27-12-2012, 02:23 PM
Introduction to Flow Injection Analysis (FIA)
[attachment=45419]
Flow Injection Analysis Principles
Flow injection analysis (FIA) is based on the injection of a liquid sample into a
moving, nonsegmented continuous carrier stream of a suitable liquid. The injected
sample forms a zone, which is then transported toward a detector that continuously
records the changes in absorbance, electrode potential, or other physical parameter
resulting from the passage of the sample material through the flow cell.
An example of one of the simplest FIA methods, the spectrophotometric
determination of chloride, is shown in Figure 2. This is based on the release of
thiocyanate ions from mercury(II) thiocyanate and its subsequent reaction with iron (III)
and measurement of the resulting red color (for details, see Experiment). The samples,
with chloride contents in the range 5 -75 ppm chloride, are injected (S) through a 30 mL
valve into the carrier solution containing the mixed reagent, pumped at a rate of 0.8
mL/min. The iron(III) thiocyanate is formed on the way to the detector (D) in a mixing
coil (0.5 m long, 0.5 mm i.d.), as the injected sample zone disperses in the carrier stream
of reagent. The absorbance A of the carrier stream is continuously monitored at 480 nm
in a micro flow-through cell (volume of 10 mL) and recorded (Figure 2b). To
demonstrate the reproducibility of the analytical readout each sample in this experiment
was injected in quadruplicate, so that 28 samples were analyzed at seven different
concentrations of chloride. As this took 14 min, the average sampling rate was 120
samples / h. The fast scan of the 75- and 30-ppm sample peaks (shows on the right in
Figure 1b) confirms that there was less than 1% of the solution left in the flow cell at the
time when the next sample (injected at S2) would reach it, and that there was no
carryover when injecting the samples at 30-s intervals.
The FIA System
The simplest flow injection analyzer (Figure 3a) consists of a pump, which is used
to propel the carrier stream through a narrow tube; an injection port, through which a
well-defined volume of a sample solution S is injected into the carrier stream in a
reproducible manner; and a microreactor in which the sample zone disperses and reacts
with the components of the carrier stream, forming a species which is sensed by a flowthrough
detector and recorded. A bypass loop allows passage of carrier when the
injection valve is in the load position. A typical recorder output has the form of a peak
(Figure 3b), the height H, width W, or area A of which is related to the concentration of
the analyte. The time span between the sample injection S and the peak maximum,
which yields the analytical readout as peak height H, is the residence time t during which
the chemical reaction takes place. A well-designed FIA system has an extremely rapid
response, because T is in the range 5 - 20 s. Therefore, a sample cycle is less than 30 s
(roughly T + tb) and thus, typically, two samples can be analyzed per minute. The
injected sample volumes may be between 1 and 200 mL (typically 25 - 50 mL), which in
turn requires no more than 0.5 mL of reagent per sampling cycle. This makes FIA a
simple, automated microchemical technique, capable of a high sampling rate and
minimum sample and reagent consumption.
Flow System Characterization
Determination of flow rates. Fill a 10 mL graduate cylinder with distilled water,
record the volume and insert the carrier tube in the cylinder. Turn on the pump and
simultaneously start a stop watch (or begin timing with a clock). Pump for five minutes,
remove the tube, and turn off the pump. Record the volume of water remaining in the
cylinder. Calculate the flow rate in milliliters per minute and record on the form at the
end of the experiment.
Perform a similar experiment for the sample flow (injector in the load position).
Finally measure the flow rate of the waste stream from the volume collected for
five minutes.
If pump tubing of equal internal diameter is used for all channels, the flow rates
should be similar. Also, the flow rate of the waste stream should equal the flow rate of
the carrier stream.
Note that the flow rate is directly proportional to the square of the internal radius
of a pump tube (i.e., flow is proportional to cross section area = pr2).
Estimation of sample loop volume. With the pump turned on, place the valve in
the load position and remove the sampling tube from the sample. This will allow the
loop to fill with air. Then insert the tube in the sample solution and with a stopwatch
measure the time from when the sample just enters the loop to when it leaves the loop.
Perform the measurement several times and take the average. From a knowledge of the
sample flow rate determined above and the measured time to fill the loop, calculate the
sample loop volume in microliters. NOTE: This is an estimate and will not include the
dead volume of the injector.
[attachment=45419]
Flow Injection Analysis Principles
Flow injection analysis (FIA) is based on the injection of a liquid sample into a
moving, nonsegmented continuous carrier stream of a suitable liquid. The injected
sample forms a zone, which is then transported toward a detector that continuously
records the changes in absorbance, electrode potential, or other physical parameter
resulting from the passage of the sample material through the flow cell.
An example of one of the simplest FIA methods, the spectrophotometric
determination of chloride, is shown in Figure 2. This is based on the release of
thiocyanate ions from mercury(II) thiocyanate and its subsequent reaction with iron (III)
and measurement of the resulting red color (for details, see Experiment). The samples,
with chloride contents in the range 5 -75 ppm chloride, are injected (S) through a 30 mL
valve into the carrier solution containing the mixed reagent, pumped at a rate of 0.8
mL/min. The iron(III) thiocyanate is formed on the way to the detector (D) in a mixing
coil (0.5 m long, 0.5 mm i.d.), as the injected sample zone disperses in the carrier stream
of reagent. The absorbance A of the carrier stream is continuously monitored at 480 nm
in a micro flow-through cell (volume of 10 mL) and recorded (Figure 2b). To
demonstrate the reproducibility of the analytical readout each sample in this experiment
was injected in quadruplicate, so that 28 samples were analyzed at seven different
concentrations of chloride. As this took 14 min, the average sampling rate was 120
samples / h. The fast scan of the 75- and 30-ppm sample peaks (shows on the right in
Figure 1b) confirms that there was less than 1% of the solution left in the flow cell at the
time when the next sample (injected at S2) would reach it, and that there was no
carryover when injecting the samples at 30-s intervals.
The FIA System
The simplest flow injection analyzer (Figure 3a) consists of a pump, which is used
to propel the carrier stream through a narrow tube; an injection port, through which a
well-defined volume of a sample solution S is injected into the carrier stream in a
reproducible manner; and a microreactor in which the sample zone disperses and reacts
with the components of the carrier stream, forming a species which is sensed by a flowthrough
detector and recorded. A bypass loop allows passage of carrier when the
injection valve is in the load position. A typical recorder output has the form of a peak
(Figure 3b), the height H, width W, or area A of which is related to the concentration of
the analyte. The time span between the sample injection S and the peak maximum,
which yields the analytical readout as peak height H, is the residence time t during which
the chemical reaction takes place. A well-designed FIA system has an extremely rapid
response, because T is in the range 5 - 20 s. Therefore, a sample cycle is less than 30 s
(roughly T + tb) and thus, typically, two samples can be analyzed per minute. The
injected sample volumes may be between 1 and 200 mL (typically 25 - 50 mL), which in
turn requires no more than 0.5 mL of reagent per sampling cycle. This makes FIA a
simple, automated microchemical technique, capable of a high sampling rate and
minimum sample and reagent consumption.
Flow System Characterization
Determination of flow rates. Fill a 10 mL graduate cylinder with distilled water,
record the volume and insert the carrier tube in the cylinder. Turn on the pump and
simultaneously start a stop watch (or begin timing with a clock). Pump for five minutes,
remove the tube, and turn off the pump. Record the volume of water remaining in the
cylinder. Calculate the flow rate in milliliters per minute and record on the form at the
end of the experiment.
Perform a similar experiment for the sample flow (injector in the load position).
Finally measure the flow rate of the waste stream from the volume collected for
five minutes.
If pump tubing of equal internal diameter is used for all channels, the flow rates
should be similar. Also, the flow rate of the waste stream should equal the flow rate of
the carrier stream.
Note that the flow rate is directly proportional to the square of the internal radius
of a pump tube (i.e., flow is proportional to cross section area = pr2).
Estimation of sample loop volume. With the pump turned on, place the valve in
the load position and remove the sampling tube from the sample. This will allow the
loop to fill with air. Then insert the tube in the sample solution and with a stopwatch
measure the time from when the sample just enters the loop to when it leaves the loop.
Perform the measurement several times and take the average. From a knowledge of the
sample flow rate determined above and the measured time to fill the loop, calculate the
sample loop volume in microliters. NOTE: This is an estimate and will not include the
dead volume of the injector.