Helium Neon Laser Intracavity Photothermal Beam Deflection Densitometer

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Abstract
A novel, compact He-Ne laser intracavity photothermal beam deflection densitometer has been developed. In this instrument, a TLC plate is placed inside the cavity of the He-Ne laser, which serves as a probe. An argon-ion pump-laser overlaps with the probe beam at the chro-matogram spot. The apparatus is 34 times more sensitive than the corresponding extracavity instrument. The technique not only was able to detect bilirubin, bilirubin ditauride, and half-bilirubin on a developed HPTLC but also provided insight into the photophysical processes of those pigments. The detection limit for bilirubin, excited by the 476-nm laser line, at 20 mW intensity, modulated at 7 Hz, is 2.5 ng.