Intro to Spectrometric Methods
• General Properties of Electromagnetic
Radiation (EM)
• Wave Properties of EM
• Quantum-Mechanical Properties of EM
• Quantitative Aspects of
Spectrochemical Measurements
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Intro to Spectrometric Methods - Analytical Chemistry II - Lecture Slides, Slides of Analytical Chemistry
Major topics of this course are: General Instrumentation, Spectroscopy Theory, Molecular Spectroscopy, Chromatography, Electrochemistry, Coulometric Methods, Voltammetric Methods. This lecture covers following points: Intro to Spectrometric Methods, Electromagnetic Radiation, Quantum-Mechanical Properties, Spectrochemical Measurements, Spectroscopy, Frequency, Amplitude, Angular Frequency, Phase Angle, Refraction
Typology: Slides
2012/2013
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- General Properties of ElectromagneticIntro to Spectrometric Methods Radiation (EM)
- Quantitative Aspects of• Quantum-Mechanical Properties of EM• Wave Properties of EM Spectrochemical Measurements
R O Y
G
B V
SpectroscopyRayGamma
FluorescenceAbsorption,X-Ray
FluorescenceAbsorption,UV-vis
SpectroscopyAbsorptionInfrared
SpectroscopyAbsorptionMicrowave
EPRNMR
Transitions Nuclear
ElectronsInner Shell
ElectronsOuter Shell
VibrationsMolecular
RotationsMolecular
EnergyLow StatesSpin
EnergyHigh
Relationship between various wave properties
C
i
i
Where
(^) = frequency in cycles/s or Hz
i = wavelength in medium i
i = refractive index of medium i
C
(^) = speed of light in vacuum
(^) (2.99 x
(^1) 0 (^10) cm/s)^
EM slows down in media other than vacuum because medium (matter)electric vector interacts with electric fields in the
(^) (^) this effect is greatest in solids &
liquids, in gases (air) velocity similar to vacuum
Wave Equation
y = A sin (
ω t + (^) α )
Where
ω A = amplitude (^) = angular frequency
α t = time (^) = phase angle
For a collection of waves the resulting y = Aposition y at a given t can be calculated by
(^1) sin (
ω (^1) t +
(^) α
(^1) ) + A
(^2) sin (
ω (^2) t +
(^) α
(^2) ) + …
At (^) α (^1)
α (^2) = 0
o adding of waves gives
Maximum Constructive Interference
(^0) o
(^180) o 360 o 540 o 720 o 900 o
Wave
(^1)
Wavedifference betweenPhase angleResultant wave Wave 2
(^1) (^) & Wave 2
is zero
(^) α (^1)
α (^2) = 0 o
Amplitude
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When
(^) α (^1)
α (^2) = (^1) 80 o or 540
o adding of waves
gives Maximum Destructive Interference
(^0) o
(^180) o 360 o 540 o 720 o 900 o
Wave
(^1)
Resultant wave Wave 2
Wavedifference betweenPhase angle
(^1) (^) & Wave 2
is (^1) 80 o (α (^1)
α (^2) = (^1) 80 )o
Amplitude
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Refraction
(^) = change in velocity of EM as it
goes from one medium to another
to surfaceNormal
Medium
(^1) (^) (air)
Velocity larger
(^) η (^) = (^1) .
Velocity smallerMedium 2 (glass)
(^) η (^) = (^1) .
rayIncident
Ф 1 Ф 2
ray Refracted
directionOriginal
normalRay bent toward
sin Equation for Refraction (Snell) (^) Ф 1
ν (^1)
η (^2)
if medium
η 2
sin
(^) Ф (^2)
ν (^2)
η (^1)
is air
(^) η (^1) = (^1) .
Magnitude of the direction change (i.e., size of equation asthe angle depends on wavelength (shown in
(^) ν ) this is how a prism works
Direction of bending depends on relative values of
(^) η (^) for each medium. Going from
low
(^) η (^) to higher, the ray bends toward the
normal. Going from higher
(^) η (^) to lower the ray
bends away from the normal.
rI
(η (^2)
η (^1) ) 2
Reflectance = R =
i I
(η (^2)
(^) η (^1) ) 2
Where I
i (^) and I
r = incident & reflected intensity
For radiation going from air (
η (^) = (^1) .00) to glass
(η (^) = (^1) .50) as shown in previous slide
R = 0.04 = 4 %
Many surfaces at 4 % each (i.e., many lenses) can generatescause serious light losses in a spectrometer. This
(^) stray radiation
(^) or
(^) stray light
.
Scattering
(^) = EM interacts with matter and changes
direction, usually without changing energy
This can be described using both the wave or
particle nature of light:
1 ) charge of matterWave – EM induces oscillations in electrical
⇒⇒^ ⇒⇒ (^) resulting in oscillating
in all directions = scattered radiationdipoles which in turn radiate secondary waves
matter to form a virtual state (lifetimeParticle (or Quantum) – EM interacts with
(^1) 0
- 1 (^4) s)
which reemits in all directions.
Raman effect = when some molecules return to a
different state
(^) ⇒
(^) change in frequency
Rayleigh Scattering – scattering by particles whose longest dimension is < 5 % to
of (^) λ (^) with no change in observed frequency
(^) π (^4) α 2
sI (^) = ------------ (
(^) + cos
(^2) θ ) I o
λ (^4) r 2
polarizability
intensityscattering
wavelength
beam& scatteredincident beamangle between
to detectorscattering centerdistance from
intensityincident beam
short wavelengths are scattered more efficientlyNotice the fourth power dependence on wavelength meaning
(^) ⇒
(^) sky is blue
Polarizability (
α ) is measure of how well a given
frequency induces a dipole in a substance
α proteins)^ Tends to be large for large molecules (e.g.,
Large Particle Scattering – particle dimensions <
(^1) 0
% (^) λ (^) to (^1) . (^) λ
Applies in techniques like turbidimetry and nephelometry
Large particles do not act as a point source & give rise to various interference phenomena
Forward scatter becomes greater than back scatter
Linearly polarized light oscillates in one plane only as it moves through space
Linearly polarized light oscillates in one plane only as it moves through space
at 90polarized and H vector is Here E vector is vertically
o in horizontal plane