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In article <718B3C23F68DD211BE950008C70D7C94B204D5@OCDFRSTEXS1>, Gould, Nick [OCDFR] <[log in to unmask]> writes >Dear Robin, >Photoacoustic detection work in a very similar way to diffuse reflection >does for the FTIR analysis of solids it is just the detection system that is >different. >The photoaccoustic detector consists of a gas tight cell (a small chamber in >fact) into which one places the ground stone which has been placed in a >small cup. >When the stone is irradiated by the IR the molecular bonds absorb energy at >particular wavelenghts and are raised to a higher vibrational state (up to >this point just the same as diffuse reflectance or absorbtion spectroscopy). >When the bonds relax back to their ground state, they release the energy. >Some of this released energy is in the form of heat. The heat thus generated >causes the gas in the photoacoustic cell to expand and hence produces an >increase in the pressure within the cell >Built into the cell is a very sensitive pressure detector (you can think of >it if you like as a highly sensitive microphone). The pressure waves >generated by the absorbtion and relaxation of light energy by the molecules >that make up the sample are then transformed into a spectral profile. The >spectral profile that you get is exactly analogous to that which you would >obtain by absorption since it is generated by the same spectral phenomenon. >Helium is used as for the gaseous environment in the cell as it has a much >higher coefficient of expansion than air. It also has the advantage that one >knows very quickly if you have a leak in the cell because everyone in the >lab starts talking with a squeaky voice. >In conclusion, I like to think of this detection system as "what you see is >what you hear". >I hope that this helps >Nick > Fascinating. So you grill the sample till it squarks then ! Presumably different stones squark differently ? Robin \/ \/ \/ \/ \/ \/ \/ /\____/\____/\____/\____/\____/\____/\____ %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%