Double-beam photometric direct-injection detector for multi-pumping flow system

Abstract

A second generation of the photometric direct-injection detector for flow analysis is presented. In the detector, a log ratio amplifier to measure the absorbance in the system of double paired emission-detection LED diodes (double-PEDD) has been applied for the first time. The detector co-works with a solenoid pulse micro-pumps system and consists of two chambers: reaction-detection and reference. The sample and reagents are injected by solenoid micro-pumps directly into the reaction-detection chamber. The carrier solution flows consecutively through the reference and then reaction-detection chambers, cleaning them. Both chambers are irradiated by two identical emission LEDs. Two identical detection LEDs, situated at the opposite ends of both chambers, are connected with the integrated precise log ratio amplifier. Thus, the output voltage is a linear function of absorbance and can be measured properly, with high accuracy, over the entire range, up to 2.2. The software developed for controlling this system is implemented with the option of auto-zeroing in each measurement cycle. It results in a higher stability of the absorbance signal and possibility of working with solutions in which incidental gas bubbles are created.The performance of the detector is characterized using a series of methylene blue dye studies and analytical reactions of different kinetics. The potential applications to the Fe(III) determination using the thiocyanate method is discussed. The system is characterized by better repeatability (R.S.D. below 1%) and lower limit of detection (0.06mgL−1) than a similar, single-beam PEDD detector. The calibration graph proved to be linear in the range of up to 15mgL−1. It was found that the optical design of the detector minimizes the Schlieren effect.The chief advantages of the presented device are: very high stability and repeatability, simple and compact construction, high sampling frequency, low volume of the solutions used and low energy consumption. Therefore, the flow system can be especially recommended for the Green Chemistry monitoring stations working in a fully automatic mode.