Laser-Based Double-Beam Thermal Lens Detector for Microcolumn Liquid Chromatography

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Abstract
A double-beam laser absorption detection system for high-performance liquid chromatography (HPLC) using the thermal lens effect has been developed. An electronic circuit is used to reduce the background noise level. The circuit is based on direct subtraction of reference and signal photocurrents under feedback control. A practical intensity signal-to-noise (S/N) ratio of 5 × 10−6 for normal absorption and 1 × 10−5 for thermal lens absorption measurements is achieved. The thermal-lens enhancement per unit laser power is 0.62 mW−1. For a 34-mW beam power at the sample cell, the absorption signal is 22 times higher in comparison to normal absorption. The system is shown to have a noise level of 2 × 10−7 AU. Amounts as low as 1 × 10−10 M 2′-Cl-4-dimethylaminoazobenzene at the flow cell or 1.5 fmol injected can be detected (S/N = 3). This corresponds to a 10-fold improvement in comparison to normal absorption measurements with the same electronic canceller and a 100-fold improvement in comparison to those for commercial absorption detectors.