Measurement Science Lab: Introduction

People Involved

Course Co-ordinator:		Dr. Joshua Edel		joshua.edel@imperial.ac.uk
		Dr. Kristelle Bougot-Robin		k.bougot-robin@imperial.ac.uk
Demonstrators:		Silvia Di-Lecce		silvia.di-lecce12@imperial.ac.uk
		Raquel Fraccari		r.fraccari12@imperial.ac.uk
		Markéta Kubánková		m.kubankova13@imperial.ac.uk
		Ali Magness		alastair.magness12@imperial.ac.uk
		Hugh Sowley		hugh.sowley12@imperial.ac.uk
Technicians:		Simon Bastians		s.bastians@imperial.ac.uk
		DeeJay Kristnah		d.kristnah@imperial.ac.uk
		Simon Turner		s.t.turner@imperial.ac.uk

Safety

The lab course is designed to be an enjoyable experience however as with any lab course safety is the number one priority and there are some simple safety rules:

1. YOU MUST WEAR A LAB COAT AND GOOGLES AT ALL TIMES IN THE LABORATORY.

1. YOU MUST NOT EAT OR DRINK IN THE LABORATORY OR BRING ANY FOOD IN WITH YOU.

1. YOU MUST NOT USE YOUR MOBILE PHONE IN THE LABORATORY.

In addition to these rules you must refresh yourself of all the safety regulations from the Foundation Laboratory Course and complete a risk assessment before carrying out any practical work.

Experimental Objectives and Timetable

The experiment is divided into four parts:

1. Building of a UV-Vis spectrometer using a combination of optical components, computer acquisition hardware, and LEGO!
2. How low can you go? Optimisation of the sensitivity and maximising the signal to noise using the dye Methylene Blue.
3. Recording of a complete UV-Vis Spectrum of Methylene Blue. Results to be compared using a commercial instrument.
4. Following a reaction to explore the kinetics of the reduction of methylene blue to leucomethylene blue.

Proposed Timetable

This lab is designed as a problem solving exercise and will run for 2 weeks. There is only one experiment in this lab that is to build and test a spectrometer based on the information held in this lab manual. The lab is designed to take the full 2 weeks and we recommend that you plan you time carefully a proposed time plan could be:

	Monday	Tuesday	Thursday	Friday
Week 1	Initial Spectrometer Design	Build and Improve Initial Design	Perform Concentration Studies	Optimise Spectrometer
Week 2	Perform concentration studies	Generate Absorbance spectra	Comparison with commercial instrument	Present final design

Assessment

This lab has been designed so that you can improve your measurement and data handling skills whilst at the same time understanding that instruments such as a UV-Vis spectrometer should not be treated as a black box. As such no formal mark will be given but rather the groups will be judged based on spectrometer design, discussions with assessor, and quality of experimental data. The winning teams will receive a departmental certificate and vouchers.

Course Materials

As well as this Wiki there is a lab manual. This wiki contains the instructions from the manual as well as downloads of iPython notebooks for use in data analysis and more information about the hardware and software available to you. Please supplement the information on the wiki and in the manual with your own research.

The Wiki Format

Creating a page:

1. In the address box, type something like wiki.ch.ic.ac.uk/wiki/index.php?title=MSL:XYZ1234
2. The characters MSL indicate a report associated with the Measurement Science Lab and XYZ1234 is your secret password for the report. It can be any length, but do not make it too long! It should then tell you there is no text in this page. If not, try another more unique password. You should now click on the edit this page link to start. Use a different address for each module of the course you are submitting.
3. It is a good idea to add a bookmark to this page, so that you can go back to it quickly.

Equipment

The exact setup for this experiment is largely left up to the student, with some guidelines provided within sections that must be completed in order to proceed. The following equipment should be at the experimenter’s disposal:

Assessment

The lab course will be assessed by judging the final spectrometer that you design, it is important that you are able to present details of your design iterations and the results that you have obtained. To do this you must keep and present a detailed lab book, the form of your final report is left up to you to decide in your group you can give a short (10 slides maximum) Powerpoint presentation, submit a written report or submit a wiki link. T

Safety

If you do not follow these simple safety instructions you will be asked to LEAVE the lab.

This lab course is designed to be one that you enjoy however safety is still paramount, please follow these simple rules to keep yourself and others safe.

• YOU MUST WEAR A LAB COAT AND SAFETY GLASSES AT ALL TIMES IN THE LAB.
• You must not eat or drink in the lab, or bring any food in with you.
• The use of mobile phones is not allowed in the lab.
• The saftey regulations given to you at the beginning of the Foundation Course still apply.
• You must carry out a risk assessment and get it signed before beginning any practical work
• test

In addition to these rules you must refresh yourself of all the safety regulations from the Foundation Laboratory Course and complete a risk assessment before carrying out any practical work.

If you do not follow these simple safety instructions you will be asked to LEAVE the lab.

The Brief

In this lab course you will build your own spectrometer that must be able to:

1. Record the absorbance spectrum for a range of concentrations;
2. Record the angular dependence of absorbance;
3. Follow a colour change reaction.

To build your spectrometer you will be supplied with a photodiode, a box of Lego, a black out cloth, a light source, a multi-meter, a power source and a Raspberry Pi. You can use as many or as few of these items as you like, you are also welcome to make additions to the kit. In some circumstance we may even be able to order additional parts at your request if you can justify their procurement. This lab course is about problem solving and therefore the lab script is not traditional and does not give you a list of steps to follow. Instead it gives an overview of the key parts of a spectrometer and the main points you need to think about in your design.

Designing a Spectrometer

All spectrometers have four basic components, a light source, a monochromator or diffraction grating, a detector and a device to read the detected signal. Before you begin building your spectrometer it is important that you design your spectrometer to make sure it will fulfil the brief’s requirements. This lab course is designed as a problem solving activity so this manual will not tell you how to build a spectrometer but will give you guidance and advice so that your spectrometer is as effective as possible.

IMAGE OF SPECTROMETER

The Lego is for you to build the structure of your spectrometer the spatial design of your spectrometer is up to you. However, remember that it must be able to record a spectrum as a function of angle. The light source is powered by a battery pack and used to provide a broadband beam, this is a fixed part and your group must decide how to include it in your design. The diffraction grating is supplied as a monochromator and should be placed after the sample but before the photodiode. The photodiode is used to convert a photon count into a voltage that can be recorded to construct a spectrum. The device that you use to record the voltage can be either a multimeter or a Raspberry Pi. We strongly advise that you build and test your device with a multimeter before moving to a Raspberry Pi.

The photospectrometer that you create must be able to approximate the extinction co-efficient for a range of substances. This is a simple process that can be calculated from the gradient of a plot of absorbance against concentration by applying the Beer-Lambert Law:

$A = \varepsilon c l$

Where $A$ is the absorbance, $\varepsilon$ is the extinction co-efficient,$c$ is the concentration and $l$ is the path length. A plot of absorbance against concentration will give a straight-line which can be fit to $y=mx$ where $m=\varepsilon l$can be obtained. The path length is the size of your cuvette and so the extinction co-efficient is readily found. The Beer-Lambert law is not applicable at all concentrations since the absorbance will plateau at some concentration rather than continue to infinity. The absorbance is related to the intensity of light:

$A = -\log_{10}\left|\frac{I}{I_0}\right| = \log_{10}\left|\frac{I_0 - I_B}{I - I_B}\right|$

The intensity of the light, $I$, is normalised to the intensity recorded for the solvent (or when the concentration is zero), $I_0$. The photodiode will produce some voltage even in the dark this is known as the dark count, $I_d$, and should be subtracted from each reading. The photodiode converts intensity or photon count into a voltage and therefore can be universally replaced with for voltage to give:

$A = \log_{10}\left|\frac{V_0 - V_B}{V - V_B}\right|$

The light source that you are using is a broadband source that emits white light this light is made up of a spectrum. As the light passes through the sample one particular frequency of light will be absorbed as it provides the required energy for an electronic transition. The light absorbed by solutions of a particular colour can be found from the chemistry colour wheel. A solution of one colour absorbs light of the opposite colour on the colour wheel. To make reduce the signal to noise ratio it is better to concentrate only the frequencies that you are interested in, so if you had a violet solution you would want to record the change in the intensity of yellow light for the optimal result. In a spectrometer the component that selects wavelength is known as a monochromator, the monochromator you will be using a diffraction grating as a monochromator. This delivers a particular wavelength, $\lambda$, based on the angle of the light passing from the grating, $\theta$, and the periodicity of the diffraction grating, $d$.

$d\sin\theta_m = m\lambda$

In this case $\pm m$ since the sine function is odd the $\pm$ symbol can be dropped and $m=1$, so the wavelength that will be transmitted after light has passed through your diffraction grating is $\lambda = d\sin\theta$. So if we had a diffraction grating with a period of 1000 nm and were interested in a violet solution we would want our photodiode to be at an angle of 35$o$ relative to the sample so that it would be detecting the change in yellow light ( 570-590 nm). You must look at the colour of the solution you are testing then position your diode at the optimal angle. This means that your diode must be fixed to a moving arm that is capable of sweeping across a range of angles something that is also vital for task 2. Using this information you should design a spectrometer, remember that science is an iterative process, so you should design, build, test and improve your spectrometer to achieve the optimal results.

Recording Voltage

To turn your spectrometer into a useful device you must connect the photodiode to something that is able to record voltage. In the first instance you should use a multimeter and record the voltage from your photodiode and then analyse the data to produce plots and fits. An iPython Notebook for this course is available from the wiki, however you are free to use any program you like. After you have completed your tests think about how to improve your spectrometer. Once you have a design that you are happy with and that satisfies the brief then you can upgrade your multimeter to a Raspberry Pi and use this to record your voltage and analyse the data on the fly.

Using a Multimeter

Connect the red input to the red diode output and black input to the black diode output. Make sure the multi-meter is set to mV and then record the voltages on the screen in your lab book.

Using a Raspberry Pi

Once you are at the stage where you are ready to use a Raspberry Pi please follow these instructions on how to use and setup a Raspberry Pi.