Homework Solutions for Chem 422. Chapter 9

Homework Solutions for Chem 422 Chapter 9 1. 3. a) A releasing agent is a substance that reacts preferentially with a chemical interference, thus preventing its interaction with the analyte. b) A protective agent is a chelating agent that prevents chemical interferences by forming stable but volatile combinations with the analyte. c) An ionization suppressor is an easily ionized metal that provides a high concentration of electrons in the flame, thereby lessening the ionization of the analyte. d) Atomization is the conversion of a sample into an atomic vapor.. e) Pressure broadening is the broadening of natural absorption line widths by collisions with other atoms or ions in the flame. f) A hollow cathode lamp is a discontinuous radiation source used for atomic absorption spectrophotometry consisting of a cathode composed of the analyte of interest. g) Sputtering is the dislodging of atoms from a surface by bombardment with high energy particles. h) Self-absorption is the absorption of radiation from excited atoms by unexcited atoms in the same sample. i) A spectral interference is the closeness of absorption/emission lines from 2 analytes unresolvable by the monochromator. j) A chemical interference is any chemical process occurring during atomization that affects the absorption by the analyte. k) A radiation buffer is a spectral interference that is added in large excess to a sample before AAS analysis to swamp the effects of the sample matrix. l) Doppler broadening is the broadening of natural absorption/emission line widths because of motion of the absorbing/emitting species. Electrothermal atomizers concentrate more of the sample in the optical path for a longer period of time. Homework Solutions for Chem 422 Chapter 9 4. The continuous emissions from a deuterium lamp and the hollow cathode lamp (HCL) emissions are both directed through the flame or furnace by a chopping mirror. The HCL emissions are absorbed by the analyte atoms and scattered or absorbed by the sample matrix. Since the bandwidth of the monochromator is large compared to the analyte absorption line width, any attenuation of the deuterium radiation is primarily a result of broad-band absorption or scattering by the sample matrix. The deuterium lamp absorption is then subtracted from the HCL signals to yield the absorption by analyte atoms. 5. Radiation from the source is modulated to distinguish it from emissions by the flame. 7. At high currents, more unexcited atoms are formed in the sputtering process. These atoms attenuate the lamp emission by self-absorption. Since these atoms generally have less kinetic energy than the excited ones, their absorption lines are Doppler broadened to a lesser extent, resulting in only the center of the emission line being attenuated. 8. To increase sensitivity: 9. Atomic emission depends on the analyte population in the excited state, which is very much dependent upon flame characteristics. Atomic absorption and atomic fluorescence measure analyte atoms in the ground state 10. Desolvation MgCl2 (aq) Volatilization MgCl2 (s) 6 MgCl2 (g) Atomization MgCl2 (g) 6 Mg (g) + Cl (g) Absorption Mg (g) 6 Mg* (g) Ionization Mg (g) 6 Mg+ (g) Complexation Mg (g) 6 MgO (g) use a higher temperature flame (acetylene/oxygen) use a solvent containing ethanol (increase nebulizer efficiency) add a releasing agent or a protective agent or an ionization suppressor 6 MgCl2 (s) Homework Solutions for Chem 422 Chapter 9 13. a) Sulfate ion forms complexes with Fe(III) that are not readily atomized, resulting in a lower concentration of iron atoms in the flame. b) Lessen sulfate interference by: • adding a releasing agent that forms complexes with sulfate that are more stable than the iron-sulfate complexes; • adding a protective agent, such as EDTA, that forms a highly stable but volatile complex with Fe(III); • using a higher temperature flame (oxygen/acetylene or nitrous oxide/acetylene). 17. During drying and ashing, volatile absorbing species may have been formed. In addition, particulate matter would appear as smoke during the ashing and would scatter the source radiation. 19. When an internal standard is used, the ratio of the intensity of the analyte line to that of the internal standard is plotted as a function of the concentration of the analyte. This intensity ratio for the unknown can be used with the calibration curve to assess the unknown concentration. If the internal standard and the analyte species are influenced in the same way by variations in the aspiration rate and temperature of the source, and if the internal standard is present at the same concentration in the calibration standards and unknown, the result is independent of these variables.