| Author | Message | ||
     Claire Laraman (Claire) |
I am trying to write my thesis for my Masters degree and am having trouble finding some band assignments. My instrument scans over 600-1900nm and I am analysing polyester resin production. In particular does anyone know where I can find out about the -COOH and -C=O bands as I can't find them in any literature for my wavelength region. I am measuring the changes in OHV and AV if that is of any interest. Thanks Claire | ||
| weyer |
If you send me your e-mail address, I will send you what I have in the way of band assignments. Mine is [email protected]. | ||
| David Hopkins (Hopkins) |
Claire, That is a very good question, and I'm interested in the answer too. The problem is, common "knowledge" is accepted that only bonds with H, as in O-H, C-H, N-H, S-H are active in NIR. That is not really true, I'm convinced, from looking at spectra of crystalline salts. You just need a sensitive instrument. I am glad that you are interested, and writing your Masters Thesis on an NIR topic, and doing good work too, I'd guess from your question. Would you mind sharing where you are going to college? Lois, do you have any further info beyond the combinations of CO and CH that you showed in Chambersburg? Best wishes, Dave Hopkins NIR Consultant, Battle Creek, MI [email protected] | ||
| hlmark |
Claire - I understand where Dave is coming from, and theoretically he's correct: there SHOULD be bands from other than hydrogenic species. The problem is as follows: the frequency of the fundamental absorbance will vary roughly with the inverse of the atomic weight of the atom involved. To see an absorbance in the NIR, therefore, you would have to be able to detect a correspondingly higher-order overtone. Overtone strength gets weaker as the order increases. To take an numerical example: consider trying to see the absorbance of C-H in the 1600-1700 nm region. Thar's twice the frequency of the fundamental at 3200-3300 nm, and the absorbance is roughly 1/10 that of the fundamental. Now consider trying to see a C-O absorbance. Since the atomic weight of oxygen is 16, the frequency of the fundamental absorbance will be (very roughly) 1/16 of the C-H frequency, and so you will be looking at a frequency 32 times that of the fundamental, to see it at the same wavelength as the C-H overtone. The absorbance strength is correspondingly less. To say "vanishingly" small becomes a pun, but is a realistic estimate of the situation. Peter Griffiths once did a study of some of these effects; I seem to reclal his giving a talk about them at Pittcon several years ago. You might try contacting him for some general guidance. Good luck in your quest. Howard | ||
| Claire Laraman (Claire) |
Thank you for your replies, am currently reading through Lois's chapter to gain some more info. In answer to your question Dave, I am studying for my MSc through the University of Surrey, UK, but it is an industry based degree. I have spent the last 20 months working for a powder coatings company and my MSc project was set up by them - to install an NIR online monitoring system for polyester resin production to measure hydroxyl and acid values. The project has so far been successful and we are at the stage now that we will be able to implement the system for certain product types. Have had a few problems along the way, but it looks like it will be a very useful tool when it is fully operational. Unfortunately the company are not keeping me on to finish the project when my temporary contract runs out in 2 months (no money apparently) so I will not see the project to its conclusion. Kind regards, Claire |