Parts of a Spectrophotometer Flashcards
Light path in a Monochromator
- polychromatic light comes in through the entrance slit
- reflected off first concave mirror to diffraction grating
- the light hits the grating and is reflected off at different angles into separate wavelengths
- separated wavelengths hit second concave mirror, get reflected down to exit slit
- wavelength(s) exit the exit slit to detector
what is a diffraction grating made of?
aluminum with silicon dioxide over it to be shiny / reflective
each wavelength is unique in how it oscillates through the air
TRUE
Types of light sources
tungsten filament lamp, deuterium arc lamp, electric discharge lamp, silicon carbide globar, laser
tungsten filament lamp
Visible range, 320 - 2500 nm, polychromatic light
deuterium arc lamp
UV, 110 - 400 nm, polychromatic light
electric discharge lamp
UV-VIS, polychromatic light
silicon carbide globar
IR, 4000 - 200 cm^-1, polychromatic light
laser
monochromatic light
types of wavelength selectors
filter and monochromator
filter
allows a small range of wavelengths to pass by blocked a section of wavelengths
types of filters
high-pass and low-pass
high-pass filter
will transmit light with longer wavelengths (look at notes)
low-pass filter
will transmit light with shorter wavelengths (look at notes)
qualities of a filter
cheap, rugged, quantitative, simple design, fixed or infrequent wavelength changes, low resolution wavelength selection
qualities of a monochromator
can continuously vary the wavelength, high resolution wavelenth selector, can be used for both qualitative and quantitative purposes
how to increase the resolution of a monochromator?
- narrow the exit slit
- select a narrower range of wavelengths or just one wavelength
- increase the distance between the grating and the second concave mirror
how do you select wavelengths in a monochromator?
change the angle of the diffraction grating
resolution
ability to separate two closely spaced peaks; (wavelength / change in wavelengths) = nN
nN
n = diffraction order
N = number of grooves in grating
the distance (d) between each groove in the grating should be bigger than the wavelength chosen?
Yes, but a smaller d gives better resolution
why does increasing the distance between the grating and the mirror increase resolution?
gives the different wavelengths more time to separate
types of detectors
photomultiplier tube (PMT) and photodiode array (PDA)
what does a PMT do?
amplifies electrical signal to get a measurable current
steps of PMT
- radiation in the form of a photon strikes the photoemissive cathode
- the photocathode produces photoelectrons
- the photoelectrons are accelerated toward a positively charge dynode
- the dynode generates photoelectrons which accelerate toward the next dynode
- collision with each dynode doubles the number of electrons
- a million or ten million photoelectrons are produced thru the dynode chain
- the packet of photoelectrons reaches the anode, generating an electrical current
formula for how many photons come out of a dynode chain?
2^n (n = dynodes)
disadvantages of PMT
does not distinguish the wavelengths of the incoming photons
semiconductors
has some electrical resitivity, sometimes made out of Si
doped semiconductor
one atom is switched out for another; n-type = extra electron is moving through the semiconductor
p-type = extra proton is moving through the semiconductor
forward bias semiconductor
(look at notes) the oppositely charge particles are attracted to each other and create a neutral area in the middle, has a current flowing through the wires
reverse bias semiconductor
(look at notes) the particles are drawn away from each other creating a depletion region, there is no current without applied energy
sketch PDA
sketch PMT
PDA
array of alternating n-type and p-type semiconductors to create depletion regions that correspond to different wavelengths of photons; once a wavelength hits, the - has enough energy to jump the depletion region to reach the + and connect the current, this will be an instrument response
PDA light
polychromatic light comes in, gets separated so only one wavelength of light is hitting a depletion region
PDA light path
- polychromatic light comes from a light source
- goes through a sample
- is reflected off a mirror to a prism or diffraction grating
- light is separated and goes to the PDA
is PDA a polychromator?
Yes
pros of PDA
fast acquisition, multiple wavelengths at a time, less issue with stray light
cons of PDA
resolution is dependent upon how well the polychromator can separate wavelengths of light
