1. Neutron Activation Analysis relies on the detection of ?-rays because: a.The
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Question
1. Neutron Activation Analysis relies on the detection of ?-rays because:
a.The only source of ?-rays in a laboratory will be from the analyte, so interferences will be minimized.
b. ?-ray detectors have improved in sensitivity since the mid-1960’s, so greater sensitivity and improved resolution are possible.
c. ?-rays are more energetic than ? particles or ? particles, have greater penetrating power, so they can be detected from all atoms within the analyte.
d. ? particles(electrons) exiting from an atom can collide with other electrons, emitting Auger electrons, whereas ?-rays will escape from the atom unchanged in energy.
e. Bremsstrahlung(radiation across a wide energy range) is produced when electrons are halted when they reach a detector, but not when ?-rays reach a detector, so there is less extraneous radiation with ?-rays.
3. An Auger electron is generated when:
a. A thermal neutron encounters the nucleus of an analyte forming a compound nucleus which decays to a radioactive nucleus with the emission of an electron.
b. The X-ray emitted when an electron moves to an electron hole in the core shell ejects an electron from the atom.
c. An incident X-ray ejects an outer shell electron, rather than a core shell electron.
d. An electron moves to fill an electron hole in the core shell and its energy is lost as an Auger electron.
e.X-ray fluorescence is used to study heavy elements.
4. The light from a hollow cathode lamp used in atomic absorption, is produced when:
a. Atoms of the analyte element, promoted to an excited electronic state by collisions with fast moving gaseous atoms, return to their ground electronic state.
b. Atoms of the analyte element, promoted to an excited state by collisions with gaseous atoms, return to their ground state.
c. Atoms of the analyte element, promoted to an excited electronic state by heating of a cathode of that element, return to their ground state.
d. Fast moving gaseous ions collide with a cathode of the analyte element, volatilizing atoms of that element in an excited electronic state from which they return to the ground state.
e. Atoms of the analyte element are ionized and accelerated to a metal cathode with which they collide and the energy released as they return to their ground state sputters atoms from the surface and is released as light.
5. Capillary electrophoresis offers better resolution than slab-gel electrophoresis because:
a. Detection of the analytes can be made “on column” using detectors either assaying “along” the column or utilizing total internal reflection to maximize the pathlength, d, in Beer’s Law, A=ecd.
b. The choice of either electrokinetic detection or pressure injection offers ggreater flexibility in loading the capillary column.
c. The number of theoretical plates is greater at the higher voltages possible with capillary electrophoresis.
d. Since the sample sizes are so much smaller, lower voltages can be used with capillary electrophoresis, so there is less thermal decomposition of the analyte.
e. Only gels can be used with slab electrophoresis, so derivatization of the stationary phase is not possible.
6. Fluorescent techniques are very useful in forensic applications because:
a. Very low concentrations of analyte can be detected because there is no other radiation to complicate the analysis.
b. The radiation used to detect the analyte is always of higher energy than that used to excite the sample.
c. The wavelength of the radiation used to excite the sample differs from that emitted by the sample.
d. The laser light gives a characteristic pattern of spots of varying intensity when passed through a diffraction grating.
e. The radiation used to detect the analyte is always of shorter wavelength than that used to excite the sample.
7. The difference between electroosmotic flow and electrophoretic flow is:
a. The migration of the analyte under the influence of an electric field.
b. The increased number of theoretical plates with the latter.
c. The increased number of theoretical plates with the former.
d. Only electroosmotic flow has a flat hydrodynamic flow profile.
e. Electroosmotic flow is cancelled out by wall effects in capillary electrophoresis because of the narrow bore of the column.
10. In the technique of flame photometry:
a. Molecules of the analyte are aspirated into a flame where a small fraction of the molecules is promoted to their excited state, from which they decay with the emission of characteristic radiation.
b. The analyte is aspirated into a flame and promoted to the excited state by irradiation with characteristic radiation, the absorption of which is measured.
c. The analyte is aspirated into a flame, in which it is atomised, and atoms, promoted to their excited state, decay to the ground state with the emission of characteristic radiation.
d. A mercury analyte is subjected first to oxidation, then reduction, and the volatile elemental mercury is vapourised in a stream of nitrogen, in which it absorbs light of a characteristic energy.
e. A sodium, potassium or calcium analyte is subjected first to oxidation, then reduction, and the volatile elemental analyte is vapourised in a stream of nitrogen, in which it absorbs light of a characteristic energy.
11. In atomic absorption spectroscopy, the analyte signal can be discriminated from that of the flame because:
a. Most of the ground state molecules are promoted to their state, so the signal is relatively intense.
b. In a fluorescence measurement, the decay from the excited state is at a lower energy than the excitation energy, so the fluorescence signal can be observed in the presence of the exciting radiation.
c. Most of the ground state atoms are promoted to their excited state, so the signal is relatively intense.
d. The exciting radiation specific to the analyte is periodically interrupted.
e. Modern detectores such as lithium-drifted detectors are much more sensitive than ionization detectors.
13. The radiation from a Coolidge X-ray generator, is produced when:
a. The paths of electrons, accelerated towards a tungsten anode change,with the emission of a continuous X-ray spectrum.
b. The paths of electrons, accelerated towards a tungsten anode change, with the emission of just the intense characteristic tungsten L? lines.
c. The paths of electrons, accelerated towards a tungsten anode change, with the emission of just the intense characteristic tungsten L? lines.
d. Tungsten atoms, fired an anode, change velocity emitting characteristic L? lines.
e. Tungsten atoms, fired an anode, change velocity emitting characteristic L? lines.
14. The principle behind the ionisation detector is:
a. Incident radition impaction on electrodes generates a current, which is measured.
b. The ions and electrons generated by the interaction of incident radiation and gas atoms at low pressure migrate to polarised electrodes, creating a current, which is measured.
c. Incident radiation promotes electrons to the conduction band of a semiconducting electrode, generating a current, which is measured.
d. Incident radiation promotes electrons to the valence band of a semiconducting electrode, generating a current, which is measured.
e. Incident radiation interacts with gas phase atoms which are accelerated to a metal cathode, sputtering metal ions into the gas phase, and these generate a current, which is measured.
15. Which of the following best describes the difference between flame emission and atomic absorption spectroscopies?
a. Emission is dependent on the fraction of atoms in their excited state, whereas absorption is from atoms in their ground state.
b. Emission is dependent on the fraction of atoms in their ground state, whereas absorption is from atoms in their excited state.
c. The population of atoms in the excited state is a function of the temperature of the flame.
d. Lithium and sodium are better detected by absorption, whereas beryllium and magnesium are better detected by emission.
e. The use of an inductively coupled plasma enables greater sensitivity because of the higher temperature available.
Explanation / Answer
Ans (4)
Correct Answer is (d).
Explanation :
Hollow cathode lamp is most commonly used as a light source for Atomic Absorption.This lamp contains an Inert Gaslike Argon or Neon at a very low pressure, which is used as a light source, inside the lamp there is a metal cathode that contains the same metal as the analyte of the analysis. In other words cathode is coated with the analyte metal of interest.
Within the lamp inert filler gas Argon OR Neon is ionized by an electric current and these ions are then attracted by the cathode. The inert gas ions bombards on the cathode due to this the metal ions coated on the cathode gets excited. This excitation of metal ions produce the emmision of radiation which wavelengths characteristic of the analyte.
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