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Sample questions Test 3-KEY 1. Circle the correct response. a. A three electrode potentiostat is used to apply two voltages simultaneously to the same working electrode to measure both the current and the resistance simultaneously to eliminate the voltage measurement error that can occur at high currents to eliminate errors in measuring currents in solutions of low resistance b. Measurement with an ion selective electrode is an example of voltametry conductometry potentiometry c. coulometry In ICP-AES the species detected is an unexcited neutral atom a neutral atom or ion in an exited electronic state an ion in the ground state an atom or ion in the ground electronic state d. Which of the following techniques has the lowest detection limits? flame AA flame emission electrothermal AA ICP AES e. Which of the following techniques is NOT routinely used for simultaneous mutli-element analysis? arc emission ICP-MS ICP-AES AA spectroscopy f. In which of the following methods is the concentration of the analyte is determined from the Nernst Equation. coulometry voltametry conductometry potentiometry g. Which technique is most useful for determining the chemical nature of a molecule on a surface Auger electron X-ray electron XPS spectroscopy fluorescence microprobe h. Chemical interference is not a serious problem in a plasma as compared to a flame because a plasma has more ions than a flame a plasma has fewer ions than a flame a plasma is much hotter than a flame a plasma is much cooler than a flame i. Mercury is commonly used as an electrode because mercury has a high overpotential for the reduction of hydrogen ions in the solvent mercury has a high density mercury is the least reactive metal mercury is easily reduced j. This technique is generally only used for the analysis of alkali metals ICP-MS differential pulse flame emission polarography k. Beam (or source) modulation is used in flame AA and flame AE to reduce the effects chemical interferences eliminate interferences caused by emissions from the flame eliminate ionization interferences 1 flame absorption Sample questions Test 3-KEY increase the intensity of the source l. Which of the following techniques would be least useful for determining a trace amount of tin in a sample of a metal? differential pulse ICP-AES AA x-ray polarography fluorescence m. For which of the following surface techniques is it sometimes possible to detect adsorbates at part per billion levels XPS Auger electron microprobe SIMS n. What is detected in ICP-MS ions in the plasma radiation from ions in the plasma atoms in the plasma radiation from atoms in the plasma o. In which of the following methods is the voltage scan preceded by a concentration phase polarography differential pulse cyclic anodic polarography voltametry stripping p. Which of the following types of probe beams can be used for high resolution surface imaging with little surface damage He ion Ar ion X-ray electron q. In which techniques are electrons used as a probe and X-rays are detected x-ray fluorescence XPS electron microprobe Auger r. Which of the following is NOT a type of voltametry (differential pulse polarography, anodic stripping, pH measurement, square wave polarography) s. Which technique tends to be more sensitive to high molecular weight atoms. XRF spectroscopy Auger electron ESCA spectroscopy t. In which of the following techniques is the sample completely destroyed ? differential pulse potentiometry coulometry polarography SIMS polarography u. In which of the following electrochemical methods is the determination of concentration based on diffusion controlled migration of analyte? conductometry pH measurement coulometry polarography v. The charging current in polarograhy is due to undesired ions in solution the electrical double layer around the electrode concentration polarization overpotential 2 Sample questions Test 3-KEY w. In which one of the following the sample is probed with X-rays and the electron emitted are analyzed. Auger the electron XRF XPS spectroscopy microprobe x. Which technique is used for simultaneous analysis and identification of multiple ions potentiometry coulometry conductometry voltametry y. A(n) (auger, photo-, thermal, free) electron is created in a two electron process in an atom. An upper level electron in an atom that is missing a core electron drops to a lower level to fill the hole and simultaneously a valance electron is ejected. z. Which of the following techniques is not used for simultaneous multielement analysis? ICP-MS ICP-AES arc emisson flame AA aa. Which is FALSE about Atomic Absorption Spectroscopy? The sensitivity for various atoms differs significantly Hollow cathode lamps are used as sources Flame AA has lower detection limits than electrochemical furnace AA The energies of the transitions are in the UV-visible region bb. Give the following two half reactions and reduction potentials, what is the spontaneous reaction and what is the cell voltage 2+ Zn + 2e- → Zn(s) Eo = -0.763 Ni2+ + 2e- → Ni(s) Eo = -0.250 2+ Zn + Ni(s) → Ni2+ + Zn(s) Eo = 0.513 V Zn2+ + Ni(s) → Ni2+ + Zn(s) Eo = -0.513 V Ni2+ + Zn(s) → Zn2+ + Ni(s) Eo = 0.513 V Ni2+ + Zn(s) → Zn2+ + Ni(s) Eo = -0.513 V cc. Which of the following is most useful for determining the chemical structure of an unknown molecule ? Auger electron spectroscopy X-ray diffraction electron microprobe Atomic force microscope dd. Which of the following is most useful for determining the atomic composition of a surface with micrometer resolution? X-ray flourescence X-ray diffraction electron microprobe Scanning tunelling microscope 2 Briefly answer the following questions. a. Why is the surface concentration of an electroactive species often different from the bulk concentration? 3 Sample questions Test 3-KEY The electrochemical processes occur at the surface of the electrodes and the concentration near the electrode can be greater than less than the bulk concentration since it take time for diffusion to equalize the concentration. b. Why is ultra high vacuum needed for most surface analysis techniques. Ultrahigh vacuum is needed to help keep the surface clean and to prevent ions and electrons from reacting with other molecules. c. What is overpotential and what is its source? Overpotential refers to the extra voltage needed, beyond that predicted by thermodynamics, to cause an oxidation or reduction to occur. It arises from the activation energy of process. Overpotential is minimized at low current. d. How are electrons analyzed and detected? The energy of an electron can be measured with an electrostatic analyzer. The electron signal is detected and amplified by an electron multiplier which functions like a photomultiplier. e. How are X-rays analyzed and detected? X-Rays are detected by the ionization that they cause. In vacuum tube designs the X-rays ionize a low pressure gas and these ions are detected. The X-ray energy can be analyzed with an X-ray monochrometer that uses a salt crystal in place of a grating. In solid state X-ray detectors, the X-rays create ions within the substrate and in this case the current pulses produced can be analyzed by height to directly determine the energy without a monochromator. 3. For each technique indicate what is measured (current, voltage, current vs. voltage, current vs. time, etc.). Also give an example of an application of each technique. coulometry differential pulse polarography potentiometry measured current vs time (charge) current vs voltage voltage application titrations multielement analysis for metals. Many organics can also be analyzed. pH measurement, ion selective electrodes 4. Describe briefly how you could analyze the following samples. Indicate the instrumental technique or techniques that you will use and what information you expect from each technique. Indicate if any sample preparation or separation is likely to be needed. Assume that all instrumental techniques are available to you. a. Test for Pb in a soil sample. These are expected to be present at the ppb level. 4 Sample questions Test 3-KEY techniques - anodic stripping, electrothermal AA, or ICP-MS. In each case, the sample would first have to be dissolved into an aqueous solution. For anodic stripping an electrolyte would have to be added. b. Quickly determine the composition of a metal ore with a precision of 10 %. X-ray fluorescence (XRF) - no sample prep. c. Determine the Ca and Mg content of some grains. AA, AE, or ICP, The sample would have to dissolved in an aqueous solution d. Determine the thickness of a thin diamond film that has been deposited on a metal surface.. SIMS or Auger with depth profiling. No preparation e. Test a cleaning solution for several metals and non metals simultaneously. ICP-AES or ICP-MS. No preparation 5 (6 ) Give a typical application for each of the following X-ray fluorescence - determination of elemental composition a pigments on a painting, determination of the major components of an ore. Nondestructive elemental analysis in air. XPS - determination of elemental composition of a metal or semiconductor surface. Must be done in UHV. SIMS - atomic or molecular analysis of a surface. Depth profiling is also possible scanning tunneling microscopy - determination of surface topography with atomic resolution scanning electron microscope - determination of surface topography with resolution better than optical microscopes 5