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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



		-----Original Message-----
		From:	Robin Marks [mailto:[log in to unmask]]
		Sent:	Wednesday, November 01, 2000 12:12 AM
		To:	[log in to unmask]
		Subject:	Re: Renal Stones (a bit late)

		In article
<718B3C23F68DD211BE950008C70D7C94B02730@OCDFRSTEXS1>, Gould,
		Nick [OCDFR] <[log in to unmask]> writes
		>This is somewhat en retard as I have just joined the list.
However, try the
		>following reference for quantitative IR analysis of renal
stones using
		>photoacoustic detection. Photoacoustic detection has the
advantage of being
		>non-destructive so you retain sometimes precious material
for followup
		>analysis if needed. It's also very quick.
		>
		>N.Gould, P.C.Hallson, G.P. Kasidad, C.T. Samuell, T.B. Weir
		>Urol Res (1995) 23: 63-69
		>
		>UCH may have done further work on this method since I was
there.
		>
		>Nick Gould
		Could you explain briefly how photoacoustic detection works
?

		Robin Marks, Halifax
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