Thermal lens technique for sensitive and nonintrusive determination of isotopic purity

A new technique in which absorption in the infrared region is sensitively monitored in the visible has been developed. This was accomplished using the visible laser to probe the thermal lens effect induced in a sample as a consequence of its absorption of 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, infrared absorption is monitored in the visible region, which is relatively less noisy, and is detected by a phase-lock-detecting method. The technique is so sensitive that it can be used to determine the isotopic purity of methanol and its 13C and deuterated samples by measuring, not directly the absorption of the C-H group but rather indirectly the absorption of the O-H group. With use of the multivariate calibration method to analyze the data, this technique can be utilized for the nondestructive, noninvasive, and sensitive determination of isotopic impurity in methanol, 13CH3OH, CD3OH, CD2HOH, and CDH2OH at concentrations as low as 10(-3)% (w/w).