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1105 Analytical Principles > Spectrophotometer > Flashcards

Spectrophotometer Flashcards

1
Q

Photometers are?

A

Coloured filters

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2
Q

Spectrophotometer use

A

Devices that create a spectrum

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3
Q

Light sources

A

Provide high intensity, stable radiation at the wavelength of interest

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4
Q

Monochromatic light

A

One specific wavelength (most methods are based off this)

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5
Q

Ideal light source

A

Provides a constant intensity over an infinite range of wavelengths (does not exist)

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6
Q

Visible light sources

A

Tungsten filament lamp, quartz halide lamp, LED, LASER

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7
Q

UV light sources

A

Deuterium lamp, xenon lamp, high pressure mercury lamp

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8
Q

Tungsten Filament Lamp

A

• produces visible light
• output is a function of filament temperature
- higher temperatures; the output shifts towards blue end of the spectrum
- lower temperatures; look yellow
• emits a continuous nonlinear spectrum
• most output is in the IR region (heat)

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9
Q

Quartz Halide Lamp AKA Tungsten-halogen Lamp

A

•Quartz bulb filled with an inert gas + halogen gas
• The presence of the halogen gas causes the tungsten that evaporated from the filament to deposit back into the filament

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10
Q

Light Emitting Diode AKA LED

A

• A semiconductor light source
• Commonly used as indicator lights
• Semiconductor material is created by a process called “doping”
• p-n junction; p-type (+) material contains excess “holes” and n-type(-) material contains excess electrons
• Has two legs; long leg(anode) and short leg(cathode)
• Higher band gap the higher energy of light emitted
• Band gap is determined by materials forming p-n junction
• Monochromatic

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11
Q

LASER

A

• Monochromatic
• Various types and wavelengths available
• Important for flow cytometry

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12
Q

Deuterium Lamp

A

• Low pressure deuterium gas inside a fused silica (quartz) bulb
• Tungsten filament excited the deuterium gas molecules to an elevated state
-when the electrons relax to their ground state, light is emitted
• 160-375nm

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13
Q

Xenon Arc Lamp

A

• Ionized xenon gas produces high intensity white light
• 250-1000nm

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14
Q

Mercury Vapour Lamp

A

• Uses mercury in an excited state to produce light
• Emits a non-continuous spectrum; 184nm, 254nm, 365nm, 405nm, 436nm, 546nm, 578nm

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15
Q

Sources of Error

A

• Dirty light source
• Power fluctuations
• Forgetting to blank after changing wavelength setting

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16
Q

Wavelength Selector System

A

Used for isolating the desired wavelength from the light source

1) filters
2) monochromators

There is always a range of wavelengths transmitted

17
Q

Bandpass (or Bandwidth)

A

The width (in nm) of the spectral transmittance curve of a filter at a point equal to one half the peak transmittance

• As bandwidth increases, more wavelengths are transmitted
• It is desirable to have a lower bandpass
Example: a bandpass of 5nm is 507.5-512.5nm

18
Q

Filters

A

Two types:
1) Absorption filters
- useful for visible wavelengths
2) Interference filters
- useful for UV and visible wavelengths

19
Q

Absorption Filters

A

• Made of coloured glass or plastic
• Designed to selectively transmit certain wavelengths
• Other wavelengths are absorbed
• Low transmission intensity and require high intensity light source
• Wide bandpass filters (25-50nm)

20
Q

Interference Filters

A

• Made if multiple thin layers
- thickness of layers determines which wavelengths are transmitted
• Other wavelengths are removed by destructive interference or by reflection
• transmit up to 90% of incident light energy
• narrow bandpass (1-15nm)

21
Q

Bandpass Fliters

A

Transmit a specific range of wavelength for analysis

on automated analyzers these filters are typically on a filter wheel

22
Q

Shortpass/Longpass Filters

A

Shortpass
• transmit shorter wavelengths (cut-off wavelength)

Longpass
• transmit longer wavelengths (cut-on wavelength)

23
Q

Dichroic Filters

A
  • Some wavelengths are transmitted
  • Some wavelengths are reflected
24
Q

Neutral Density Filters

A

• Used to reduce light intensity across a wide range of wavelengths
• Not used to isolate specific wavelengths

25
Q

Monochromators

A

Devices that are able to select a desired wavelength from a range of wavelengths (can choose any in the visible spectrum)

Two types :
1) Prisms
2) Diffraction gratings

26
Q

Prisms

A

• Disperse white light into a continuous spectrum by refraction (bending of light)

• Short wavelengths are bent the most

• Long wavelengths are bent the least

27
Q

Diffraction Gratings

A

• Produce a spectrum by diffraction
• Made by etching a series of parallel lines or grooves onto a surface

Transmission gratings - produce a spectrum by passage of light through the grating

Reflection gratings - have lines etched onto a reflective surface so that the spectrum is produced by reflection of the incident light

• Each line on a grating produces its own spectrum

• because the lines are so close together, these spectra interfere with each other and a series of spectra(orders) are produced

28
Q

Diffraction Gratings vs Prisms

A

• diffraction gratings are capable of better resolution (can better separate two wavelengths)

• Diffraction Gratings can be used for all wavelengths (visible and UV)

• Very low bandwidth

29
Q

Monochromator Optics

A

Another component assisting in wavelength selection

• Lenses are used to collimate the light rays so that they are parallel when they strike the prism/grating surface
• Mirrors are used to divert or focus light (focusing the light onto the cuvette)
• Filters absorb unwanted wavelengths
(Shortpass/Longpass completely absorb one side of the spectrum)
• Entrance slit provides a narrow source of radiation
• Exit slit selects a narrow band of the spectrum

30
Q

Wavelength Calibration

A

• Absorbance peaks of chemical solutions
• chemical solutions have characteristic spectra with specific peaks
• the solution should be scanned across a range of wavelengths (look for expected peaks and that there are no extra peaks)
• mercury lamps are the most accurate method of wavelength calibration

31
Q

Cuvettes

A

• Holds the sample
• Sets the path length, b

Quartz or Fused Silica - visible/UV
Borosilicate Glass - visible only
Plastic - visible and near-UV

32
Q

Photon Detectors

A

• converts the light energy transmitted by sample into an electrical signal that can be recorded

• All photon detectors are a form of transducer (a device that converts one form of energy into a different form)

• Uses photoelectric effect
1) Photoemmissive effect
2) Photoconductive effect

33
Q

Photoconductive Effect

A

• conductivity of detector changes due to absorption of light energy
• electrons are not released from the photosensitive material
• similar to photoemissive effect

34
Q

Properties of Photon Detectors

A

• respond to light energy over a broad wavelength range
• be sensitive to low levels of radiant energy
• exhibit a fast response time
• produce an electrical signal that is directly proportional to the amount of light detected
• produce minimal output signal in absence of illumination

35
Q

Types of Photon Detectors

A

1) Vacuum/Gas Phototube

2) Photomultiplier Tube (PMT)

3) Solid State Photodiode

4) Diode Arrays

36
Q

Vacuum Phototube

A

• Photoemissive

Two main parts:
- emitter(cathode) ; photosensitive material, generates electrons when light hits surface
-collector(anode) ; positively charged wire, collects electrons ejected from cathode

• flow of electrons generates current

• number of electrons generates is proportional to the intensity of the light

37
Q

Photomultiplier Tube (PMT)

A

• highly sensitive photon detector

• consists of:
- photosensitive cathode emitter
- secondary emitters: DYNODES
- anode

• cascade effect;
- each dynode emits several electrons (3-6) for each electron that strikes its surface
- one photon striking the cathode can end up producing 10^8-10^17 electrons
- number of electrons is directly proportional to the intensity of light hitting cathode

38
Q

Photodiode vs LED

A

• LEDs convert electrical energy into light
• photodiodes convert light into electricity

• LEDs principle: electroluminescence
• Photodiode principle: photoconduction

• LEDs intensity of light proportional to applied voltage
• Photodiode current generated is proportional to intensity of light striking p-n junction

1105 Analytical Principles (1 decks)

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