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Sigene
11-19-2005, 09:39 AM
What's the purpose of the laser in an FTIR? It obviously isn't the source of the IR light. I believe it has something to do with alignment of the mirror in the interferometer. Is it used to precisely measure the position of the mirror? I'm pretty clueless on the Fourier Transform, I just recognize it as some mysterious mathematical (not mechanical or optical) algorithm that transforms the incoming squiggles to a usable spectra.

I'm not sure how the laser fits in here?

t-bonham@scc.net
11-19-2005, 03:30 PM
Maybe if you told us what the hell an "FTIR" is...

Mr. Blue Sky
11-19-2005, 03:41 PM
Fourier Transform Infrared Spectroscopy?

Declan
11-19-2005, 03:45 PM
What's the purpose of the laser in an FTIR? It obviously isn't the source of the IR light. I believe it has something to do with alignment of the mirror in the interferometer. Is it used to precisely measure the position of the mirror? I'm pretty clueless on the Fourier Transform, I just recognize it as some mysterious mathematical (not mechanical or optical) algorithm that transforms the incoming squiggles to a usable spectra.

I'm not sure how the laser fits in here?


Did you mean FLIR ?

Declan

mnemosyne
11-19-2005, 05:41 PM
I'm afraid I can't give you much detail (I don't know a lot about IR instruments, and what I know is from memory from classes a couple years ago) but the laser (a Helium-Neon one) in an FTIR is used as a reference source, which helps control the sampling rate of the IR detector. It also provides an automatic calibration of the wavelengths being detected, since the wavelength of the laser and it's distance from the interferometer is known. This is known as "Connes Advantage".

Sadly, I can't explain this better, but Google seems to have a lot on Connes advantage! Good luck!

mnemosyne
11-19-2005, 05:48 PM
Oh, and although I'm pretty sure this isn't what you're asking, some systems do use a laser as the IR source: The Carbon-dioxide laser source is used to measure the atmospheric concentrations of pollutants or determining absorbing species in aqueous solutions. It's useful in the 900-1100cm-1 range. It is very good at detecting things like ammonia, benzene, ethanol, nitrogen dioxide, etc. The spectrum of the laser actually consists of several lines in that range, so it can be tuned very specifically, and its radiant power tends to be greater than the other common blackbody sources.

Honestly, I have no idea how common this source is, or whatever. My Anal. Chem. book is kind of old and somewhat unreliable, in my experience (Profs, do NOT subject your students to Skoog, Holler and Nieman!)

Antonius Block
11-20-2005, 03:53 AM
As mnemosyne said, it's to act as a an internal calibration for the Fourier Transform InfraRed Spectrometer.

In its simplest form, an FTIR instrument has an IR source, a sample cell or chamber, a beamsplitter, a fixed mirror, a moving mirror, and a detector (this is for absorption spectroscopy; for emission spectroscopy, the IR Source is removed and the light from the sample under study is detected instead).

It is crucial to any FTIR experiment to know exactly where the moving mirror is at any given moment. Most modern instruments have this mirror mounted to a voice-coil (similar to that used in an audio loudspeaker). The instrument's resolution is dependent on the "throw" of the mirror, i.e. the distance moved during a given sweep. The instrument's bandwidth is dependent on the sampling rate, i.e. the inverse of the distance that the mirror moves between adjacent samples measured at the detector. For high resolution you need a long-throw voicecoil, and for wide bandwidth you need to be able to measure the mirror position very accurately.

As mentioned above, most commercial FTIR instruments use a Helium-Neon laser to determine the moving-mirror position at any given time. The beam intensity is usually measured at two points in the interferometer. As the mirror moves, the intensity at these two detectors rises and falls due to interferometric enhancement and cancellation of the HeNe beam paths, producing an approximate sinewave of intensity vs mirror position. By counting the number of "fringes" in the sinewave pattern, the instrument knows exacty how far the mirror has moved, and the relative phase of the sinewave at the two HeNe detectors tells the instrument in which direction the mirror is moving. Typically, measurements at the IR detector (the one looking at the sample in absorption or emission) are taken at specific phase-pair points of the two HeNe reference sinewaves; for lower-resolution studies, sampling may take place at every "N" HeNe fringes rather than at every fringe.

Although a given FTIR instrument may be able to be fitted with optics and sources to record both infrared and visible spectra, no single spectrum can extend across the wavelength of the reference laser (usually the HeNe line at 632.8nm). So, in any experiment one can do visible (wavelength<632.8nm) or infrared (wavelength>632.8nm) spectroscopy. [There are ways around this limitation, but they are not available in any commercial instruments AFAIK.]

In some time-resolved studies, in which the chemical species under observation only exists for a short period of time (e.g. nanoseconds, microseconds or milliseconds), Step-Scan FTIR may be used. In this case, the moving mirror of the FTIR instrument is held at a certain position while (typically) a pulsed laser is fired, producing or exciting the species under observation. The transient IR waveform is digitized (at sampling rates greater than 1 Gigasample/second in certain cases), and then the FTIR movable mirror is "stepped" to the next stable location at which the two HeNe reference beams have the selected phase relationship (usually a multiple of 316.4nm, i.e. half of the HeNe wavelength), and so on. [The mirror may stay at a given position for many pulses of the excitation laser, accumulating sufficient transient waveforms to produce the desired signal/noise ratio.] The result, after Fourier Transform, is a 3-D dataset of Intensity vs Time vs Wavelength. Such experiements may take several hours.

Hope this helps!

[Antonius Block, former modifier of FTIR instruments for enhanced time resolution.]