FOR MORE DETAILED INFORMATION CONSULT TUTORIAL
Principle - Flow Injection
For more information on theoretical and practical aspects of
flow injection consult the complimentary CD available
Almost all reagent based assayscan be downscaled and automated by Flow Injection Analysis. FIA was first described in a patent filed in Denmark by Ruzicka and Hansen in 1974. Since then the technique has grown into a discipline covered by 21 monographs and almost 20,000 research papers. The scope of the method grew from a tool for automation of serial assays to a method for enhancing the performance of spectroscopic and electrochemical instruments. The first generation of FIA methodology, the Flow Injection (FI) is based on continuous flow, as described in this section. The second generation, the Sequential Injection (SI) utilizes flow programming to optimize the assay protocol.
The Principle
D
The FI technique is based on injection of sample solution into a continuously moving carrier solution that transports the assayed species through the reactor and into the detector. The assay protocol comprises the following steps :
A) Sample injection is designed to meter an exact volume of analyte solution into a flowing stream of reagent.
B) As the sample zone (red) moves downstream, the dispersion process mixes sample with reagent forming a reaction product (yellow). The extent of mixing and the length of reaction time is controlled by the flow rate, by channel volume, and by channel geometry.
C) The reaction mixture flows through the detector yielding an analytical readout. Since all standards and samples to be analyzed are individually processed in exactly the same way, the calibration curve is valid for samples to be assayed.
D) The peak height recorded by the detector is proportional to the analyte concentration.
Flow diagram for colorimetric assay of chloride
Colorimetric Assay of Chloride.
Each sample injected four times.
In this, the simplest configuration, a chloride sample is injected,
by means of a two position valve, into a continuously flowing
carrier stream of reagent. As the injected zone moves
downstream, the sample solution disperses within the reagent, forming a reaction product. A flow through detector placed downstream records absorbance, yielding a readout within 15 seconds after sample ( S1) injection.
Example: Nitrate Assay
FI manifold for automated assay of nitrate with integrated cadmium column using sulfanilamide reagent.. The calibration record of 0, 2, 5 and 8 ppm nitrate is followed by a routine run of nitrate assay extracted from soil samples.
Example: Hydride Generation for Atomic Spectroscopies ( AA and ICP)
This method is routinely used for trace analysis of As, Bi, Ge, Hg, Pb, Se, Sn and Te. In addition, assay of volatile compounds of Ag, Co, Cu, Ni, and Zn has been reported. Separation of the trace metals from complex matrices, analyte enrichment, fast reaction speed, and ease of automation were first demonstrated by Astrom (1982) in his pioneering work on the FI-based hydride AA assay of bismuth. By combining an acidified sample stream with a strong
reducing agent (sodium borohydride), hydrogen and metal hydride are rapidly released, and the gaseous phase is separated with the aid of purging gas (air or argon) and swept into AA or ICP for detection.
Flow Injection or Air Segmented ( Auto Analyzer ) Technique?
Both techniques share the ability to process a wide range of wet chemical assays, expediently, while using an experimental setup that provides full automation of the solution handling operations. Automation removes the burden of manual repetitive tasks that otherwise must be carried out in a routine laboratory and improves reproducibility of serial assays.
There are several reasons why FI replaced the Air Segmented ( Auto analyzer) technique and became the method of choice for routine and research laboratories. These are :
High sampling frequency: typically 2 samples / minute – or up to 6 samples / minute.
Short residence time: readout available within less than 30 seconds after sample injection.
Low sample consumption: typically 50 to 200 miroliters
Low reagent consumption: typically 1ml per assay
Small footprint: about 1/5 of a typical Auto Analyzer bench space .
Transparency: the entire flow path, including flow cell can be visually inspected during the assay
Enclosed System: no air segmentation. Processed solutions are isolated from ambient air.
Low cost replaceable components: No glass coils, no debubblers, only PEEK or Teflon tubing.
Fast Startup and Short Close Down Times: because the internal volume of FI system is less that 1/10th that of a typical Auto Analyzer system.
