The SpecUP educational spectrophotometer Dr Patricia Forbes Department of Chemistry University of Pretoria, South Africa

The SpecUP educational spectrophotometer

Dr Patricia ForbesDepartment of Chemistry

University of Pretoria, South Africa

http://web.up.ac.za/index.asp

The motivation

The analytical instrument

The motivation


Results

The motivation


Results

The motivation


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


Sample

Lens

GratingSlit

Yellow incident light

Possible components of a DIY spectrophotometer- moveable slit

Light source

Aperture


Sample

Lens

GratingSlit

Green incident light

Possible components of a DIY spectrophotometer- moveable slit

Light source

Aperture


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


Sample

Lens

GratingSlit

…or coloured filters

Light source

Aperture


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


50-160 mm 110-240 mm50-160 mm 110-240 mm


400 mm


290 mm 100 mm 340 mm 20 mm


10-125 mm 70-200 mm

Cuvette

30 mm

Top view

Side view

400 mm


290 mm 100 mm 340 mm 20 mm


10-125 mm 70-200 mm

Cuvette

30 mm400 mm


290 mm 100 mm 340 mm 20 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!

Documents

The SpecUP educational spectrophotometer Dr Patricia Forbes Department of Chemistry University of Pretoria, South Africa