert gas (argon or neon). The current is generated (of about 5-15 mA) as ions and electrons migrate to the electrodes. • -if potential is large enough the gaseous cations gather enough kinetic energy to dislodge the metal atoms from the cathode surface and produce an atomic cloud in the process known as sputtering. • -The excited metal atoms (a portion of those sputtered) emit their characteristic radiation as they return to ground state • -Efficiency of the cathode depends on its geometry and the operating potential: • High potentials (and thus high currents) greater intensities • -A down-fall to high currents is that they produce an increased number of unexcited atoms in the cloud which have the potential of absorbing the radiation emitted from the excited atoms (Self-absorption) • -This leads to lower intensities Electrodeless Discharge Lamps What are they made of? • Sealed quartz tube • Filled with an inert gas (Ar) • Small amount of metal or its salt What does it use? • Uses radio frequency • Or microwave radiation to energize it What happens? • The gas is ionized by the frequency • Once enough energy is obtained it excites the atoms of metal • The metal spectrum is the desired spectrum. What it provides • Provides radiant intensities in greater supply than a Hollow-Cathode Lamp (HCL) • Not as reliable as the (HCL) • But better for elements such as – Se, As, Cd, Sb Source modulation • Emitted radiation is removed via the monochromator • It is necessary to adjust the output the source so intensity will fluctuate at a constant frequency • Detector receives 2 signal – An alternating from the source – Continuous from the flame. These signals are then converted into electrical responses A high pass RC filter (section 2B-5) can be used to remove unadjusted signals • Adjusting the emission can be done by inserting a circular metal disc (chopper) into the system between the source and the flame • Rotation of this disk at a constant rate will create a beam that is “chopped” to the desired frequency • Tuning forks with vanes attatched to alternately allow the beam to pass and to not pass is another technique • An alternative is the power supply being designed for intermittent or ac operation so the source can be switched oin and off at the desired frequency AA Spectrophotometer See Figure 9-13 for block diagrams • Instrument must be capable of providing a sufficiently narrow bandwidth to isolate the line chosen for the measurement • Glass filter – alkali metals – Only a few widely spaced resonance lines in the visible region • Separate filter and light source for each element • Most use photomultiplier tubes Single-Beam • • • • Several hollow- cathode sources Chopper or pulsed power supply Atomizer Simple grating spectrometer with a photomultiplier transducer • 100% transmittance is set with a blank • The blank is replaced with samples to determine absorbance and transmittance Double Beam • Beam from hollow-cathode source is split by a mirrored chopper • One half passes through the flame and the other half goes around it • 2 beams recombine by a half silvered mirror and passed into a Czerny-Turner grating monochromator • Photomultiplier = transducer • Output = input to a lock-in amplifier • Ratio between reference and sample is amplified and fed to the readout • Since reference beam is not passed through the flame it cannot correct for loss of radiant power due to absorption or scattering by the flame Chapter 9 Section C Megan Seeger, Andrea Lando, Joe Bailey, and Sarah Duncan Spectral Interferences • Can be caused by overlapping lines but is very rare due to the emission lines of the hollowcathode sources being so narrow • Can also result from the presence of combustion products that exhibit broadband absorption or particle products that scatter radiation • Both reduce the power of the transmitted beam and lead to positive analytical errors Continued • A more troublesome problem occurs when the source of absorption or scattering originates in the sample matrix • Interferences because of scattering by products of atomization is most often encountered when concentrated solutions containing elements such as Ti, Zr, and W are aspirated in the flame Continued • Interferences caused by scattering may also be a problem when the sample contains organic species or if organic solvents are used to dissolve the sample • Flame atomization spectral interferences by matrix products are not widely seen and can be avoided by variations in the analytical variables Radiation Buffer • When an excess of the interfering substance is added to the sample and standards • If the concentration added is large compared to the concentration in the sample matrix then the contribution from the sample matrix is insignificant Two-Line Correction Method • Uses a line from the source as a reference it should be as close as possible to the analyte line • This makes any decrease in power of the reference line from that observed during the calibration arises from absorption or scattering and is then used to correct the absorbance of the analyte line The Continuum-Source Correction Method • Deuterium lamp provides a source of continuum radiation throughout the ultraviolet region • The radiation from the continuum source and the hollow cathode lamp are passed alternately through the electrothermal atomizer, the absorbance from the deuterium radiation is then subtracted from the analyte beam Chemical Interferences • More common than spectral interferences • Can be minimized by suitable operating conditions Most Common Interferences • Occurs when anion form low-volatility compounds with the analyte – only a fraction of analyte is atomized and the outcome is low results – Ex. Decrease in calcium absorbance with increasing concentrations of sulfate or phosphate Common Interferences Con’t • Cation Interferences – Outcome: low results – Ex: Aluminum causes low results when determining magnesium (forms a heat stable compound) Solutions to Interferences • When caused by formation of species of low volatility, interference can be eliminated by use of higher temps • Releasing Agents: cations that react preferably with the interferent and prevent analyte interaction • Protective Agents: prevent interferences by forming a stable, volatile species with the analyte Background Correction Based on the Zeeman Effect p.242-243 Zeeman Effect • When an atomic vapor is exposed to a strong magnetic field, a splitting of electronic energy levels of the atoms takes place that leads to the formation of several absorption lines for each electronic transition. The sum of the absorbencies of the lines is equal to exactly the value of the original line from which they were formed. • A,B,C -------------- A --------B --------C --------- Splitting Pattern • Most common type of splitting: – Central line (π) and two equally spaced satellite lines (σ). This is observed with a singlet but for more complex transitions these lines will be split further. • The π line absorbs only plane polarized light in a parallel direction to the magnetic field • The σ lines absorb only polarized radiation at a 90 degree angle to the magnetic field How it works • Turn to page 243 in textbooks • Radiation from a cathode tube • Rotating polarizer – Separates the beam into two parts that are polarized at 90 degrees to each other • These go into a graphite furnace that splits the energy levels into three peaks (D) • This information then goes to a monochromator, photomultiplier tube, and into a data analysis system. • This system subtracts the perpendicular cycle from the parallel half cycle giving a background correction. Background Corrections with Source Self Reversal • Also known as the Smith-Hieftje method – Based on the self reversal or self absorption of radiation from a cathode lamp • the absorbance is collected at periods where the lamp is running at a low current • The background is collected when the lamp is at high voltage • High currents = high number of nonexcited electrons that will absorb the radiation of the excited species Dissociation Equilibria • Dissociation reactions involving metal oxides and hydroxides play an important role in determining the emission and absorption spectra for an element. MO M + O The M is the analyte atom and the OH is the hydroxyl radical. Dissociation Equilibria • Dissociation equilibria which involve anions other than oxygen may also influence flame emission and absorption. • Line intensity for Na is decreased by presence of HCl NaCl Na +Cl Adding HCL decreases Na concentration thereby lowering line intensity. Dissociation Equilibria • V, Al, and Ti interact with such species as O and OH. These are represents as Ox. These are always present in flames. VOx V + Ox AlOx Al +Ox TiOX Ti + Ox Ionization Equilibria • Ionization of atoms is small in combustion mixtures that involve air as the oxidant, it is often neglected. • Ionization is important in higher temp. flames where oxygen or nitrous oxide is the oxidant. There are free electrons produced by the equilibrium. M M+ + e- Ionization Equilibria • The equilibrium constant K for the reaction: K= [M+][e- ]/ [M] Degree of Ionization of metals at flame temps. Table 9-2 pg. 246 Ch. 9 Atomic Absorption Spectrometry Section D Atomic Absorption Analytical Techniques Sample Preparation 1. Flame Spectroscopic Methods Sample materials: • Soils • Animal tissues • Plants • Petroleum products • Minerals Common problem: most are insoluble in aqueous solutions so preliminary treatment to the sample is required Preliminary Treatments Decomposition of material – Rigorous treatment of the sample at high temperatures Con: risk losing the analyte by volatilization or as particulates in smoke – Treatment with specific reagents Con: can cause chemical and spectral interferences or can cause the analyte to appear as in impurity in the solution Common Decomp. Methods 1. Treatment with hot mineral acids 2. Oxidation with liquid reagents (sulfuric, nitric, or perchloric acids: wet ashing) 3. Combustion in an oxygen bomb (or other closed container) 4. Ashing at high temperatures 5. High temperature fusion with reagents ( boric oxide, sodium carbonate, sodium peroxide, and potassium pyrosulfate) Sample Preparation 2. Electrothermal Atomization Sample Types 1. Liquid Samples: blood, petroleum products, and organic solvents. * liquid solvents can be pipetted directly into the furnace for ashing and atomization. 2. Solid Samples: plant leaves, animal tissues, and inorganic substances. * solids can be weighed directly into a cup-type atomizer or into specific containers for introduction into a tube type furnace. Sample Introduction by Flow Injection • Introduce samples into a flame atomic absorption spectrometer • Peristaltic pump and valve arrangements help insure efficiency while conserving the sample • Carrier system: Deionized water or diluted electrolyte are used to provide continuous flushing of the flame atomizer • This reduces build up from samples containing high levels of salts or suspended solids Organic Solvents Low Molecular-weight organic solvents: 1. Alcohols 2. Esters 3. Ketones Why Organic Solvents? 1. Increased nebulizer efficiency- increases the amount of sample that reaches the flame 2. Rapid evaporation of the solvent Solvent Ratios: Leaner fuel-oxidant ratios must be used to offset the presence of any added organic material • This produces lower flame temperatures, which can increase the potential for chemical interferences Organic Solvents (cont.) Immiscible Solvents ex: Methyl isobutyl ketone • These solvents extract chelates of metallic ions – • • • The resulting extract in then nebulized directly into a flame Enhance absorption lines Only small amounts are required to extract from relatively large volumes of aqueous solutions Enhance the sensitivity of the sample, which reduces interferences Common Chelating Agents1. Ammonium pyrrolidinedithiocarbamate 2. Diphenylthicarbazone 3. 8-hydroxyquinoline 4. Acetylacetone Calibration Curves Should Follow Beer’s Law: A= abc A: absorption (L/ g cm) a: absorptivity b: path length through medium c: concentration ● Calibration Curves (cont.) • Should cover range of concentration found in the sample • 1 standard solution should be measured after each time an analysis is performed. Using 2 standards that bracket the analyte concentration would be more efficient in identifying any uncontrolled variables that result from atomization and absorbance measurements Application of AAS • Sensitive men for the quantitative determination of more than 60 metals or metalloid elements Table 9-3 shows Detection Limits Columns 2 & 3 present detection limits for a number of common elements by flame and electrothermal atomic absorption Detection Limits 1. Flame Atomization: 0.001 – 0.020 ppm 2. Electrothermal Atomization: 2 x 10-6 – 1 x 10-5 ppm Accuracy Relative error • • Flame Analysis: 1-2 % Electrothermal Analysis: errors extend flame errors by a factor of 5-10 9D ATOMIC ABSORPTION ANALYTICAL TECHNIQUES 9D-1 Sample Preparation • Sample has to be introduced into the excitation in the form of a solution (disadvantage). Many materials are not soluble in common solvents; extensive treatment is required. – Treatment with hot minerals, oxidation with liquid reagent, ashing at high temperature, etc. • Some minerals can be atomized directly. Solid samples are weighed into cup-type atomizers (advantage). 9D-2 Sample Introduction by Flow Injection • FIA – Introduces samples into a flame atomic absorption spectrometer. • Carrier stream of the FIA system provides continuous flushing of the flame atomizer (advantage). 9D-3 Organic Solvents • The effect of organic solvents is attributable to increased nebulizer efficiency. More rapid solvent evaporation also contribute to the effect. • Use of immiscible solvents is the most important analytical application of organic solvents to flame spectroscopy. • Resulting extract is nebulized into the flame (sensitivity increases). • Part of the matrix components remain in the solvent (advantage). 9D-4 Calibration Curves • Theory is that calibration curves should follow Beer’s Law which does not happen very often • Absorbance should be directly proportional to concentration • Use two standards that bracket the concentration of the analyte. 9D-5 Standard Addition Method • Should use method found in Section 1D-3 • Need to compensate for chemical and spectral interferences of the sample 9D-6 Application of AAS • A sensitive way of determining 60 metals and metalloid elements Detection Limits • Flame Atomization Atomic Absorption Spectroscopy are in the range of 1-20ng/mL,or .001-.020ppm • Electrothermal Atomization are in the range of .002.01ng/mL or 2x10-6 - 1x10-5ppm Accuracy • Error in Flame Ionization Atomic Absorption Spectroscopy 1-2% • Electrothermal Atomization increase by a factor of 5-10 Atomic Fluorescence Spectroscopy Keri Franz Kyle Howard Lauren Kaminsky Fluorescence Atomic Fluorescence Spectroscopy • Useful and convenient for quantitative determination of many elements • Not used as often as atomic emission and atomic absorption • Useful for determining elements that form vapors and hydrides- Pb, Hg, Cd, Zn • Fluorescence instruments are generally harder to maintain and thus more expensive Instrumentation Sources • Sample container is usually a flame or electrothermal atomization cell, glow discharge, or an inductively coupled plasma • Continuum source is ideal • Hollow cathode lamps were used frequently but now EDLs (electrodeless discharge lamps) are more common • EDLs have greater intensity than hollow cathode lamps • Lasers are good sources despite increased Dispersive Instrumentation • Contains: – Modulated Source – Atomizer (flame or nonflame) – Monochromator or Interference Filter System – Detector – Signal Processor – Readout Nondispersive Instrumentation • Contains: – Source – Atomizer – Detector • Advantages: – Low Cost & Simplicity – Multi-element Analysis Adaptability – High Sensitivity – Simultaneous Collection of Energy • For accuracy, make sure source output lacks Applications • Determination of Metals – Lubricating Oils – Seawater – Geological Samples – Clinical Samples – Environmental Samples – Agricultural Samples