Measuring Infrared Absorption in the Visible: Sensitive Determinations of Chemical and Isotopic Purity of Solvents by the Thermal Lens Effect

Abstract

A new technique has been developed in which the absorption in the infrared region is measured in the visible. This was accomplished by use of the visible (probe) laser to monitor the thermal lens effect that was induced in a sample as a consequence of its absorption of the radiation in the infrared. The sensitivity of the technique is much higher than that of conventional transmission measurements because, in addition to its inherent ultrasensitivity, the infrared absorption is measured in the visible region, which has relatively less noise, and is detected by a phase-lock detecting method. With the use of the partial least-squares method to analyze the data, this technique can be used for the nondestructive, noninvasive, and sensitive determination of the chemical and isotopic purity of samples. Specifically, it is capable of measuring water in D₂O and in tetrahydrofuran at levels as low as 3.75 × 10⁻⁴ and 1.20 × 10⁻⁴% (w/w), respectively.