Module 7 Flashcards
(32 cards)
What do manufactured light sources require?
Well regulated power supply
Cool burning lamp/cooling system
Mechanism to allow exact alignment when replaced
What types of light sources are there and what are their characteristic wavelengths?
Regular tungsten- IR
Tungsten-halogen- visible
H and deuterium- more intense UV
Hg and xenon arc- very high intensity UV
LEDs- very specific wavelengths
Lasers- nearly monochromatic, high intensity light
What types of natural light sources are there?
Metals/alkali metals- flame emission photometry
Molecules that fluoresce when excited
Bioluminescent substances
What is a monochromator used for?
Isolating a characteristic wavelength, ideally a single wavelength.
What is nominal wavelength and bandwidth?
Nominal- the desired wavelength, peak light intensity
Bandwidth- range of wavelengths on either side of the nominal, width of spectral transmittance curve at half peak.
Want high peak intensity and narrow bandpass.
What is spectral resolution?
The ability of a device to select narrow wavelengths.
Why is monochromatic light desired for spectrophotometry?
Beer’s law is obeyed
Increased sensitivity
Decreased interference
What types of filters are there and what are they used in?
Used in photometers.
Glass absorption- absorbs undesirable wavelengths, 5-20% efficient, bandwidth 25-50nm
Interference- dialectic material between glass, thickness determines wavelength selected, 40-90% efficient, bandwidth 1-15nm
What device uses monochromators and what are the parts?
Used in spectrophotometers.
Parts:
Entrance slit
Dispersing device
Exit slit
What types of dispersing devices are there for monochromators?
Prisms- use refraction to create a nonlinear spectrum, glass (visible) or Quartz/silica (UV)
Diffraction gratings- etched lines diffract light into a linear spectrum, more lines = better resolution, bandwidth 0.5-20nm
What types of diffraction gratings are there?
Transmission- spectrum is obtained as light passes through
Reflection- spectrum is obtained as light is reflected (more efficient)
What types of cuvettes are there?
Borosilicate glass- visible
Quartz/silica- UV
Plastic- UV and visible
Flow through- solution is pumped in and out
What is the purpose of a photodetector?
Detects light and generates a proportional signal.
What is the photoelectric effect?
Some metals become excited and cause current to flow when hit by radiant energy.
Photoconductive effect- electrons are ejected.
Photoemissive effect- electrons are ejected from the metal’s surface.
What is spectral response?
Range of wavelength over which the photodetector operates.
What is dark current?
Current that flows even when no light is present.
What types of photodetectors are there?
Vacuum and glass photodiodes- inert gases are ionized by electrons released
Photomultiplier tubes- electrons are passed down a series of diodes to produce a photocurrent
Solid state photodiodes- electrons generated pass into external current
Linear diode arrays- collection of photodiodes that each detect very narrow ranges of the spectrum, many wavelengths can be measured at once, reverse optics (monochromator after sample)
What does a signal processor do and what types are there?
It amplifies and cleans the electrical signal to convert it into useable data.
Difference amplifier- rejects noise
Analog to digital conversion (ADC)
Where do photometry error signals come from and what types are there?
Voltages that come from sources within the system.
Drift- change in output voltage over time
Noise- any electrical fluctuations that obscure the signal
Artifact- random electrical impulse with similar characteristics to the signal
What is the function of the display/output device and what types are there?
Converts the electrical signal into a system of units.
Panel meter- analog
LED- selective illumination
Video display terminals
Chart recorders
Printers
What are single beam photometers and what sources of inaccuracy do they have?
A single light path penetrates the sample and hits the detector.
Sources of inaccuracy:
Variations in light intensity
Drift in dark current
How are inaccuracies in single beam photometers compensated for?
Addition of second photodetector to monitor light source output.
Splitting the beam and passing them through a sample and reference.
What types of double beam photometers are there?
Dichroic mirror- half the light is allowed to pass through the sample, the rest is reflected through the reference cuvette
Photochopper- rotating mirror alternately directs light
Double beam in time- single detector used, beams are alternately pulsed
What is the purpose of wavelength calibration?
To ensure the wavelength of light striking the sample is the one selected.
