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automating production: the use of home-made monochromatic spectrophotometers? -drone

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dormouse:
Author  Topic:   automating production: the use of home-made monochromatic spectrophotometers? 
drone 342
Member   posted 11-17-98 05:18 PM          
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One very bright bee, in the mass production discussion, made the astute observation that in order to maximize pilot plant-scale production, automation ought to be utilized whenever possible. Perhaps one of the seemingly simplest means of making life a lot sweeter would be to use simple monochromatic spectroscopy to monitor a reaction's progress. A common practice in industry; by selecting a wavelength primarily absorbed by the starting product, shooting a beam through your reactor vessel, and measuring the absorbance on the other side with a photomultiplier tube, one can successfully measure the concentration of reactants, products, etc, and thus monitor a reaction's rate and progress.
By using this technology, a reaction's time can be minimized, while the final product and its purity can be maximized as well. The question is: how?By using UV/IR spectroscopies of products and their starting materials, one can select which wavelengths to use to monitor each species, but how does one go abot constructing a home-made monochromator and transducer? Aside from purchasing expensive filters and other overpriced equiptment, is there a cheap way of doing this? While I do know my chemistry, I must admit I'm no expert in electronics. I suspect though there might be a few out there reading this. How would you do this? I feel this technology would greatly advance our cause.-drone #342
Rhenium
Member   posted 11-18-98 07:29 AM          
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Drone,
I do remember reading a while back in an article (I forget where) that a handy spectrometer grating for the visible spectrum can be made using a common CD. Because the grooves are the correct ditance apart you get the diffraction of the light. I'm not sure of the exact range of wavelengths, but this would depend upon the spacings.
I don't know whether the hassle would be worth it though, in my undergraduate days we used to use the old Varian 20's which covered most of the visible and IR pretty well, and they didn't look particularly expensive.
How exactly you would program the automated facility to operate I don't know. Maybe instuct the machine to continue stirring, heating or whatever until the peak at (~1400 cm-1) disappears (or where ever the ketone comes).
Actually, maybe another idea (IMHO) would be automated TLC (Thin Layer Chromatography). Every hour or so the machine takes a sample (via syringe), drops it onto a plate and lowers the plate (mechanical arm or whatever) into an automated developing tank (fills and empties on computer command). The plate develops over x number of minutes and is taken out (arm again) and sprayed with nihydrin or examined under UV light. Computer takes a snapshot with digital camera, and emails it to your anonymous account on the other side of town. By doing many small scale synthesis, you could pretty much tell straight away what you've got, and this could be used in paralell with automated UV-vis/IR. Now that would be the ultimate in long distance synthesis. Maybe someone with a bit of robotics knowledge could suggest an improvement, I'm just thinking of the automated sample dispensor which operates on the GC-MS here, it's pretty damn nifty, yet incredibly simple. But this is just an idea.Take care,Rhenium
Rhenium
Member   posted 11-18-98 08:28 AM          
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Drone,
I've just had a look at what I wrote, and I'm sorry for getting off the topic. I'm much more of an ideas person than a practical application person. Hope you understand.
Anyway, for the monochrometer, you have two main options, a diffraction grating (Czerny-Turner type) versus a Prism (Bunsen) type.
For totally at home I would recommend the prism type, since prism are relatively easier to obtain then good diffraction gratings.
Both types require either focussing mirrors or lenses, and collimating slits. Since the width of the collimating slit will determine the width of the wavelengths you will be looking at, the smaller the better. Making tiny regular width slits at home wouldn't be enormously fun.
Alternatively, use a photodiode array with a simple grating and a polychromatic source. This will do scan the whole spectrum in a very quick period of time, but photodiodes are cheap.
Writing the software or making the hardware to contol any of these is something I wouldn't like to think about.Take care,Rhenium
drone 342
Member   posted 11-18-98 03:41 PM          
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The software is not actually that big a deal, nor would you even really need any. By hooking up your photomultiplier tube to a simple voltmeter, you get a useful readout. The part I think would suck would be angling the prisms, etc. until you managed to find the the wavelength you want to select.
Alternatively, I know of a few companies that sell monochromatic filters. Perhaps puting one of these in front of a garden-variety incandescent is all you'd need?Another thing: which would be better for spectroscopic monitoring -- IR or UV?-drone #342
ChemHack
Member   posted 11-18-98 08:15 PM          
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That probably depends on the substance but remember that IR will add heat, especially if it is contantly on for realtime monitoring.
Using a visable wavelength would be ideal because you could do the rxn in the dark and use a simple photodetector switch possibly even diverted from consumer products such as nightlight or yard lighting. Connect the switch to a relay and the software requirement goes down even lower.Computer takes a snapshot with digital camera, and emails it to your anonymous account on the other side of town.
Nice digital cameras for use in control systems have OCR software that could be configured to make the determination for you. Encapsalate the OCR component in a OLE server and then, when the rxn is done, take the next action like turning off heat, flooding to a certain level, etc...
drone 342
Member   posted 11-18-98 09:45 PM          
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IR *may* add heat, but not much. We're talking about the kinetic energy gained by the absorbance of light from a monochromatic spectrographic source -- the quantity is *ridiculously* negligible. Aside from photosensetive situations and maybe microscale chemistry, I can't think of a single situation where the effect of this amount of light would amount to a hill of beans.
Imagine with me for a moment, what complexity would go into automated TLC and hooking that up to a computer: the act of TLC is a mechanicly complex process, meaning a robotic component may be needed. The equiptment needed to take a snapshot and everything else is expensive by anybody's budget. Since such a procedure isn't continuous, and wouldn't be able to automatically know exactly when the reaction is complete, I really am not sure if this labor-0saving devise would be much use.But, for arguement's sake. Let's say there were a situation where it would be practical for you to remotely observe a reaction's progress. Using the system I described, with continuous monitoring, and simply an op amp hooked to a voltmeter for measuring the progression of the reaction. A computer monitoring the voltmeter has a program that essentially reads "If the voltage is greater than X, send an e-mail to crackhead@chemfreak.com". Somewhere on the other side of the planet, our chemical hero is sitting by a computer, when a little "beep" signals that incoming mail has arrived. Its the news she was looking for! Even if our chemist isn't remotely observing the reaction, (that is to say, that they're in the lab), this still would be quite nice, since it would tell the chemist precisely when the reaction was completed -- always a useful, time-saving thing. This cuts production cost, production time, and improves yield.-drone #342
Wizard X
Moderator   posted 11-18-98 11:18 PM          
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I understand what you want to achieve and I can help you in the IR detectors and the IR monochromator , but the IR radiation source is difficult to build and the sample holding cell ( pure NaCl or NaBr crystal ). For UV the phototube detector is very difficult to build , the UV source and UV monochromator are not to difficult. Look at secondhand shops. Give me a little time and I will write-up the info. My E- mail wizardx14@hotmail.com
 
ChemHack
Member   posted 11-18-98 11:28 PM          
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I disagree that a UV detection system is always hard to build. The question is what wavelength are you interested in? Detectors for UV and even X-Ray are often constructed by passing the wave through a substance that absorbs that wavelength and emits visible light in return. Then you can concentrate only on detecting the visible light. This is how industrial non-distructive X-Ray examination is done...
 
drone 342
Member   posted 11-19-98 04:45 PM          
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Wizard X,
The sample cell need not be KBr nor NaCl -- quartz, among several other common materials, will still do nicely. In fact, wouldn't garden-variety borosilicate work?Looking around on the net, its not difficult to find dozens of sites selling high-performance monochromatic filters, some site even accept credit cards. Seeing that even a regular incandescent light bulb, coupled with one of these filters, would work effectively as a light source, it looks like making one of these thing-a-ma-jiggies would be cake.Chemhack,Non-destructive X-ray spectroanalysis, eh? Sort of like high-energy Raman spectroscopy? Very clever. The question is, how would you begin looking for an appropriate wavelength of UV/X-rays to use, and which visible wavelengths to look for?
Judging by the posts in the industrial discussion, so far the only reactions we've been able to establish with certainty as current candidates for viable industrial production are the Wacker oxidation and the subsequent reductive amination, this seems alike a fine set of reaction to turn our interest to here. (I recognise that other reactions, inculding halosafrole-related chemistry, *may* at some time be useful, their chemistry isn't as thoroughly established.) Let's use these as hypothetical models for our discussion.-drone #342
Mobius
Member   posted 11-20-98 08:27 PM          
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Here's a note from the bright (well I'm not bright, only very lazzy) bee (thanx drone) that said that automatizating production was the way to go:
Before geting into chemistry, I (probably like many of us here) was... a nerd... I was at this age (12-16) facinated by lasers and robotics... I was building small robots that I was able to controle using my Commodore 64 (tm)... Here are my advices to you fellow bees...1) Using a narrow band monochromatic filter would be a waist of money... Wide band filters are cheap and available in a wide range of colors (wave lenght)! Wide range filters = Colored acetates... Deep purple ones are cool for near UV monitoring...2) No need to be precise here... most absorbtion peeks are not sharp... Any change in absorption around a given frequency could be used to monitor reaction evolution...3) Ever tought of using a pair of optical fibers (inert to most solvents; most are borosilicate-quartz) immersed in the reaction medium ? One send in some light the other catches it... Use Radio-shack IR photo-couple or any other proper emiter-reciver couple... Any change in transmitance-absorbtion could be used as a reference... Just keep an eye on the set-up the first time you do it ;-)...I think an other way to do it (I think I prefer this one) would be to use a small semi-conductor laser (Radio shack againg) and to send the beem trough the mix and right on a photo detector... If I remember well as soon as the nature of the coumpounds in the solution are going to change, it's refraction indice is going to change too, which will bend the beem more and send it out (or at a different spot) of the detector sight, triggering whatever action...Any other ideas...-Moby: reminding you that... Auto is the way to go !------------------
 
Arthur Dent
Member   posted 11-23-98 12:35 AM          
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Great idea guys. I'd like to suggest that if one wanted to set up a reliable and extremely versatile monitoring system, you could implement a LC pump which would feed into a quartz flow cell in a desktop spectrophotometer. I think that, with some creative sourcing, this might be done for under US$350. You would'nt need constant calibration as with a home made instument. You would also be able to use the setup for any reaction you like, having a variable wavelength spec. I know, sounds expensive. It doesn't have to be.
There are lots of wierd places to scrounge small flow pumps from. Just be sure to use the right tubing to connect it. The quartz flow cells are made by all manufacturers of desktop specs as accessories. Whatever spec you get, contact the maker.Used desktop specs are easy to find. They're one of the most common instruments in labs. A lot of internet resellers stock lab equipment from analytical labs that go under. And beleive me, there's always a lot of 'em going out of business. When they go under their creditors have a fire sale and the not-so-high-tech equipment like specs end up in the wierdest places. Among other places is dantiques.com - they sell a variety of used lab equipment. You won't believe their prices on some of this stuff.BTW- another cool result of using a desktop spec flowcell is that they're pretty much standardized and fit into all kinds of other detectors . AD
Wizard X
Moderator   posted 11-24-98 08:56 PM          
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Drone
It will take me too long to write up the information you require as work commitments are overwhelming.
So, read the sections in Vogel 5th Ed on IR/UV spectrometers. All circuits, differential amplifiers, current amplifiers can be found in the " Encyclopedia of Electronic Circuits ", by Graf Vol 1-6 , look under Instrument amplifiers.
A very basic circuit is a Wheatstone bridge circuit ( basic comparator circuit which compares the sample transmitted radiation with the reference transmitted radiation ) can be constructed easy.
IR , the monochromator prism and sample cells and all optics must be made of alkali salts (NaCl , KBr) as glass or fused silica transmit IR very little.
UV , glass prisms and lenses can be used in the UV region , but fused silica and quartz can not.
If you need more info or can't find the Wheatstone bridge circuit , then E-mail me. Good Luck !
 
drone 342
Member   posted 11-28-98 03:40 PM          
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I really think that using a narrow pass filter is the way to go; true, they can be spendy, but what we're looking at are some pretty narrow bandwidth of interest.
-drone #342
 

Bozakium:
Thanx for the point here, lowtech!,
WOW, I never knew this thread existed, as this has been one of my pet projects for a while. I'm currently hunting down IR spec schematics, and I do have the electronics background to build one. IR, I believe would be most useful as a spec curve gives the relative proportions of the functional groups, and hence allows rxn monitoring, and comparison to published spec data. An interface/data aquisition board for the PC will allow much of the electronics to be replaced with programming. An infrared LED and IR phototransistor(s) are the "working guts" of the device, the method of frequency sweep can be done in a few different ways (electronically or optically)and is the focus of my currernt research. I missed an oportunity to get a commercial model for free and am still kicking myself in the ass. I should hopefully bee done by this time next year, as my life is in a shambles right now and I have other projects on the burner, but it WILL happen. Now where can I get the specs of cadaverine and putrecine so I can monitor decomposition for the the correct time to harvest?Heh, heh--Bz138

UTFSE:
exactly what do you mean by "IR spec schematics" cause I might have some refs for you. they are older homebrew spectophotometers if that will help.
always appreciate your time and consideration

Bozakium:
I am looking specifically for the electronic circuit diagram, commonly known as a schematic, so that I can understand HOW the circuit works so that I may adapt it to build my own. I was hoping someone had the servicemans's paperwork for their own unit, which would contain as well as the schematic a listing or diagram of voltages, waveforms and logic states at various points in the circuit which are used for troubleshooting purposes. One could easily build a "ghetto" visible spectrophotometer with a couple of prisms and comparing a reference spectrum(rainbow) side-by-side with the spectrum of light sent through the sample.This ghetto job is impractical for me for a number of reasons:this necessitstes a large sample so it ain't good for rxn monitoring, I don't know jack about visible spectroscopy, and even if I did, I still think IR is the most useful. I never liked UV for a reason....
  Any info is appreciated, even if it is homebrew. I don't see why Cole-Palmer(sp?) wouldnt send my electronics repairman the info if he's got my spec in his shop, for example. Perhaps I should try the direct approach....

PolytheneSam:
I'm working on a page about Ir specs.  Typical IR diodes and IR transistors which are used in IR remote controls won't work with IR spectrophotometers.  All the answers are in books the library.  Not all electronic schematics work with all spectrophotometers, either.

http://www.geocities.com/apis_mellifica2002/IRspecs.html
http://www.geocities.com/dritte123/PSPF.html

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