Spectrometry Flashcards
- Meas. light intensity in a much narrower WL using a device (prism/grating) to
disperse the light source into a cont. spectrum
Spectrophotometry
Quantitation of subs of spectrophotometry
meas. amt of light absorbed after appropriate treatment
spectrophotometry advantage
- Therefore, it gives a relatively high sensitivity, greater ease of rapid meas.
compared to visual colorimetry
-High degree of specificity reacts the subs of interest w/ proper rgts = diff colors
(analytical separation prior to color formation rxns)
Filter Photometry
- meas light intensity of multiple WL
- uses filter to isolate part of spectrum
Parts of Spectrophotometer:
Source→(EnS)→monochromator→(ExS)→cuvet→detector→meter/read out device(Galvanometer)
provides radiant energy to monochromator—separates into discrete WL
Light Source
is a form of electromagnetic energy Transmitted via electromagnetic waves Characterized by frequency and wavelength
Light
is the distance of 2 successive waves (nanometers). Waves are measured by nanometers (wavelength).
Wavelength
is number of vibrations of wave motions per second;
Frequency
Wavelength is _______ proportional to the frequency and energy.
inversely
Shorterwavelength,
↑Freq,↑Energy
LongerWavelength
↓Freq,↓Energy
Visible spectrum – wavelength
400-700 nm
Invisible spectrum ranges below 400 nm
UV Region
Invisible spectrum ranges below 700 nm
Infrared Region
below 400 nm
- Gamma Rays
- X-Rays
- UV
greater 700 nm
Infrared and Radio Waves
Spectrum of energy from a short wavelength
of Gamma Rays and X-rays to a longer wavelength of Infrared and Radio waves
Between the invisible Region, the Visible light falls in those regions
Spectrum of energy from a short wavelength
of Gamma Rays and X-rays to a longer wavelength of Infrared and Radio waves
Spectrophotometric measurement is related to the principle of beer’s law
Beer’s Law
The concentration of a substance is Beer’s Law
o Directly proportional to amount of light
absorbed
o Inversely proportional to amount of transmitted
light
The amount of light absorbed, which is proportional to concentration
Not directly measured by spectrophotometer; derived from percent transmittance
Absorbance
measures amount of
transmitted light or percent transmittance
Spectrophotometer
Directly proportional to the inverse logarithm of
percent transmittance
Absorbance
-Ratio of transmitted light through the sample
(aka the light that is not absorbed by the sample) and the intensity of light striking the sample (from the light source) multiplied by 100
-As light passes through the sample, some of them are absorbed while the remainder passes through
- Light strikes the light detector and is converted to an electrical signal (% transmittance)
% T r a n s m i t t a n c e
Measures light transmitted by a solution to determine the concentration of substance in the solution
Spectrophotometry
Components of a Spectrophotometer
Light Source
Entrance Slit
Monochromator
Exit slip
Sample Cell
Photodetector
Read-out Device
Provide incident light for the system
Light Source
For visible and near infrared spectrum
(wavelengths ranging from 320-700 nm)
Incandescent Tungsten or Tungsten iodide
lamp
For UV Spectrum
To provide UV radiation in analytic
spectrometers (wavelengths ranging from
165-350 nm)
Deuterium-discharge lamp and Mercury arc
lamp
For infrared spectrum
Silicone carbide
Exclude unwanted or “stray light”
Before the monochromator
Entrance Slit
o Wavelength outside the band transmitted by the monochromator
o Does not originate from the light source
o Causes absorbance error
Isolates specific/individual wavelength of light that is desired for measurement
In short, it selects and isolates desired wavelength that will strike the sample
Monochromator
- Based on constructive interference of waves
- Simple, useful, least expensive but not precise
- Passes a wide band of radiant energy (even the unwanted wavelengths can pass through)
Interference Filter
Separates white light into a continuous
spectrum
It can be rotated allowing only the
desired wavelength to pass through/exit
Prism
Wed-shaped pieces of glass, quartz or
sodium chloride (NaCl)
When light strikes, it separates light into component wavelengths
Diffraction gratings
Better resolution than Prism
Diffraction gratings
Most commonly used monochromator Made of cutting or parallel groups or slits and produce a widely dispersed
spectrum
Diffraction gratings
Controls width of light beam or band pass
Allows only a fraction of light to reach the
sample
Exit slip
Band pass is total range of wavelength transmitted
The narrower the band pass, the greater the resolution
Exit slip
After the monochromator
Exit slip
Aka Cuvette absorption cell or analytical cell
Holds the solution of which the absorption is to be measured
Sample Cell
For visible range
Glass Cuvette
For UV range
Quarts or Fused Silica
Converts transmitted radiant energy into an equivalent amount of electrical energy
Detects amount of light that passes through the sample
Photodetector
-Simplest and least expensive
-Generates electromotive force (no
-external voltage)
-Relies on internal electron transfer
-Output is not amplified
Photocell (Barrier layer cell, selenide cell)
