Flow Injection Analysis

Batch analysis typically requires 10–30 minutes per sample, includes human labor for pipetting and mixing, and consumes large amounts of reagents. FIA analyzes a sample in seconds with fully automated operation, consuming just microliters of reagent. This reduces cost per analysis, improves throughput, and eliminates inter-operator variability. The tradeoff is that FIA must be optimized for each specific analyte.

FIA quantifies by comparing the peak area or height of unknown samples to a calibration curve established using known standards, all analyzed under identical conditions (same flow rates, injection volume, tubing geometry, temperature). Because all injections are treated identically, the signal is reproducibly proportional to analyte concentration even if the reaction is incomplete. This is possible because every sample spends the same time in the system, achieving the same degree of reaction conversion.

Carryover results from residual analyte or reagent remaining in the tubing and detector after the previous sample, which contaminates the next injection. Minimization strategies include increasing wash volume between injections (using carrier solution or rinsing solution), extending wash time, using tubing with smaller diameter (less volume), and selecting injection valve designs with minimal dead volume. Validation runs (analyzing blanks after high-concentration samples) confirm carryover is negligible.

Peak signal depends on: (1) reagent concentration—increase for faster or more complete reaction; (2) flow rate—slower flow allows longer reaction time; (3) mixing coil length—longer coils increase mixing and reaction time; (4) injection volume—larger injections increase peak height (but may affect baseline). Optimization involves systematically varying these parameters and selecting settings that maximize signal-to-noise ratio and peak resolution.

FIA uses multiple peristaltic pumps to simultaneously deliver carrier, reagents, and sample into a manifold, creating a continuous stream. SIA uses a single pump and stores all liquids in ports, aspirating them in programmed sequences into a single flow line. SIA offers greater flexibility (easier to change methods, fewer reagent lines), uses even smaller sample/reagent volumes, and can perform multiple assays per sample. FIA offers simpler manifold design and is faster for single high-volume assays.