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TUTORIAL
Chemistry plays a central role in most FIA methodologies. Its purpose is to convert the analyte into a detectable species compatible with the detector in use. Getting the chemistry right for a FIA methodology is often a challenging, and sometimes daunting task. However, the task is made easier by the fact that there is a rich resource of colorimetric analytical methods in the literature which were developed decades ago when colorimetric analyses were widely used. In most cases, these methods can be adapted to FIA. This is one of the reasons that colorimetric FIA methodologies are very common. Other common manual wet chemistry techniques have also been applied to FIA. Of these, titrations are an interesting group that will be discussed in a future lesson. The variety of chemistries that are used in FIA, or could potentially be used in FIA, is too vast to be covered in our tutorial. What we will attempt to do is provide some guidelines to the reader on how to go about selecting a chemistry for a new application he/she has in mind. The first step should be to search the FIA literature to determine whether a FIA method has already been developed for your analyte of interest. With the 10000+ publications on FIA/SIA, there is a good probability that some work has already been done which could at least serve as a starting point for developing your method. Fortunately, there is a free searchable FIA/SIA database on the Internet at the address, www.fia.unf.edu. This database is constantly being updated and expanded. Several books on FIA also contain a considerable
number of method descriptions. The books are referenced at the
end of this chapter. Adapting a manual method to FIA/SIA, or even modifying an existing FIA method, requires a good deal of skill. This can only be gained by experience. For a beginner, a good place to start is to set-up the well-established and tested "BTB method". This FIA method is described on page 301-302 of Ruzicka and Hansensı book listed in the references. BTB is bromthymol blue, an acid base indicator. The BTB method is a simple, single stream optical method that is often used for teaching FIA, as well as testing new FIA instruments functionally and for measuring performance. Practice with the BTB method will acquaint you with the FIA process, and provide a good starting point for developing your own FIA method. Most FIA methods are more complex than the BTB method. Some very complex, multi-step chemistries can be performed with FIA. In fact, some FIA methodologies, such as some involving unstable reaction intermediates, are unique to FIA and cannot be performed, or are difficult to perform, with manual or other analytical techniques. One of the most important points to keep in mind in adapting a manual method to FIA is that manual methods are generally equilibrium based while FIA methods are non-equilibrium based. To understand this, the beginner should study our earlier chapter on an introduction to dispersion thoroughly, and then use the BTB method and the more detailed explanation and exercises on dispersion in Ruzicka and Hansensı book to get hands-on experience with how parameters such as sample size, flow rate, reactor dimensions, and manifold configuration affect dispersion and chemistry. This should lead to a good working knowledge on how to use these parameters to manipulate dispersion and chemistry, and with that, you should be ready to start developing your own FIA method. The most common manifold configurations are what is often called single-stream, two stream, and three stream manifolds. A single-stream manifold is depicted schematically in the following figure.
In this scheme, a stream containing reagent is pumped through the system. A volume of sample is injected into the stream and dispersion causes mixing of the reagent with the sample zone leading to chemistry between analyte and reagent as the zone passes through the reactor and detector. The next figure depicts a two-stream FIA manifold.
The two-stream manifold can be used in two different schemes. The first is with a single stage chemistry, similar to that described for the single-stream application. However, in the two-stream approach, the sample is injected into a reagentless carrier, and the reagent stream is merged with it downstream. This provides a uniform mixing of the reagent with sample over the length of the sample zone, and often provides better sensitivity and performance compared to the single-steam approach. For this reason, the two-stream manifold is often preferred over the single-stream manifold for simple chemistries. The second scheme involves two stage chemistries. The sample is injected into the first stream containing reagent 1. The analyte reacts with the reagent, generating an intermediate. On merging with stream two downstream, the intermediate reacts with reagent 2, forming the product that is measured by the detector. An example of a two-stream system is the widely used FIA methodology for determining cyanide based on the König reaction, depicted in the next Figure.
The first stream contains the reagent, chloramine-T. On injecting the sample into this stream, the cyanide reacts with the chloramine-T to form cyanogen chloride. The second stream contains a mixture of sodium isonicotinate (INA) and 3-methyl-1-phenyl-2-pyrazollin-5-one (PZ). On merging with this stream, the cyanogen chloride reacts with the PZ to form a highly colored purple product. The next figure depicts a three stream manifold.
Three stream manifolds are generally used for two stage or three stage chemistries. When used for two stage chemistries, the sample is injected into a reagentless carrier, and merges downstream with the first reagent. Further downstream, it merges with the second reagent. For three stage chemistries, the sample is injected into a stream containing the first reagent. The product of this reaction merges with the second reagent downstream to form a second intermediate. This intermediate then merges with the third reagent downstream to form the final product that is measured by the detector. An example is a widely used chemistry for the determination of trace levels of ammonia, depicted in the next Figure.
The ammonia sample is injected into the carrier containing the first reagent, alkaline phenate. Downstream, the sample zone merges with a stream of hypochlorite, and the combination of the two reagents react with ammonia to form highly colored indophenol. Merging with a stream of nitroprusside further downstream leads to the formation of a complex that enhances the color intensity and increases sensitivity. The foregoing examples show that FIA can perform some complex, multi-step analytical chemistries. As mentioned earlier, there are a significant number of cases where FIA can perform an analysis that is difficult, if not impossible, by conventional wet chemical techniques. These generally involve unstable reagents, which can be generated on-line by FIA, or unstable chemistry intermediates that can be measured by FIA. An example of the latter is the cyanide method discussed above. The second stage of the Chemistry produces an intensely colored purple product that has a half-life of only 90 sec. This is long enough to capture a peak in FIA, but much too short for a conventional colorimetric technique. In the conventional manual technique, a blue final product of much lower color intensity is measured, giving the manual method a lower sensitivity. REFERENCE BOOKS MANUAL SPECTROPHOTOMETRIC
METHODS This completes this session of our Web Tutorial. © Global FIA, Inc, 2003 Home | About Us | Buy | Tutorial | Library | What's New | Database | Contact Us | Search | Checkout |
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