Public Lab Research note

The importance of infrared radiation:INFRARED SPECTROSCOPY

by homechemist | | 5,335 views | 6 comments |

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homechemist was awarded the Basic Barnstar by warren for their work in this research note.

What I want to do

As we Know ,Spectroscopy is a very powerful technique for analytical chemistry, it help us to understand how light interacts with a certain compound or element allowing us to identify it, there are several types of spectroscopy, the most common one is visible spectroscopy which allows us to analyze in the visible range, (from 400-700 nm) this is good specially for colorimetric analysis and determination of the amount of analyte in solution, for element detection or specific compounds which emits/absorbs in visible light.., but one of the most important types of spectroscopy is infrared spectroscopy which works in the infrared region, infrared light is associated with vibrations, that’s why we usually associated infrared light with heat(thermal motion) , so let's go farther and lets imagine a molecule as a spring, this molecule is able to vibrate among the different bonds between atoms, some bonds are able to rotate , while others cannot depending on the electronic overlapping of the atoms that are bond to form the molecule, so let´s consider a vibration of atoms with respect to a certain bond, if a pulse of infrared radiation reaches the compound this vibration can be absorbed producing great variations on the infrared spectrum of the compound so we can conclude that infrared light is as a consequence the most cheap and non destructive analytical method that we can use to detect functional groups, and consequently to detect the presence of organic molecules among other analytes.The problem is that infrared spectroscopy instruments are extremely expensive , so the aim of this note would be to develop a powerfull but cheap spectroscope in order to be able to detect more infrared radiation than what we can detect with nowaday´s spectroscopes.

My attempt and results

As an example I want to show you two different solutions shown bellow, they are complete different from the chemical composition point of view, as you can see both interacts with the light in the same way in the visible range(both are blue). That´s meaning that they interact with photons in the visible range exactly in the same way, and as a consequence at our eyes they seem to be almost the same solution, so with visible spectroscopy we cannot notice any difference at all.


Let´s see now, the same solutions but with a near infrared camera which is able to see from 700-1000nm , a range that our eyes cannot see


As we can notice, one of the solution is black under the infrared , while the other is colorless, that´s indicating us that one solution is absorbing all the infrared light that is coming to the test tube and additional vibrations are produced, however the other solution cannot absorb the infrared radiation at this range so all of the photons between 700-1000 nm can pass trough it without been absorbed , remember that a photon is absorbed if its energy coincides with the difference in energy between two energetic levels involved in the electronic transition. In fact, the solution on the right is simply an aqueous solution of an organic dye, while the solution in the left is also an aqueous solution but in this case of an inorganic slat, cooper sulfate penta hydrated, when cooper sulfate is dissolved in water , it dissociates into Cu 2 + ions as well as sulfate ions, then the cation gets solvated and it´s surrounded by water molecules , this is what makes the solution blue, however cooper is a metal , and metallic solutions tend to block infrared light , that´s why it seem black under the infrared cam, while the other solution is colorless.

From a more technical point of view, the spectrum obtained for the transmitted light that passed through the solution is the following.


Blue plot represents the transmitted light of the solution that contains a blue dye

Red plot represents s the transmitted light of the solution that contains the copper sulfate aqueous solution.

As we can see form the picture both solutions interact with electromagnetic radiation in the same way in the visible range that´s why at our eyes they seem exactly the same solution, However above 700 nm more or less the cooper sulfate solution absorbs almost all the light in the infrared region , while the other solution transmitted almost all the radiation coming out from the bulb. That´s why infrared is so important for detection of functional groups.

The work range of this very simple device that I have designed is okay, but is not large enough to detect most of the functional groups present , the idea is to start a project to try to go farther and be able to look at lower frequencies, so we would be able to detect in the infrared range much more contaminants and chemicals in solution , than what we can do nowadays, It would be a nice idea a collaboration in between the spectroscopy and the infrared cam projects to try to achieve a camera able to detect more infrared light than what at the moment our cams are able to see.

As another example which for me is more interesting is the result that I have obtained when comparing chloroform and water, both are completely colorless, and in fact they behave almost equal at any wavelength, but I could notice a small difference in the infrared region.


If we magnify the infrared region we can even appreciate it better


This small difference would be enough to detect the presence of dangerous solvents in solution or other organic solvents , which would be very hard to identify with any other conventional technique

Questions and next steps

As future projects related with this one I would like to contribute with this research looking for more variations on the near infrared spectrum, of several compounds that I have available such as ketones , alcohols, esters and aldehydes and then compare it with the reference ( water) to try to stay if there is a significance difference in the spectrum or not in order to be able to detect a certain functional group in our problem solution.

It would be very nice as I have said a collaboration between the infrared cam project and the spectroscopy one to contribute to the design of a more sensitive instrument able to detect wider region of the infrared spectrum.

If you have any suggestion , comment or question please send me a comment or get in contact with me



metallic solutions tend to block infrared light

Really, even under 1000nm? Could this help to detect heavy metal contamination in water, or does it have to be very concentrated, or perhaps you couldn't tell different metals apart?

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not large enough to detect most of the functional groups present

What range would we need? I understood that the important fingerprint range is 1600-2000nm, but I'm surprised to see you can detect chloroform under 1000. What is your device like? Was it based on the Public Lab spectrometer?

Thanks for sharing!

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The device is based on the public´s lab one, is one of the project that i posted time ago, the only thing that changes is the calibration(I used two lasers) and the software.

About the metal detection, I have to test more samples( I want to test iron , nickel and silver at least) but the solution that I show you in the photo is not concentrated, so I suppose that you can detect the presence of metals in solution, however unless this metals were rare metals which has very specific fingerprint you probably won't be able to differentiate in between metals, if you want to detect metals specifically, I will suggest to use a selective precipitation method, is a really very well-known method, is very accurate and very simple and we can detect almost all the most common metals with some basic chemicals.

About the other result i have obtained with the chloroform, I am also very surprised but i have done it twice and i obtained the same result, I have even obtained another interesting difference between ice and water, I have looked on net and there is supposed to be a small difference due to the short or large range order arrangement of water molecules in the sample,but the difference is much smaller than the one that i have obtained with the chloroform and the water(it might also be an experimental error)

About the range , ideally it would nice to reach around 2000 nm at that point , theoretically some characteristic peaks of multiple bonds will be significant enough to detect some functional groups.

Very interesting. What was the concentration of the copper solution?

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Well, actually i cannot tell you exactly the concentration because i have prepared the solution roughly , but i think is more or less between 0,30 and 0,40 molar.

Anyway, tomorrow if I have time I am going to upload a new research note on chromatography that i think it will be really interesting for detecting and isolating compounds,and coming back to the metal detection i have carried out an specific quantitative analysis of metallic cations in solution some time ago and as i have told you before i would suggest you to use them, they are very simple chemical reactions but at the same time very precise, and not only for metal cations but there are also well-Known specific reactions to detect the presence of specific groups such as phenols , that produce a color change in the visible range that would allow us to determine not only the presence but also the amount of analyte in solution.

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