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This paper reports the design and ground-based testing of a multiplexed colorimetric solid-phase extraction (MCSPE) platform for the rapid determination of multiple water quality parameters in a simple set of operational steps. Colorimetric solid-phase extraction (C-SPE) is an analytical platform that combines impregnated colorimetric reagents on a solid-phase extraction membrane and diffuse reflectance spectroscopy to quantify trace analytes in water. In extending C-SPE to MC-SPE, a filter holder that incorporates discrete analysis channels and a jig that facilitates the concurrent operation of multiple syringes have been designed, enabling the simultaneous determination of three different measures of water quality. Separate, single-parameter membranes, placed in a readout cartridge create unique, parameter-specific addresses at the exit of each channel. Following sample exposure, the diffuse reflectance spectrum of each address is collected serially and the Kubelka-Munk function is used to quantify each water quality parameter via calibration curves. Performance evaluations of the MC-SPE platform were conducted using sample pH, silver(I), and nickel(II). Determinations of silver(I) (0.05-0.5 ppm) and nickel(II) (1.8-5.0 ppm) follow established C-SPE methods on reversed-phase extraction membranes using 5-(p-dimethylaminobenzylidene)rhodanine and dimethylglyoxime, respectively, as colorimetric reagents. Sample pH (2.5-5.0) is measured using an anion-exchange membrane impregnated with fluorescein. These determinations require ~120 s to complete using a total sample volume of 3.0 mL. The extension of MC-SPE to the determination of a greater number of analytes and its potential application to space and earth-bound monitoring needs are briefly discussed.

A new, rapid methodology for trace analysis using solid-phase extraction is described. The two-step methodology is based on the concentration of an analyte onto a membrane disk and on the determination by diffuse reflectance spectroscopy of the amount of analyte extracted on the disk surface. This method, which is adaptable to a wide range of analytes, has been used for monitoring ppm levels of iodine and iodide in spacecraft water. Iodine is used as a biocide in spacecraft water. For these determinations, a water sample is passed through a membrane disk by means of a 10-mL syringe that is attached to a disk holder assembly. The disk, which is a polystyrene--divinylbenzene composite, is impregnated with poly(vinylpyrrolidone) (PVP), which exhaustively concentrates iodine as a yellow iodine--PVP complex. The amount of concentrated iodine is then determined in only 2 s by using a hand-held diffuse reflectance spectrometer by comparing the result with a calibration curve based on the Kubelka--Munk function. The same general procedure can be used to determine iodide levels after its facile and exhaustive oxidation to iodine by peroxymonosulfate (i.e., Oxone reagent). For samples containing both analytes, a two-step procedure can be used in which the iodide concentration is calculated from the difference in iodine levels before and after treatment of the sample with peroxymonosulfate. With this methodology, iodine and iodide levels in the 0.1-5.0 ppm range can be determined with a total workup time of ~60 s with a RSD of ~6%.

Working at pH 2.4 with uncoated silica capillaries has the advantage that electroosmotic flow is virtually eliminated. Excellent separations of protonated organic bases were obtained when ethanesulfonic acid was added to the running electrolyte to coat the capillary surface by a dynamic equilibrium. The effect of adding a new surfactant, sulfonated Brij-30, to the acidic electrolyte was also investigated. Use of this surfactant in acidic organic - aqueous solutions changes the elution order of many organic cations and also permits the separation of neutral organic compounds. Excellent resolution of a mixture of 19 PAHs and similar compounds was obtained in 40% organic solvent in only 20 min. The largest organic compounds form the most stable association complexes with the sulfonated Brij-30 and, thus, have the shortest migration times. It is shown that the type and concentration of surfactant, as well as the composition of the aqueous-organic solution, are conditions that can be varied over a broad range to obtain superior separations of both neutral and cationic organic compounds. The type of organic solvent is yet another condition that can be manipulated advantageously. For example, the use of equal volumes of acetonitrile and 2-propanol in water - organic solutions can give better resolution of neutral organic analytes than either solvent used alone.

A nonionic surfactant (Brij-30) was treated with chlorosulfonic acid to form [C.sub.12][H.sub.25][(OC[H.sub.2]C[H.sub.2]).sup.4]OS[O.sub.3]-[Na.sup.+]. Addition of a low concentration of this material to an aqueous solution-acetonitrile electrolyte makes it possible to separate nonionic organic compounds by capillary electrophoresis. Separation is based on differences in the stabilities of analyte-additive complexes in solution. Migration times of analytes can be varied over an unusually large range by varying the additive concentration and the proportion of acetonitrile in the electrolyte. This greatly expands the scope of neutral organic compounds that can be separated by CE. Several examples are given. Some very difficult separations were achieved, such as the baseline resolution of four methylbenz[a]anthracene isomers and the separation of normal and deuterated acetophenone.

A low concentration of a surfactant added to a mobile phase containing 60% acetonitrile (+ 40% water) significantly reduces the capacity factors of organic analytes. An even greater reduction in k[prime] resulted from the use of binary additive systems, particularly those containing a cationic and an anionic surfactant. It is believed that an ion pair is formed that is more effective in forming solution complexes with organic analytes than either additive alone. Other binary additive systems included mixtures of a neutral and a charged additive or a mixture of two anionic surfactant additives. A ternary mixture of additives was more effective than any of the binary mixtures in modifying the chromatographic behavior of organic solutes in HPLC.

Excellent separations of nonionic organic compounds were obtained by adding sodium dioctyl sulfosuccinate (DOSS) to an acetonitrile ([approximately] 40% v/v)-water electrolyte. Separation is based on differences in the strength of analyte-DOSS association 'complexes' in solution, which results in differences in effective electrophoretic mobility. Micelle formation is not believed to occur. The effects of varying apparent pH, applied voltage, acetonitrile concentration, and DOSS concentration were studied with regard to electroosmotic mobility and effective electrophoretic mobility. Under optimum conditions, excellent separations of 23 organic compounds were obtained.

It has long been held that chromatographic separation of carboxylic acids requires (a) the use of a gel-type column of fairly large dimensions and (b) an acidic aqueous eluent to repress ionization of the analytes and thereby give sharp peaks. The present work shows that neither of these conditions is necessary. Excellent separations of aliphatic carboxylic acids were obtained on macroporous resin columns using a predominantly aqueous eluent with no added acid. Addition of an alcohol, such as 1-butanol, to an aqueous mobile phase has a dramatic effect on the chromatographic separation of carboxylic acids. The alcohol is believed to coat the surface of a macroporous polystyrene-divinylbenzene resin and establish a dynamic equilibrium between the mobile and stationary phases. This increases the hydrophilicity of the resin surface and reduces the hydrophobic attraction of the analytes for the resin phase. The eluent systems have a very low background conductance, thus giving a very sensitive conductometric detection. The presence of carbonic acid (from atmospheric carbon dioxide) in the eluent was found to play a role in the retention mechanism of the carboxylic acid analytes.

The retention mechanism of electrostatic ion chromatography (EIC) is currently under debate and is the focus of this paper. A comprehensive set of retention data has been obtained on a C18 column coated with the zwitterionic surfactant 3-(N,N-dimethylmyristylammonio)propane-sulfonate used with a range of mobile phases in which both the mobile-phase anion and cation have been varied systematically. Electro-osmotic flow measurements were also obtained on fused-silica capillaries coated with the zwitterion (and also some monofunctional surfactants) and were used to evaluate the nature of the surface charge on the layer of adsorbed surfactant in the presence of various background electrolytes. A new retention mechanism for EIC was developed on the basis of these data. This mechanism proposes that equilibration of the bound zwitterions with a mobile phase containing a suitable electrolyte causes the establishment of a charged layer created by the terminal sulfonate groups of the zwitterion, which acts as a Donnan membrane. The magnitude and polarity of the charge on this membrane depends on the nature of the mobile-phase ions. The Donnan membrane exerts weak electrostatic repulsion or attraction effects on analyte anions. A second component of the retention mechanism is chaotropic interaction of the analyte anion with the quaternary ammonium functional group of the zwitterion. This interaction exerts the major effect on the separation selectivity of EIC, such that analyte anions are eluted in order of increasing chaotropic interactions in accordance with the Hofmeister series.

James D. Fritz [*] John F. Grubb [*] Ronald F. Polinski [*] Newly developed stainless steels provide increased corrosion resistance, improved mechanical properties, easier forming and fabrication, and/or improved economy. […]