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Old 11th March 2006, 12:53 PM   #11
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Location: Llanddewi Brefi, NJ
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.
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Old 11th March 2006, 12:59 PM   #12
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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.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 11th March 2006, 01:04 PM   #13
Join Date: Mar 2006
Location: VA
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.
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Old 19th May 2006, 02:23 PM   #14
Join Date: Mar 2006
Location: VA
I made a web page showing pictures of the original detector from the Perkin Elmer 337 and one from a Pye Unicam IR spectrophotometer.

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 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
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).
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Old 8th November 2009, 12:44 PM   #16
Join Date: Mar 2006
Location: VA
Geocities free pages are gone so I put it here.

antiquesci index
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Old 21st November 2009, 12:11 PM   #17
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Location: VA
A copy is here, too, with some information about PE 700 series IR specs.
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Old 21st November 2009, 04:23 PM   #18
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Location: Monroe Township, NJ
Good old PE intruments. Yes, I too triturated solids in "nujol" before apply the paste to salt windows and producing a graph. Organic Chemistry was good for my QP average.
Eli D.
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