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TUTORIAL
It may seem surprising, but FIA can be used not only to dilute, but also to enrich (pre-concentrate) an analyte. Furthermore, the same device, the membrane sampling device, (MSD) used for dilution can also be used for enrichment. For enrichment, the MSD is used in a unique configuration with a sample injection valve. As illustrated in the following figure, a tube-in-a-shell design MSD is installed across two adjacent ports of the valve.
A by-pass loop is installed across the two opposite adjacent ports. One of the remaining ports is used as an inlet for the carrier stream, and the other as an outlet. The sample stream containing the analyte to be enriched is pumped through the shell of the MSD. Enrichment and analysis are carried out in two timed steps, called the collection and flush steps. In the collection step, the valve is switched to the position that directs the carrier stream through the by-pass loop. During this phase, carrier is static in the MSD tube. With the sample stream flowing through the MSD, fresh analyte is continuously presented to the membrane surface to permeate or diffuse through the walls for capture by the carrier. This process causes the analyte to accumulate and concentrate in the carrier. After an elapsed time adequate to sufficiently enrich the analyte in the carrier, the valve is switched to initiate the flush step. The carrier now flows through the MSD, flushing the sample zone downstream for chemistry and/or detection. The enrichment/analysis can be a continuous automated process if a timer is used to control the valve. Another common technique for enrichment employs a mini-column packed with a material that retains the analyte. A typical configuration is shown in the next figure.
The mini-column is installed across opposite ports of a six-port injection valve. With the valve in one position, the sample stream flows through the column to waste, and analyte is sorbed onto the column. The valve is then switched, and eluent then flows through the column as depicted in the figure, eluting analyte from the column. Analyte originally contained in several mLs of sample can be eluted in a volume of only a few hundred µL. The concentrated analyte zone flows downstream, merging with a reagent stream (if needed) for chemistry, and then detection. Enrichment factors of up to 500 for trace metals have been achieved with this technique. An excellent reference for details on enrichment and pre-concentration by FIA is given below. REFERENCES 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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