Preconcentration preconcentrator

Ma, M. and F.L. Cantwell. 1999. Solvent microextraction with simultaneous back-extraction for sample cleanup and preconcentration Preconcentration into a single microdrop. Anal. Chem. 71 388-393. 

Our goal was to determine if the biuret reagent would be effective at low concentrations and coupled with preconcentration. Preconcentration with derivatization requires (1) derivatization chemistry that does not produce extraneous compounds that are also preconcentrated and give rise to background peaks, (2) derivatization chemistry that is compatible with weak mobile phases (aqueous solutions for reversed-phase EC), and (3) derivatization chemistry that has adequate kinetics and specificity to work quantitatively at trace concentrations. 

Two frequently encountered analytical problems are (1) the presence of matrix components interfering with the analysis of the analyte and (2) the presence of analytes at concentrations too small to analyze accurately. We have seen how a separation can be used to solve the former problem. Interestingly, separation techniques can often be used to solve the second problem as well. For separations in which a complete recovery of the analyte is desired, it may be possible to transfer the analyte in a manner that increases its concentration. This step in an analytical procedure is known as a preconcentration. 

Two examples from the analysis of water samples illustrate how a separation and preconcentration can be accomplished simultaneously. In the gas chromatographic analysis for organophosphorous pesticides in environmental waters, the analytes in a 1000-mL sample may be separated from their aqueous matrix by a solid-phase extraction using 15 mb of ethyl acetate. After the extraction, the analytes are present in the ethyl acetate at a concentration that is 67 times greater than that in... 

Compton, T. R. Direct Preconcentration Techniques. Oxford Science Publications Oxford, 1993. 

Trace metals in sea water are preconcentrated either by coprecipitating with Ee(OH)3 and recovering by dissolving the precipitate or by ion exchange. The concentrations of several trace metals are determined by standard additions using graphite furnace atomic absorption spectrometry. 

When the sample contains <0.1% selenium or if interfering substances are present, selenium may be preconcentrated by distillation from a bromine—hydrobromic acid mixture ... 

Eeed should be close to saturation limit before cooling to maximize potential recovery (consider preconcentration step to remove excess solvent). 

Generally best when crystallising component is large percentage of feed (consider preconcentration step if dilute). 

Because a preconcentration step is probably needed to make the final sequence more economical, it is logical to start with the opportunistic separation. This separation produces one of the products, pure water, as the underflow and a concentrated distillate appropriate for feed into either strategic separation. Arbitrarily choosing pervaporation first, the retentate has a composition on the 2-propanol-rich side of the azeotrope, whereas the permeate is pure water. No further strategic separations are required. 

If an opportunistic preconcentration of the feed is used instead, an entirely different flow sheet results. In this case the MSA composition is a two-phase mixture of methylene chloride and water. Detailed simulations ate requited to determine which of these (or other) 2-ptopanol dehydration flow sheet alternatives is the economically advantaged process. 

Ion Removal and Metal Oxide Electrodes. The ethylenediamine ( )-functional silane, shown in Table 3 (No. 5), has been studied extensively as a sdylating agent on siUca gel to preconcentrate polyvalent anions and cations from dilute aqueous solutions (26,27). Numerous other chelate-functional silanes have been immobilized on siUca gel, controUed-pore glass, and fiber glass for removal of metal ions from solution (28,29). 

Frequently, preconcentration of an analyte is necessary because the detector used for quantitation may not have the necessary detectabiUty, selectivity, or freedom from matrix interferences (32). Significant sample losses can occur during this step because of very small volume losses to glass walls of the recovery containers, pipets, and other glassware. 

Total solar salt, NaCl, produced in the world is 90 million tons. Well over that amount of salt is produced in preconcentration ponds as an intermediate step in the production of other chemicals such as potassium chloride. For example, the Dead Sea faciUties produce 40 million tons of salt each year but sell none because of the high cost of transportation to markets. 

Production of KCl at the Wendover, Utah operation employs a large 7000 acre complex of solar ponds. Both shallow brine wells and deeper wells are used to pump brine into the pond complex. In the preconcentration ponds water is evaporated and sodium chloride is crystallized. Later the brine is transferred to production ponds where sylvinite is deposited. Brine is then transferred to other ponds where camaUite is crystallized. Sylvinite is removed from drained ponds with self-loading scrapers and taken to the plant were KCl is separated by flotation with an amine oil collector. The camaUite,... 

Fig. 19. Separation of ethanol and water from an ethanol—water—benzene mixture. Bottoms and are water, B is ethanol, (a) Kubierschky three-column sequence where columns 1, 2, and 3 represent the preconcentration, azeotropic, and entrainer recovery columns, respectively, (b) Material balance lines from the azeotropic and the entrainer recovery columns, A and E, respectively, where represents the overall vapor composition from the azeo-column, 2 1SP Hquid in equiUbrium with overhead vapor composition from the azeo-column, Xj, distillate composition from entrainer...

Ydibierschky Three-Column Sequence. If only simple columns are used, ie, no side-streams, side-rectifiers/strippers etc, then the separation sequence can be completed by adding an entrainer recovery column, column 3 in Figure 19a, to recycle the entrainer, and a preconcentrator column (column 1) to bring the feed to the azeotropic column up to the composition of the binary azeotrope. 

---