19 terms

Exam 2 Short Answer

Describe the differences between hydrogen and deuterium discharge lamps as sources for UV radiation
-Hydrogen and deuterium lamps differ only in the gases that are used in the discharge.
-Deuterium lamps generally produce higher intensity radiation
Describe the differences between filters and monochromators as wavelength selectors
-Filters provide low resolution wavelength selection often suitable for quantitative work, but not for qualitative analysis or structural studies. -Monochromators produce high resolution (narrow bandwidths) for both qualitative and quantitative work.
Describe the differences between photovoltaic cells and phototubes as detectors for electromagnetic radiation
-A phototube is a vacuum tube equipped with a photoemissive cathode and a collection anode. The electrons emitted as a result of photon bombardment are attracted to the positively charged anode to produce a small photocurrent proportional to the photon flux. Phototubes are generally more sensitive and have a greater wavelength range.
-A photovoltaic cell consists of a photosensitive semiconductor sandwiched between two electrodes. An incident beam of photons causes production of electron-hole pairs which when separated produce a voltage related to the photon flux. Photovoltaic cells are in general simpler, cheaper, and more rugged. They do not require external power supplies.
Describe the differences between photodiodes and photomultiplier tubes
-A photodiode consists of a photo-sensitive pn-junction diode that is normally reverse-biased. An incident beam of photons causes a photocurrent proportional to the photon flux. Require low voltage power supplies and are better suited for small, portable instruments because of their size and ruggedness.
-A photomultiplier tube is a vacuum tube consisting of a photoemissive cathode, a series of intermediate electrodes called dynodes, and a collection anode. Each photoelectron emitted by the photocathode is accelerated in the electric field to the first positively charged dynode to give rise to multiple electrons. The result is a cascade multiplication of 10 to the 6th or more electrons per emitted photoelectron. Photomultipliers are more sensitive than photodiodes, but require a high voltage power supply compared to the low voltage supplies required by photodiodes.
Describe the differences between double-beam-in-space and double-beam-in-time spectrophotometers
-Both types of spectrophotometers split the beam into two portions. One travels through the reference cell and one through the sample cell.
-With the double-beam-in-space arrangement, both beams travel at the same time through the two cells. They then strike two separate photodetectors where the signals are processed to produce the absorbance. This arrangement is simpler, but requires two matched photodetectors
-With the double-beam-in-time arrangement, the two beams travel at different times through the cells. They are later recombined to strike one photodetector at different times. This arrangement is a little more complicated mechanically and electronically, but uses one photodetector.
Describe the differences between spectrophotometers and photometers
-Spectrophotometers have monochromators or spectrographs for wavelength selection. Can be used for wavelength scanning or for multiple wavelength selection.
-Photometers generally have filters and use an LED source for wavelength selection. Restricted to one or a few wavelengths
Describe the differences between single-beam and double-beam instruments for absorbance measurements
-A single-beam spectrophotometer employs one beam of radiation that irradiates one cell. To obtain the absorbance, the reference cell is replaced with the sample cell containing the analyte. Single-beam instruments have the advantages of simplicity and lower cost.
-With a double-beam instrument, the reference cell and sample cell are irradiated simultaneously or nearly so. These instruments have the advantages that fluctuations in source intensity are cancelled as is drift in electronic components. The double-beam instrument is readily adapted for spectral scanning
Describe the differences between conventional and multichannel spectrophotometers
-Multichannel spectrophotometers detect the entire spectral range essentially simultaneously and can produce an entire spectrum in one second or less. They do not use mechanical means to obtain a spectrum. These instruments have the advantage of speed and long-term reliability.
-Conventional spectrophotometers use mechanical methods (rotation of grating) to scan the spectrum. An entire spectrum requires several minutes to procure. These instruments can be of higher resolution and have lower stray light characteristics
Why does a deuterium lamp produce a continuum rather than a line spectrum?
In a deuterium lamp, the lamp energy from the power source produces an excited deuterium molecule that dissociates into two atoms in the ground state and a photon of radiation. As the excited deuterium relaxes, its quantized energy is distributed between the energy of the photon and the energies of the two atoms. The latter can vary from nearly zero to the energy of the excited molecule. Therefore, the energy of the radiation, which is the difference between the quantized energy of the excited molecule and the kinetic energies of the atoms, can also vary continuously over the same range. Consequently, the emission spectrum is a spectral continuum.
Why can photomultiplier tubes not be used with infrared radiation?
Photons from the infrared region of the spectrum do not have enough energy to cause photoemission from the cathode of a photomultiplier tube
Why is iodine sometimes introduced into a tungsten lamp?
Tungsten/halogen lamps often include a small amount of iodine in the evacuated quartz envelope that contains the tungsten filament. 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 form the filament and causes the metal to be redeposited, thus adding to the life of the lamp
Describe the origin of shot noise in a spectrophotometer. How does the relative uncertainty vary with concentration if shot noise is the major noise source?
Shot noise has its origin in the random emission of photons from a source and the random emission of electrons from the electrons from the electrodes in phototubes and photomultiplier tubes. When shot noise is the most important source of noise, the relative concentration uncertainty goes through a minimum as the concentration increases
Describe how a monochromator, a spectrograph, and a spectrophotometer differ from each other.
-A monochromator is a dispersive instrument with an entrance slit and an exit slit. It is designed to isolate a single band of wavelengths.
-A spectrograph has an entrance slit, but no exit slit. It is designed to image an entire spectrum at its focal plane. Spectrographs are used with multichannel detectors like CCD arrays and diode arrays.
-A spectrophotometer is an instrument with a monochromator or spectrograph designed to obtain the ratio of two beam intensities to calculate absorbances and transmittances in absorption spectroscopy.
Why do quantitative and qualitative analyses often require different monochromator slit widths?
-Quantitative analyses can usually tolerate rather wide slits because the measurements are often made on an absorption maximum where there is little change in absorptivity over the bandwidth. Wide slit widths are desirable because the radiant powers will be larger and the signal-to-noise ratio will be higher.
-On the other hand, qualitative analysis requires narrow slit widths so that fine structure in the spectrum will be resolved
Explain the difference between a fluorescence emission spectrum and a fluorescence excitation spectrum. Which more closely resembles an absorption spectrum?
-In a fluorescence emission spectrum, the excitation wavelength is held constant and the emission intensity is measured as a function of the emission wavelength.
-In an excitation spectrum, the emission is measured at one wavelength while the excitation wavelengths are scanned. The excitation spectrum closely resembles an absorption spectrum since the emission intensity is usually proportional to the absorbance of the molecule
Why is spectrofluorometry potentially more sensitive than spectrophotometry?
-For spectrofluorometry, the analytical signal F is proportional to the source intensity Po and the transducer sensitivity.
-In spectrophotometry, the absorbance A is proportional to the ratio of Po to P. Increasing Po or the transducer sensitivity to Po produces a corresponding increase in P or the sensitivity to P. Thus the ratio does not change.
-As a result, the sensitivity of fluorescence can be increased by increasing Po or transducer sensitivity, but that of absorbance does not change
Why do some absorbing compounds fluoresce but others do not?
Compounds that fluoresce have structures that slow the rate of nonradiative relaxation to the point where there is time for fluorescence to occur. Compounds that do not fluoresce have structures that permit rapid relaxation by nonradiative processes
Discuss the major reasons why molecular phosphorescence spectrometry has not been as widely used as molecular fluorescence spectrometry?
The triplet state has a long lifetime and is very susceptible to collisional deactivation. Thus, most phosphorescence measurements are made at low temperature in a rigid matrix or in solutions containing micelles or cyclodextrin molecules. Also, electronic methods must be used to discriminate phosphorescence from fluorescence. Not as many molecules give good phosphorescence signals as fluorescence signals. As a result, the experimental requirements to measure phosphorescence are more difficult than those to measure fluorescence and the applications are not as large
Quinine is one of the best known fluorescent molecules, and the sensitivities of fluormeters are often specified in terms of the detection limit for this molecule. The structure of quinine is given below. Predict the part of this molecule that is most likely to behave as the chromophore and the fluorescent center
The fluorescent center is the rigid quinoline ring, which is rich in pi electrons