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MEDL 210 > Module 7 > Flashcards

Module 7 Flashcards

1
Q

What do manufactured light sources require?

A

Well regulated power supply

Cool burning lamp/cooling system

Mechanism to allow exact alignment when replaced

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2
Q

What types of light sources are there and what are their characteristic wavelengths?

A

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

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3
Q

What types of natural light sources are there?

A

Metals/alkali metals- flame emission photometry

Molecules that fluoresce when excited

Bioluminescent substances

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4
Q

What is a monochromator used for?

A

Isolating a characteristic wavelength, ideally a single wavelength.

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5
Q

What is nominal wavelength and bandwidth?

A

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.

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6
Q

What is spectral resolution?

A

The ability of a device to select narrow wavelengths.

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7
Q

Why is monochromatic light desired for spectrophotometry?

A

Beer’s law is obeyed

Increased sensitivity

Decreased interference

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8
Q

What types of filters are there and what are they used in?

A

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

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9
Q

What device uses monochromators and what are the parts?

A

Used in spectrophotometers.

Parts:
Entrance slit

Dispersing device

Exit slit

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10
Q

What types of dispersing devices are there for monochromators?

A

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

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11
Q

What types of diffraction gratings are there?

A

Transmission- spectrum is obtained as light passes through

Reflection- spectrum is obtained as light is reflected (more efficient)

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12
Q

What types of cuvettes are there?

A

Borosilicate glass- visible

Quartz/silica- UV

Plastic- UV and visible

Flow through- solution is pumped in and out

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13
Q

What is the purpose of a photodetector?

A

Detects light and generates a proportional signal.

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14
Q

What is the photoelectric effect?

A

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.

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15
Q

What is spectral response?

A

Range of wavelength over which the photodetector operates.

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16
Q

What is dark current?

A

Current that flows even when no light is present.

17
Q

What types of photodetectors are there?

A

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)

18
Q

What does a signal processor do and what types are there?

A

It amplifies and cleans the electrical signal to convert it into useable data.

Difference amplifier- rejects noise

Analog to digital conversion (ADC)

19
Q

Where do photometry error signals come from and what types are there?

A

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

20
Q

What is the function of the display/output device and what types are there?

A

Converts the electrical signal into a system of units.

Panel meter- analog

LED- selective illumination

Video display terminals

Chart recorders

Printers

21
Q

What are single beam photometers and what sources of inaccuracy do they have?

A

A single light path penetrates the sample and hits the detector.

Sources of inaccuracy:
Variations in light intensity

Drift in dark current

22
Q

How are inaccuracies in single beam photometers compensated for?

A

Addition of second photodetector to monitor light source output.

Splitting the beam and passing them through a sample and reference.

23
Q

What types of double beam photometers are there?

A

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

24
Q

What is the purpose of wavelength calibration?

A

To ensure the wavelength of light striking the sample is the one selected.

25
Q

What is used to determine wavelength accuracy?

A

Didymium (visible) or holium oxide (UV and visible) glass filters- sharp absorption peaks at specific wavelengths

Deuterium or Hg vapour lamps- sharp emission lines at several wavelengths

Chemical solutions with sharp absorption peaks (cobalt chloride, nickel sulfate, potassium chromate)

26
Q

What is the purpose of photo metric linearity?

A

Ensures that for a given change in concentration there is a proportional change in absorbance.

Ensures Beer’s law is obeyed.

Uses neutral density filters of solutions known to follow Beer’s law.

27
Q

What is stray light and what does it do?

A

All radiant energy that doesn’t pass trough a sample.

Produces a decrease in sensitivity and apparent decrease in absorbance by falsely increasing transmittance.

28
Q

What is the purpose of photometric accuracy?

A

Verifies the absorbance displayed is the sample’s true absorbance.

Checked using the same solution and filters used for wavelength accuracy and linearity.

29
Q

How is sensitivity measured in photometry?

A

The slope of a calibration curve.

Linearity is assessed using standards of accurately known concentrations.

30
Q

What does instrument calibration do?

A

Defines the reference points which set the range over which the instrument operates.

31
Q

What is assay calibration?

A

Creation of a calibration curve.

32
Q

What are the parts of a photometer?

A

Light source

Wavelength selector (entrance slit, monochromator, exit slit)

Sample holder/cuvette

Photodetector

Signal processor

Display

MEDL 210 (12 decks)

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