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Homework Solutions for Chem 422 Chapter 13 13. g) A single-beam spectrophotometer employs a fixed beam of radiation that irradiates first a reference solution then the analyte solution. In a double-beam instrument, the reference and analyte solutions are irradiated nearly simultaneously by using a rotating chopping mirror. The main advantage of the double-beam instruments is the freedom from problems arising from fluctuations in the source intensity and electronic drift in the detector/amplifier. The single-beam instruments offer the advantages of simplicity and lower cost. h) Diode-array spectrophotometers can detect the entire spectral range essentially simultaneously and can produce a spectrum in less than a second. Conventional instruments require several minutes to perform the same task. Consequently, diodearray spectrophotometers can be used to monitor processes that occur on very fast time scales. 16. In a deuterium lamp, the input energy from the power source produces an excited D2 molecule that dissociates into two atoms in the ground state and a photon of radiation. As the excited deuterium molecule relaxes, its quantized energy is distributed between the energy of the photon and the kinetic energies of the two deuterium atoms. The later can vary from nearly zero to the energy of the excited molecule. Consequently, the photon energy can vary continuously over the same range and the emission spectrum is a continuum. 17. IR photons do not have sufficient energy to cause photoemission from the cathode of a photomultiplier. 18. The iodine prolongs the life of the lamp and permits it to operate at a higher temperature. The iodine combines with gaseous tungsten that sublimes from the filament and causes the metal to be redeposited, thus adding to the life of the lamp. Homework Solutions for Chem 422 Chapter 13 20. (a) The dark current is the small current that develops in a radiation transducer in the absence of radiation. (b) A transducer is a device that converts a physical or chemical quantity into an electrical signal. (c) Scattered radiation in a monochromator is unwanted radiation that reaches the exit slit as a result of reflection and scattering. Its wavelength is usually different from that of the radiation reaching the slit directly from the dispersing element. (d) Source flicker noise is caused by variations in experimental variables that control the source intensity, such as power supply voltages and temperature. It can also be caused by mechanical variations such as vibrations. (e) Cell positioning uncertainty is caused by our inability to position the cell in the same exact place each time. A random variation is introduced because the incident beam is imaged onto slightly different portions of the cell walls each time causing differences in the reflection, transmission and scattering characteristics of the cell. (f) A beamsplitter is a device that causes an incident beam to divide into two beams at its output. It can be made from mirrors, rotating choppers, or optical materials that cause a beam to be split into two beams.