Spectrophotometers Flashcards
Why do we use both Deuterium Arc and Tungsten lamps for the entire spectrum UV/Vis spectrum?
Tungsten (common) - visible light & near IR
Deuterium Arc - UV light
What wavelengths does a Xenon Arc lamp cover?
175-1000 nm
How does a monochromator work, in general?
Disperses light into component wavelengths and selects narrow band of wavelengths to pass on
What is diffraction grating? How does it work?
What: bending of light by an optical element with closely spaced lines
How: 1. Radiation made into parallel rays
2. Different wavelengths diffracted at different angles
3. Each wavelength focussed at different different point in focal plane
4. Rotation of grating allows different wavelengths to pass through exit slit
What are the differences between optical glass and quartz cuvettes?
Optical Glass (OG) - 340-2500nm
Visible light, less expensive than quartz
Quartz (Q) - 190-2500nm
UV light and Visible, expensive
Why don’t we want to touch the cuvettes on the clear side? What is the impact on results?
When you touch cuvettes you transfer fingerprints - fingerprints scatter and absorb light causing inconsistent readings
Why can we use two cuvettes simultaneously for a double beam spectrophotometer?
A double beam spectrophotometer splits the light into two beams (able to read 2 cuvettes at the same time)
What is the difference between a single and double beam spectrophotometer? What about the operation?
Single only has one beam of light while double (only read one cuvette)
Double beam has two (can read two cuvettes)
ability to compensate for drift in:
- intensity of the source
- the response of the detector with time and wavelength
2 reasons we use a double spectrophotometer?
- more reliable and accurate measurements
- ability to compensate for drift in intensity and response
How do phototube, photomultiplier tube, and photodiode array detectors work? How do they differ? Why would I choose one or the other?
Phototube - produces electric signal when struck by photons (current is proportional to radiation intensity)
Photomultiplier - electron emitted from photosensitive surface strike a seconds surface (dynode) (low light intensities become measurable electric signals
Photodiode - conventional spectroscopy scan through a spectrum one wavelength at a time (causes scattered reflections of unwanted orders)
Why is stray light a problem?
- can come through monochromator (causes scattered reflections of unwanted orders)
- can come from outside the instrument when the sample compartment not fully sealed
- causes deviations from beers law that lead to analysis error
How does a fluorometer differ from UV/Vis? Why are they designed this way?
UV/Vis - grating in the excitation monochromator rotated to scan excitation wavelengths during acquisition of absorbable spectrum
Fluorometer - uses an emission monochromator
Grating in the emission monochromator is static during aquisiton of excitation spectrum
