Spectro, Questions
What is an absorption spectrum?
A graph of absorbance vs. wavelength
What is the role of the beam chopper in a spectrophotometer?
A rotating mirror that alternately directs light to the sample or reference
What is an isobestic point?
A specific wavelength at which two chemical species have the same molar absorptivity (ε) or -more generally- are linearly related.
What is the difference between fluorescence and phosphorescence?
Fluorescence is emission of light with no change in electronic spin state of the molecule (ex. singlet to singlet). In phosphorescence, the electronic spin does change during emission ( ex. triplet ot singlet). Phosphorescence is less probable, so molecules spend more time in the excited state prior to phosphorescence than to fluorescence (phosphorescence has a longer lifetime than fluorescence). Phosphorescence also comes at lower energy (and longer wavelength) than fluorescence, because the triplet excited state is at lower energy than the singlet excited state.
Why is it most accurate to measure absorbances in the range A = 0.3 - 2?
If absorbance is too high, too little light reaches the detector for accurate measurement. If absorbance is too low, there is too little difference between sample and reference for accurate measurement.
Why does an optical fiber still work when it is bent?
If the bending angle is not too great, the angle of incidence will still exceed the critical angle and light will not leave the core
When are isobestic points observed and why?
If the spectra of two compounds with a constant total concentration cross at any wavelength, all mixtures with the same total concentration will go through that same point. The appearance of isobestic points in a chemical reaction is good evidence that we are observing one main species being converted to one other major species.
What is the difference between a fluorescence excitation spectrum and a fluorescence emission spectrum?
In an excitation spectrum, the exciting wavelength is varied while the detector wavelength is fixed. In an emission spectrum, the exciting wavelength is held constant while the detector wavelength is varied.
Which variables increase the resolution of grating?
Increases in proportion to the number of grooves that are illuminated (grooves can be increased with a finer ruled grating and a longer grating) and to the diffraction order (optimized by appropriate choice of the blaze angle of the grating)
What is the role of the amplifier in a spectrophotometer?
Increases the detector signal for display
A silicon carbide globar is associated with what type of radiation?
Infrared Radiation (12000 nm to 40000 nm)
What does LASER stand for?
Light Amplification by Stimulated Emission of Radiation
How does an optical fiber work?
Light inside the fiber strikes the wall at an angle greater than the critical angle for total reflection. Therefore, all light is reflected back into the core and continues to be reflected from wall-to-wall as it moves along the fiber.
What is the difference between luminescence and chemiluminescence?
Luminescence is light given off after a molecule absorbs light. Chemiluminescence is light given off by a molecule created in an excited state in a chemical reaction.
What are the properties of laser light?
Monochromatic, bright, collimated, polarized, coherent
Which variable increase dispersion of the grating?
Proportional to the diffraction order and inversely proportional to the spacing between lines in the grating (the closer the lines, the greater the dispersion)
What is the role of a filter in a grating monochromator?
Remove higher order diffraction (different wavelengths) at the same angle as the desired diffraction
What is the role of the monochromator in a spectrophotometer?
Selects a narrow band of wavelengths to pass on to the sample. As the experiment progresses, the monochromator scans through a desired range of wavelengths.
Explain how beam chopping reduces line noise and noise from source drift.
The beam is alternately directed through the sample and reference cells. Slow drift of source intensity should be cancelled because the intensity seen by the sample and reference are almost the same. More rapid flicker of the lamp might not be cancelled
Why does a compound whose visible absorption maximum is at 480 nm (blue-green) appear to be red?
The color of transmitted light is the complement of the color that is absorbed. If blue-green light is absorbed, red light is transmitted.
Does a fluorescence excitation spectrum or a fluorescence emission spectrum resembles an absorption spectrum?
The excitation spectrum because emission intensity is proportional to the absorption of the exciting radiation.
Explain the difference between transmittance, absorbance, and molar absorbtivity
Transmittance is the fraction of incident light that is transmitted by a substance. Absorbance is logarithmically related to transmittance. When all light is transmitted, absorbance is zero. When no light is transmitted, absorbance is infinite. Absorbance is proportional to concentration. Molar absorptivity is the constant of proportionality between absorbance at a particular wavelength.
A deuterium arc lamp is associated with what type of radiation?
Ultraviolet Radiation (110 nm to 400 nm)
A tungsten lamp is associated with what type of radiation?
Visible Radiation (320 nm to 2500 nm)
How does a laser generate light?
an excited state of the lasing material is pumped to a high population by light, an electric discharge or other means. Photons emitted when the excited state decays to a less populated lower state stimulate emission from other excited molecules. The stimulated emission has the same energy and phase as the incident photon. In the laser cavity, most light is retained by reflective end mirrors. Some light is allowed ot escape from one end.
What is the role of the sample cuvet in a spectrophotometer?
an identical cell containing pure solvent or a reagent blank
What is the advantages of decreasing monochromator slit width?
increased ability to resolve closely spaced spectral peaks
What isthe disadvantages of decreasing monochromator slit width?
more noise because less light reaches the detector
What is the role of the mirror after the reference cuvet and semitransparent mirror in a spectrophotometer?
pass both beams of light through to the detector, which could be a photomultiplier tube that generates an electric current proportional to the photon irradiance.
What is the role of the light source in a spectrophotometer?
to provide ultraviolet, visible, or infrared radiation
