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The SpecUP educational spectrophotometer
Dr Patricia ForbesDepartment of Chemistry
University of Pretoria, South Africa
The motivation
The analytical instrument
The motivation
The analytical instrument
Results
The motivation
The analytical instrument
Results
The motivation
The analytical instrument
And also:
•High student numbers•Cost considerations
The motivation
The concept
Third year analytical chemistry students build their own spectrophotometer using components from a kit provided.
They use their instrument to conduct experiments, ranging from fundamental to applied.
The practical session should ideally follow on from or be run in parallel to the presentation of a series of lectures on spectroscopy.
Cost of the SpecUP:
• ~R600 as compared to ~R30 000 for a commercial spectrophotometer
• Thor Labs educational spectrophotometer ~R12 000
Possible components of a DIY spectrophotometer:
Possible components of a DIY spectrophotometer
Light source
Aperture
Detectorlinked toamplifier & voltmeter
Detector = light dependent resistor. Resistance decreases when more light falls on it, thus current increases.
Sample
Lens
GratingSlit
Possible components of a DIY spectrophotometer
Light source
Aperture
Detectorlinked toamplifier & voltmeter
Sample
Lens
GratingSlit
Yellow incident light
Possible components of a DIY spectrophotometer- moveable slit
Light source
Aperture
Detectorlinked toamplifier & voltmeter
Sample
Lens
GratingSlit
Green incident light
Possible components of a DIY spectrophotometer- moveable slit
Light source
Aperture
Detectorlinked toamplifier & voltmeter
Sample
Lens
GratingSlit
The spectrum produced by the grating is projected onto graph paper to produce a wavelength scale. Calibration is preformed by eye, using a table of colour wavelength ranges.
..or a moveable grating
Light source
Aperture
Detectorlinked toamplifier & voltmeter
Sample
Lens
GratingSlit
…or coloured filters
Light source
Aperture
Detectorlinked toamplifier & voltmeter
Sample
Lens
GratingSlit
Colour filters
Disadvantages: Limited data points and low intensities
Main limitation of this design: most components are fixed…
The spectrophotometer showing LED, LDR, amplifier and sample cuvette
Tavener, S.J. and Thomas-Oates, J.E., 2007, Education in Chemistry, 44, 151-154.
…and the liquid sample is on the electric circuit board…
Which spectrophotometer design?
Depends on the target audience:• Primarily analytical chemistry students• Other disciplines which use spectrophotometry include:
• Physics• Pharmacy• Geography (e.g. sun photometer)• Environmental science• Food science• Biochemistry• Electrical engineering• Computer science
Electronics
Spectrophotometer circuit diagram
Yeh, T.-S., S.J. and Tseng S-S., 2006, Journal of Chinese Chemical Science, 53, 1067-1072.
Electronics
Spectrophotometer circuit diagram
Final design…the SpecUP
But what is inside?....
The SpecUP
Components of the SpecUP
50-160 mm 110-240 mm
LED Lens Grating Cuvette LDR
400 mm
LED Lens Grating LDR
290 mm 100 mm 340 mm 20 mm
Fully retracted: 800 mmFully contracted: 540 mm
10-125 mm 70-200 mm
Cuvette
30 mm
Top view
Side view
50-160 mm 110-240 mm
LED Lens Grating Cuvette LDR
50-160 mm 110-240 mm50-160 mm 110-240 mm
LED Lens Grating Cuvette LDR
400 mm
LED Lens Grating LDR
290 mm 100 mm 340 mm 20 mm
Fully retracted: 800 mmFully contracted: 540 mm
10-125 mm 70-200 mm
Cuvette
30 mm
Top view
Side view
400 mm
LED Lens Grating LDR
290 mm 100 mm 340 mm 20 mm
Fully retracted: 800 mmFully contracted: 540 mm
10-125 mm 70-200 mm
Cuvette
30 mm400 mm
LED Lens Grating LDR
290 mm 100 mm 340 mm 20 mm
Fully retracted: 800 mmFully contracted: 540 mm
10-125 mm 70-200 mm
Cuvette
30 mm
Top view
Side view
Components of the SpecUP
Two modes of operation….
1. Coloured LED with no diffraction grating
Table 1: LEDs to use for different colour solutions.
Solution colour LED colour
Green Red
Blue or purple Yellow
Red Green
Yellow or orange Blue
Two modes of operation….
2. White LED with diffraction grating & manual adjustment
Table 2: Wavelengths of colours.
Colour observed
Wavelength Range (nm)
Mean wavelength(nm)
Violet 400-430 410
Blue 430-490 470
Green 490-570 520
Yellow 570-595 580
Orange 595-650 610
Red 650-700 650
http://www.colour.org.uk/spectrum_chart%201.jpg
Alternatively a colour chart can be used, for example:
Cost of the SpecUP:
• Main cost components of the SpecUP are the:• Aluminium plate• Batteries• Multi-meter• Circuit board
Applications
Absorbance calibration & Beer Lambert Law
• Molar absorption coefficient is determined from the slope of line of concentration vs A for standard solutions
• Then determine concentration of
unknown samples Calibration plot of absorbance versus concentration for solutions
of KMnO4 (Tavener & Thomas-Oates, 2007)
SpecUP results
Green food colourant
SpecUP results
Red food colourant
SpecUP results: Construction of a spectrum
KMnO4 solution
Vzero, Vwater & Vsample must be measured at each wavelength to calculate absorbance
Or spectrum obtained using coloured LEDs
KMnO4 (2.09x10-3 M)
Colour LED Wavelength (nm)Absorbanc
eBlue 470 0.21
Green 520 1.02Yellow 580 0.45
Red 650 0.20
Reaction kinetics…the iodine clock reaction
• In the first step, iodine is generated from the iodide ion by reaction with persulphate
2 I− + S2O82− → I2 + 2 SO4
2− (1)
• In the second step, the iodine reacts with thiosulphate
I2 + 2 S2O32− → 2 I− + S4O6
2− (2)
• Yellow colour of iodine is detected using blue LED
]][[ 82 IOSkrate
Additional applications
Determination of metal ion concentrations• Environmental chemistry applications (waste water
testing)• Based on absorption of coloured metal complexes• Suitable wavelength LEDs are employed• Interference effects can be studied
Hauser, P.C., and Rupasinghe, T.W.T., 1997, Fresenius J. Anal. Chem., 357, 1056-1060.
Concepts to be covered in all experiments:
• Resolution (eg: relationship between slit width and spatial resolution)
• Sensitivity (eg: relationship between slit width and spectral intensity)
• Selectivity (eg: differences between diffraction orders)• Accuracy (comparison to commercial instruments)• Precision and repeatability• Limitations and sources of error
Results of repeatability experiment:
Educational outcomes include:
• Hands-on experience wrt workings of the instrument & its components (including setup and adjustment)
• Experience with calibrating the instrument• Understanding of relationship between absorption of light
& concentration• Understanding of analytical concepts of resolution;
selectivity; sensitivity; accuracy & precision• Specific outcomes for each application experiment
Focus is on inquiry-based learning
Conclusion
Advantages of the SpecUP:• Low cost• Simple to construct• Open design• Moving components• Generates useable analytical results• Allows for inquiry-based learning
Implementation of the SpecUP
Workshop at UP, November 2013
Workshops in Tunisia, March 2014 & 2015
Implementation of the SpecUP in the Analytical Chemistry III course at UP
• Forty students• Work in groups of 3 students• Mix of commercial spectrophotometer & SpecUP• Some limitations identified and improvements made
Patricia B.C. Forbes and Johan A. Nöthling, Shedding light on spectrophotometry: the SpecUP educational spectrophotometer, South African Journal of Science, 2014, 110 (1/2), 1-5,
http://dx.doi.org/10.1590/sajs.2014/20130096
Questionnaires
Before SpecUP use
Q5: Have you ever used a commercial spectrophotometer?Q6: Have you ever used the SpecUP spectrophotometer?
Q7: Have you been taught the theoretical aspects of spectrophotometry in lectures?
Before SpecUP use
Q3: Spectrophotometers do not have moving components.Q5: The only thing that is needed to be done to obtain a result from a spectrophotometer
is to place the sample cuvette inside the instrument.Q9: I would recommend that all chemistry students get hands-on experience
with spectrophotometers.
After SpecUP use
Q2: The SpecUP helped me to understand spectrophotometry.Q3: I enjoyed moving the components of the SpecUP to see the effect it had on the results.
Q5: The SpecUP made me think about how a spectrophotometer works.
Acknowledgements
• CSIR National Laser Centre & the African Laser Centre• Dr Paul Motalane• Prof. Andrew Forbes• Thomas du Plooy
• Prof Mourad Zghal• Leon Engelbrecht, Nico Van Vuuren (University of Pretoria)• Phakama Botha• Monné van der Linde• Prof. Thomas-Oates (University of York)
ChromSAAMS 2012 Conference, South Africa
Thank you!