Parts of a Spectrophotometer

Question 1

Light path in a Monochromator

Answer 1

1. polychromatic light comes in through the entrance slit
2. reflected off first concave mirror to diffraction grating
3. the light hits the grating and is reflected off at different angles into separate wavelengths
4. separated wavelengths hit second concave mirror, get reflected down to exit slit
5. wavelength(s) exit the exit slit to detector

Question 2

what is a diffraction grating made of?

Answer 2

aluminum with silicon dioxide over it to be shiny / reflective

Question 3

each wavelength is unique in how it oscillates through the air

Answer 3

TRUE

Question 4

Types of light sources

Answer 4

tungsten filament lamp, deuterium arc lamp, electric discharge lamp, silicon carbide globar, laser

Question 5

tungsten filament lamp

Answer 5

Visible range, 320 - 2500 nm, polychromatic light

Question 6

deuterium arc lamp

Answer 6

UV, 110 - 400 nm, polychromatic light

Question 7

electric discharge lamp

Answer 7

UV-VIS, polychromatic light

Question 8

silicon carbide globar

Answer 8

IR, 4000 - 200 cm^-1, polychromatic light

Question 9

laser

Answer 9

monochromatic light

Question 10

types of wavelength selectors

Answer 10

filter and monochromator

Question 11

filter

Answer 11

allows a small range of wavelengths to pass by blocked a section of wavelengths

Question 12

types of filters

Answer 12

high-pass and low-pass

Question 13

high-pass filter

Answer 13

will transmit light with longer wavelengths (look at notes)

Question 14

low-pass filter

Answer 14

will transmit light with shorter wavelengths (look at notes)

Question 15

qualities of a filter

Answer 15

cheap, rugged, quantitative, simple design, fixed or infrequent wavelength changes, low resolution wavelength selection

Question 16

qualities of a monochromator

Answer 16

can continuously vary the wavelength, high resolution wavelenth selector, can be used for both qualitative and quantitative purposes

Question 17

how to increase the resolution of a monochromator?

Answer 17

1. narrow the exit slit
2. select a narrower range of wavelengths or just one wavelength
3. increase the distance between the grating and the second concave mirror

Question 18

how do you select wavelengths in a monochromator?

Answer 18

change the angle of the diffraction grating

Question 19

resolution

Answer 19

ability to separate two closely spaced peaks; (wavelength / change in wavelengths) = nN

Question 20

nN

Answer 20

n = diffraction order
N = number of grooves in grating

Question 21

the distance (d) between each groove in the grating should be bigger than the wavelength chosen?

Answer 21

Yes, but a smaller d gives better resolution

Question 22

why does increasing the distance between the grating and the mirror increase resolution?

Answer 22

gives the different wavelengths more time to separate

Question 23

types of detectors

Answer 23

photomultiplier tube (PMT) and photodiode array (PDA)

Question 24

what does a PMT do?

Answer 24

amplifies electrical signal to get a measurable current

Question 25

steps of PMT

Answer 25

1. radiation in the form of a photon strikes the photoemissive cathode
2. the photocathode produces photoelectrons
3. the photoelectrons are accelerated toward a positively charge dynode
4. the dynode generates photoelectrons which accelerate toward the next dynode
5. collision with each dynode doubles the number of electrons
6. a million or ten million photoelectrons are produced thru the dynode chain
7. the packet of photoelectrons reaches the anode, generating an electrical current

Question 26

formula for how many photons come out of a dynode chain?

Answer 26

2^n (n = dynodes)

Question 27

disadvantages of PMT

Answer 27

does not distinguish the wavelengths of the incoming photons

Question 28

semiconductors

Answer 28

has some electrical resitivity, sometimes made out of Si

Question 29

doped semiconductor

Answer 29

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

Question 30

forward bias semiconductor

Answer 30

(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

Question 31

reverse bias semiconductor

Answer 31

(look at notes) the particles are drawn away from each other creating a depletion region, there is no current without applied energy

Question 32

sketch PDA

Question 33

sketch PMT

Question 34

PDA

Answer 34

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

Question 35

PDA light

Answer 35

polychromatic light comes in, gets separated so only one wavelength of light is hitting a depletion region

Question 36

PDA light path

Answer 36

1. polychromatic light comes from a light source
2. goes through a sample
3. is reflected off a mirror to a prism or diffraction grating
4. light is separated and goes to the PDA

Question 37

is PDA a polychromator?

Answer 37

Yes

Question 38

pros of PDA

Answer 38

fast acquisition, multiple wavelengths at a time, less issue with stray light

Question 39

cons of PDA

Answer 39

resolution is dependent upon how well the polychromator can separate wavelengths of light