Transcript الشريحة 1
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An introduction to Ultraviolet/Visible Absorption Spectroscopy
Chapter 13
2 In this chapter, absorption by molecules, rather than atoms, is considered. Absorption in the ultraviolet and visible regions occurs due to electronic transitions from the ground state to excited state. Broad band spectra are obtained since molecules have vibrational and rotational energy levels associated with electronic energy levels. The signal is either absorbance or percent transmittance of the analyte solution where:
An Introduction to Ultraviolet/Visible Molecular Absorption Spectrometry Absorption measurements based upon ultraviolet and visible radiation find widespread application for the quantitative determination of a large variety species.
Beer’s Law: A = -logT = logP 0 /P =
bc
A = absorbance = molar absorptivity [M -1 cm -1 ] c = concentration [M] P 0 = incident power 3 P = transmitted power (after passing through sample)
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Measurement of Transmittance and Absorbance:
The power of the beam transmitted by the analyte solution is usually compared with the power of the beam transmitted by an identical cell containing only solvent. An experimental transmittance and absorbance are then obtained with the equations.
T
P solution
P solvent P P
0
A
P solvent
log
P solution
log
P
0
P
P 0 and P refers to the power of radiation after it has passed through the solvent and the analyte.
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Beer ’s law and mixtures
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Each analyte present in the solution absorbs light! The magnitude of the absorption depends on its
A total = A 1 +A 2 + …+A n A total =
1 bc 1 +
2 bc 2 + …+
n bc n If
1 =
2 =
n then simultaneous determination is impossible Need n
l
’s where
’s are different to solve the mixture
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Limitations to Beer ’s Law
• Real limitations • Chemical deviations • Instrumental deviations
1.
Real Limitations a.
Beer ’s law is good for dilute analyte solutions only. High concentrations (>0.01M) will cause a negative error since as the distance between molecules become smaller the charge distribution will be affected which alter the molecules ability to absorb a specific wavelength. The same phenomenon is also observed for solutions with high electrolyte concentration, even at low analyte concentration. The molar absorptivity is altered due to electrostatic interactions.
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b.
In the derivation of Beer ’s law we have introduced a constant (
). However, e is dependent on the refractive index and the refractive index is a function of concentration. Therefore, e will be concentration dependent. However, the refractive index changes very slightly for dilute solutions and thus we can practically assume that e is constant.
c.
In rare cases, the molar absorptivity changes widely with concentration, even at dilute solutions. Therefore, Beer like methylene blue.
’s law is never a linear relation for such compounds,
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2.
Chemical Deviations
This factor is an important one which largely affects linearity in Beer ’s law. It originates when an analyte dissociates, associates, or reacts in the solvent. For example, an acid base indicator when dissolved in water will partially dissociate according to its acid dissociation constant: 12
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Chemical deviations from Beer ’s law for unbuffered solutions of the indicator Hln. Note that there are positive deviations at 430 nm and negative deviations at 570 nm. At 430 nm, the absorbance is primarily due to the ionized In- form of the indicator and is proportional to the fraction ionized, which varies nonlinearly with the total indicator concentration. At 570 nm, the absorbance is due principally to the undissociated acid Hln, which increases nonlinearly with the total concentration . 17
Calculated Absorbance Data for Various Indicator Concentrations
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3.
Instrumental Deviations
a.
Beer ’s law is good for monochromatic light only since is wavelength dependent. It is enough to assume a dichromatic beam passing through a sample to appreciate the need for a monochromatic light. Assume that the radiant power of incident radiation is P o and P o ’ while transmitted power is P and P ’. The absorbance of solution can be written as: 20
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The effect of polychromatic radiation on Beer ’s law. In the spectrum at the top, the absorptivity of the analyte is nearly constant over Band A from the source. Note in the Beer ’s law plot at the bottom that using Band A gives a linear relationship. In the spectrum, Band B corresponds to a region where the absorptivity shows substantial changes. In the lower plot, note the dramatic deviation from Beer ’s law that results.
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Therefore, the linearity between absorbance and concentration breaks down if incident radiation was polychromatic. In most cases with UV-Vis spectroscopy, the effect small changes in wavelengths is insignificant since e differs only slightly; especially at the wavelength maximum.
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b.
Stray Radiation Stray radiation resulting from scattering or various reflections in the instrument will reach the detector without passing through the sample. The problem can be severe in cases of high absorbance or when the wavelengths of stray radiation is in such a range where the detector is highly sensitive as well as at wavelengths extremes of an instrument. The absorbance recorded can be represented by the relation: A = log (P o + P s )/(P + P s ) Where; P s is the radiant power of stray radiation.
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Instrumental Noise as a Function in Transmittance
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Therefore, an absorbance between 0.2-0.7 may be advantageous in terms of a lower uncertainty in concentration measurements. At higher or lower absorbances, an increase in uncertainty is encountered. It is therefore advised that the test solution be in the concentration range which gives an absorbance value in the range from 0.2-0.7 for best precision.
However, it should also be remembered that we ended up with this conclusion provided that sT is constant. Unfortunately, sT is not always constant which complicates the conclusions above.
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EFFECT OF bandwidth WIDTH
Effect of bandwidth on spectral detail for a sample of benzene vapor. Note that as the spectral bandwidth increases, the fine structure in the spectrum is lost. At a bandwidth of 10 nm, only a broad absorption band is observed.
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Effect of slit width (spectral bandwidth) on peak heights. Here, the sample was s solution of praseodymium chloride. Note that as the spectral bandwidth decreases by decreasing the slit width from 1.0 mm to 0.1 mm, the peak heights increase. 34
Effect of Scattered Radiation at Wavelength Extremes of an Instrument
Wavelength extremes of an instrument are dependent on type of source, detector and optical components used in the manufacture of the instrument. Outside the working range of the instrument, it is not possible to use it for accurate determinations. However, the extremes of the instrument are very close to the region of invalid instrumental performance and would thus be not very accurate. An example may be a visible photometer which, in principle, can be used in the range from 340-780 nm. It may be obvious that glass windows, cells and prism will start to absorb significantly below 380 nm and thus a decrease in the incident radiant power is significant.
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36 B: UV-VIS spectrophotometer A: VIS spectrophotometer
EFFECT OF SCATTERED RADIATION
Spectrum of cerium (IV) obtained with a spectrophotometer having glass optics (
A
) and quartz optics (
B
). The false peak in A arises from transmission of stray radiation of longer wavelengths.
The output from the source at the low wavelength range is minimal. Also, the detector has best sensitivities around 550 nm which means that away up and down this value, the sensitivity significantly decrease. However, scattered radiation, and stray radiation in general, will reach the detector without passing through these surfaces as well as these radiation are constituted from wavelengths for which the detector is highly sensitive. In some cases, stray and scattered radiation reaching the detector can be far more intense than the monochromatic beam from the source. False peaks may appear in such cases and one should be aware of this cause of such peaks.
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Instrumentation
• • Light source l - selection • Sample container • Detector 38 • Signal processing • Light Sources (commercial instruments) – D 2 lamp (UV: 160 – 375 nm) – W lamp (vis: 350 – 2500 nm)
Sources
Deuterium and hydrogen lamps (160 – 375 nm) D 2 + E e → D 2 * → D’ + D’’ +
h
Excited deuterium molecule with fixed quantized energy Dissociated into two deuterium atoms with different kinetic energies E e = E D2* = E D’ + E D’’ + hv Ee is the electrical energy absorbed by the molecule. E D2* energy of D 2* , E D’
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and E D’’ is the fixed quantized are kinetic energy of the two deuterium atoms.
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Sources
Deuterium lamp UV region (a) A deuterium lamp of the type used in spectrophotometers and (b) its spectrum. The plot is of irradiance
E
λ (proportional to radiant power) versus wavelength. Note that the maximum intensity occurs at ~225 m.Typically,
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Visible and near-IR region
(a) A tungsten lamp of the type used in spectroscopy and its spectrum (b). Intensity of the tungsten source is usually quite low at wavelengths shorter than about 350 nm. Note that the intensity reaches a maximum in the near-IR region of the spectrum (~1200 nm in this case).
The tungsten lamp is by far the most common source in the visible and near IR region with a continuum output wavelength in the range from 350-2500 nm. The lamp is formed from a tungsten filament heated to about 3000 o C housed in a glass envelope. The output of the lamp approaches a black body radiation where it is observed that the energy of a tungsten lamp varies as the fourth power of the operating voltage. 43
Tungsten halogen lamps are currently more popular than just tungsten lamps since they have longer lifetime. Tungsten halogen lamps contain small quantities of iodine in a quartz envelope. The quartz envelope is necessary due to the higher temperature of the tungsten halogen lamps (3500 o C). The longer lifetime of tungsten halogen lamps stems from the fact that sublimed tungsten forms volatile WI2 which redeposits on the filament thus increasing its lifetime. The output of tungsten halogen lamps are more efficient and extend well into the UV.
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Sources
Tungsten lamps (350-2500 nm)
Why add I 2 W + I 2 in the lamps?
→ WI 2 Low limit: 350 nm 1)Low intensity 2)Glass envelope
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3. Xenon Arc Lamps
Passage of current through an atmosphere of high pressured xenon excites xenon and produces a continuum in the range from 200-1000 nm with maximum output at about 500 nm. Although the output of the xenon arc lamp covers the whole UV and visible regions, it is seldom used as a conventional source in the UV-Vis. The radiant power of the lamp is very high as to preclude the use of the lamp in UV-Vis instruments. However, an important application of this source will be discussed in luminescence spectroscopy which will be discussed later 46
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Sample Containers
Sample containers are called cells or cuvettes and are made of either glass or quartz depending on the region of the electromagnetic spectrum. The path length of the cell varies between 0.1 and 10 cm but the most common path length is 1.0 cm. Rectangular cells or cylindrical cells are routinely used. In addition, disposable polypropylene cells are used in the visible region. The quality of the absorbance signal is dependent on the quality of the cells used in terms of matching, cleaning as well as freedom from scratches.
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Instrumental Components Source
l
- selection (monochromators) Sample holders
Cuvettes (b = 1 cm typically)
1. Glass (Vis) 2. Fused silica (UV+Vis)
– –
Detectors Photodiodes PMTs
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1. Single beam
–Place cuvette with blank (i.e., solvent) in instrument and take a reading
100% T –Replace cuvette with sample and take reading
% T for analyte (from which absorbance is calc ’d)
Instrumentation
Most common spectrophotometer: Spectronic 20.
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2.
3.
4.
5.
On/Off switch and zero transmission adjustment knob Wavelength selector/Readout Sample chamber Blank adjustment knob Absorbance/Transmittanc e scale
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End view of the exit slit of the Spectronic 20 spectrophotometer pictured earlier 53
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Single-Beam Instruments for the Ultraviolet/Visible Region
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• Single-Beam Computerized Spectrophotometers Inside of a single-beam spectrophot ometer connected to a computer.
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Types of Instruments Instrumental designs for UV-visible photometers or spectrophotometers. In (a), a single-beam instrument is shown. Radiation from the filter or monochromator passes through either the reference cell or the sample cell before striking the photodetector.
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2. Double beam (most commercial instruments)
– Light is split and directed towards both reference cell (blank) and sample cell – Two detectors; electronics measure ratio (i.e., measure/calculate absorbance) 58 – Advantages: • Compensates for fluctuations in source intensity and drift in detector • Better design for continuous recording of spectra
General Instrument Designs Double Beam: In - Space
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Needs two detectors
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General Instrument Designs Double Beam: In - Time
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Merits of Double Beam Instruments 1.Compensate for all but the most short term fluctuation in radiant output of the source 2.Compensate drift in transducer and amplifier 3.Compensate for wide variations in source intensity with wavelength
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Location of Sample cell
In all photometers and scanning spectrophotpmeters described above, the cell has been positioned after the monochromators. This is important to decrease the possibility of sample photodecomposition due to prolonged exposure to all frequencies coming from the source. However, the sample is positioned before the monochromator in multichannel instruments like a photodiode array spectrophotometer. This can be done without fear of photodecomposition since the sample exposure time is usually less than 1 s. Therefore, it is now clear that in UV-Vis where photodecomposition of samples can take place, the sample is placed after the monochromators in scanning instruments while positioning of the sample before the monochromators is advised in multichannel instruments.
3. Multichannel Instruments
• Photodiode array detectors used (multichannel detector, can measure all wavelengths dispersed by grating simultaneously).
• Advantage: scan “snapshot” < 1 sec.
spectrum very quickly • Powerful tool for studies of transient intermediates in moderately fast reactions.
• Useful for kinetic studies.
• Useful for qualitative and quantitative determination of the components exiting from a liquid chromatographic column.
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Multi-channel Design
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A multichannel diode-array spectrophotometer, the Agilent Technologies 8453. 67
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4. Probe Type Instruments
These are the same as conventional single beam instruments but the beam from the monochromators is guided through a bifurcated optical fiber to the sample container where absorption takes place. The attenuation in reflected beam at the specified wavelength is thus measured and related to concentration of analyte in the sample.
A fiber optic cable can be referred to as a light pipe where light can be transmitted by the fiber without loss in intensity (when light hits the internal surface of the fiber at an angle larger than a critical angle). Therefore, fiber optics can be used to transmit light for very long distances without losses. A group of fibers can be combined together to form a fiber optic cable or bundle. A bifurcated fiber optic cable has three terminals where fibers from two separate cables are combined at one end to form the new configuration.
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70 Fiber optic probe
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Double Dispersing Instruments
The instrument in this case has two gratings where the light beam leaving the first monochromators at a specified wavelength is directed to the second grating. This procedure results in better spectral resolution as well as decreased scattered radiation. However, double dispersing instruments are expensive and seem to offer limited advantages as compared to cost; especially in the UV-Vis region where exact wavelength may not be crucial. 72
73 Optical diagram of the Varian Cary 300 double dispersing spectrophotomet er. A second monochromator is added immediately after the source.
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Molar absorptivities
= 8.7 x 10
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P A
•
A: cross section of molecule in cm
2
(~10
-15
)
•
P: Probability of the electronic transition (0-1)
–
P>0.1-1
allowable transitions
–
P<0.01
forbidden transitions
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Molecular Absorption
M +
h
M* M* (
absorption 10 -8 sec)
M +
heat (relaxation process)
M*
A+B+C (photochemical decomposition)
M* M +
h
(
emission)
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Visible Absorption Spectra
• The absorption of UV-visible radiation generally results from excitation of bonding electrons.
• can be used for quantitative and qualitative analysis 78
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Molecular orbital
results.) is the nonlocalized fields between atoms that are occupied by bonding electrons. (when two atom orbitals combine, either a low-energy bonding molecular orbital or a high energy antibonding molecular orbital
Sigma (
) orbital
The molecular orbital associated with single bonds in organic compounds
Pi (
) orbital
The molecular orbital associated with parallel overlap of atomic P orbital.
n electrons
No bonding electrons 79
Molecular Transitions for UV-Visible Absorptions
What electrons can we use for these transitions? 80
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MO Diagram for Formaldehyde (CH
2
O)
H H
= C n = O
=
Singlet vs. triplet
• •
In these diagrams, one electron has been excited (promoted) from the n to
* energy levels (non-bonding to anti-bonding). One is a Singlet excited state, the other is a Triplet.
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Type of Transitions
• σ → σ*
High energy required, vacuum UV range CH 4 :
l
= 125 nm
• n → σ*
Saturated compounds, CH 3 O H etc (
l • n → * and → *
Mostly used!
l
= 200 - 700 nm = 150 - 250 nm)
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Examples of UV-Visible Absorptions LOW!
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UV-Visible Absorption Chromophores
Effects of solvents
• Blue shift (n *) (Hypsocromic shift) – Increasing polarity of solvent better solvation of electron pairs (n level has lower E) – peak shifts to the blue (more energetic) – 30 nm (hydrogen bond energy) • Red shift (n and * and
excited states
– *) (Bathochromic shift) – Increasing polarity of solvent, then increase the attractive polarization forces between solvent and absorber, thus decreases the energy of the unexcited with the later greater – peaks shift to the red – 5 nm 86
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UV-Visible Absorption Chromophores
Typical UV Absorption Spectra Chromophores?
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The effects of substitution
Auxochrome function group Auxochrome is a functional group that does not absorb in UV region but has the effect of shifting chromophore peaks to longer wavelength as well As increasing their intensity.
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Now solvents are your “container”
They need to be transparent and do not erase the fine structure arising from the vibrational effects
Polar solvents generally tend to cause this problem Same solvent must be Used when comparing absorption spectra for identification purpose.
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Summary of transitions for organic molecules
n
* transition in vacuum UV (single bonds) * saturated compounds with non-bonding electrons l ~ 150-250 nm
n
*, ~ 100-3000 ( not strong) * requires unsaturated functional groups (eq. double bonds) most commonly used, energy good range for UV/Vis l
n
~ 200 - 700 nm * : *: ~ 10-100 ~ 1000 – 10,000 92
93 List of common chromophores and their transitions
Organic Compounds
• Most organic spectra are complex • Electronic and vibration transitions superimposed • Absorption bands usually broad • Detailed theoretical analysis not possible, but semi-quantitative or 94 qualitative analysis of types of bonds is possible.
• Effects of solvent & molecular details complicate comparison
Rule of thumb for conjugation
CH CH 3 2 CH 2 If greater then one single bond apart are relatively additive (hyperchromic shift) l constant CH =CHCH 2 2 CH=CH CH 2 2 CH=CH 2 l max = 184 l max =185 max = ~10,000 max = ~20,000 H 2 If conjugated - shifts to higher l ’s ( red shift ) C=CHCH=CH 2 l max =217 max = ~21,000 95
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Spectral nomenclature of shifts
•
What about inorganics?
Common anions n
* nitrate (313 nm), carbonate (217 nm)
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Most transition-metal ions absorb in the UV/Vis region.
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In the lanthanide and actinide series the absorption process results from electronic transitions of 4f and 5f electrons.
•
For the first and second transition metal series the absorption process results from transitions of 3d and 4d electrons.
– The bands are often broad.
– The position of the maxima are strongly influenced by the chemical environment.
– The metal forms a complex with other stuff, called
ligands
. The presence of the ligands splits the d-orbital energies.
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Transition metal ions
Charge-Transfer-Absorption
A charge-transfer complex consists of an electron-donor group bonded to an electron acceptor. When this product absorbs radiation, an electron from the donor is transferred to an orbital that is largely associated with the acceptor.
1) Large molar absorptivity ( ε max >10,000) 2) Many organic and inorganic complexes 99
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