The design and characteristics of a dynamically growing and falling liquid drop based optical detection system are described for use with flow injection analysis (FIA). Several aspects of the analytical potential of a liquid drop as an optical cell are demonstrated by the detection of sulfate through the precipitation reaction between barium ion and sulfate ion in a FIA system. Deposition of precipitate on the windows of a conventional flow cell usually constitutes a vexing problem; this is avoided due to the windowless nature of the drop based cell. Gradient dilution techniques are conveniently implemented without precise external timing: With small drops, a single FIA peak is spread over a multitude of drops. Each individual drop represents a different dilution ratio and the dynamic range of measurement can be greatly increased based on the choice of the specific drop used for quantitation. Both absorbance/turbidimetric detection and nephelometric detection can be made on a single drop using a single optical fiber for light collection. Different configurations for mixing and reaction of the sample and reagents in a drop based system has been studied.
Figure 1. (a) Schematic diagram of the drop-cell detection arrangement. h is the vertical distance between the drop head and the centerline of the optical fiber, d is the insertion depth of the fiber beyond the chamber wall; (b) the bottom view of the drop head.
Figure 3. Apparent absorbance signal and the photocurrent registered by the detector photodiode during a FIA cycle (comprising of 8 sequential drops) for a 400 mg /l sulfate standard. The inset shows the FIA peak reconstructed from the absorbance minima.