L15. Chapter 20a_1fe765409a48531f7e693c43c2f595ef.pdf

4/27/2020 Developed by Alan Walsh (below) of the CSIRO in early 1950s. Advantages of Atmoic spectroscopy • High sensitivity Analyte is measured at parts per million to parts per trillion levels. • High selectivity Ability to distinguish one element from another in a complex sample • High throughpu...

4/27/2020 Developed by Alan Walsh (below) of the CSIRO in early 1950s. Advantages of Atmoic spectroscopy • High sensitivity Analyte is measured at parts per million to parts per trillion levels. • High selectivity Ability to distinguish one element from another in a complex sample • High throughput Ability to perform simultaneous multielement analyses and many samples can be automatically analyzed. • Good precision The precision is typically a few percent (depending on the type of sample and matrix), which is not as good as that of some wet chemical methods. Disadvantages: expensive but widely available 1 4/27/2020 Three forms of atomic spectroscopy: absorption, emission, and fluorescence In atomic absorption, a liquid sample is aspirated (to fine droplets) into a flame whose temperature is 2 000–3 000 K. Liquid droplets evaporates and the remaining solid is atomized (broken into atoms) in the flame. The pathlength of the flame is typically 10 cm. The hollow-cathode lamp at the left has an iron cathode. When the cathode is bombarded with energetic Ne+ or Ar+ ions, excited Fe atoms vaporize and emit light with the same frequencies absorbed by analyte Fe in the flame. A detector measures the amount of light that passes through the flame. 2 4/27/2020 hollow-cathode lamp in AAS measuring concentration • It is possible to measure the concentration of an absorbing species in a sample by applying the Beer-Lambert Law:  I  Abs   log  Io  Abs  cb  = extinction coefficient • But what if  is unknown? • Concentration measurements can be made from a working curve after calibrating the instrument with standards of known concentration. 3 4/27/2020 Atomic fluorescence • Atoms in the flame irradiated by a laser to promote them to an excited electronic state. • Atoms fluorescence to return to the ground state • Atomic emission • Widely used. A plasma is a gas that is hot enough to contain ions and free electrons • Collisions in the hot plasma promote some atoms to excited electronic states from which they can emit photons to return to lower energy states. • No lamp is required. • Emission intensity is proportional to the concentration of the element in the sample. • Emission from atoms in a plasma is now the dominant form of atomic spectroscopy. 4 4/27/2020 M+XSolution M+XMist Evaporation MX Solid Vapourisation Thermal M*(g) excitation M (gas) X (gas) Dissociation Flame emission h Measure for flame emission spectroscopy MX Gas Absorption of radiant energy h Measure for atomic absorption spectroscopy M*(g) Re-emit radiation at 90o Measure for atomic (Atomic fluorescence) fluorescence spectroscopy Process by which gaseous atoms are produced in flames 9 An important difference between atomic and molecular spectroscopy : the width of absorption or emission bands. Absorption and emission spectra of liquids and solids typically have bandwidths: 10 - 100 nm Visible absorption spectrum of the complex (ferrozine)3Fe(II) used in the colorimetric analysis of iron. Spectra of gaseous atoms consist of sharp line spectra with widths of 0.001 nm (Figure 20-3). Lines are so sharp that there is usually little overlap between the spectra of different elements in the same sample. Therefore, some instruments can measure more than 70 elements simultaneously. 5 4/27/2020 In atomic spectroscopy, analyte is atomized in a flame, an electrically heated furnace, or a plasma. • Most flame spectrometers use a premix burner, (fuel, oxidant, and sample are mixed before introduction into the flame). • Sample solution is drawn into the pneumatic nebulizer by the rapid flow of oxidant (usually air) past the tip of the sample capillary. • Liquid breaks into a fine mist as it leaves the capillary. • The spray is directed against a glass bead, upon which the droplets break into smaller particles. • The formation of small droplets is termed nebulization • A fine suspension of liquid (or solid) particles in a gas is called an aerosol. Premix burner 6 4/27/2020 • The nebulizer creates an aerosol from the liquid sample. • The mist, oxidant, and fuel flow past baffles that promote further mixing and block large droplets of liquid. • Excess liquid collects at the bottom of the spray chamber and flows out to a drain. • Aerosol reaching the flame contains only about 5 % of the initial sample. Premix burner • The most common fuel-oxidizer combination is acetylene and air, which produces a flame temperature of 2400–2700 K • If a hotter flame is required to atomize high boiling elements (called refractory elements), acetylene and nitrous oxide are usually used. • gas entering the preheating region is heated by conduction and radiation from the primary reaction zone (the blue cone in the flame). • Combustion is completed in the outer cone, where surrounding air is drawn into the flame. • Flames emit light that must be subtracted from the total signal to obtain the analyte signal. • Droplets entering the flame evaporate; then the remaining solid vaporizes and decomposes into atoms. • Many elements form oxides and hydroxides in the outer cone. Molecules do not have the same spectra as atoms, so the atomic signal is lowered. • Molecules also emit broad radiation that must be subtracted from the sharp atomic signals. 7 4/27/2020 • If the flame is relatively rich in fuel (a “rich” flame), excess carbon tends to reduce metal oxides and hydroxides and thereby increases sensitivity. • A “lean” flame, with excess oxidant, is hotter. Different elements require either rich or lean flames for best analysis. • The height in the flame at which maximum atomic absorption or emission is observed depends on the element being measured and the flow rates of sample, fuel, and oxidizer. • • • • Graphite furnace is more sensitive than a flame and requires less sample. 1 - 100 µL of sample are injected into the furnace through the hole at the center Light from a hollow-cathode lamp travels through windows at each end of the graphite tube. To prevent oxidation of the graphite, Ar gas is passed over the furnace and the maximum recommended temperature is 2550 C for not more than 7 s. • In flame spectroscopy, the residence time of analyte in the optical path is 1 s as it rises through the flame. However in the graphite furnace the atomized sample remains in the optical path for several seconds, thereby affording higher sensitivity. • Whereas 1–2 mL is the minimum volume of solution necessary for flame analysis, as little as 1 µL is adequate for a furnace. • Precision is rarely better than 5–10% with manual sample injection, but automated injection improves reproducibility to 1%. 8 4/27/2020 • When you inject sample, the droplet should contact the floor of the furnace and remain in a small area. If you inject the droplet too high, it splashes and spreads, leading to poor precision. • Compared with flames, furnaces require more operator skills. • The furnace is heated in three or more steps to properly atomize the sample. Example: To measure Fe in the iron-storage protein ferritin • 10 µL of sample containing 0.1 ppm Fe are injected into the furnace at 90 oC. The furnace is programmed to dry the sample at 125 C for 20 s to remove solvent. • Drying is followed by 60 s of charring at 1400 C to destroy organic matter. Charring is also called pyrolysis, which means decomposing with heat. Charring creates smoke that would interfere with the Fe determination. • After charring, the sample is atomized at 2100C for 10 s. • Absorbance reaches a maximum and then decreases as Fe evaporates from the oven. • The analytical signal is the time-integrated absorbance (the peak area) during atomization. • After atomization, the furnace is heated to 2500 oC for 3 s to clean out remaining residue. • The furnace is purged with Ar or N2 during each step except atomization to remove volatile material. A skilled operator interprets which signal is due to analyte so that the right peak is integrated. 9 4/27/2020 Furnace (solid sample) • In direct solid sampling, a solid is analyzed without sample preparation (Fig. 20-9). • Example: trace impurities in tungsten powder used to make components for industry can be analyzed by weighing 0.1 to 100 mg of powder onto a graphite platform. • Because so much more sample is analyzed when solid is injected than when liquid is injected, detection limits can be 100 times lower than those obtained for liquid injection. • Example, Zn could be detected at a level of 10 pg/g (10 parts per trillion) in 100 mg of tungsten. • Calibration curves are obtained by injecting standard solutions of the trace elements and analyzing them as conventional liquids. • Results obtained from direct solid sampling are in good agreement with results obtained by laboriously dissolving the solid. • Other solids that have been analyzed by direct solid sampling include graphite, silicon carbide, cement, river sediments, hair, and vegetable matter Matrix modifiers for Furnace • Everything in a sample other than analyte is called the matrix. • Ideally, the matrix decomposes and vaporizes during the charring step. • A matrix modifier is a substance added to the sample to reduce the loss of analyte during charring by making the matrix more volatile or the analyte less volatile. • The matrix modifier ammonium nitrate can be added to seawater to increase the volatility of the matrix NaCl. • NH4NO3 plus NaCl give NH4Cl and NaNO3, which cleanly evaporate instead of making smoke. • The matrix modifier Pd(NO3)2 is added to seawater to decrease the volatility of the analyte Sb. • In the absence of modifier, 90% of Sb is lost during charring at 1250 C. • With the modifier, seawater matrix can be largely evaporated at 1400C without loss of Sb. • The matrix modifier Mg(NO3)2 raises the temperature for atomization of Al analyte. • At high temperature, Mg(NO3)2 decomposes to MgO(g) and Al is converted into Al2O3. • At sufficiently high temperature, Al2O3 decomposes to Al and O, and Al evaporates. • Evaporation of Al is retarded by MgO(g) • When MgO has evaporated, Reaction 20-1 no longer occurs and Al2O3 decomposes and evaporates. 10 4/27/2020 11