Design and construction of a new configuration of crossed-beam thermal lens spectrometer operating at high flow velocity and its application for Cysteine determination in human serum and saliva

Thermal lens spectrometry (TLS) is a high sensitive method for trace determination of non-fluorescent materials [1]. Previous thermal lens spectrometers suffer from operating limitations at high flow velocities, arising from taking the heated element off the probe beam direction which results in a decrease in the thermal lens (TL) signal [2, 3]. Herein, we introduce a new optical configuration of crossed-beam thermal lens in transversal flow mode in which the propagating direction of the probe beam and liquid sample flow azimuth are concentric (CBTC). The system consists of a microfluidic cell with a volume of lower than 3 μL. In the current optical configuration, using 1-(2-pyridylazo)-2-naphthol (PAN) in ethanol as a test solution, by increasing the sample flow velocity and without increasing chopping frequency the reduction in sensitivity is less pronounced. Under 15 Hz chopping frequency, the optimum sample flow velocity is about 2 cm s-1 which is among the highest reported values achieved to date for photothermal lens spectrometers. Although, the system operates at higher flow velocities and lower chopping frequencies compared to the collinear configuration, it provides comparable analytical limit of detection. This optical configuration has been successfully employed for highly sensitive and selective determination of cysteine in human serum and saliva samples through a competitive complexation reaction with Cu-PAN as colorimetric probe. The detection limit of this method (9.5 nM) shows a significant enhancement (726-times) in comparison to UV-vis measurements.