Comparison of collinear and transverse photothermal deflection spectroscopy for trace analysis of pesticides in water

Summary

A fiber optic modified photothermal deflection spectrometer (PTDS) was developed and tested for trace analysis of pesticides in water. A UV argon laser beam (λ=363.8 nm) was delivered to a sample cuvette either directly, or through a multimode optical fiber of 400 μm core diameter. The HeNe-probe laser beam was guided to the same sample cuvette by a monomode optical fiber of 5 μm core diameter and focussed not only transverse within an angle of 90°, but also collinear within an angle of approximately 0° to the Ar⁺-excitation beam. In transverse dual beam experiments a limit of detection (LOD) of 400 ppt for the pesticide DNOC (4,6-dinitroorthocresol) was achieved. The LODs for 2,4-dinitrophenol, dinoseb and dinosebacetate were 2 ppb, 18 pbb and 6 ppb, respectively. These LODs obtained with the transverse PTDS are up to 2 orders of magnitudes lower than the values taken with the Cary 2400 spectrophotometer. In collinear PTDS experiments with the pesticide dinosebacetate the enhancement factor of the LOD is about 4, when compared to the transverse PTDS using 1×1 cm² diameter sample cuvettes.