Double beam thermal lens as detector system for capillary electrophoresis

The properties and performances of the Thermal Lens detector, based on a double beam absorption scheme, were studied in a capillary electrophoresis system with different types of pollutants, e.g. pesticides. The setup of the detector system is miniaturized using smallest deflection pathlengths between the cell and the pinhole (5mm). The probe laser beam (HeNe laser, 633nm) and the excitation beam (Ar+ ion laser, 364 nm, 457 nm, 488 nm, 514 Mn) with a crossed setup are directed by mirrors into two microscope objectives, which focused the beam to a 15 mu m waist inside the capillary. The detection volume is in the range of 75 nl using an 75 mu m capillary. The change of the intensity of the probe beam is detected behind a pinhole with a photodiode, which is protected with different band pass interference filters. The excitation laser can be used in the multiline order. Different types of hydrodynamic and electrokinetic probe injections are compared to optimize the probe volume. Micellar electrokinetic methods are used for separation of pesticides. The performance of the detector in capillary electrophoresis was assessed by different types of capillaries and compared with a conventional absorption detector. The limit of detection is at least one order of magnitude better than the absorption detector.