Quiz 30 Flashcards
- The hydrogen or deuterium discharge tube can be used as a source of continuous ultraviolet radiation for spectrophotometers because of
the characteristics of chopper-modulated radiation
pressure broadening of hydrogen or deuterium emission lines
the great sensitivity of photomultiplier tube
the narrow band pass of modern grating of monochromators
pressure broadening of hydrogen or deuterium emission lines
- Chopping the source beam in conjunction with the use of a tuned AC amplifier in an atomic absorption spectrophotometer accomplishes the following
a recording potentiometer can be used instead of a voltmeter for the readout
a less sensitive detector can be used instead of the usual photomultiplier tube
a cooler can be used without decreasing the population of ground-state atoms
radiation emitted by excited atoms in the flame will not interfere with the absorbance measurement
a less sensitive detector can be used instead of the usual photomultiplier tube
- Line spectra are emitted by
hot solids
excited polyatomic molecules
molecules in the ground electronic state
excited atoms and monatomic ions
excited atoms and monatomic ions
- Recording spectrophotometers sometimes operate with feedback loops that vary the power of the reference beam until it matches the power of the beam through the sample. This mode of operation
eliminates the need for a continuous source
requires two monochromators
eliminates the need for nonabsorbing solvents
makes the detector a null device, with the result that nonlinear response to radiant power would not be deleterious
makes the detector a null device, with the result that nonlinear response to radiant power would not be deleterious
- Which of the following best explains why atomic absorption is sometimes more sensitive than flame emission spectroscopy?
At the temperature of a typical flame, the population of ground-state atoms is much greater than the population of excited atoms
Detectors employed in absorption work are inherently more sensitive than those used to measure emission.
Hollow cathode discharge tubes have a much greater radiant power output than do ordinary flames .
An absorption line in a flame is always much sharper than an emission line because of the Doppler effect.
At the temperature of a typical flame, the population of ground-state atoms is much greater than the population of excited atoms
- The method of standard addition compensates for matrix effects provided
the addition does not dilute the sample appreciably and does not itself introduce appreciable quantities or interfering substances
the addition dilutes the sample enough that the concentrations of interfering substances are lowered to negligible values
the addition contains none of the substance being determined
the addition contains a large enough quantity of some substance to swamp out sample variations
the addition does not dilute the sample appreciably and does not itself introduce appreciable quantities or interfering substances
- In chromatography, a substance for which the distribution of the coefficient, K, is zero may be used to estimate
the volume within the column occupied by the packing materials
the total volume of the column
the volume within the pores of the packing material
the volume within the column available to the mobile phase
the volume within the column available to the mobile phase
- The purpose of the solid support material in a GLC column is to
immobilize the stationary liquid phase
adsorb sample components that are insufficiently soluble in the stationary liquid phase
provide a “backup” stationary phase in the event that the liquid is lost by evaporation
remove impurities from the carrier gas
immobilize the stationary liquid phase
- Helium, rather than nitrogen, is sometimes used as the carrier gas in GLC because
being lighter than nitrogen, helium elutes the sample components more rapidly
helium is less expensive than nitrogen
nitrogen has stable isotopes which separate and cause anomalous column behavior
of its much higher thermal conductivity
of its much higher thermal conductivity
- An important feature of open-tubular GLC column is
they can accept much larger samples than can packed column because of their great length
solute partitioning between stationary and mobile phases is very rapid and the C term in the Van Deemter equation is relatively small
they permit the use of a wider variety of carrier gases
solute partitioning between stationary and mobile phases is very rapid and the C term in the Van Deemter equation is relatively small
- Raising the column temperature in GLC decreases solute retention times primarily because
solute diffusion coefficients in the liquid phase decrease with increasing temperature
van der Waals interactions between solutes and stationary phase are stronger at higher temperatures
gases are generally less soluble in liquids at higher temperatures
detector sensitivity is a function of temperature, especially with a thermal conductivity cell
gases are generally less soluble in liquids at higher temperatures
- Which of the following would have practically no effect upon the retention volume of a solute in GLC?
changing the carrier-gas flow-rate
increasing the stationary liquid loading of the column packing from 5 to 10% by weight
increasing the column temperature
changing the chemical nature of the stationary liquid
changing the carrier-gas flow-rate
- The separation factor, S, in chromatography depends upon
the length of the column
the square root of the length of the column
the nature of the stationary liquid phase
the number of theoretical plates in the column
the nature of the stationary liquid phase
- In GLC, interaction of solutes with the solid support will often cause
unusually narrow elution bands
asymmetric elution band
excessive eddy diffusion
decreased detector sensitivity
asymmetric elution band
- A neutral molecule such as ethanol or sugar which has found its way into the pores of a typical anion-exchange resin can be eliminated
only by replacement with a cation
only by replacement with an anion
only if replaced by another organic molecule on a one-for-one exchange basis
by flushing out the water
by flushing out the water
