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MSC701S- MOLECULAR SPECTROSCOPY AND CHEMICAL SEPERATION METHODS -2ND OPP - JULY 2022 |
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-
NAMIBIA UNIVERSITY
OF SCIENCE AND TECHNOLOGY
FACULTY OF HEALTH, APPLIED SCIENCES AND NATURAL RESOURCES
DEPARTMENT OF NATURAL AND APPLIED SCIENCES
QUALIFICATION: BACHELOR OF SCIENCE
QUALIFICATION CODE: 07BOSC
LEVEL: 7
COURSE CODE: MSC701S
COURSE NAME: MOLECULAR SPECTROSCOPY AND
CHEMICAL SEPARATION METHODS
SESSION: JULY 2022
DURATION: 3 HOURS
PAPER: THEORY
MARKS: 100
SUPPLEMENTARY/SECOND OPPORTUNITY EXAMINATION QUESTION PAPER
EXAMINER(S) | DR JULIEN LUSILAO
MOpDERATOR: | DR STEFAN LOUW
INSTRUCTIONS
1. Answer ALL the questions in the answer book provided.
2. Write and number your answers clearly.
3. All written work MUST be done in blue or black ink.
PERMISSIBLE MATERIALS
Non-programmable calculators
ATTACHMENTS
List of useful formulas and constants
THIS QUESTION PAPER CONSISTS OF 8 PAGES (Including this front page and attachments)
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Question 1
[25]
1.1 Define the following terms
(a) Sensors
(2)
(b) A readout device
(2)
(c) Absorbance
(2)
1.2 It is critical in UV-Vis to measure the 100% transmittance (100%T). This measurement
is always carried out with a sample blank. Provide a clear explanation of the
relevance of 100%T in UV-Vis and the reason why a blank is used for that
measurement.
(5)
1.3 The following diagram describes different phenomena that occur when the
electromagnetic radiation interacts with the matter
B
a Cc
Thermal, electrical,
or chemleal energy
Ionctideent
a ASE
Treasndsimziitiotned ;
ey7es
Po
aSae g
5 Ffaadciiataioln }q
Name the phenomena described in A to C and explain your choices.
(6)
1.4 Differentiate between
(a) Continuum and line sources of electromagnetic radiation.
(4)
(b) Monochromator and polychromator.
(4)
Question 2
[25]
2.1 The following diagram represents a spectrophotometer used for measuring
Phosphorescence:
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Excitating Sample
Measuring Emission
from
\\*
n:
from
“Ke
source
open
at
source — blocked
blocked
to
detector
choppers
open
to
detector
Briefly explain how it operates.
(5)
2.2 The following questions are related to the use of plasmas as thermal sources of
energy
(a) Use the Boltzmann distribution equation to explain why a plasma provide a better
atomization as compared to both flame and graphite furnace.
(5)
(b) Besides improving excitation and ionization efficiencies, what other advantage
does the higher temperature of plasmas have over flames and furnaces?
(2)
(c) Name three types of plasma sources used in optical emission techniques.
(3)
2.3 The burner assemblies of atomic absorption spectrometers (AAS) are known to
provide a long optical pathlength as well as a stable flame and they can also move
horizontally and _vertically. Explain the importance of the underlined properties in
the statement above.
(4)
2.4 The table below shows atomic emission lines for a Cr hollow cathode lamp.
A
(nm)
357.9
425.4
429.0
520.5
520.8
Slit width
(nm)
0.2
0.2
0.5
0.2
0.2
mg Cr/L giving
A=0.20
2.5
12
20
1500
500
Po
(relative)
40
85
100
15
20
(a) Which analytical technique uses hollow cathode lamps and why?
(2)
(b) For the Cr hollow cathode lamp in the table above, which wavelength provides
the best sensitivity? Explain your answer.
(2)
(c) When analyzing samples containing =10 mg Cr/L, which wavelength(s) would you
expect to provide a reasonable sensitivity with less uncertainty in the measured
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absorbance? Explain your choice.
(2)
Question 3
[25]
3.1 How do you adjust the selectivity, a, in gas chromatography (GC) and in liquid
chromatography (LC)?
(4)
3.2 The figures below show how the mobile phase pH affect both (a) the retention time
of the solutes, t;, and (b) their selectivity, a.
(a) 50-
1
40-7
e=
=
E=
=co
2wo
T
3.5
I:benzoic acid
2: terephthalic acid
3: p-hydroxybenzoic acid
4: p-aminobenzoic acid
(b) 2.0-
1.8
1.64
==
14 4
1.24
T
T
T
4.0
4.5
5.0
PH of mobile phase
1.04
T
T
55
3.5
a
1: benzoic acid
23
2: terephthalic acid
3: p-hydroxybenzoic acid
4: p-aminobenzoic acid
Ch24
PH windows
(34
Tb, atl
T
T
T
T
4.0
45
5.0
5.5
PH of mobile phase
(a) What unwanted situations would occur if the analysis was carried using a mobile
phase
(i) at pH between 5.0 and 5.5?
(1)
(ii) at pH 3.5?
(1)
(b) Figure (b) is also called a window diagram and is used to find the optimum
separation by plotting a for each pair of solutes. Explain, using this figure, what
would the optimum mobile phase pH be to obtain the best chromatographic
separation of these 4 components.
(4)
3.3 The following figure shows the effect of flow rate on the column’s plate height.
Flow rate (mL/min)
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(a) In the Van Deemter Equation: H = A + B/u + Cu, which parameter is affected by
changing the flow rate of a chromatographic system and what is that parameter
called?
(2)
(b) Explain the change (i.e. decreasing or increasing trend) observed in the size
of the plate height when increasing the flow rate.
(2)
(c) Which optimum flow rate would you use to obtain the best efficiency with the
three particle sizes in the figure above and why?
(2)
(d) Assuming you have two columns with particle sizes 3 um and 10 um. Which one
would you choose to carry out your analysis (assuming all other characteristics
are similar for both columns) and why?
(3)
3.4A 1.5 m column has 45000 plates per meter. Analyte A has a retention time of 16.3
minutes, analyte B has a retention time of 17.1 minutes and the dead (or void) time
is 5.14 minutes.
(a) What is the resolution for A and B on this column?
(4)
(b) What is the selectivity of the column ?
(2)
Question 4
[25]
4.1 Briefly explain how solutes separate in a mixture when using the following
chromatographic techniques
(a) Adsorption chromatography
(2)
(b) Partition chromatography
(2)
(c) lon-exchange chromatography
(2)
(d) Size-exclusion chromatography
(2)
4.2 Name and briefly explain the different injection systems used in GC.
(6)
4.3 Two HPLC systems are defined below, and the following compounds are to be
separated
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HO
OH
OH
NO»
Resorcinol
Phenol
Nitrobenzene
HPLC system 1: uses silica stationary phase with hexane as the mobile phase.
HPLC system 2: uses Cig stationary phase with a mobile phase consisting of 30 %
water and 70% acetonitrile.
a) Name the mode of separation (normal phase/ reverse phase) using system 1. (1)
b) Name the mode of separation (normal phase / reverse phase) using system 2.
(1)
c) Predict the elution order for the three compounds in a chromatogram using
system 2 and explain your choice.
(3)
4.4 Given the diagram below, explain the elution order of cations, neutral solutes and
anions in electrophoresis.
cation © —Ve_ of —Vep
Vtot
neutral @_ —— Veof—
Vtot
ani.on 6 : = VtotVeof Vep
(6)
END
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Physical Constants
Gas constant
Boltzmann constant
Planck constant
Faraday constant
Avogadro constant
Speed of light in vacuum
Mole volume of an ideal gas
Elementary charge
Rest mass of electron
Rest mass of proton
Rest mass of neutron
Permitivity of vacuum
Gravitational acceleration
Conversion
1W
1J
Factors
1 cal
leV
1Latm
1atm
1 bar
aL
1 Angstrom
1 micron (1)
1 Poise
1 ppm
Selected Formulae
R
k
h
F
LorNa
Cc
Vin
e
Me
Mp
Mn
Eo
g
= 8.315) K+ molt
= 8.315 kPa dm? K? mol
= 8.315 Pam? K? mol?
= 8.206 x 10% Latm K? mol?
= 1.381 x 1073 J K?
= 6.626 x 1074J s?
= 9.649 x 10*C mol?
=6.022 x 1073 molt
= 2.998 x 10®8ms?
= 22.41 L mol (at 1 atm and 273.15 K)
= 22.71 L mol (at 1 bar and 273.15 K)
= 1.602 x 107°C
= 9.109 x 1034kg
= 1.673 x 10°’ kg
= 1.675 x 10°’ kg
= 8.854 x 10°C? J+m? (or F m+)
= 9.807 ms?
=i1js4
=0.2390 cal=1Nm=1VC
= 1Pam?=1kgm?s?
= 4.184J
= 1.602 x 10°79)
= 101.3 J
= 1.013 x 10° N m? = 1.013 x 10° Pa
= 760 mmHg
=1x10°Pa
= 107% m? =1 dm?
=1x107m=0.1 nm = 100 pm
=10°m=1um
=0.1 Pas=0.1 N sm?
= 11g g*=1mgkg*
= 1 mgL (dilute aqueous solutions only)
R.=
‘B _ mn
=
2A1,
Ris = vN Xx cal
= 0.5(w,+w,) wiztw,
4
a
x ts
L-+ &
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1—
'
“of
t
ba
bn
ak
k,
tn,B —t m
Fea Fm
2
N=16/+t
w
q=nF
AG =-nFE
1=E/R
E = E°—RT/nF In [B]®/[A]?
E (for ISE): Ecen = K + 0.05916/z log[A]
E = hv (or E = hc/A)
A=-logT=logPo/P and A-=ebc
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