FTIR Conclusion

Welcome back to the conclusion of how a Fourier Transform Infrared Spectrophotometer works.
 
First I would like to give you a pep talk in that even in the sciences, very few scientists how actually use the FTIR really have a good idea of how it works.  Kind of like a car driver, they can drive the car, but might not know very much about what is going on with all the parts.  So, if you do grow to  understand then you can count yourself in the top 1 percent.
  
Now If you were to try to detect infrared light directly, it IS a wave, so in itself, you could get a rising and falling signal, it is just we have no detectors fast enough(to detect the individual risings and fallings of the infrared light wave).  

  Now we do have detectors like our cell phones are detectors, they detect frequencies around 2 or 3 gigahertz, which means 2 or 3 billion 'waves' per second.   I think Infrared is just out of reach, at 100 or 1000 gigahertz.  Sorry I am getting a little bogged down in the details, now the picture shows how using 'half silvered' mirrors, you can get the infrared light to split off, half of it going one way, and half going the other way.  Then you have a linear slide, where one of the mirrors, you can precisely move along the slide, it may move about the speed that an ant crawls.
 
Now imagine just one single frequency.  Like in the picture, it is showing that wave shape, so you can see, as the moving mirror moves....
I think it is good for me to quote wikipedia's article on FTIR:
 
"The beam described above is generated by starting with a broadband light source—one containing the full spectrum of wavelengths to be measured. The light shines into a Michelson interferometer—a certain configuration of mirrors, one of which is moved by a motor. As this mirror moves, each wavelength of light in the beam is periodically blocked, transmitted, blocked, transmitted, by the interferometer, due to wave interference. Different wavelengths are modulated at different rates, so that at each moment, the beam coming out of the interferometer has a different spectrum."
 
So, if we imagine only a single frequency, it will alternately shine bright, then dim, then bright, over and over again, as you move the mirror. What is important here, is now this particular wavelength of light is producing a brightness signal that rises and falls, which is a wave itself.  Now this signal is easily manageble, its frequency, or cycles per second is definitely, I think a million beats per second or less, and it corresponds to a particular infrared frequency.
 
OK, now, lets double the frequency. That means two waves happen in the time, as well as the distance of the first one.  So the brightness and dimness cycle is twice as fast as before.  Just to reiterate for clarity, as that brightness shines on the light detector, the voltage is higher, and as the light dims, the voltage becomes dimmer.  this is all happening because there is a motor that slowly moves the mirror on the slide further away, then back close again, kind of like breathing.  Again, the original ifrared light is oscillating at hundreds of billions of 'wiggles' or waves per second, and we cannot detect that directly.  But with the sliding mirror setup, we CAN get a signal that is easily measured and stored.
 
OK, now if you throw a pebble into the water, you see the nicely neat shaped waves radiating out from where the pebble hit.  This is like I was talking about above, a single frequency at a time.  The waves are nicely shaped, nice and rolling.
 
Now if your bad little brother comes along with a handful of pebbles, and throws them all at once, then the water becomes choppy looking, as all those waves mix together.
 
This is like in real life, the hot object that glows orange, which is the source of the infrared light, like a camp fire glowing red, and feeling so hot when your face is near it, yes, your eyes cannot see infrared light, in fact the name, 'infra' means 'beyond', I think in latin? , yes beyond what your eyes can see.  The red, or orange colors which you do see, are actually a little bit of the visible light being given off by the hot object.
 
When the hot material, that means the little atoms are banging around furiously, their electrons and protons shake about enough to create the infrared  waves, since electromagnetic radiation, comes from Plus and Minus charges shaking about. As something gets hotter and hotter, like the sun, then the atoms are shaking so furiously, when they crash together, the wave produced is rapid making a shorter wavelength, and a faster frequency, enough so that chemical reactions can happen in our eye to create an pulse on a nerve fiber to send the signal to our brains.
 
You know, I am going to briefly digress, it takes an eye sensor something like 80 chunks of light, coming from individual atoms wiggling, 80 'photons' each second, to get the eye sensing cell to emit a pulse of nerve electricity to send a signal to the brain.  Otherwise, say it only got 30 photons each second, your eye will say 'DARK' . It wont be able to see anything.  This is why the Hubble space telescope in space can 'see' better than our eyes.  It can detect a single incoming photon at a time, and it can stay pointed at the same spot in space for a week at a time, and slowly log in each photon trickling in from distant space, to  finally 'see' those mysterious galaxies, that are out there in the darkest patches of space, as seen by our naked eyes.
 
OK, back to the lesson.  We left off with the little brother making a chaotic splash.  This is like the light coming from our hot infrared light source. All sorts of different frequencies at once.   Well, it is also like music.  You have low frequency base, and then higher tenor notes, all coming together.  And thene there is the guitar, with the strings vibrating to make the different chords or combinations of notes.  
 
And then there are the modern phone calls. The song, or voice, is  spoken into a microphone, which turns the sound waves into voltage waves in the wires.  Then there is an 'analog' (which means the natural wavy voltages) to 'digital' converter.  The ADC, or Analog to Digital Converter changes the voice or song, into a succession of numbers, like points on a graph.  When you re-graph the numbers, you get back your analog wave by connecting the dots.  Today's ADC's, can economically work at speeds of 500 million samples per second.  Now once the wave is in the form of numbers, you can get math involved.
 
There was a mathematician, Mr Joseph Fourier who lived in the times of 1770-1830's, and he put down on paper a mathematical way to convert any choppy signal, or song or voice, and break it down into the various frequencies, and how strong each frequency is, so you get a graph of frequencies.  So if you were to sing one pure note, the fourier transform graph would be a single peak at that frequency.  The natural graph, that would come from the microphone, would be a wave happening as time marches forward.  It would be like a dolphin jumping out of the water, going down, then shooting back up, over and over again as the seconds tick forwards.
 
So the mathematical tricks that Fourier came up with can be calculated using fast computers, so as the voice is happening, it first gets turned into numbers by the ADC, then the computer calculates the Fourier transform, which shows the graph of the frequencies, you know, a peak here, another there of a higher height, meaning it is louder.  A base note would appear at the start of the graph, and a high note would be at the end.
 
Now, as time goes by, lets say in a song, and the low note stops, and the high note is still there, then the Fourier graph would have the lower peak shrink away, and the higher peak still there.
 
Now, you probably have noticed that when you have a bad phone connection, the voice kind of sounds like it has been turned into  some 'notes'  Yes, that is what they now send over the phone, is maybe a hundred 'notes' and how strong each note is, and then every twentieth or so of a second, they update how the strength of the various notes change, as the words are spoken.  When the signal gets bad, only a few notes get through, and that is when you can tell something unusual is happening.
 
In the old days, back in the seventies, when the signal went bad, you got this hissing static sound, as the actual microphone signal was transmitted and used to directly drive a speaker on the other end.
 
OK, how does this relate to the FTIR?  Well, for each infrared frequency, which by itself is too high to measure, the moving mirror set-up, called the michelson interferometer, named after a man named Souza No just kidding, of course Mr Michelson!!! well now we can use his mirror setup to make a much lower frequency that is directly related to the presence of a particular frequency of infrared light.  Now, all these infrared frequencies are all happening at once, and they all get turned into individual lower measurable frequencies by the Interferometer,
and you connect your ADC to this signal from the detector and you  use the fourier transform to turn it all into a graph of frequencies.
 
Great, this is the various frequencies given off by the hot infrared light source.  the graph would be a gradual curve, meanind each frequency is there, but as the frequencies get higher, those slowly get less strong.
 
Well, what is that good for?  By itself, not much, but then you shin the infra red light through, say a sample of blood, or a leaf, and the leaf or blood will absorb some frequencies of the infra red light, due to the molecules inside, and then let other frequencies pass by untouched. So when you put your 'sample'  or leaf or whatever in the path of the michelson interferometer, as shown in the picture, now you get a unique 'graph' of the various frequencies that pass thru, (the peaks) and other areas where the light gets absorbed, usually because it interacts with certain bonds that make up the molecules ( The low spots)
 
Now in math, you can 'flip' it, so the low spots become the high spots, and I think that is what they do, because if the light just passes on thru, well, that is boring, since we are looking for where the light gets sucked up by our sample, you know, which frequencies get absorbed.  Looking at the graph altogether, it is like a 'finger print' for each different substance, and is a way for a scientist to 'tell' what is there.
 
OK, almost finished!!! pat yourself on the back.  Just a brief note on how they used to do it.  Well, they used to still have the hot infrared light source, but then they made it shine on a 'prism', you know, those glass prisms that bend white light into a rainbow of colors, each color bent a little more or less, so as to separate each frequency.
 
Well they used to do that with infra red light, they just had to make the prism out of salt!, because the salt did not absorb the infra red light, as glass would.  So the infra red light is spread out in a rainbow, and you would move a slit to catch just one 'color' or frequency or 'wavelength' of light to pass thru the sample, and then the detector would see how strong the light was with and without the sample in the way, to get a 'percent absorbance' value, then they would plot it.  This way was much slower, because you were looking only at one little piece of light at a time, and the rest was wasted, so things were very dim.  The old machine would pass once thru the rainbow, and repeat if necessary, to collect a new graph.
 
 
In comparison to the new fourier transform method, all the original light frequencies get changed to a much lower frequency caused by the precisely moving mirror, and each one of these new lower frequencies match up with, or 'represents' one of the original frequencies.  Plus, all the new lower frequencies are being 'sung' all at once, kind of like a hundred voices in a choir singing a single sustained chord.
 
So that leaf or blood sample, the light frequencies taken all together is like your cell phone when you have your frend on the other end (hey this is supposed to be poetry)  your friends singing a sustained 'aaaaaaaaaaaaaaaah' then those frequencies are broken down using the fourier transform and wa-lah, you get a graph.
 
 
OK, the one thing among many, that I did not tell you about is how the fourier transform actually does the transformation from actuall wiggling frequencies to a single non-moving graph of those frequencies.    
 
That is one thing to point out.  the 'natural' waves are all alive and wiggling, and like people singing, the vibrations will hit you and your ears and your ear hairs are wiggled and you hear the sounds.
OK, sorry your ears have lots of these hairs of successively shorter lengths, and it takes higher and higher sound notes to make the shorter and shorter hairs wiggle, and when they wiggle, they make the nerve cells make nerve electrical pulses sent to your brain, so each nerve line is sending you frequency information.
 
I did not really appreciate how my ear worked until some loud music was playing and I had short pants on and the vibrating hairs on my leg tingled with each note coming and going, and I said, OK, thats what is going on in my ear!
 
OK, I wish you all a happy evening and a good cheers until next time, thank you for attending my classs,  
 
Aloha