Preconcentration for Trace Element Determination in Aqueous Samples

Report Donald E. Leyden Department of Chemistry University of Denver Denver, Colo. 80208

Wolfhard Wegscheider Institute for General Chemistry, Microand Radiochemistry Technical University of Graz Graz, Austria

Preconcentration for Trace Element Determination in Aqueous Samples The need for preconcentration of trace elements or ions in aqueous solution results from the fact that instrumental analytical methods often do not have the necessary selectivity, sensitivity, or freedom from matrix interferences. Certainly it is observed that analytical methods, instrumental and otherwise, often do not meet the specifications claimed by early developers and proponents. This is not to be taken as a condemnation of the other analytical techniques that have been developed in recent decades, but only a statement that, as much as analytical chemists strive to develop the ultimate methodologies, they have to date fallen short of this goal. In fact, to contradict an earlier statement, the chemistry is not gone from analytical chemistry (2). As an illustration of the point, during a recent lecture attended by one of us, the speaker made a great issue over the fact that the standards for his powerful emission spectroscopic technique were prepared without any need to match the matrix of his intended samples. This is indeed an attractive feature of a method. However, as the author described the digestion, the redissolution, and the other procedures involved in preparing the sample, it became increasingly clear that the sample was matrix matched to the standard. Again, this is not to criticize the 0003-2700/81 / A 3 5 1 -1059$01.00/0 © 1 9 8 1 American Chemical Society

need for such actions, as the results justified the entire procedure. The point of these comments is to illustrate that there is a need to combine chemical procedures with the powerful instrumentation we use. In our particular research, X-ray spectrometry is used on a routine basis. This technique offers the advantage of well-understood spectroscopic principles and the capability of providing simultaneous multielement determinations. On the other hand, it has the distinct disadvantage that it does not have sufficient sensitivity for the lowlevel determinations necessary for many of today's analytical problems. The combination of such a well-understood instrument with preconcentration techniques extends the range of application of this instrument in a rather substantial manner. Taking an example such as the inductively coupled plasma optical emission spectrometer, which does provide great sensitivity and large linear dynamic range, one often finds that it is necessary to isolate the analyte from the matrix. The chemical techniques used in preconcentration can provide, in many cases, analyte isolation, as well as enrichment factors. Examples will be given later of the extension of such powerful techniques, both in range of concentration and in the types of matrices that can be investigated.

The concept of sample preconcentration prior to determination should imply many capabilities other than just concentration enrichment. As mentioned above, the minimization of matrix effects may be accomplished. For example, methods that will be discussed below are applicable to the isolation of trace elements from seawater at very low concentrations indeed. Many of the methods of sample enrichment can also be used simultaneously as sampling techniques. Applications of stationary phase concentration tools have been made in the sampling process for the determination of trace elements in seawater. In some cases these stationary phases may be used for the analytical determination directly, and in other cases the elements must be stripped from them. In many cases preconcentration carries a connotation of eliminating the possibility of automated analyses. On the contrary, in some cases preconcentration/sampling techniques can actually lead to the application of simplerinstrumentation in the final measurement, thereby facilitating automated analyses. The purpose of this REPORT is not to discuss preconcentration as an alternative to the use of modern instrumentation; rather it is to offer it as a supplement to these methods. This paper will present some of the common methods employed

ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981 ·

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today for sample preconcentration of trace ions in an aqueous solution prior to their determination. We will at­ tempt to present examples of the ap­ plication of preconcentration tech­ niques prior to the utilization of a va­ riety of analytical finishes, and will also offer critical comments con­ cerning the various preconcentration methods discussed. Evaporation of Solvents Perhaps the simplest method of precpncentration of ions from solution is evaporation of the solvent. This technique has been applied to X-ray spectrometry using conventional wavelength dispersive techniques (2) and proton induced X-ray emission (3). In the case of commercial wave­ length dispersive X-ray spectrometry, the potential of evaporation tech­ niques is very limited. Considering a nominal detection limit of 0.1-0,01 μg on a thin film sample, greater than 10-100 mL of water must be evapo­ rated in a confined spot to achieve precise analyses at the ppb level. It is not necessary to take the sample to dryness; however, it is common prac­ tice for convenience. In the case of X-ray methods applied to samples where one element is high but variable in concentration (such as iron in cer­ tain river waters), evaporation would not be suitable because of the effect of the major element upon the intensity of emission from more minor constitu­ ents. Fractional crystallization can lead to microscopically inhomogeneous residues. Matrix effects may be amplified rather than minimized. Therefore, for practical analyses of water samples, evaporation techniques offer little promise, whether X-ray or other instrumental methods are used. Electrodeposition A method that would appear to be extremely useful for preconcentration of metal ions is electrodeposition. This technique offers many of the proper­ ties described above for a desirable method pf preconcentration. Electro­ deposition found wide applications several years ago and was used exten­ sively for electrogravimetry (4), an early preconcentration method. Depo­ sition of metals into mercury cath­ odes, followed by distillation of the mercury to yield the concentrated metals, is one useful approach. Per­ haps one of the most valuable and widely applied electrochemical pre­ concentration techniques is anodic stripping analysis (5). Anodic strip­ ping analysis combines the electro­ deposition of metals into a mercury cathode (concentration step) with a current measurement when the mer­ cury electrode is scanned in the anodic direction and the concentrated metal

is oxidized. The advantages of electrodeposition are that few reagents need be added and the instrumentation is simple. In some cases, internal elec­ trolysis may be accomplished by pre­ paring a galvanic cell, using the sam­ ple solution with an inert electrode as one cell and an electrode of a more electropositive metal as the second cell. Spontaneous deposition of the sample ions occurs when the two elec­ trodes are shorted and the solutions joined by a salt bridge (6). The major disadvantage of the electrodeposition techniques is that they are, by and large, very slow, although very little operator attention is required during the deposition. When fresh water is used, an inert supporting electrolyte must be added to increase the currentcarrying capability of the solution. If high currents are forced through the solution to speed up the deposition, gas evolution occurs and loose, mossy deposits of the metal may result. There have been some successful re­ ports of preconcentration on graphite using laboratory-simulated freshwater samples (7), and using an in situ depo­ sition of metals from sea water (8). In the first study, 2-40 /tg each of copper, mercury, zinc, nickel, cobalt, and chromium in 15 mL of solution were electrodeposited on 1 cm-diameter graphite rods, sealed in plastic heatshrink tubing so that only the end sur­ faces of the rods were exposed to the solutions. By use of a simple constant current source, a series of solutions was electrolyzed simultaneously. The standard deviation for replicate sam­ ples (90-min electrolysis time) was 1-40%. The method was not tested on "real" samples, and it has several im­ portant limitations. The current effi­ ciency of a complex sample will be un­ known a priori, and therefore trial runs may be required to determine the electrolysis time required for exhaus­ tive deposition. To achieve reasonable deposition rates a supporting electro­ lyte must be added to freshwater sam­ ples, which risks contamination. Elec­ trodeposition from freshwater samples for preconcentration thus would not appear to be a technique of first choice. Electrodeposition from seawater ap­ pears more promising. The saline na­ ture of ocean water provides an excel­ lent, natural electrochemical cell. Small self-contained electrodeposition units have been designed for in situ sampling of seawater (8). Graphite electrodes of various design have been placed in flow cells that can be sus­ pended in the sea at a desired depth. Using the flow technique, exhaustive electrolysis is not required as the rate of deposition of each metal will be proportional to the concentration of that metal. Using a constant current

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and flow rate for fixed time periods, reproducible deposition from seawater is possible. The major problem is to prepare reference standards that match the samples. The electrodes used for each run may be returned to the laboratory for analysis. X-ray spectrometry or neutron activation techniques can be used directly on the electrodes. Also, the metals can be redissolved by anodic stripping, or the electrodes ashed in low temperature plasma devices and the metals taken up in acid. Either way, techniques such as atomic absorption spectrome­ try may then be used. Attempts to se­ lectively deposit ions directly in a graphite furnace rod for atomic ab­ sorption have been reported (9,10). It is well known that enhanced cur­ rents may be observed when the electroactive component adsorbs on the electrode surface. This can lead to er­ roneous analytical results when the adsorption is not reproducible, or when it is significantly affected by other compounds in solution. Recent­ ly, however, this phenomenon has been exploited by placing a chemical functional group such as a metal complexing agent on or in the electrode (11). The electroactive analyte is im­ mobilized at the electrode and thereby concentrated from solution. Selective trace element preconcentration may thus be effected simply by inserting the electrode in the sample solution, even at a remote site. The electro­ chemistry can be performed later on. The procedure is similar to anodic stripping voltammetry, but the depo­ sition step is chemical rather than electrochemical. Liquid-Liquid Extraction One of the most extensively studied and most widely used techniques of preconcentration is liquid-liquid ex­ traction, in which a water-immiscible solvent and an aqueous solution con­ taining a chelated metal ion are brought into contact. Most often, the complexing agent forms a nonionic or neutral chelate, which is more soluble in the organic solvent than in water. The result is an extraction of the metal from the water into the organic solvent. The procedure is simple and, if the principles and practice of the technique are well understood by the analyst, liquid-liquid extraction can be powerful both as a separation and as a concentration method. A variety of chelating agents may be used. Some, such as 8-hydroxyquinoline (oxine), diethyldithiocarbamate (DDTC), ammonium pyrrolidinedithiocarbamate (APDC), and /3-diketones (acetylacetone [AcAc]), are very general chelating agents which will ex­ tract a variety of metal ions. Some others, such as dimethylglyoxime, are

highly specific. It is interesting that a great deal of research has been performed to develop methods for specific extractions with reagents such as oxine (12). Much of this work was performed because' the instrumentation at that time was not sufficiently specific and prior separations were required. These methods are still required in some cases. For example, interferences in atomic absorption determinations are not uncommon, and separations by solvent extraction are required. However, for use in conjunction with X-ray fluorescence, a very general extraction procedure would be more useful, as the spectrographic selectivity is excellent. There is no single chelating agent that is a Universal extractant. However, Pohl (13-15), and others (16,17) have proposed several multicomponent systems that cover a wide range of elements. The concentration factor reasonably achieved by solvent extraction on a batch extraction basis has practical limits. Considering that a 100-mL sample extracted with 1 or 2 mL of organic solvent is a practical limit, a concentration factor of 100 is about what one can achieve. Of more potential value to water analysis is some form of continuous or countercùrrent extraction, of the type that has been used in preeoncentration of organic compounds. In these procedures, a fixed amount of organic solvent is equilibrated with a large volume of sample taken continuously from a reservoir or perhaps directly from a river or stream. In this way a large volume of sample may be extracted by relatively small volumes of organic soL· vent. The area of continuous extraction would appear to hold much promise. A countercùrrent flow system based upon differences in solvent density for in situ sampling would be a powerful concentration method. Some rather novel yet simple devices have been designed (18). Such devices should be adaptable to water analysis. A further example is the use of fluoro/3-diketones in solvent extraction combined with gas-liquid chromatography (19) or ICP-AES (20). These reagents form volatile complexes with many metal ions. Extracts containing these complexes may be injected into a gas chromatograph or gas cell. These techniques have not been sufficiently explored in applications to water pollution analyses to accurately assess their potential. However, they appear to be significant. Some novel developments in solvent extraction are worthy of note, One of the limitations of the preeoncentration factor realized with batch solvent extraction is the difficulty in obtaining a good separation of a small volume of the organic phase from the

aqueous sample. Significant portions of the organic solvent may adhere to the walls of the vessel, requiring repeated washings, which decrease the concentration factor. Emulsions may form that are slow to separate, An interesting solution to this problem has been suggested and used in fundamental studies (21, 22) and limited analytical applications (22, 23). The organic phase is one that melts a few degrees above room temperature. A conventional liquid-liquid extraction is carried out at a temperature above the melting point of the organic material, Once the extraction is complete, the mixture is allowed to cool and the organic material solidifies. If the organic material is carefully selected, a solid bead will form upon cooling, and the extract will be retained. The small head (as little as a few milligrams) may be physically removed from the mixture for determination of the elements. Flaschka (23) has used this technique in conjunction with a capillary cell for the spectrophotometric determination of cobalt. Surface Adsorption The concept of solvent extraction can be easily translated into that of surface adsorption. Two such applications have been reported. Vanderborght and Van Grieken (24) have used the adsorption of neutral 8-hydroxyquinoline (oxine) metal complexes onto the surface of activated carbon as a means of preconcentrafion of trace ions. The activated carbon bearing the complexes is filtered and analyzed by X-ray fluorescence spectrometry, lylore recently, oxine complexes formed in seawater samples were adsorbed on a column bed packed with Cie-bonded silica gel (25). The adsorbed complexes were eluted with methanol. Concentration factors of about 200 were attained, and the saline matrix was eliminated. Inductively coupled plasma atomic emission spectrometry was utilized in the analytical finish. Precipitation A method of preconcentration of ions that is of limited use to those analytical techniques requiring liquid samples, but of great value in X-ray spectrometry and neutron activation analysis, is precipitation or coprecipitation. Precipitation is one of the oldest chemical techniques for the separation and concentration of ions. A massive effort has been made over the years to find specific precipitation reagents and to minimize coprecipitation. However, for trace element techniques with high inherent selectivity, selective precipitation is usually not necessary or desirable. The completeness of the precipitation of the ions of

1062 A · ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981

interest is much more important. The solubility of many common precipitates is sufficiently great to prevent quantitative precipitation of the ions of interest at concentrations less than a few ppm. For example, nickel in this concentration range cannot be precipitated quantitatively with dimethylglyoxime. In many cases, however, quantitative precipitation may be achieved by the addition of a coprecipitant (26-28). When precipitates are formed in exceedingly dilute solutions, several factors may contribute to lack of quantitative recovery in addition to the solubility of the precipitate. The particle size of thé precipitate may be very small and the particles may pass through the filter unless much care is used. A supersaturated solution may exist and particles may not form. One of the easiest ways to overcome these problems is to add a carrier ion or coprecipitate that will not only precipitate itself, but will aid in the recovery of the ion of interest. Luke (26) has devised an extensive scheme for both general and selective precipitation of metal ions using a variety of reagents. Puschel (27) and Watanabe, Berman, and Russell (28) have proposed additional schemes using organic précipitants. The major advantages of precipitation methods are that they are simple, and that uniform deposits on a membrane filter can be obtained. A further advantage of precipitation methods is the inherent chemical capacity available. This becomes an important factor when one Or more elements is present at a relatively high concentration. With the possible exception of continuous liquid-liquid extraction, none of the above techniques provides concentration factors of more than 1 or 2 orders of magnitude. It is difficult to determine a Concentration factor for precipitation or electrodeposition, as the ions are removed from a homogeneous medium and placed on the surface of a filter or electrode. For X-ray spectrometry, the conversion of the sample to a thin film is a bonus of preconcentration. However, if the sample is to be redissolved for analysis by some other technique, the net gain in concentration may not be great. For example, components can be electrodeposited from 15-25 mL of solution onto a graphite or carbon electrode. However, one may need at least 0.1 mL of solution to dissolve the deposit or to receive the ions by anodic stripping. Therefore, a preconcentration factor of about 250 is achieved. A similar argument can be made about precipitation and batch liquid-liquid extraction. Ion-Exchange A series of preconcentration tech-

niques that has been extensively stud­ ied and that has potential for much larger concentration factors is that based upon ion-exchange reactions. The major advantage of most ion-ex­ change methods is that the functional group is immobilized on some type of solid substrate, therefore providing the potential to either batch extract ions from solution or to use the ionexchange material in a column. Ionexchange provides the most universal preconcentration technique, as the ions may be analyzed directly upon the solid matrix using X-ray spec­ trometry (29), neutron activation analysis (30), or isotopic dilution tech­ niques (31). Also, the ions may be eluted from the column and the analy­ sis completed on the resulting solution using methods such as atomic spec­ trometry (32) or inductively coupled plasma emission spectrometry (33). Ion-exchange is capable of recovering hydrated ions, charged complexes, and solution components complexed by la­ bile ligands. The recovery efficiency depends upon the distribution ratio of the ion on the ion-exchange material (i.e., the affinity of the material for the ion) and the stability constants of any complexes present in solution. These factors must be carefully con­ sidered and tested before use of ionexchange preconcentration methods (29). For a well-chosen ion-exchange material, very large preconcentration factors may be achieved. For example, as much as 4 L of seawater may be passed through a column prepared from 100 mg of ion-exchange material, yielding a concentration factor of ~ 4 Χ 104 (34). The conventional ion-exchange resin formed by a cross-linked styrene-divinylbenzene or other matrix containing ionogenic functional sites is the most obvious ion-exchange ma­ terial to use. Both batch extraction (35) and column techniques (36) have been used. The first resins used were the common, commercially available cation and anion exchange materials. These resins operate essentially on an ion association basis, and selectivity and partitioning is frequently a prob­ lem. For example, concentration of transition metal ions from seawater is difficult because sodium ions may compete well with the less concentrat­ ed transition metal ions. In cases in which alkali metal ions are not of in­ terest, chelating ion-exchange resins may be very effective. These resins contain functional groups that form chelates with the metal ions. Unfortu­ nately, few commercial resins are available, although many varieties have been synthesized (37). One che­ lating ion-exchange resin that is com­ mercially available and has proved very useful in the preconcentration of

metal ions from solution is Chelex-100 (Bio-Rad Laboratories). The resin contains iminodiacetic acid functional groups, and its chemical behavior is very similar to ethylenediaminetetraacetic acid (EDTA). This resin has been applied to X-ray spectrometric analysis of geological materials (38) and analysis of seawater by X-ray spectrometry (39), atomic absorption (32), and inductively coupled plasma atomic emission spectroscopy (40). Chelex-100 offers several advantages as an ion-exchange concentration ma­ terial. It generally has very high distri­ bution ratios, which implies that ions (particularly transition metal ions as well as others such as Hg 2 + , P b 2 + , and Bi 3 + ) may be quantitatively recovered from solution using a batch process. It covers a wide range of metal ions, and its chemistry can be accurately pre­ dicted by analogy with EDTA. How­ ever, there are severe limitations to the applications of Chelex-100. There are samples, such as seawater, or ironand calcium-laden fresh water, in which major elements will occupy the ion-exchange sites to the exclusion of trace elements. To prevent this, mask­ ing agents can be used, or pH or other parameters must be manipulated. At times, control of these parameters may be an advantage. One example is the determination of Cr(III) and Cr(VI) using Chelex-100. Cr(III) is re­ covered by Chelex-100, whereas Cr(VI) as chromate is not. Therefore Cr(III) may be determined directly by extraction with Chelex-100, and total chromium can be determined on a sec­ ond aliquot treated with a reducing agent (41). Inductively coupled plas­ ma atomic emission spectrometry was employed with a poly(dithiocarbamate) resin for a similar determina­ tion (42). Ion-Exchange Impregnated Materials An interesting modification of the ion-exchange preconcentration ap­ proach is the use of ion-exchange im­ pregnated materials. Two major types of materials are used. Ion-exchange resin impregnated papers have found widest use. Campbell and co-workers have performed in-depth studies of the distribution of ions when solutions are filtered through paper disks im­ pregnated with ion-exchange resins (43). These materials are very easy to use as the solution is simply filtered through (often repeatedly) the filter disk held in a suitable holder (44). Standards are prepared in the same manner as the samples. The disks are well suited for X-ray spectrometric measurements. A second type of ion-exchange im­ pregnated material is ion-exchange membranes. These materials are char-

acterized by low capacity and slow rates of ion recovery. In most labora­ tory analyses they offer no significant advantage over other materials. How­ ever, one application of interest has been to take advantage of the slow up­ take to essentially integrate the aver­ age concentration of metals in a stream over a time period (45). The membrane, in principle, will recover ions from solution at a rate propor­ tional to the concentration of the ion in solution. Therefore, exposure of the membrane to a changing concentra­ tion should "time average" the con­ centration. However, careful evalua­ tion of many possible secondary ef­ fects must be performed before these membranes can be considered reliable. Competition effects, pH changes, and other parameters must be evaluated. Also, the low capacity of the mem­ brane requires an analytical method with very low limits of detection. Immobilized Reagents Several recent research efforts have been made to find methods of immo­ bilizing ion-exchange sites to simplify modification of the active functional groups on the sites. Again, a desirable feature is to be able to conduct deter­ minations either directly on the solid ion-exchange material or on solutions eluted from the material. One very im­ portant advantage is that the material may be used on a one-time basis and need not have long-term stability. One approach is to absorb a desired chelat­ ing agent in a suitable matrix. Some workers have used styrene-divinylbenzene gels (46). Commercially available resin beads (Bio-Beads, Bio-Rad Lab­ oratories) are swelled using an organic solvent. The beads are treated with solutions of chelating agents that are absorbed into the pores of the gellike beads. The pores are shrunk by spraying the gel into cold water. The gel may then be used to concentrate metal ions as the ions diffuse into the pores and are trapped by the chelating reagent. This is obviously a simple ap­ proach to the preparation of tailormade immobilized chelating materials. A major limitation of this method is that a very low exchange capacity (1-10 /ueq/g) is obtained. There should be some concern given to the possibili­ ty of leaching the metal chelate from the gel, as it is not attached by a chemical bond. A similar concept has been proposed by Braun and co-work­ ers (47, 48). These workers used re­ agents absorbed into open cell polyurethane foam and sealed with plasticizers. These foams are very easily prepared and are effective concentrat­ ing agents. Again a major limitation is the very low capacity of 23.4 /teq/g re­ ported for the dithizone-loaded foam (48). These materials are recently re-

ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981 · 1063 A

ported, and a critical evaluation of their potential is not possible. How­ ever, it is readily observed that they offer simplicity in the preparation of immobilized reagents and meet a vari­ ety of needs. The advantages of these foams have been compared with sever­ al other techniques of "trapping" dithizone on substrates (47). A chemical reaction that has found much use in recent years for a variety of purposes is silylation. This reaction has been used to form volatile deriva­ tives of organic compounds for gas chromatographic work and to immo­ bilize enzymes on substrates. A variety of silylation reagents is available. The reaction is a simple one that can be conducted in water or an organic sol­ vent such as benzene. The reaction may be represented as OH OH +

(HO)8—Si—R

OH

(1) 5% w/v silane In toluene (2) 8 0 °C, 3 h

ο Ο — S i — R H- 3H 2 0

ο where the polyhydroxyl species may be silica gel or a solid substrate such as glass. Some of the available silylation reagents contain chelating or coordinating functional groups, and modifications are possible. The modifications are best made after

ο OH-

Ο

S i — ( C H 2 ) 3 — N H 2 + CS2

ο ο H

S

Ο—Si—(CH2)3—Ν—C—S-

ο

the silylation reaction has been per­ formed. Recent applications of this re­ action have shown potential for preconcentration. Hercules and co-work­ ers have silylated fiberglass cloth with a reagent containing an amine and converted the amine to the dithiocarbamate (49). Solutions containing as little as 20 ppb of lead were filtered through the treated glass, and the re­ covered lead was detected by ESCA. The major limitations are a low capac­ ity and the difficulties in quantitation by ESCA. The silylation reaction has been used extensively in the immobili­ zation of chelating agents such as ethylenediamine, ethylenediaminedithiocarbamate, and ethylenediaminetriacetic acid on silica gel (50). Chromatographic-grade silica gel is silylated, and modification reactions are con­ ducted. The silica gel may then be used in a batch or column made for concentration, similar to ion-exchange resins. Capacities of 0.5-0.9 meq/g are obtained and, by passing solutions through a column made of 100 mg of the material, metals in the range of 1 ppb may be determined by X-ray spectrometric measurements directly on the packing made into a pellet. Other Methods There are many other preconcentration methods of a somewhat special­ ized nature. Some of these represent interesting applications of chemical reactions and are not strictly preconcentration methods. To attempt a comprehensive list would be of little value, but a few examples may stimu­ late the reader to think of new solu­ tions to problems. Siggia and co-work­ ers have reported a concentration technique specific to Ag(I), Cu(I), Hg(I), and Hg(II) using precipitation chromatography (51). A column pre­ pared with 20% 1-pentadecyne coated on Anakrom AB is treated with an ac­ etate solution of the above ions. These ions form metalloacetylides and are retained on the column. They may be eluted by nitric acid and analyzed. A further example of the use of chemical manipulation to facilitate analysis in­ cludes a "chemical amplifier." Orthophosphate is frequently determined in water samples" using the blue heteropoly acid known as molybdenum blue. A rapid method to determine phos­ phate is to extract the phospho-molybdate complex from excess molybdate (52) and use the 12:l-Mo:P ratio in the complex to determine the mo­ lybdenum. Using a silylated silica gel reagent discussed above to recover the molybdate, as little as 15 ppb phos­ phate may be determined by X-ray fluorescence determination of the ex­ tracted molybdenum. Janauer and co­ workers have used an interesting con­ cept which they call "reactive ion ex-

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change" (53, 54). One example pre­ sented (53) is the use of an anion ex­ change resin to adsorb chromate ions, followed by in situ reduction of the chromate to chromic ions. The cationic chromic ion is then easily removed by a backwash of the resin. There will probably be many other examples of this technique in the future. X-ray spectrometry has been employed in the determination of organic nitrogen by converting the nitrogen to ammo­ nia using the Kjeldahl reaction and distilling the ammonia from the di­ gested solution. The ammonia distil­ late is passed through a filter saturat­ ed with Nessler's reagent. The excess reagent is washed from the filter and the residual mercury determined by X-ray spectrometry. As little as 0.2 μ% of nitrogen may be determined (55). The above comments represent the tip of the iceberg in terms of the methods and applications of preconcentration techniques as applied to trace element determinations. The po­ tential of automated analyses and speciation determination has yet to be fully exploited. It is also important to realize that X-ray spectrometry, atomic absorption spectrometry, neu­ tron activation analysis, and induc­ tively coupled plasma atomic emission spectrometry have each been touted as all-but-ultimate solutions to analyt­ ical problems in elemental determina­ tions. Yet, we see in the above brief overview that new applications of these methods to selected analytical problems have been coupled to preconcentration techniques, used as such or as a means toward eliminating undesirable matrix problems. In sum­ mary, the ultimate method—highly sensitive, completely selective, with infinite linear dynamic range, capable of determining all elements in their different oxidation states and chemi­ cal forms, with no matrix effects—still eludes us. The use of chemical meth­ ods of concentration, matrix effect minimization, and species isolation provides an essential supplement to the powerful instrumental techniques available. One naturally hopes to avoid the need for these procedures. Yet the hallmark of a true analytical chemist is the ability to recognize that no two analytical problems are exactly the same and to master the versatility with which they must be approached. References (1) Liebhafsky, H. A. Anal. Chem. 1962, 34(7), 23 À. (2) Stone, R. G. Araaiyst 1963,88,56. (3) Walter, R. L.; Willis, R. D.; Gutknecht, W. F.; Joyce, J. M. Anal. Chem. 1974,46, 843. (4) Lingane, J. J. "Electroanalytical Chemistry", 2nd éd.; Interscience: New York, 1958. (5) Figura, P.; McDuffie, B. Anal. Chem. 1980,52,1433.

(6) Clarke, B. L.; Wooten, L. Α.; Luke, C. L. Ind. Eng. Chem. Anal. Ed. 1936,8, 411. (7) Vassos, B. H.; Hirsch, R. F.; Letterman, H. Anal. Chem. 1973,45, 792. (8) Mark, Η. Β. "Abstracts of Papers," 162nd National Meeting of the American Chemical Society, Washington, D.C., September 1971. (9) Torsi, G. Ann. Chim. Rome 1977,67, 557. (10) Torsi, G.; Desimoni, E.; Palmisano, F.; Sabbatini, L. Anal. Chim. Acta 1980, 124,143. (11) Cheek, G. T.; Nelson, R. F. Anal. Lett. 1978,11, 393. (12) Morrison, G. H.; Freiser, H. "Solvent Extraction in Analytical Chemistry"; In­ terscience: New York, 1967. (13) Pohl, F. A. Z. Anal. Chem. 1953,139, 241. (14) Pohl, F. A. Z. Anal. Chem. 1953,139, 423. (15) Pohl, F. A. Z. Anal. Chem. 19SZ.141, 81. (16) Chichilo, P.; Sprecht, A. W.; Whittaker, C. W. J. Assoc. Off. Anal. Chem. 1955,38,903. (17) Silvery, W. D.; Brennan, R. Anal. Chem. 1962,34,784. (18) Pan, S. C. Sep. Sci. 1974,9,227. (19) Ross, W. D.; Sievers, R. E. Talanta 1968, 15, 87. (20) Black, M. S.; Browner, R. F. Anal. Chem. 1981,53,249. (21) Kuznetsov, V. I.; Seryakova, I. V. Zh. Anal. Khim. 1959,14,161. (22) Fujinaga, T.; Kuwamoto, T.; Nakayma, E. Talanta, 1969,16,1225. (23) Flaschka, Η. Α.; Barnes, R.; Paschal, D. Anal. Lett. 1972,5,253. (24) Vanderborght, Β. Μ.; Van Grieken, R. E. Anal. Chem. 1977,49, 311. (25) Watanabe, H.; Goto, K.; Taguchi, S.; McLaren, J. W.; Berman, S. S.; Russell, D. S. Anal. Chem. 1981,53,738. (26) Luke, C. L. Anal. Chim. Acta 1968, 47,237. (27) Puschel, R. Talanta 1969,16, 351. (28) Watanabe, H.; Berman, S.; Russell, D. S. Talanta 1972,19,1363. (29) Blount, C. W.; Leyden, D. E.; Thom­ as, T. L.; Guill, S. Anal. Chem. 1973,45, 1045. (30) Noakes, J. E.; Harding, J. L.; Spann­ ing, J. D. 8th Annual Conference of the Marine Technology Society, Preprints, ρ 415. (31) Stokes, W. M.; Fish, W. Α.; Hickey, F. C. Anal. Chem. 1955,27,1895. (32) Riley, J. P.; Taylor, D. Anal. Chim. Acta 1968,40,479. (33) Barnes, R. M.; Genna, J. S. Anal. Chem. 1979,51,1055. (34) Leyden, D. E.; Patterson, Τ. Α.; Al­ berts, J. J. Anal. Chem. 1975,47, 733. (35) Van Nickerk, J. N.; DeWet, J. F.; Wybenga, F. T. Anal. Chem. 1961,33, 213. (36) Collin, R. L. Anal. Chem. 1961,33, 605. (37) Helfferich, F. "Ion Exchange"; McGraw-Hill: New York, 1962, ρ 47. (38) Leyden, D. E. In "Advances in X-ray Analysis; Grant, C. L.; Barrett, C. S.; Newkirk, J. B.; Ruud, C. O., Eds.; Ple­ num Press: New York, 1974; Vol. 17, ρ 293. (39) Kingston, H.; Pella, P. A. Anal. Chem. 1981 53 223 (40) Berman, S. S.; McLaren, J. W.; Willie, S. N. Anal. Chem. 1980,52,488. (41) Leyden, D. E.; Channel), R. E. Anal. Chem. 1972,44,607. (42) Miyazaki, Α.; Barnes, R. M. Anal. Chem. 1981,53,364. (43) Campbell, W. J.; Spano, E. F.; Green, T. E. Anal. Chem. 1966,38,987. (44) Hooten, Κ. Α. Η.; Parsons, M. L. Anal. Chem. 1973,45,436. (45) Lochmuller, C. H.; Galbraith, J.; Wal-

ter, R. L.; Joyce, J. D. Anal. Lett. 1972, 5, 943. (46) Sekizuka, Y.; Kojima, T.; Yano, T.; Ueno, K. Talanta 1973,20, 979. (47) Braun, T.; Farag, A. B. Anal. Chim. Acta 1974,69, 85. (48) Braun, T.; Farag, A. B. Anal. Chim. Acta 1974, 71,133. (49) Hercules, D. M.; Cox, L. E.; Onisick, S.; Nichols, G. D.; Carver, J. C. Anal. Chem. 1973,45,1973. (50) Leyden, D. E.; Luttrell, G. H. Anal. Chem. 1975,47,1612. (51) Zeronsa, W. P.; Dobkowski, G.; Siggia, S. Anal. Chem. 1974,46,309. (52) Leyden, D. E.; Nonidez, W. K.; Carr, P. W. Anal. Chem. 1975,47,1449. (53) Pankow, J. F.; Janauer, G. E. Anal. Chim. Acta 1974,69,97. (54) DeLayette-Mills, J.; Karm, L.; Ja­ nauer, G. E.; Chan, P.-K.; Bernier, W. E. Anal. Chim. Acta 1981,124, 365. (55) Mathies, J. C ; Lund, P. K.; Eide, W. Anal. Biochem. 1962,3,408.

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Wegscheider Donald Leyden received a BS in chemistry from Kent State Universi­ ty and MS and PhD degrees in ana­ lytical chemistry from Emory Univer­ sity. He is currently Phillipson Pro­ fessor of Environmental and Mining Chemistry at the University of Den­ ver. Ley den's research interests in­ clude chelating ion-exchange resins, NMR studies of proton exchange re­ actions, preconcentration of trace ions prior to X-ray fluorescence anal­ ysis, and the chemical modification of surfaces. Wolfhard Wegscheider received his doctorate in technical sciences from the Technical University of Graz, Austria, and worked later under Ley­ den as a Fulbright scholar. His re­ search interests include sample prep­ aration techniques for multielement analyses and mathematical tech­ niques in analytical chemistry. He is currently universit&tsdozent for ana­ lytical chemistry at the Technical University of Graz.

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