Transcript UV/VIS Spectrometry And Atomic Absorption By: Morgan Biehn

UV/VIS Spectrometry
And
Atomic Absorption
By: Morgan Biehn
What’s Ahead…
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Introduction to UV/VIS and AA
Procedure
Results
Error Analysis
Conclusions and Recommendations
Q&A
Introduction
Ultraviolet/Visible (UV/VIS) Spectrometry:
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A form of colorimetric analysis
Passes light through a cuvette containing
solution
Referenced to a solution that absorbs no
light (distilled water)
Beer-Lambert Law: A = αcl
In absence of α, use calibration curve
Introduction (contd.)
Atomic Absorption (AA) Spectrometry:
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Samples must undergo desolvation and
vaporization in a flame
When atoms absorb light, they transition to
higher electronic energy levels
Concentration determined from amount of
absorption
Flame AA can increase path length which
increases absorption by Beer-Lambert Law
Procedure
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Objective: Determine concentration of an
unknown solution.
Potassium Permanganate (KMnO4) solutions
5 standards prepared with concentrations of
0.1, 0.09, 0.08, 0.07, and 0.06 g/L. Each
standard tested twice
Perkin Elmer UV/VIS SP Spectrometer with
1 cm long cuvettes
Perkin Elmer AA spectrometer
Unknown solution tested five times for each
method
Results – UV/VIS
Absorbance
2
1.8
0.1 g/L
1.6
0.07 g/L
1.4
0.08 g/L
1.2
0.09 g/L
0.06 g/L
1
Unknown
0.8
0.6
0.4
0.2
0
250
300
350
400
Wavelength, nm
Figure 1: UV/VIS spectra for five standards
and unknown.
450
500
Results – UV/VIS (contd.)
1.200
1.100
Absorption @ 310 nm
1.000
Average
absorption
for unknown
at 310 nm is
0.889
0.900
0.800
0.700
y = 11.809x - 0.0328
R2 = 0.909
0.600
Calibration
0.500
0.400
0.06
Linear (Calibration)
0.065
0.07
0.075
0.08
0.085
Concentration (g/L)
Figure 2: UV/VIS Calibration Curve.
0.09
0.095
0.1
Results – UV/VIS (contd.)
Table 1: UV/VIS unknown concentration analysis.
Unknown
Calculated Concentration,
Run
Absorbance
g/L
1
0.888
0.078
2
0.889
0.078
3
0.889
0.078
4
0.889
0.078
5
0.890
0.078
0.889
0.078
Average
σ
0.053
95% CI
0.104
Actual Concentration
0.078 g/L ± 0.104 g/L
Results – AA
0.3
Absorption @ 310 nm
0.25
0.2
0.15
0.1
y = 2.99x - 0.0494
R2 = 0.9884
0.05
Calibration
Linear (Calibration)
0
0.06
0.065
0.07
0.075
0.08
0.085
Concentration (g/L)
Figure 3: AA Calibration Curve.
0.09
0.095
0.1
Results – AA (contd.)
Table 2: AA unknown concentration analysis.
Run
Unknown Concentration, g/L
1
0.070
2
0.072
3
0.076
4
0.072
5
0.075
Average
0.073
σ
0.010
95% CI
0.019
Actual Concentration
0.073 g/L ± 0.019 g/L
Error Analysis
Four types of error:
1.
2.
3.
4.
Instrument error
Fit error
Dilution error
Operator error
Error Analysis (contd.)
1. Instrument Error:
Sample standard
deviation
2. Fit Error: Use
calibration curve
linear regression

2
inst

_
1

( xi  xi ) 2
N 1
2
fit
_

1

( y  y ) 2
N
Error Analysis (contd.)
3. Dilution Error: Combine
final concentration
equation with error
propagation equation
2
2
2
 tot
  inst
  2fit   dil
2
 dil
(
c f
ci
ci ) 2  (
c f
V f
ciVi
Vf
cf 
 G 2  [
i
V f ) 2  (
c f
Vi
G
 yi ]
dyi
Vi ) 2
4. Operator Error: From comparison of previous
users’ data and use of sample standard deviation
Error Analysis (contd.)
Table 3: Summary of error analysis for both experiments.
Experiment
Error Type
Instrument Error, σ2
UV/VIS
AA
4.64E-08
7.10E-05
0.003
2.09E-05
6.44E-09
6.44E-09
Operator Error, σ2
0.021
0
Total Error, σ2
0.003
9.19E-05
σ
0.053
0.010
With 95% CI
0.104
0.019
Fit Error, σ2
Dilution Error, σ2
Conclusions
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UV/VIS and AA spectra were compared
Bad UV/VIS calibration curve produced
large error
AA data provided smaller standard
deviation than the UV/VIS
Unknown has concentration of 0.073 g/L
± 0.019 g/L (95% confidence)
AA provides a quick, easy, and relatively
painless method for determining
concentrations
Recommendations
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Monitor standards to reduce the risk of
contamination
Store standards in a dark place
Try to conduct both experiments on the
same day
Do as many tests as possible
Where We’ve Been…
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Background information on UV/VIS and
AA experimental methods
Objective for this specific experiment
and procedure to obtain objective
Discussion of Results
Discussion of error and which method
produced more accurate results
Provided recommendations
Questions?
References
Schwedt, George. The Essential Guide to Analytical Chemistry.
Wiley and Sons: Hoboken, NJ, 1997.
Tissue, Brian M. “Atomic-Absorption Spectroscopy (AA).”
http://elchem.kaist.ac.kr/vt/chem.-ed/spec/atomic/aa.htm.
Last updated 8/21/96.
Walpole, Ronald E., Myers, Raymond H., Myers Sharon L.
Probability and Statistics for Engineers and Scientists,
6th Edition. Prentice Hall: NJ, 1998.