Samenvatting
Samenvatting instrumental analysis
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- Rijksuniversiteit Groningen (RuG)
Samenvatting instrumental analysis
[Meer zien]Voorbeeld 4 van de 54 pagina's
Voorbeeld 4 van de 54 pagina's
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Voorbeeld van de inhoud
INSTRUMENTAL ANALYSISSPECTROMETRY
Spectroscopy: the science that studies the interaction of different kinds of radiation
with matter. Types of radiation – electromagnetic, acoustic, ion or electronic radiation
Spectrometry: intensity of radiation (light) is measured and converted to an electrical
signal.
- Measurement of absorbed light (absorbance) and emitted light (emission)
Light as a wave (sinusoidal shape) –
Wavelength (λ): crest-to-crest distance
Amplitude (A): length of electrical vector at the crest (maximum) of the sine curve
Frequency (ν): number of complete oscillations that the wave makes each second
The relationship between the
frequency and the wavelength of an
electromagnetic wave:
Speed = frequency x wavelength
→ Vi = ν x λi
The speed of light in air is constant:
Vi = c = 3.00 x 108 m/s
So, a ray of light in air with a higher ν must have a smaller λ, and vice versa (ν x λ = c).
Ultraviolet light: 10 nm – 400 nm
Visible light: 400 nm – 800 nm
Light as a particle (the photon) –
Quantum:
- The smallest indivisible amount or unit of something, especially energy
- Amounts of energy occur only as multiples, n, of a quantum (n = 1, 2, 3, ..)
Light quantum = photon
The relationship between the frequency/wavelength and energy of a photon (Planck
equation):
∆E = h ν = hc / λ
∆E: photon energy (in J)
h = Planck constant = 6.62 x 10-34 J s
1
, - Wavelength (λ) decreases → photon energy (∆E) increases
- Light frequency (ν) decreases → photon energy (∆E) decreases
Energy states are related to the motion of electrons, e-, around positively charged
nuclei → electronic state.
Ground state: lowest electronic state. Most chemical compounds exist in the ground
state at room temperature.
Energy can be transferred to electrons by light (photons). The energy state of the
chemical compound will then be converted from the ground state to an
excited state.
The energy state of a chemical species changes when it loses or gains an
amount of energy (photon) which exactly overlaps with the difference in
energy between 2 energy states. Absorption of a photon having an energy
which does NOT exactly correspond to the energy difference between two
states is IMPOSSIBLE.
The relation between molecular energy levels and absorbed photon frequency
(wavelength):
E1 – E0 = h ν = hc / λ → photon energy
E1 = molecular energy state with higher energy
E0 = molecular energy state with lower energy
h = Planck constant = 6.62 × 10-34 J s
Absorption refers to the process by which a material absorbs photons (light). The
energy of the absorbed photon is used to promote electrons from lower energy levels
(ground state) to higher energy levels (excited state) within the material. These
energy levels are quantized, meaning they can only take on round values.
Electronic state > vibration level > rotation level
- Eelectronic - large ∆E → short wavelength → UV photons and visible photons
(Thus, for UV and visible light: the energy of the absorbed photon will mostly lead to
electronic transitions)
- Evibration – medium ∆E → medium
wavelength → infra-red photons
- Erotation – small ∆E → long wavelength →
microwave photons
NOTE: an electronic transition is always coupled
to a change in vibration and rotation state.
2
,The vibrational levels are related to inter-atomic vibrations, while the
rotational levels correspond to the rotation of molecules.
Notation for the purple transition: S0 (V1, y1) → S1 (V2, y2)
Born-Oppenheimer: E = Eelectronic + Evibration + Erotation
For every electronic transition, Eelec, many Evib exist, and even more
Erot.
Transmission (T) is the fraction of
P
transmitted light: T = P0
P0
Absorption: A = log ( P ) = - log (T)
Lambert-beer: A = ελ b c
ελ = molar absorptivity (M-1 cm-1) – wavelength specific!
c = concentration of the absorbing analyte (M)
b = pathlength of light through sample (cm)
Absorbance spectrum of ferrozine –
Molar absorptivity is directly proportional to absorbance,
so the strongest absorbance corresponds to the largest ε
value. In this case, at λ = 562 nm.
When molecules collide (botsen) with each other, they can
transfer energy, causing a range of energy levels within the sample. This leads to
broader absorption lines in the spectrum → absorbance peaks are smeared out.
Fluorescence emission is the process by which a material emits photons (light) when
it returns from an excited state to its ground state. It is the reverse process of
absorption. During this transition, it emits a photon with energy equal to the energy
difference between the excited and ground states. This emitted photon typically has a
longer wavelength (lower energy) than the absorbed photon. The wavelengths of
these photons are typically in the visible range, giving rise to a visible light
(fluorescence). The emission of a photon happens within a very short timescale.
Lifetime fluorescence – 10-8 – 10-4 S
Phosphorescence is a process where a molecule absorbs photons and re-emits them
as lower-energy photons, similar to fluorescence. However, unlike fluorescence, the
emission process in phosphorescence is delayed, occurring over a much longer
timescale. In phosphorescence, the excited electron remains in an excited state for a
longer duration before returning to the ground state and releasing energy as a
photon. Phosphorescence is less common.
3
, Lifetime phosphorescence – 10-4 – 100 S
Vibration relaxation: molecules lose energy and
return to the lowest energy-vibration state in the
same electronic state (Si, Vn → Si, V0). This
energy is released as heat. This ALWAYS
happens when the molecule is just in their
excited state, so it can return to the lowest state
and fluorescence can possibly happen.
Relaxation causes thus a loss of energy. No light
emission takes place during this process.
Internal conversion: conversion from the lowest vibration level of an excited state (Si,
V0) to a certain vibration level of 1 lower energy state (Si-1, Vn). Happens more often.
Intersystem crossing: a crossing from the
lowest vibration level of an excited state (S1, V0)
to a certain vibration level of another excited
state (T1, Vn). Happens less often.
Thus, internal conversion and intersystem
crossing provide changes in energy state, but
the energy level remains the same, so no
energy is released during these actions.
Energy release as:
- Light
- Heat
Fluorescence: intensity x 1
Phosphorescence: intensity x 10
Which means that the peak of phosphorescence is
actually 10 times smaller than represented in the graph.
Fluorescence – emission intensity peak at shorter wavelength, thus higher energy
Phosphorescence – emission intensity peak at longer wavelength, thus lower energy
Since phosphorescence takes more time, it also has a longer lifetime (10 -4 – 100 ms)
compared to fluorescence (10-8 – 10-4).
4
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