UV/VIS Spectrometry

And

Atomic Absorption

By: Morgan Biehn

What’s Ahead…

Introduction to UV/VIS and AA Procedure Results Error Analysis Conclusions and Recommendations Q&A

Introduction

Ultraviolet/Visible (UV/VIS) Spectrometry:

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

Atomic Absorption (AA) Spectrometry:

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

Introduction (contd.)

Procedure 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

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

250 300 350 400 450 500

Wavelength, nm

Ab

sorb

ance

0.1 g/L

0.07 g/L

0.08 g/L

0.09 g/L

0.06 g/L

Unknown

Figure 1: UV/VIS spectra for five standards and unknown.

Results – UV/VIS (contd.)

Average absorption for unknown at 310 nm is 0.889

Figure 2: UV/VIS Calibration Curve.

y = 11.809x - 0.0328

R2 = 0.909

0.400

0.500

0.600

0.700

0.800

0.900

1.000

1.100

1.200

0.06 0.065 0.07 0.075 0.08 0.085 0.09 0.095 0.1

Concentration (g/L)

Ab

sorp

tio

n @

310

nm

Calibration

Linear (Calibration)

Results – UV/VIS (contd.)

RunUnknown

AbsorbanceCalculated Concentration,

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

Average 0.889 0.078

σ 0.053

95% CI 0.104

Actual Concentration 0.078 g/L ± 0.104 g/L

Table 1: UV/VIS unknown concentration analysis.

Results – AA

Figure 3: AA Calibration Curve.

y = 2.99x - 0.0494

R2 = 0.9884

0

0.05

0.1

0.15

0.2

0.25

0.3

0.06 0.065 0.07 0.075 0.08 0.085 0.09 0.095 0.1

Concentration (g/L)

Ab

so

rpti

on

@ 3

10 n

m

Calibration

Linear (Calibration)

Results – AA (contd.)

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

Table 2: AA unknown concentration analysis.

Error Analysis

Four types of error:

1. Instrument error 2. Fit error 3. Dilution error 4. Operator error

Error Analysis (contd.)

1.Instrument Error: Sample standard deviation

2. Fit Error: Use calibration

curve linear

regression

2_

2 )(1

1iiinst xx

N

2_

2 )(1

yyNfit

2222dilfitinsttot

Error Analysis (contd.)

4. Operator Error: From comparison of previous users’ data and use of sample standard deviation

3.Dilution Error: Combine final concentration equation with error propagation equation

][2yi

i iG dy

G

f

iif V

Vcc

2222 )()()( ii

ff

f

fi

i

fdil V

V

cV

V

cc

c

c

Error Analysis (contd.)  Experiment

Error Type UV/VIS AA

Instrument Error, σ2 4.64E-08 7.10E-05

Fit Error, σ2 0.003 2.09E-05

Dilution Error, σ2 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

Table 3: Summary of error analysis for both experiments.

Conclusions

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

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…

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?

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.

References