Direct and indirect detection of liquid chromatography by infrared thermal lens spectrometry

A novel, sensitive and universal detector for liquid chromatography has been developed. This detector is based on the measurement of infrared absorption of effluents by the thermal lens effect. In this instrument, the chromatographic effluent was excited by infrared radiation derived from a solid-state tunable F-center laser. The heat generated as a consequence of the sample absorption of the infrared radiation was measured by a He-Ne monitoring laser whose beam was collinearly overlapped with the excitation infrared beam in the sample. The technique is universal because it can be used for the detection of compounds which have absorption in the infrared region (by direct detection) as well as non-absorbing compounds (by indirect detection). The sensitivity of the technique is directly proportional to the excitation laser power, and with the used of laser power of only 4.5 mW, the technique is at least ten times more sensitive than corresponding absorption detectors. A detection limit of picograms was achieved for phenol and its chloro substituents.