4130

4130 INORGANIC NONMETALS BY FLOW INJECTION ANALYSIS*
4130 A. Introduction
1. Principle
Flow injection analysis (FIA) is an automated method of
introducing a precisely measured portion of liquid sample into a
continuously flowing carrier stream. The sample portion usually
is injected into the carrier stream by either an injection valve
with a fixed-volume sample loop or an injection valve in which
a fixed time period determines injected sample volume. As the
sample portion leaves the injection valve, it disperses into the
carrier stream and forms an asymmetric Gaussian gradient in
analyte concentration. This concentration gradient is detected
continuously by either a color reaction or another analyte-
specific detector through which the carrier and gradient flow.
When a color reaction is used as the detector, the color
reaction reagents also flow continuously into the carrier stream.
Each color reagent merges with the carrier stream and is added
to the analyte gradient in the carrier in a proportion equal to the
relative flow rates of the carrier stream and merging color
reagent. The color reagent becomes part of the carrier after it is
injected and has the effect of modifying or derivatizing the
analyte in the gradient. Each subsequent color reagent has a
similar effect, finally resulting in a color gradient proportional to
the analyte gradient. When the color gradient passes through a
flow cell placed in a flow-through absorbance detector, an ab-
sorbance peak is formed. The area of this peak is proportional to
the analyte concentration in the injected sample. A series of
calibration standards is injected to generate detector response
data used to produce a calibration curve. It is important that the
FIA flow rates, injected sample portion volume, temperature, and
time the sample is flowing through the system (“residence time”)
be the same for calibration standards and unknowns. Careful
selection of flow rate, injected sample volume, frequency of
sample injection, reagent flow rates, and residence time deter-
mines the precise dilution of the sample’s original analyte con-
centration into the useful concentration range of the color reac-
tion. All of these parameters ultimately determine the sample
throughput, dynamic range of the method, reaction time of the
color reaction discrimination against slow interference reactions,
signal-to-noise ratio, and method detection level (MDL).
2. Applications
FIA enjoys the advantages of all continuous-flow methods: There
is a constantly measured reagent blank, the “base line” against
which all samples are measured; high sample throughput encour-
ages frequent use of quality control samples; large numbers of
samples can be analyzed in batches; sample volume measurement,
reagent addition, reaction time, and detection occur reproducibly
without the need for discrete measurement and transfer vessels,
such as cuvettes, pipets, and volumetric flasks; and all samples share
a single reaction manifold or vessel consisting of inert flow tubing.
Specific FIA methods are presented as Sections 4500-Br
⫺
.D,
4500-Cl
⫺
.G, 4500-CN
⫺
.N and O, 4500-F
⫺
.G, 4500-NH
3
.H,
4500-NO
3
⫺
.I, 4500-N.B, 4500-N
org
.D, 4500-P.G, H, and I,
4500-SiO
2
.F, 4500-SO
4
2⫺
.G, and 4500-S
2⫺
.I.
4130 B. Quality Control
When FIA methods are used, follow a formal laboratory quality
control program. The minimum requirements consist of an initial
demonstration of laboratory capability and periodic analysis of
laboratory reagent blanks, fortified blanks, and other laboratory
solutions as a continuing check on performance. Maintain perfor-
mance records that define the quality of the data generated.
See Sections 1020 and 4020 for the elements of such a quality
control program.
*Approved by Standard Methods Committee, 2004.
Joint Task Group: 20th Edition—Scott Stieg (chair), Bradford R. Fisher, Owen B.
Mathre, Theresa M. Wright.
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