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Botswana International University of Science & Technology
Faculty of Science
Department of Chemical and Forensic Science
CHEM 302
Instrumental Analysis I
Test 1
Date: 09/12/2019
Time: 09: 00 10: 00 hours
Duration: 1 hour
Venue: Block 122/ 005-006
Total: 60 marks
This question paper has FIVE questions in 10 pages and the PERIODIC TABLE attached
Please answer ALL questions in the spaces provided. Marks are assigned per question as
shown in brackets
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Question 1
Differentiate between the following
a) Normal and Resonance Raman Spectroscopy
In normal Raman Spectroscopy, the excitation wavelength is normally well
distinguished from an absorption band to minimize fluorescence. On the other hand,
in Resonance Raman Spectroscopy, the intensities of the Raman lines are enhanced
by excitation with wavelengths that closely approach that of an electronic absorption
band of the analyte
b) Singlet and triplet states of molecules
A singlet state of a molecule is one in which all the electron spins are paired, and no
splitting of energy levels occur when the molecule is exposed to a magnetic field. In
the triplet states, the two spins of the electrons are unpaired and parallel
c) Elastic and inelastic collisions in Raman Spectroscopy
Elastic collisions between photons and molecules in Raman Spectroscopy are result in
no change in energy, inelastic collisions, on the other hand, bring about a net change
in energy.
d) Bending and stretching vibrations in IR spectroscopy
Stretching vibrations are those rhythmical movements along the bond axis such that
the interatomic distance is increasing or decreasing while bending vibrations may
consist of a change in bond angle between bonds with a common atom or the
movement of a group of atoms with respect to the remainder of the molecule without
movement of the atoms in the group with respect to one another
e) Transmittance and absorbance
The transmittance of a medium is the fraction of the incident radiation that is
transmitted by the medium while absorbance refers to that portion that is absorbed by
the medium
(10 marks)
Question 2
a) What important requirement must an analytical sample fulfil to be analyzed by
Atomic Spectroscopy?
The sample understudy must be in solution and must be easily volatilized and
atomized
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(2 mark)
b) Give 3 advantages of using atomic spectrometry for chemical analysis
High sensitivity, Rapid, Convenient, high selectivity (anyone of these three)
(3 marks)
c) Describe the processes that occur when solutions of metals are introduced into the
flame of an atomic absorption spectrometer (AAS)
First, the solvent evaporates, leaving behind formula units of the formerly
dissolved salt.
Next, dissociation of the formula units of salt into atoms occursthe metal
ions atomize or are transformed into atoms.
Then, if the atoms are easily raised to excited states by the thermal energy of
the flame, a resonance process occurs in which the atoms resonate back and
forth between the ground state and the excited states
As these atoms drop back to the ground state (a natural process), the emission
spectrum is emitted. A color characteristic of the element that is in the flame.
For quantification, Beer’s law, the width of the flame being the pathlength
.
(5 marks)
d) Give 3 advantages of using a plasma source for AES
High stability, low noise, low background, freedom from interferences (any 3 of
these)
(3 marks)
e) List the 5 steps for developing a method for quantitative analysis using AES
Choosing a source for atomization and excitation
Selecting a wavelength and a slit width
Preparing the sample for analysis
Minimizing spectral and chemical interferences
Selecting a method of standardization
(5 marks)
f) List 2 disadvantages of using AES for analysis of samples
Expensive equipment, Procedures are complicated in comparison to absorption methods,
high operation costs (any 2 of these)
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(2 marks)
Question 3
a) Draw a block diagram showing the major components of Atomic Fluorescence
Spectroscopy (AFS) instrument
See notes!!
(5marks)
b) State three problems associated with analysis using AFS
Precision and accuracy are highly dependent on the atomization step, light source,
Molecules, atoms and ions are all in heated medium thus producing three different
atomic emission spectra
(3 marks)
c) Define quenching in fluorescence
Quenching refers to non-radiative energy transfer from an excited species to another
species
(2 marks)
Question 4
a) What important feature/characteristic must a molecule have for it to reflect IR
absorptions?
For a molecule to show infrared absorptions it must possess a specific feature, i.e. an
electric dipole moment of the molecule must change during the vibration
(1 mark)
b) Define the fingerprint region in an IR spectrum
The complex area in the IR spectrum showing many overlapping bands with
wavenumbers ranging from 600-1400 cm
-1
. It’s the region of most single band signals
(2 marks)
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c) Define the group frequency region in an IR spectrum
This is the region of the IR spectrum where most functional groups of interests are
found. This covers the range of the spectrum from 4000 to 1300 cm
-1
.
(2 marks)
d) Give the wavenumbers for the position of each of the following functional groups in
an IR spectrum
i) Carbonyl -1690 -1760 cm
-1
ii) -OH-
3590-2500 cm
-1
iii) -C=C-
1500-1600 cm
-1
iv) -C=N-
2210-2280 cm
-1
v) -C-C-
900-1100 cm
-1
(5marks)
v) Give 3 ways in which attenuated total reflection IR is better than FTIR sampling
mode IR
ATR mode is simpler and faster to use, requires minimal sample preparation, involves
a one step process and very easy to clean up
(3 marks)
Question 5
a) Explain the principle upon which Raman spectroscopy is based
Raman spectroscopy is based on the irradiation of a sample with monochromatic
incident radiation of given frequency in the visible and near-IR spectral region. The
radiation is scattered by the molecules of interest in the sample at various angles due
to momentary absorption and excitation to virtual states of higher energy. The
molecule then relaxes and re-emits a photon of higher or lower energy. The resultant
spectrum consists of lower frequency emissions known as Stokes scattering and
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higher frequency emissions called anti-Stokes scattering. Elastically scattered
radiation of the same frequency as the excitation beam called Rayleigh scattering is
also emitted.
In Raman spectroscopy, incident radiation of given frequency is scattered by the
molecule at various angles due to momentary to virtual states and reemission. The
energy absorbed by the molecule from the photon of light is not quantized and causes
transition to an infinite number of virtual states. This is then followed by re-emission
(5marks)
b) Why is Raman spectroscopy considered a complementary technique to IR
spectroscopy?
Raman spectroscopy offers additional information about the molecular structure of the
analytical sample and allows for analysis using water as a solvent
(2 marks)
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c) List three factors that affect the intensity of Raman peaks
Polarizability of the molecule
Intensity of the source, frequency of the source
Concentration of the active group
(3 marks)
d) Name the radiation source that is commonly used in Raman spectroscopy
Lasers
(1 mark)
e) Name the detector that is commonly used for Raman Spectroscopy applications
Charge-coupled devices (CCDs)
(1 mark)
f) List 3 areas/fields in which Raman Spectroscopy can be applied
Investigations of the structures of inorganic molecules
Determination of probable structures of many species
Determination of dissociation constants of strong acids
Provide useful information on the structure, composition and stability of coordination
compounds
Study of biological systems (List any 3 of these)
g) Give 2 disadvantages of using Raman Spectroscopy for chemical analysis
High cost of equipment
Matrix effects limit applicability for quantitative analyses
(2 marks)
TOTAL: 60 marks
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Unformatted Attachment Preview

Botswana International University of Science & Technology Faculty of Science Department of Chemical and Forensic Science CHEM 302 Instrumental Analysis I Test 1 Date: 09/12/2019 Time: 09: 00 – 10: 00 hours Duration: 1 hour Venue: Block 122/ 005-006 Total: 60 marks This question paper has FIVE questions in 10 pages and the PERIODIC TABLE attached Please answer ALL questions in the spaces provided. Marks are assigned per question as shown in brackets Question 1 Differentiate between the following a) Normal and Resonance Raman Spectroscopy In normal Raman Spectroscopy, the excitation wavelength is normally well distinguished from an absorption band to minimize fluorescence. On the other hand, in Resonance Raman Spectroscopy, the intensities of the Raman lines are enhanced by excitation with wavelengths that closely approach that of an electronic absorption band of the analyte b) Singlet and triplet states of molecules A singlet state of a molecule is one in which all the electron spins are paired, and no splitting of energy levels occur when the molecule is exposed to a magnetic field. In the triplet states, the two spins of the electrons are unpaired and parallel c) Elastic and inelastic collisions in Raman Spectroscopy Elastic collisions between photons and molecules in Raman Spectroscopy are result in no change in energy, inelastic collisions, on the other hand, bring about a net change in energy. d) Bending and stretching vibrations in IR spectroscopy Stretching vibrations are those rhythmical movements along the bond axis such that the interatomic distance is increasing or decreasing while bending vibrations may consist of a change in bond angle between bonds with a common atom or the movement of a group of atoms with respect to the remainder of the molecule without movement of the atoms in the group with respect to one another e) Transmittance and absorbance The transmittance of a medium is the fraction of the incident radiation that is transmitted by the medium while absorbance refers to that portion that is absorbed by the medium (10 marks) Question 2 a) What important requirement must an analytical sample fulfil to be analyzed by Atomic Spectroscopy? The sample understudy must be in solution and must be easily volatilized and atomized (2 mark) b) Give 3 advantages of using atomic spectrometry for chemical analysis High sensitivity, Rapid, Convenient, high selectivity (anyone of these three) (3 marks) c) Describe the processes that occur when solutions of metals are introduced into the flame of an atomic absorption spectrometer (AAS) • • • • • First, the solvent evaporates, leaving behind formula units of the formerly dissolved salt. Next, dissociation of the formula units of salt into atoms occurs—the metal ions atomize or are transformed into atoms. Then, if the atoms are easily raised to excited states by the thermal energy of the flame, a resonance process occurs in which the atoms resonate back and forth between the ground state and the excited states As these atoms drop back to the ground state (a natural process), the emission spectrum is emitted. A color characteristic of the element that is in the flame. For quantification, Beer’s law, the width of the flame being the pathlength . (5 marks) d) Give 3 advantages of using a plasma source for AES High stability, low noise, low background, freedom from interferences (any 3 of these) (3 marks) e) List the 5 steps for developing a method for quantitative analysis using AES • Choosing a source for atomization and excitation • Selecting a wavelength and a slit width • Preparing the sample for analysis • Minimizing spectral and chemical interferences • Selecting a method of standardization (5 marks) f) List 2 disadvantages of using AES for analysis of samples Expensive equipment, Procedures are complicated in comparison to absorption methods, high operation costs (any 2 of these) (2 marks) Question 3 a) Draw a block diagram showing the major components of Atomic Fluorescence Spectroscopy (AFS) instrument See notes!! (5marks) b) State three problems associated with analysis using AFS Precision and accuracy are highly dependent on the atomization step, light source, Molecules, atoms and ions are all in heated medium thus producing three different atomic emission spectra (3 marks) c) Define quenching in fluorescence Quenching refers to non-radiative energy transfer from an excited species to another species (2 marks) Question 4 a) What important feature/characteristic must a molecule have for it to reflect IR absorptions? For a molecule to show infrared absorptions it must possess a specific feature, i.e. an electric dipole moment of the molecule must change during the vibration (1 mark) b) Define the fingerprint region in an IR spectrum The complex area in the IR spectrum showing many overlapping bands with wavenumbers ranging from 600-1400 cm-1. It’s the region of most single band signals (2 marks) c) Define the group frequency region in an IR spectrum This is the region of the IR spectrum where most functional groups of interests are found. This covers the range of the spectrum from 4000 to 1300 cm-1. (2 marks) d) Give the wavenumbers for the position of each of the following functional groups in an IR spectrum i) Carbonyl -1690 -1760 cm-1 ii) -OH3590-2500 cm-1 iii) -C=C1500-1600 cm-1 iv) -C=N2210-2280 cm-1 v) -C-C900-1100 cm-1 (5marks) v) Give 3 ways in which attenuated total reflection IR is better than FTIR sampling mode IR ATR mode is simpler and faster to use, requires minimal sample preparation, involves a one step process and very easy to clean up (3 marks) Question 5 a) Explain the principle upon which Raman spectroscopy is based Raman spectroscopy is based on the irradiation of a sample with monochromatic incident radiation of given frequency in the visible and near-IR spectral region. The radiation is scattered by the molecules of interest in the sample at various angles due to momentary absorption and excitation to virtual states of higher energy. The molecule then relaxes and re-emits a photon of higher or lower energy. The resultant spectrum consists of lower frequency emissions known as Stokes scattering and higher frequency emissions called anti-Stokes scattering. Elastically scattered radiation of the same frequency as the excitation beam called Rayleigh scattering is also emitted. In Raman spectroscopy, incident radiation of given frequency is scattered by the molecule at various angles due to momentary to virtual states and reemission. The energy absorbed by the molecule from the photon of light is not quantized and causes transition to an infinite number of virtual states. This is then followed by re-emission (5marks) b) Why is Raman spectroscopy considered a complementary technique to IR spectroscopy? Raman spectroscopy offers additional information about the molecular structure of the analytical sample and allows for analysis using water as a solvent (2 marks) c) List three factors that affect the intensity of Raman peaks Polarizability of the molecule Intensity of the source, frequency of the source Concentration of the active group (3 marks) d) Name the radiation source that is commonly used in Raman spectroscopy Lasers (1 mark) e) Name the detector that is commonly used for Raman Spectroscopy applications Charge-coupled devices (CCDs) (1 mark) f) List 3 areas/fields in which Raman Spectroscopy can be applied Investigations of the structures of inorganic molecules Determination of probable structures of many species Determination of dissociation constants of strong acids Provide useful information on the structure, composition and stability of coordination compounds Study of biological systems (List any 3 of these) g) Give 2 disadvantages of using Raman Spectroscopy for chemical analysis High cost of equipment Matrix effects limit applicability for quantitative analyses (2 marks) TOTAL: 60 marks Name: Description: ...
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