Flow Injection Analysis (FIA) is an automated continuous-flow technique in which a precisely measured plug of sample is injected into a continuously flowing stream of carrier reagent. Unlike Segmented Flow Analysis (SFA), no air bubbles are used to separate sample from reagent. Instead, reproducibility is achieved through highly controlled injection volumes, pump speeds, tubing dimensions, and timing, so that each sample plug disperses in an identical and predictable way as it travels toward the detector.
FIA was introduced by Ruzicka and Hansen in the mid-1970s as a faster alternative to SFA for situations where high throughput of a single analyte is needed. Because there is no waiting for reactions to reach equilibrium — measurements are taken as the sample zone passes through the detector — cycle times can be very short, and throughputs of 100 to 300 injections per hour are achievable for simple determinations.
The trade-off versus SFA is that FIA typically offers less mixing of sample and reagent (since air-driven churning is absent), so it is less suited to analytes that require extended reaction times or complex multi-reagent chemistries. For well-characterised single-parameter work, however, it is highly competitive.
FIA is widely used for in-line or at-line process monitoring — for example, monitoring free cyanide in a gold leach circuit, or measuring ammonia in a wastewater treatment plant. The short cycle time and fast response make it attractive for time-sensitive process control.
The FS3700 Chemistry Analyser supports FIA alongside SFA and related methods on the same platform, allowing a laboratory to configure the instrument for either mode depending on the analyte and throughput requirement.
Key Points
- Sample plug injected into a flowing reagent stream with no air segmentation
- Very fast cycle times: 100 to 300 injections per hour for simple determinations
- Reproducibility depends on precise control of timing, flow rate, and injection volume
- Best suited to single-parameter, high-throughput, or process monitoring applications
- Complementary to SFA on the same instrument platform (FS3700)
Relevant Standards
- ISO 15923-1 (FIA general principle)
- Various ASTM and EPA methods adapted for FIA
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Frequently Asked Questions
When should I use FIA instead of SFA?
FIA is a good choice when you need the highest possible throughput for a single analyte and the chemistry is straightforward enough that extended reaction time is not needed. It is also preferred for in-line or at-line process monitoring where fast response matters. For complex multi-reagent chemistries or multi-parameter batch work, SFA is often more reliable.
Does FIA provide as good precision as SFA?
For well-optimised methods and well-controlled conditions, FIA can match SFA precision. In practice, SFA tends to be more forgiving of minor variations in flow conditions because the air segmentation ensures consistent mixing. FIA requires tighter control of all variables to achieve equivalent reproducibility.