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TUTORIAL
The final dilution technique we will cover is membrane sampling. Membrane sampling devices (MSD) can be used not only for dilution, but also for a range of other sample processing operations, such as matrix modification or elimination, sampling of gas streams, solvent extraction, and analyte enrichment. Because they are so versatile, we will digress briefly, to look at what membrane sampling devices are and how they work. Then we will discuss how they are used for dilution; later in the Tutorial, their application to other sample processing operations will be covered. MSDs are designed with channels for two flowing streams separated by a thin membrane, as depicted in the following figure.
One of the streams is called the donor stream, which can be either a liquid or a gas stream. The other is the receiving stream. The donor stream contains the analyte, and can either be a dispersed sample zone injected into a carrier stream, or the sample stream itself. If a suitable membrane is selected, when the donor stream containing the analyte flows past the membrane, some of the analyte, A, will be transferred by mass transport across the membrane to the receiving solution. Generally, only a fraction will be transferred, but with fixed, reproducible conditions, the fraction will be constant. This allows calibration for quantitative analysis. A number of different types of membranes have been studied as membrane sampling devices for FIA and reported in the literature, but most of them fall into the three categories listed in the following figure.
The main difference that is of relevance to FIA between these types of membranes is the mode of mass transport across the membrane wall. With micro porous membranes, mass transport occurs by diffusion of the analyte through the pores. Generally, these membranes are used with volatile analytes, such as dissolved CO2, NH3, H2S, and HCN. In the case of nonporous membranes, the analyte actually dissolves in the membrane and diffuses through the wall structure to the receptor stream side. Only neutral molecules have any appreciable solubility in silicone rubber, the most commonly used nonporous membrane, so this membrane does not work with ionic species. For this type of membrane to effectively transfer analyte from the walls of the membrane to the receptor stream, the analyte must have a much greater solubility in the receptor stream than in the membrane, or become converted to a species with this property. Generally, this is achieved using a chemical reaction with a reagent in the receptor stream that converts the analyte to a soluble ionic species. For example, for transport of NH3, an acidic receptor stream will convert the ammonia to NH4+. The third type of membrane listed is ionic, where ionic transport moves the analyte across the membrane wall. Obviously, this type of membrane is used for ionic species. The most common membrane of this type is Nafion. Nafion is a perfluoronated hydrocarbon polymer with pendant sulfonic acid groups. It readily transports small, monovalent cations, such as H+ and NH4+. The rate of mass transport across membrane walls is dependent upon, among other things, the wall thickness. Therefore, thin-walled membranes are generally used in FIA, usually with wall thicknesses in the range of 0.01 mm to 0.2 mm. A number of different designs of membrane sampling devices have been published in the literature, but most are either a flat plate type or a tube-in-a-shell type. The flat plate designs, an example of which is illustrated in the next figure, use planar membranes.
Channels are cut on the faces of the flat plates for the donor and receptor streams, and the planar membrane is sandwiched between the plate channels. The tube-in-a-shell design, an example of which is shown in the next figure, employs a hollow fiber membrane with a concentric shell surrounding it. Generally, the receptor stream is pumped through the hollow fiber, and the donor stream is pumped through the shell.
The use of membrane sampling devices for dilution is relatively simple. A donor stream is flowed on one side of the membrane, and a receptor stream on the other. The sample is injected into the donor stream, and as the sample segment passes over the membrane, a fraction of it becomes transported to the receptor stream. The dilution factor depends on the sample volume injected, the thickness of the membrane, the surface area of membrane, the channel dimensions in the membrane sampling device, and the flow rates of the two streams. 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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