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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.