Check out this tubed infrared spectrophotometer.
http://chemapparati.topcities.com/PE337/PE337-1.htm
There's links to the schematic and other information on the 5th page. There is a chopper (rotating sector mirror) in the spec which creates a 13 Hz multiplex signal (sample beam and reference beam signals multiplexed). The circuit includes an impedance matching transformer between the thermocouple sensor and the preamp, some AC coupled circuits similar to typical audio circuits, a demultiplexer circuit and drive circuit for a two phase servo motor that drives the chart recorder and reference beam attenuator. The instrument is an optical null spectrophotometer.
Also see http://www.geocities.com/apis_mellifica2002/IRramble.html
http://chemapparati.topcities.com/PE337/PE337-1.htm
There's links to the schematic and other information on the 5th page. There is a chopper (rotating sector mirror) in the spec which creates a 13 Hz multiplex signal (sample beam and reference beam signals multiplexed). The circuit includes an impedance matching transformer between the thermocouple sensor and the preamp, some AC coupled circuits similar to typical audio circuits, a demultiplexer circuit and drive circuit for a two phase servo motor that drives the chart recorder and reference beam attenuator. The instrument is an optical null spectrophotometer.
Also see http://www.geocities.com/apis_mellifica2002/IRramble.html
Cut my teeth on one of those. Candle for a source. Drum turned by a long stick with a donkey.
During my post-doc days, I worked for a prof who wouldn't even look at an FT-IR spectrum- that wasn't real data like the stuff on the graph paper. He would tell me, "FT-IR is not a well-established technique, use the Perkin-Elmer." This despite having a first-rate IBM FT-IR for department use... I hated using that damn dispersive instrument. But the prof would look at the FT-IR spectra blankly and say, "I'm sorry, I don't know how to read these."
Anyway, I interviewed at a company that made FT stuff, and the chief scientist suggested that I grab some Perkin-Elmer graph paper, use the FT-IR, set the plotter parameters to the P-E paper dimensions, plot on it, then lie to my prof. I tried that, showed the prof the spectra without mentioning what instrument I used. He exclaimed, "These are the finest spectra I've ever seen! Why can't the grad students get spectra as clean as these? They're BEAUTIFUL!!!"
During my post-doc days, I worked for a prof who wouldn't even look at an FT-IR spectrum- that wasn't real data like the stuff on the graph paper. He would tell me, "FT-IR is not a well-established technique, use the Perkin-Elmer." This despite having a first-rate IBM FT-IR for department use... I hated using that damn dispersive instrument. But the prof would look at the FT-IR spectra blankly and say, "I'm sorry, I don't know how to read these."
Anyway, I interviewed at a company that made FT stuff, and the chief scientist suggested that I grab some Perkin-Elmer graph paper, use the FT-IR, set the plotter parameters to the P-E paper dimensions, plot on it, then lie to my prof. I tried that, showed the prof the spectra without mentioning what instrument I used. He exclaimed, "These are the finest spectra I've ever seen! Why can't the grad students get spectra as clean as these? They're BEAUTIFUL!!!"
SY said:
it also helps (or did 30 years ago in my case
)if you have a good idea of what the answer should be.i was fascinated by IR spectra as the Perkin Elmer spectrometer was the niftiest thing in the lab -- and as an undergrad spent hours and hours just looking at the plots in journals --
jackinnj said:It also helps (or did 30 years ago in my case
Similarly, the best bit of advice I ever received was, "You should never measure anything unless you already know the result." The second good bit of advice (from a seriously good lecturer) was, "So long as you are twenty minutes ahead of the students, you're OK."
Jack, I was lucky enough to snag my first job out of school on the design team of the top-of-the-line Nicolet FT-IR, then got to spend a year playing with it and publishing papers. Interestingly, the servo system used to drive the interferometer looked exactly like that of a servo woofer (but with optical rather than accelerometer sensing) right down to the LM1875 chip amp. In fact, it really WAS a servo woofer, but with a mirror rather than a cone.
No tubes, though.
No tubes, though.
I never used an FT-IR spectrophotometer, but I have used a number of dispersive IR spectrophotometers at a community college and a university I went to. I used a Perkin Elmer Infracord (137?) in 1980 for one class on advanced organic synthesis. It drew a complete spectrum on one sheet of chart paper from what I can remember. The 337 draws half the spectrum on one sheet of chart paper and the second half on another sheet of chart paper. The 337 is good if you like working with tube electronics and optics, as well as chemistry. The optics are high quality. The lowest price I've seen for a new dispersive IR spectrophotometer is over eight thousand dollars (US). FT-IR spectrophotometers are even more expensive. Complicated solid state electronics is harder to figure out and service, too. You can sometimes buy a 337 or similar spec on Ebay for less than $100. Shipping is usually at least $50. They're heavy.
Here's the schematic in gif format. The gif image is smaller than the pdf file.
http://chemapparati.topcities.com/PE337/Schematic.htm
Here's the schematic in gif format. The gif image is smaller than the pdf file.
http://chemapparati.topcities.com/PE337/Schematic.htm
SY said:
Sounds like its similar to a Golay detector.
http://www.acreo.se/templates/Page____226.aspx
there has been only one time in the past 35 years in which I wish I had access to some of the analytical chemistry tools I used in Qualitative Chemistry -- to figure out the formula for Ciba's (Ilford) bleach in their reversal photography process -- a Cibachrome print still beats anything that a high end digital printer can produce.
The Golay detector is used principally for gases. PAS uses a standard spectrometer (in the case of IR, generally an FT instrument) and is the method of choice for intractable solids, the stuff we polymer guys call "brick dust." It's easiest to understand the method by considering a dispersive measurement, i.e., frequency by frequency.
The sample is put into a small sealed chamber that has a thin port to a mike diaphragm. Chopped light impinges the sample, with the chopping frequency generally in the audio band. At a light frequency where the sample has a high absorbance, it will alternately heat and cool at the chopping frequency. That causes the boundary layer of gas near the sample surface to act as a piston, which produces a sound. The greater the sample absorbance, the louder the sound.
Although PAS is generally thought of as a modern technique, the original effect was observed by Alexander Graham Bell in his so-called photophone. The effect was rediscovered and applied to spectroscopy by Alan Rozencwaig (sp?) in the 1970s. A very smart fellow named Warren Vidrine (the guy who suggested the cheat I described a few posts back!) realized that since the interferogram modulation in FT-IR is in the audio band for normal mirror speeds, PAS was a natural partner for FT-IR.
The sample is put into a small sealed chamber that has a thin port to a mike diaphragm. Chopped light impinges the sample, with the chopping frequency generally in the audio band. At a light frequency where the sample has a high absorbance, it will alternately heat and cool at the chopping frequency. That causes the boundary layer of gas near the sample surface to act as a piston, which produces a sound. The greater the sample absorbance, the louder the sound.
Although PAS is generally thought of as a modern technique, the original effect was observed by Alexander Graham Bell in his so-called photophone. The effect was rediscovered and applied to spectroscopy by Alan Rozencwaig (sp?) in the 1970s. A very smart fellow named Warren Vidrine (the guy who suggested the cheat I described a few posts back!) realized that since the interferogram modulation in FT-IR is in the audio band for normal mirror speeds, PAS was a natural partner for FT-IR.
Here's an interesting article on signal averaging. Its not immediately obvious, but apparently in the 337 the signal is averaged to improve the S/N ratio, ie. by alternately sampling the sample and reference beam signals and passing the combined signals through a low pass filter to the demodulator. The slower the scan speed, the more the samples and the better the resolution.
http://www.jce.divched.org/cgi-bin/JCE/jce-idx.pl?type=goto&volume=63&issue=7&page=648
http://www.jce.divched.org/cgi-bin/JCE/jce-idx.pl?type=goto&volume=63&issue=7&page=648
I made a web page showing pictures of the original detector from the Perkin Elmer 337 and one from a Pye Unicam IR spectrophotometer.
http://chemapparati.topcities.com/PE337/Detectors.html
The Pye Unicam was made later and I figured that the detector might be more sensitive than the Perkin Elmer detector and could be adapted for use in the Perkin Elmer 337. I put a detector from an old Perkin Elmer model 710B IR spectrophotometer in the 337 since the original detector in the 337 was bad. The 710B detector works, but appears to be weak; explained here http://chemapparati.topcities.com/PE337/PE337-5.htm under "Repairs and Modifications" note 6. The Pye Unicam detector has a built in preamp and a power supply needs to be set up in the 337 to power it. A voltage divider comprising a 220K resistor and a 15K resistor between the 210V regulated supply and the regulated ground would provide about 13 volts. Compare to R242 and R243 in the schematic http://chemapparati.topcities.com/PE337/Schematic.htm
Maybe I could power the Pye Unicam detector from that and have the signal from the detector go into the 500K pot gain control R201 (tube preamp disconnected).
http://chemapparati.topcities.com/PE337/Detectors.html
The Pye Unicam was made later and I figured that the detector might be more sensitive than the Perkin Elmer detector and could be adapted for use in the Perkin Elmer 337. I put a detector from an old Perkin Elmer model 710B IR spectrophotometer in the 337 since the original detector in the 337 was bad. The 710B detector works, but appears to be weak; explained here http://chemapparati.topcities.com/PE337/PE337-5.htm under "Repairs and Modifications" note 6. The Pye Unicam detector has a built in preamp and a power supply needs to be set up in the 337 to power it. A voltage divider comprising a 220K resistor and a 15K resistor between the 210V regulated supply and the regulated ground would provide about 13 volts. Compare to R242 and R243 in the schematic http://chemapparati.topcities.com/PE337/Schematic.htm
Maybe I could power the Pye Unicam detector from that and have the signal from the detector go into the 500K pot gain control R201 (tube preamp disconnected).
The pages disappeared at topcities so I started them over here:
http://www.geocities.com/antiquesci/PE337/PE337-1.htm
http://www.geocities.com/antiquesci/PE337/PE337-2.htm
http://www.geocities.com/antiquesci/PE337/PE337-3.htm
http://www.geocities.com/antiquesci/PE337/PE337-4.htm
http://www.geocities.com/antiquesci/PE337/PE337-5.htm
The detectors are shown here:
http://www.geocities.com/antiquesci/PE337/Detectors.html
http://www.geocities.com/antiquesci/PE337/PE337-1.htm
http://www.geocities.com/antiquesci/PE337/PE337-2.htm
http://www.geocities.com/antiquesci/PE337/PE337-3.htm
http://www.geocities.com/antiquesci/PE337/PE337-4.htm
http://www.geocities.com/antiquesci/PE337/PE337-5.htm
The detectors are shown here:
http://www.geocities.com/antiquesci/PE337/Detectors.html
A copy is here, too, with some information about PE 700 series IR specs.
http://antiquesci.50webs.com/index.htm
http://antiquesci.50webs.com/index.htm
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