Stray Light Analysis And Suppression In Fluorescence Detector

Fluorescence detector is one of the common detectors for high performance liquid chromatography (HPLC), it has the following advantages: high selectivity, only response to the fluorescent substances; high sensitivity, extremely low detection limit. Fluorescence detector can be applied for trace analysis of polycyclic aromatic hydrocarbons and various fluorescent substances, and has been widely used in petrochemical, organic synthesis, physiology and biochemistry, medicine and other fields [1-4]. Meanwhile, the sample volume of the fluorescence detector is extremely small (usually only 12μL), thus the fluorescence signal is very weak and is very sensitive to the stray light and the background light,The presence of stray light will not only increase the background intensity, flooding the weak fluorescence signal, lowering the signal to noise ratio, thereby hoist the detection limit, but also will undermine the monochromatic nature of the instrument, reducing the measurement accuracy of the fluorescence detector.This makes the study of stray light suppression in fluorescence detector very meaningful.Work of this paper was conducted on the double-scanning dual-monochromator-based fluorescence detector platform developed by our laboratories. Firstly, fluorescence detector control and data processing programs have been designed; Secondly, the performance of the fluorescence detector was tested and analyzed; thirdly, The fluorescence detector was modeled by the optical software ASAP and its accuracy has been verified; Finally, several stray light suppression structures have been proposed, their stray light suppression effects have been examined via simulations and experiments.The main work of this paper including:1) Control software and programs have been developed for the double-scanning dual-monochromator-based fluorescence detector. Programs coded via C language and assembly language on the IAR platform were compiled and written to the MSP430 microcontroller to accomplish: hardware control, interrupt response, communication with PC, etc.; User control interface was coded by MATLAB, it can transmit data with microcontroller via serial port, performing data processing and analysis, realizing real-time mapping and process control, etc.;2) Calibration and performance measurement has been performed, the fact that the key indicator of the fluorescence detector is low were analyzed and discussed, and corresponding experiments have been designed and performed, the test results indicate that the key factor which constraints the performance of the fluorescence detector is the stray light and noise;3) The optical analysis software ASAP has been used to create the fluorescence detector model, the contribution to stray light of the monochromator faces were analyzed and compared, and corresponding experiments were designed to verify the simulation results, the experimental results agree well with the simulation results;4) Proposed several structural designs for stray light suppression which are suitable for the monochromators in fluorescence detector, including the leaf structure, light block rings structure, and baffle structure. These stray light suppression structures were added into the ASAP model and their stray light suppression effects were verified via simulation, corresponding experiments were designed to inspect the simulation results, the emission monochromator was severed as the detector for the excitation monochromator, to distinguish the rays emits from the excitation monochromator; comparing the simulation results with the experimental results of the excitation monochromator shows that: vanes and apertures are very effective in suppression of stray light, with all structures installed in the monochromator at the same time, the stray light suppression ratio reaches over 80%. All in all, with all the stray light suppression structures installed in the fluorescence detector, the measured water Raman signal to noise ratio can reach up to 117.