Title: MOLECULAR SPECTROSCOPY section 1: Absorption and fluorescence spectra section 2: Determination of quinine sulphate in tonic water using fluorescence spectroscopy section 3: Quenching interference in fluorescence spectroscopy Full name: NAUMAN MITHANI Student no.: 301016320 Sections: LA02: group C Date of expt.: Jan. 24, 2008 ABSTRACT: A series of three molecular absorption-emission spectroscopy experiments were carried out with quinine sulphate as the primary object/analyte. MS 1: The objective was to verify quinine sulphate’s molar absorptivities using Beer’s law with the molecular absorption spectroscopy technique, calculated to be 51871, 12812, 7248 and 9280 L mol -1 cm -1 for wavelengths of 210, 250, 316 and 346 nm respectively. MS 2: The objective was to determine the concentration of quinine sulphate in a sample of tonic water, it was determined to be 3.371!10 -7 mol L -1 ; the technique employed was fluorescence spectroscopy. MS 3: The effect of sodium halides as quenching agents on fluorescence spectroscopy was observed.
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Title: MOLECULAR SPECTROSCOPY
section 1: Absorption and fluorescence spectra
section 2: Determination of quinine sulphate in tonic water using
fluorescence spectroscopy
section 3: Quenching interference in fluorescence spectroscopy
Full name: NAUMAN MITHANI
Student no.: 301016320
Sections: LA02: group C
Date of expt.: Jan. 24, 2008
ABSTRACT:
A series of three molecular absorption-emission spectroscopy experiments
were carried out with quinine sulphate as the primary object/analyte.
MS 1: The objective was to verify quinine sulphate’s molar absorptivities
using Beer’s law with the molecular absorption spectroscopy technique, calculated to
be 51871, 12812, 7248 and 9280 L mol-1
cm-1
for wavelengths of 210, 250, 316 and
346 nm respectively.
MS 2: The objective was to determine the concentration of quinine sulphate
in a sample of tonic water, it was determined to be 3.371!10-7
mol L-1
; the technique
employed was fluorescence spectroscopy.
MS 3: The effect of sodium halides as quenching agents on fluorescence
spectroscopy was observed.
Nauman Mithani
! "!
INTRODUCTION:
The experiment was comprised of three sub-experiments on molecular
absorption- fluorescence spectroscopy. It involves irradiating a substance/analyte
with a beam of radiation and measuring the particular magnitudes of energies
(corresponding wavelengths) absorbed and/or emitted as the substance returns to a de-
excited state.
MS 1: The first section explored the application of Beer’s law of molecular
absorption spectroscopy, it states
A = ! b c
A ! absorption
! ! molar absorptivity (L mol-1
cm-1
)
b ! path length in medium (cm-1
)
c ! concentration of medium (mol L-1
)
Quinine sulphate of varying concentrations was used as the analyte, and the
absorbance measurements were conducted with a UV-visible spectrophotometer.
MS 2: Fluorescence spectroscopy of quinine sulphate-containing liquid was
conducted using a spectrofluorometer. A sample of tonic water was analysed in order
to determine the concentration of quinine sulphate in it.
MS 3: Interference of quenching agents on the fluorescence spectra of quinine
sulphate was investigated using varying concentrations of aqueous sodium halides.
The data was processed as per the Stern-Volmer equation (adapted):
!
Ii
I f=1+Ksv Q[ ]
! #!
Ii ! intensity/rate of fluorescence without quencher
If ! intensity/rate of fluorescence with quencher
Ksv ! Stern-Volmer coefficient
[Q] ! concentration of quencher
EXPERIMENTAL:
MS 1: The sub-experiment was commenced by weighing out (5.1 ± 0.1) mg
of quinine sulphate and diluting it to mark in a volumetric flask of 50 mL with 0.05
mol L-1
H2SO4 (aq). This resulted in a quinine sulphate solution of 100 ppm
concentration (1.277 ! 10-4
mol/L). (5 ± 0.02) mL of the solution, and subsequent
solutions, were diluted to the 50 mL mark with the H2SO4 (aq) in new volumetric
flasks resulting in quinine sulphate solutions of 10, 1, 0.1, 0.01, 0.001 and 0.0001
ppm concentrations respectively.
The next step was the commencement of the molecular absorption
spectroscopy, for which the HP-8453 UV-visible spectrophotometer was employed in
conjunction with the UV-visible HP-8453 software program. A quartz cuvette (frosted
opposite faces) of 1 cm thickness (path length) was filled with the H2SO4 (aq) and
placed in the spectrophotometer for calibration (setting the baseline / zero mark) and
determination of the limit of detection. Six measurements (spectra) were taken, the
last five were deemed a consistent detector response and so the experiment moved
ahead. The cuvette was removed from the spectrophotometer, rinsed with the lowest
concentration quinine sulphate solution of 0.0001 ppm, filled with it, placed in the
! $!
spectrophotometer and its absorption spectra taken. Absorption spectra of the other
quinine sulphate solutions were taken in increasing order of concentration.
Next, quinine sulphate solutions were subjected to fluorescence spectroscopy.
A clear quartz cuvette (thickness / path length of 1 cm) was rinsed then filled with the
0.1 ppm quinine sulphate solution, placed in the spectrofluorometer and its spectra
were recorded. The emission scan parameters were 200 to 800 nm with the optimal
wavelength set at 350 nm and a step size of 2 nm. An excitation scan of the same
solution was conducted in the spectrofluorometer and its spectrum recorded; the
parameters were the same as before except the optimal wavelength, which was set at
450 nm.
MS 2: The spectrofluorometer’s parameters were changed so as to perform
time-based scans, the scan length was 40 seconds, the data acquisition rate was 1
point/second. The excitation and emission wavelengths were set at 350 and 450 nm
respectively. The first time-based scan was performed on the 0.05 mol/L H2SO4 (aq)
then the quinine sulphate (aq) in increasing concentration. A clear quartz cuvette was
used for this sub-experiment; it was first rinsed with the solution to be analysed then
filled with before being inserted in the spectrofluorometer.
A 1 mL sample of tonic water (containing an unknown concentration of
quinine sulphate (aq)) was diluted to the mark with the 0.05 mol/L H2SO4 (aq) in a
100 mL volumetric flask. A sample of this solution was placed in the
spectrofluorometer and its spectrum, fluorescence intensity were recorded.
MS 3: Fifteen 0.5 mL extractions from the 100 ppm solution of quinine
sulphate were added to new 50 mL volumetric flasks. 0.5, 1, 2, 3 and 4 mL of NaCl
(aq), NaBr (aq) and NaF (aq) of each were added to the new volumetric flasks and
diluted to the mark with the 0.05 mol/L H2SO4 (aq). This resulted in 1 ppm quinine
sulphate solutions with halide concentrations of 0.005, 0.01, 0.02, 0.03 and 0.04
! %!
mol/L (corresponding to 0.5, 1, 2, 3 and 4 mL respectively); (3 halides ! 5
concentrations each = 15 solutions). NOTE: plastic volumetric flasks were used for
quinine sulphate (aq) containing NaF (aq). Fluorescence intensity spectrum of each of
the solutions was recorded with the spectrofluorometer; the parameters were time-
based scan with the scan length of 30 seconds and data collection at 1 point/second.
! &!
DATA and RESULTS:
MS 1: --------------------------------------------------------------------------------!