Primer on Flow Injection Analysis

Product Information

Components

Specifications

A Primer on Flow Injection Analysis

Last update: September 4, 2008

Flow injection analysis (FIA) was originated in 1975 by scientists who were trying to process large numbers of samples and became frustrated with the bottleneck to their productivity caused by their segmented flow analyzers (SFA). Their initial innovations leading to FIA included -

Eliminating the bubbles from the analytical stream
Decreasing the inner diameter of the reactor tubing
Precisely injecting samples into the analytical stream

The result was analytical peaks with very rapid rise and recovery times and complete inter-sample washout.

As the technology developed, many other aspects of FIA were discovered that have become uniquely beneficial to routine analytical laboratories.

Startup and shutdown times are so short that it is very practical to change over between analytical methods rather than dedicating channels as is required with SFA.

Short analysis times, enabled by the highly reproducible sample volumes and residence times, allow samples to be analyzed in near real-time.

Finally, due to the near real-time sample analysis, data quality can be monitored and corrective actions taken as the analyses proceed, not at the end of the tray when it is too late.

FIA is really more analagous to modern HPLC than to SFA. In fact, FIA has been called "HPLC without the column and high pressures". The following table supports this analogy:

Comparison of Characteristics of FIA, HPLC and SFA

Parameter	FIA	µHPLC	SFA
Sample introduction	Injection	Injection	Aspiration
Sample volume	µL	µL	mL
Analytical stream	Unsegmented	Unsegmented	Segmented
Manifold conduits	<1 mm i.d.	<1 mm i.d.	1-2 mm i.d.
Lag phase	None	None	Significant
Normal data reduction	Integration	Integration	Height
Mixing conditions	Laminar	Laminar	Turbulent

FIA Productivity Characteristics

Fast Startup:	~5 minutes
Rapid Analysis:	20 to 60 seconds is typical
High Sample Throughput:	60 to 120 samples per hour is typical
Broad Working Ranges:	Parts per trillion to percents
Complete Baseline Resolution:	No carryover between samples
Wide Dynamic Range:	2 to 3 decades is typical
Fast Shutdown:	~5 minutes
Rapid Method Changeover:	~10 minutes
Intelligent Auto Dilution:	Off-scale samples are automatically diluted using the correct ratio with no operator intervention
Data Quality Control:	Real-time closed-loop control of data quality

Over 450 Methods Developed Over the Past 20 Years - Both Simple and Complex!

Lachat's library of QuikChem methods, all developed using flow injection analysis technology, provide a powerful array of analytical tools to measure over 70 different analytes in nearly 30 sample matrices. To provide some insight into the level of complexity that can be accommodated by FIA, we have developed a suite of methods which perform both in-line sample preparation followed by determination of cyanides, phenolics, total nitrogen, total phosphorus and surfactants. Here is a sample schematic of one of the in-line nitrogen methods:

As you can see, flow injection analysis is very capable of accommodating complex analytical processing requirements.

Comparison of the Operating Characteristics of FIA and SFA*

	FIA	SFA
Startup Time	3-5 minutes	30 minutes
Analysis Time	30-60 seconds	5-20 minutes
Nutrient Detection Limits	<1 ppb	<1 ppb
Baseline Resolution (between samples)	Yes	No
Bubbles & Surfactant Required	No	Yes
Shutdown Time:	2-5 minutes	10-40 minutes
Method Changeover Time	10 minutes	60-80 minutes
Closed-Loop Data Quality Control	Yes	No
Intelligent Auto Dilution	Yes	No
Post-Run Carryover Correction Required	No	Yes
Performs Complex Chemistries	Yes	Yes

*Data presented in this table are from actual users of FIA and SFA systems.

Sensitivity

Since FIA processes samples so rapidly and does not normally bring the reaction to completion, a common question is "How does FIA achieve the same minimum detection limits as SFA?". The answer is that, due to the absence of air bubbles, the noise contributed by the analytical stream is minimal so the signal-to-noise ratio in FIA is better than that in SFA. But rather than speak in the abstract or theoretical, let's look at an actual method - the determination of orthophosphate in waters and wastewaters.

Method Detection Limit for orthophosphate using 1 µg P/L standard
MDL= 0.1 µg P/L
Standard Deviation (s) = 0.04 µg P/L, Mean (x) = 0.04 µg P/L, Known value = 1.0 µg P/L

This level of detection is achieved without the need for reaching steady state nor using a specially crafted and expensive long path length flow cell as are required with SFA. In addition, interference from silica is minimized compared to SFA as the reaction time with orthophosphate is much faster and, since FIA does not go to steady state, the formation of the silico-molybdate complex is kinetically discriminated against - a unique feature of FIA.

Perhaps most importantly, methods based on flow injection analysis are accepted for use in communities regulated by the USEPA, ISO, DIN, EN and others. The consensus organizations also have started to accept these methods. For example, methods based on FIA appear in the 20th Edition of Standard Methods for the Examination of Water and Wastewater.

Brackish Water Applications

Samples having varying degrees of salinity offer a special challenge to automated analysis as refractive index effects can mask or deform the peak shape. Several techniques have been used over the years with SFA including matrix matching, nutrient-depleted seawater, artificial seawater and subtraction of reagent blanks. To avoid these complex and time-consuming approaches, QuikChem brackish water methods use two simple parameters:

Injection of a relatively large sample volume to achieve a physical steady state in the flow cell
Use of user-set integration times to capture peak area, free from interference from the refractive index perturbations.

The above discussion presents just a few examples of how flow injection analysis technology brings powerful and flexible capabilities to automated ion analysis. As an analog to HPLC, FIA uses modern principles of fluid dynamics to effect sample introduction, mixing, reaction, heating, dialysis, digestion and extraction. The proof of the efficacy of any analytical technique is not in an extended discussion of theory, but the presentation of empirical data. Every one of the large library of QuikChem methods includes Method Support Data demonstrating accuracy, precision, method detection limit and carryover.