Following DIY spectrometry on a tap water sample and as pursue our work on our own spectrogprah Let's work on our own Spectrograph Software version, bellow we provide the fundamental knowledge
Spectroscopy is the study of the interaction of light and matter, and so there’s a lot of forms of spectroscopy that we can use to gather data about what the structure of a molecule might be.
We’re going to look at IR spectroscopy. IR stands for infrared, as an infrared light.
Infrared light refers to electromagnetic radiation with wavenumber ranging wavelength from 0.78 – 1000 μm (corresponding from 13000 – 10 cm-1). Infrared region is further divided into three subregions: near-infrared (13000 – 4000 cm-1 or 0.78 – 2.5 μm), mid-infrared (4000 – 400 cm-1 or 2.5 – 25 μm) and far-infrared (400 – 10 cm-1 or 25 – 1000 μm)
As any other analytical techniques, infrared spectroscopy works well on some samples, and poorly on others. It is important to know the strengths and weaknesses of infrared spectroscopy so it can be used in the proper way. Some advantages and disadvantages of infrared spectroscopy are listed in table
| Advantages | Disadvantages |
|---|---|
| Solids, Liquids, gases, semi-solids, powders and polymers are all analyzed. The peak positions, intensities, widths, and shapes all provide useful information. Fast and easy technique. Sensitive technique (Micrograms of materials can be detected routinely). Inexpensive | Atoms or monatomic ions do not have infrared spectra. Homonuclear diatomic molecules do not posses infrared spectra. Complex mixture and aqueous solutions are difficult to analyze using infrared spectroscopy |
Source 13.1.16: How to Interpret An Infrared Spectrum, Libre Text Chemistry, CC BY SA NC 4.0
And our best advantage with a mobile spectroscope: We can carry out analyses directly in the field with fresh samples and a very short waiting time.
In a cuvette we load our liquid sample, then closing our dark chamber and we irradiate this sample with infrared light.
That light is going to interact in some way with the compounds in the sample, and then it’ll eventually reach a detector (our camera in the device) and could be interpreted in our software.
Some light will be absorbed by the sample, some will not, some will pass through, and this is information that we can use to figure out something about the structure of the molecule.
The reason that we use infrared light is because we know that molecules move around. They have translational motion.
We know that heat energy is converted into kinetic energy of motion. So molecules in a liquid are moving around.
We also know that bonds are rotating all the time. So there there is rotational motion for all of the all of the sigma bonds. And chemical bonds are also doing other things. As it happens, they can do these other things that that occur specifically when they are irradiated with a very specific wavelength. We want to look after thoses wavelengths using our software.
Having infrared light, compodonds can do something like a symmetric or an asymmetric stretch.
The covalent bonds can be contracting and expanding with a particular vibration, depending on whether it has absorbed this particular photon of UV light.
We have all of these different kinds of vibrational motions happening in a molecule and they occur at very particular energies. So they will occur when a wavelength of a very specific infrared light is absorbed and it will vary depending on the functional group, i.e RO−H or saturated CH.
So the identity of these atoms will will affect the photon of absorption. So this is information that we’re going to use to generat an IR spectrum.
There is a particular functional group in the molecule that is in the sample that absorbs IR light of that wave number and that is information that tells us about what kind of bonds are in that compound.
Looking across our graphical output, we can see that here’s some important data. Here are some important data.
There’s long tables of every functional group that we can imagine in a molecule and we can look after the particular wave number of IR light that is absorbed by that functional group.
This type of tables shows the wave numbers where you would tend to find a
functional groups. The exact peak depends on the exact substance. For example, the peak for the C to O bond is slightly different depending on whether we’ve got a primary or secondary alcohol and an extended large table will help us distinguish between those.
Then we look at the functional group region and we can see various peaks in our output graph and we can start interpreting and understanding what these peaks are actually telling us.
It’s not telling us the precise structure. But this could be useful, specially when when set our Absorption mode in our spectroscope. The frequencies at which absorption lines occur, as well as their relative intensities, primarily depend on the electronic and molecular structure of the sample. The frequencies will also depend on the interactions between molecules in the sample, the crystal structure in solids, and on several environmental factors (e.g., temperature, pressure, electric field, magnetic field).
We must also look at so-called fingerprint region (5-10 µm, 1000-2000 cm−1), it’s a portion of the IR spectrum. But we’ve got to be very careful about drawing too much information out of the fingerprint region.
We want to get from this very basic introduction to spectroscopy is that functional groups that absorb IR light at specific wave numbers will provide very specific peaks. Not only are they occurring at specific wave numbers, but they have characteristic shapes, and so we can take and IR spectrum and we can know roughly what kind of functional groups are in a molecule.
Like as we already did in Brussels, in Berlin and in Rennes.
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