Preconcentration

Preconcentration of metals from water samples prior to atomic absorption spectrophotometric analysis increases the sensitivity of the analytical technique. It is only necessary when the level of metal required to be determined in the unconcentrated sample is below that which can be confidently measured by direct aspiration. 

There are four widely used methods for preconcentrating trace metals from water, namely evaporation, chelation—solvent extraction, ion-exchange and coprecipitation. 

In this method preconcentration is achieved by evaporating the sample to low volume under a heat lamp or on a hot plate. An obvious disadvantage of this technique is that in addition to the analyte being concentrated, other species which might interfere in later analysis will be concentrated with it. The method is also subject to contamination throughout the evaporation period and furthermore, losses of volatile elements may occur. Notwithstanding, the technique has an application in water analysis where waters are relatively clean with a low total dissolved solids content. 

The most effective solvents for use in atomic absorption are medium weight, low volatile aliphatics, alcohols and ketones. Frequently used solvents are methyl isobutyl ketone (MIBK) and ethyl propionate. These solvents have viscosities and surface tensions such that the efficiency of nebulisation is increased. 

Reagents. Solvent methyl isobufyl ketone (MIBK). Chelating agent ammonium pyrrolidine dithiocarbamate (APDC). Make up a 1% (m/v) solution of APDC in water. Prepare fresh daily. 

Both MEMS gas analyzer systems described above have recently demonstrated preconcentration gains of over 100 after a sampling time of 4 s, and separation of eight analytes in less than 4 s. We will now describe some details of the measurements obtained with the PHASED MG A system. 

PHASED 2Q+20, lOsm/lOqum/lOO nm-NGE Sampte Gas Na or 720 ppm C6-ln-Air Gas Velocity 2.2 rrVs, Reverse Flow 

20 stages and 20 °C, respectively. In summary, even these preliminary results show the unprecedented gain, speed and injection pulse width achievable with such MEMS preconcentrators. 

In collaborating with LLNL, R. Synovec s team at the University of Washington demonstrated very repeatable separations of alkanes and other analytes with the SWNTs grown in LLNL s 50-cm channels, under both isothermal and temperature ramped (as high as 60 °C s ) conditions, with H2 as carrier gas and 3 ms injection pulses [7]. The four-compound test mixtures were separated within 

For speed and resolution comparison, the nearest GC separahon on carbon nanotubes known to the authors is the one by Saridara and Mitra at the N.J. Inst. Tech. [13], which took 60x longer, or 4 min, to achieve a separahon of C -C alkenes and a comparable peak capacity of 16. They used a 300-cm capillary column with 500- tm ID, temperature ramping of 50 °C min , and a carrier gas velocity of -85 cm s . We will return later to the importance of peak capacity and its influence on FAR in the section on FAR (Sechon 9.3.5.4). 

For volatiles and semivolatiles with vapor pressures too low to be detected by direct IMS introduction methods, preconcentration techniqnes are often anployed. The most common approach is to pack a glass tube or stainless steel column with an adsorbing material such as Tenax or Carbosieve. Other methods, such as micro-fabricated adsorbents, have also been proposed, bnt these microfabricated concentrators are relatively new, with limited field exposnre. Nevertheless, for handheld and mini-IMS instruments, these preconcentration devices appear promising, offering rapid concentration and desorption with low power.  

For a typical preconcentration operation, a vapor sample is pulled through an adsorption material at a flow rate much larger than is possible with direct injection into an IMS. For example, the flow rates used in a standard preconcentrator are on the order of 160 L/s. With a miniaturized preconcentrator, these flows are approximately 2 L/s. For preconcentrators used for explosive detection in airports, the filters that are used are known as metal felt. Metal felt consists of a high-density mesh of metal filaments that provides minimal restriction to the sample airflow bnt efficiently adsorbs organic vapors in the air sample. After the adsorption step, the metal felt is heated to 200°C, volatilizing the adsorbed species into a clean carrier gas, which is directed into the IMS. The carrier gas flow is very low compared with the sampling gas flow. Thus, the net result of the adsorption/desorption operation is to increase the concentration of the analyte by a factor of 10 or more. A comprehensive review of 

While most preconcentration methods focus on the collection and concentration of analytes from vapor samples, the purge-and-trap method is used for collection and concentration of volatile and semivolatile analytes from liquid (usually aqueous) samples. 

FIGURE 3.3 (a) Glass flask for extracting volatile organic compounds from water samples. 

Equation 3.2 holds for analytes that are not absorbed on the surface of the container and for perfect mixing of the analyte. The actual behavior of analytes, especially polar molecules, would show a slower decrease in signal intensity than that predicted by true exponential dilution. 

The various sample preparation methods mentioned in Section 5.6 have been tried in microfluidic devices off-line for biological and environmental matrices followed by analysis of the extracted samples by NLC and NCE. The available papers in the literature on this subject are discussed in the following sections. 

Ion chromatography is frequently used to determine anions and cations at very low concentration levels, often in the low xg/L (ppb) range. In the electric power industry the water used in steam generators must be almost free of Na , Cl and other ions to avoid stress corrosion cracking. The ionic content of ultrapure water used in the electronics industry must be kept to extremely low levels. Semiconductor chip manufacturers require clean-rooms with utility impurities of no more than 1 ppb for 0.35 pm devices [1]. 

Although several valve arrangements may be used, the basic configuration is illustrated by Fig. 9.1. In the load mode the sample flows through the concentrator column 

Figwc 9.1. Configuration for a Dionex Low Pressure 4-Way Valve and a Concentrator Column (Courtesy Dionex Corp). 

The sample breakthrough volume from the concentrator column needs to be measured in order to know how large a sample may be used. The sample must be of low ionic strength ( 50 xS), otherwise the sample itself can act as an eluent for the sample ions. A good discussion of the use of concentrator columns in IC is available [2]. 

Many methods are available for removal of organic material from aqueous samples by off-Hne SPE [2]. Hydrophobic organic material is best extracted by solid poly(styrene-DVB) polymers or reversed-phase sihca extractants. PolyvinylpyrroH-done (PVP) is an appropriate choice for removal of humic acids, hgnins and tannins from water samples. As stated before in connection with syringe filters, one should be careful of ion contamination with SPE columns. The columns should be pre-rinsed with de-ionized water. 

In the manufacture of semiconductor integrated circuits, it is of the utmost importance to manufacture in a contaminant-free environment. The wafers and chips containing integrated circuits will fail if a particulate or ion contaminant shorts out any one of the circuit connections. As more and more integrated circuits are placed on the silica wafer the distance between the circuit connections become less. This means contamination is becoming more of an issue so that it has been proposed to monitor the air for airborne ionic contaminants. 

Lue and coworkers have described a method of monitoring acidic airborne contaminants in clean rooms [3], In this method, acidic contaminants were adsorbed on sihca gel tubes by passing a known volume of air through the tubes. The adsorbed impurities were extracted by a solution of carbonate and hydrogen car- 

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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. 

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