Fundamentals of Flow Injection Analysis
- Lesson 1: Introduction
- Lesson 2: Fundamentals of Flow Injection Analysis
- Lesson 3: Membrane Sampling Devices
- Lesson 4: Dispersion
- Lesson 5: Enrichment
- Lesson 6: Chemistry
- Lesson 7: Sequential Injection Analysis
- Lesson 8: Zone Fluidics
DEFINITION OF FIA
FIA is both simple in concept, and difficult to define. Due to the great versatility and diversity of FIA, it is difficult to capture its broad scope and many facets in a single sentence definition. There are, in general, two types of definitions in use, which one might categorize as the "academic" and "industrial" definitions. An example of an academic definition describes FIA as:
"Information gathering from a concentration gradient formed from an injected, well-defined zone of a fluid, dispersed into a continuous unsegmented stream of a carrier"
A typical industrial definition describes FIA as:
"a simple and versatile analytical technology for automating wet chemical analysis, based on the physical and chemical manipulation of a dispersed sample zone formed from the injection of the sample into a flowing carrier stream and detection downstream"
Those most interested in the fundamentals of the FIA process will probably prefer the academic definition. Those who are interested in the analytical utility of FIA can probably relate better to the industrial definition.
We should add that FIA has advanced far beyond automating existing wet chemical analyses. Entirely new wet chemical analyses have been created with FIA, many of which are impossible to perform, or not readily performed, by conventional batch techniques.
Bo Karlberg, a pioneer in the development of FIA, suggested in his famous quote, an alternative to wrestling with the many definitions and descriptions of FIA,
"Flow injection analysis should not be explained.
We agree; when this is possible, there is no better way.
Until the technology becomes readily available to demonstrate FIA on the Internet, we will attempt to describe it.
GENERIC DESCRIPTION OF FIA
We will begin with a generic description of FIA in terms of the analytical functions it combines to perform a method. The schematic below groups the FIA process into three stages to help visualize how the technique performs a method or analysis.
First is sampling, where the sample is measured out and injected into the flowing carrier stream (thus, the name Flow Injection Analysis). This step is generally performed with a sample injection valve. The second stage is what we call sample processing. The purpose of this step is to transform the analyte into a species that can be measured by the detector and manipulate its concentration into a range that is compatible with the detector, using one or more of the indicated processes. The third stage is detection where the analyte, or a derivative of it, generates a signal peak that is used to quantify the compound being determined. As indicated, a large variety of detectors can be used in FIA.
The power of FIA as an analytical tool lies in its ability to combine these analytical functions in a wide variety of different ways to create a broad range of different methodologies, and perform these methodologies rapidly and automatically with minute (?L) amounts of sample. The first and last stages are, largely, conventional technology. It is the second stage, sample processing, that is the heart of FIA. A number of the most common analytical sample processing functions that can be performed by FIA are depicted in the box. For example, FIA can dilute by factors up to tens of thousands, and can enrich by several hundred. It can perform chemistry on an analyte to generate a detectable species. It can transfer an analyte from one medium to another, for example from a gas sample to a FIA carrier, and vice versa. It can do solvent extraction, and matrix modification or matrix elimination.
In the following sections, we will look in more detail at the aforementioned analytical functions performed in the three stages of the FIA process.
The device most commonly used to measure out the sample and insert it into the FIA carrier stream is a two-position sample injection valve. For years, HPLC valves were used for this purpose, but this is now considered an expensive overkill, since HPLC valves are designed for high pressures, whereas FIA is a low pressure technique. Low-pressure valves, such as the Valco C22 and C24 series, are now available for use in FIA. Important features that valves must have to be suitable for FIA include high precision, fast switching, pressure limits of about 100 psi, and ability to inject sample volumes from a few micro liters to several hundred micro liters, and in some cases fractions of a micro liter.
There are actually several different FIA techniques for dilution, the most common of which are listed below.
- Electronic Dilution
- Zone Sampling
- Gradient Chamber
- Membrane Sampling
Dilution by dispersion takes place in every FIA run.Controlled dispersion is one of the key processes in FIA, which disperses the sample into the carrier, both diluting it and mixing it with reagent. This technique is, without doubt, the simplest and most precise FIA technique for dilution. The smaller the sample, the greater the dilution; therefore, the key to large dilutions is the capability to precisely inject very small samples. The most precise tool for injecting extremely small volumes is a 2-position sample injection valve, such as the Valco C24 series, with a fixed, internal loop. Volumes as small as 0.2 ?l can be injected, thus giving dilution factors of several hundred-fold. A disadvantage of the internal loop model is that the volume is fixed, so that changing the volume requires changing the valve. A 2-position sample injection valve with an external loop allows the volume to be varied. Generally, the volume is varied by manually changing the length or ID of the external loop. However, the Valco C22 and C22Z series with its micro-electric actuator has the unique capability of "timed injection" switching under software control, which allows partial loop, variable volume injection with a fixed sample loop. Volumes from about 1 ?l up to the total volume of the loop can be injected. A Global FIA Technology Note, "Partial Loop (Timed) Injection" describes how the valve is used in this mode.
Electronic dilution is a novel and simple technique for dilution that is also based on dispersion. Electronic dilution simply refers to a technique of measuring the analytical signal somewhere along the detector peak where the signal is on-scale, as depicted in the next Figure. Generally, the signal is measured at the peak maximum, depicted as t1.
However, t2 or t3, which represents points in the sample zone where the sample is more highly dispersed or diluted, are equally valid points to take the analytical signal. The timing of the signal measurement must be precisely controlled for this technique to work well. "Electronic dilutions" up to approximately 100-fold can be achieved. While this technique is simple, it is probably the least precise of the dilution methods due to errors inherent in measurement on a sloping signal, and errors due to slight shifts in the peak position.
Zone Sampling is a relatively simple and powerful technique for dilution. It is similar to the "heart-cut" technique used in chromatography. It requires two sample injection valves set-up as depicted in the following figure.
With the first valve, a relatively large sample is injected, which then flows downstream dispersed by the reaction coil. As the dispersed sample zone fills the smaller sample loop of the second valve, an "aliquot" of the zone is injected into a second carrier stream. The second carrier can also pass through a reaction coil for further dispersion or dilution.
The following figure depicts how this technique removes a slice of the peak from the first injection for injection into the second carrier. Dilutions by a factor of several hundred can be achieved. Precise timing of the two valves is important, requiring software or an electronic timer for optimum precision.
A gradient chamber is a simple device for dilution in FIA and SIA, as depicted in the following figure. The gradient chamber is a small mixing vessel, generally with a stirring bar, and an inlet and outlet for the carrier stream. The volume of the chamber is generally 1 ml or less. Sample is injected into the carrier and undergoes a large dispersion as it mixes with the much greater volume of the carrier in the gradient chamber. Dilutions by a factor of several hundred can be achieved with good precision.
This completes this session of our Web Tutorial. The next lesson will begin with a description of membrane sampling devices and their use in dilution, as well as other applications.