I could’ve sworn I asked this question years ago, but I can’t find it in the search function. I know I’ve asked it a couple of times in my life, but seem to forget the reason.
Why are there no Fourier Transform UV-Vis spectrophotometers? I’m sure there are a few specialty ones, but why aren’t they the standard, most accepted type…Infrared spectrometers are typically FT; what’s the technical reason why FT-UV or FT-Vis types aren’t suitable?
I don’t know the details of the specific devices you’re asking about, but in general, the higher the frequency, the harder it is to treat light (or any other phenomenon) as a wave. Mostly, this is because you need to use structures which are smaller than, or at least comparable to, the wavelength.
I think it might be because the physical motions required would be so fine, like 10 to 30 times finer than on FTIR. But don’t know for sure.
Ehr… I may be misremembering, but IIRC, part of the advantage of FT-spectrometers is that, unlike traditional ones, those do not require moving mechano-optical parts (you need moving parts only at the sample tray). The whole spectrum to be analized is “shined” at once, and the absorption at each wavelength is determined mathematically. In a traditional spectrometer, you use mirrors, lenses and filters to shine light in very narrow wavelength ranges, sweeping through the whole range that you want to analyze.
Three things come to mind; one is that scanning dispersive spectroscopy, like the typical UV-vis, just doesn’t work that well for IR, though I’m not sure what the problem is. The second thing is that since dispersive scanning worked fine for the UV-visible range, the instruments that did it were already in place when the computer power for FT became available. Finally, it’s my experience that UV and visible spectra aren’t the “be all and end all”; while it’s sometimes useful to scan a range, e.g., for ID purposes, a lot of work is done at a single wavelength, and direct measurement through the simplest possible optics is desirable
And I may misremember, too, but I thought there was a moving mirror and a split signal path, half of which travels an extra distance to the mirror and back. This forms an interferometer, and the recombined signal strength has amplitude sinewaves of different frequencies depending on the wavelengths in the signal and on the mirror velocity. I must go look it up…
The Wikipedia article on FTIR explains the moving mirror and split signal path and has a nice diagram.
Do you have particular applications in mind? I’m not too familiar with FTIR, and which applications it’s used in.
Michelson interferometers are used for visible light in some applications. In my field (solar physics) it’s often used for wide-field imaging spectroscopy; the latest and greatest example is the Helioseismic and Magnetic Imager on the Solar Dynamics Observatory. But they’re only used when there is a need for a very wide field of view; for narrower field of view, a Fabry–Pérot interferometer is usually preferred.
UV is a bit more tricky. Mechanical tolerances and optical figure requirements tend to scale with wavelength, as Chronos noted above. Also, conventional designs for Fourier transform spectrometers require beamsplitters, which is usually a transmissive element. Depending on wavelength, your choice of optical material becomes very limited.
Still, there is some research going into all-reflective Fourier-transform optics. Heterodyne spectroscopy using a diffraction grating as a beamsplitter, for example.
Apologies for late posting. Just came across this.
In the 80s an astronomer in the Physics Department at Imperial College built a vacuum FT-UV spectrometer. It was the size of a desk. Chelsea Instruments built a few which were learned out to labs, including, I think, the NBS as it was then. As a test they examined a triplet in the spectrum of iron and said they were unimpressed as, they said, it had failed to resolve it. In fact, it had resolved it so well that the triplet appeared as three distinct and well separated peaks.
There’s no mention of it on the Chelsea Instruments web-site, but it is referred to on the Imperial College Space Physics web-site:
http://www.sp.ph.ic.ac.uk/~julietp/FTS/instrument.htm
So to answer the question, why no FT-UV? Not necessary for most applications, and it’s so expensive to get an instrument that will do it. If someone had unlimited resources I’m sure they would like to update and improve Mavrodineanu.
You can build a device like a traditional FTIR with no moving parts, and which will work in the Visible or UV. In that case, the arguments against using it because of the diffioculties on movement go away. I’ve worked on such a device.
One of my professors once tralked (not entirely tongue-in-cheek) about a rule of “Conservation of Complexity” – that if you try an end-run around the obviouis problems in a situation, it’ll be paid for by the appearance of some new and unexpected problem. You do pay a price for not having any moving parts in your “fixed” FT spectrum system, but it’s not insoluble.
Greavsie, no need to apologize. One of the situations where we welcome a bump of a long-dormant thread is when someone has new information, or can definitively answer a question. The problem is mostly with bumps to just say “Me, too”, or the like.
I am glad tgo see you interested in approaching UV with an FT instrument. When I first learned about FTIR, about 40 years ago, I was doing chromatography with a UV detector, single wavelength, of course back then. Later DADS came along but I was sure a FTUV was in order. My friends from ISU, an excellent department of Analytical Chemistry, told me it was impossible. I didn’t believe them, but I couldn’t argue as I knew so little.
Later I saw an FTUV chromatography detector from Gorton. It couldn’t have been a big success as I don’t see them advertised any more and an Internet search comes up pretty lean. All I remember from Gorton’s literature was no moving parts and concentric circles pattern. I would guess the pattern was Newton’s Rings and they had made use of a Fabry-Perot interferometer. So you could make one and with today’s electronics, you could probably make it cheap enough if it had advantages.
Fellgett’s advantage makes FTIR so good. Lots of light multiplexed into the detector increases the signal to noise. The type of predominate noise for IR and UV differ. The difference kills FTUV as a great idea. UV noise is proportional to signal unlike IR. Read the Wikipedia article on Fellgett’s advantage for a better understanding than I am writing. Keep up the ideas, the more you work your brain, the more the ideas, and usually some of them are useful.
Wow, I think this finally got the right answer, five years later. So if I understand correctly:
A far-IR spectrometer typically uses something like a pyroelectric detector (i.e., a crystal that develops a voltage across it when its temperature changes slightly, because it gets heated by the incident light). This detector has noise that’s independent of the signal amplitude. With such a detector, by using an interferometer instead of a monochromator (which throws the undesired wavelengths away), you increase the total power that you successfully measure, and improve SNR. Very roughly, that’s Fellgett’s advantage.
A shorter-wavelength (near IR to UV) spectrometer typically uses something like a photomultiplier tube, which is kind of like counting photons. That gives you shot noise, which is proportional to the square root of the signal. In such a case, Fellgett’s advantage no longer exists, so you don’t get that improvement in performance over just using a monochromator, so there’s not much reason to incur that complexity.
Am I close? Does this mean e.g. that a far-IR spectrometer using a chilled semiconductor detector instead of a thermal/pyroelectric detector also wouldn’t exhibit Fellgett’s advantage?
Probably because unlike IR spectra, UV provides very little qualitative information, but works quite well for quantitative detection. Current detector designs (i.e. monochromator with diode-array detection) work quite well, and haven’t changed in decades. Making an FT-UV would require much more computational power than current systems, while offering no particular benefit.
However, upon reflection, my experience is biased toward chromatographic applications. On these timescales, the response time is relatively long (1-10 Hz). If you were studying extremely fast phenomena, an FT system might offer an advantage; I honestly have no idea how quickly they can scan.
The computational effort to convert an interferogram to a spectrum is completely trivial, and has been for many years. Think, for example, of all the Fourier transforms that a typical video or image codec performs, in real time on your phone.
Did you read BigFish_43’s comment about how the different noise characteristics of typical UV/Vis vs. far-IR detectors negates Fellgett’s advantage? That’s supported in the literature, and it makes sense. I’m pretty sure that’s the answer.
Good point about advantages in computing power; I cut my teeth on FT-MS a while ago, at which time, scans had to be processed off-line, so my experience is a little dated.