When samples are placed in the near infrared laser beam of a Fourier Transform Raman Spectrometer, the temperature can rise. Even a very slight absorption can cause a rise of many degrees. The problem is exacerbated by the practice of focussing the laser unit into the sample – the brightness at the sampled point can be very high indeed (up to 103 Watts/sq.cm). If the sample absorbs significantly the temperature rise at the small illuminated point can increase catastrophically causing in bad cases decomposition and even burning. Several solutions have been proposed including making potassium bromide discs or mulls sandwiched between potassium bromide windows [ Y.West Internet J. Vib. Spect., [www.ijvs.com] 1, 1, 5 (1996) and P.J. Hendra, Internet J. Vib. Spect., [www.ijvs.com] 4, 3, 2 (2000) Both methods rely on the alkalihalide to dissipate the heat. A far better solution is to move the sample slowly and continuously under the laser beam say by rotation. Ventacon manufactures a general purpose sample rotator to minimise sample heating (Model SRP-1). The sample liquid, slurry or solid held in a vertical 5mm NMR tube is slowly rotated.
In this way, the laser illuminates a strip around the specimen rather than a point. If
sample heating does occur, the temperature rise is dissipated before the sample is
re-illuminated. In general use, it is found that considerably higher laser powers can be
used with this device improving the sensitivity of the measurement. |
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