Perchlorate analysis using solid-phase extraction cartridges

Perchlorate is a compound of increasing concern as an environmental contaminant and is being regulated at increasingly stringent levels. Reliable methods are needed to consistently analyze perchlorate at low concentration levels. This research investigates the use of solid-phase extraction cartridges as an alternative to large-volume injection loops to achieve low-level (microg/L level) perchlorate quantitation. The method involves commercially available strong anion exchange (SAX) cartridges. Water samples are filtered (100 to 1000 mL) using the cartridge, which removes the perchlorate from the solution by anion exchange. Then, after the desired volume is filtered, the perchlorate is extracted using 4 mL of 1% NaOH. In addition, a cleanup method is developed to remove competing anions (chloride, sulfate, and carbonate) that are often found in environmental samples. Analyses are performed with an ion chromatograph using a 10-microL injection loop, yielding a perchlorate method detection limit (MDL) of 210 microg/L. One-liter volumes of a 2-microg/L perchlorate spiked deionized water solution are filtered with SAX SPE. Following extraction and analysis, an MDL of 0.82 microg/L is obtained, comparable to that found for 1-mL injection loop systems (reported as low as 0.53 microg/L). MDL studies are then conducted on perchlorate-amended groundwater (solution concentration of 70 microg/L) and surface water (solution concentration of 10 microg/L) using a filtration volume of 200 mL. The MDLs are 6.7 microg/L for the groundwater and 2.4 microg/L for the surface water.     

QUANTITATION OF PHOTOSYSTEM II ACTIVITY IN SPINACH CHLOROPLASTS. EFFECT OF ARTIFICIAL QUINONE ACCEPTORS

Quantitation of photosystem II (PSII) activity in spinach chloroplasts is presented. Rates of PSII electron-transport were estimated from the concentration of PSII reaction-centers (Chl/PSII = 380:1 when measured spectrophotometrically in the ultraviolet [ΔA₃₂₀] and green [ΔA_540–550] regions of the spectrum) and from the rate of light utilization by PSII under limiting excitation conditions. Rates of PSII electron-transport were measured under the same light-limiting conditions using 2,5-dimethylbenzoquinone or 2,5-dichlorobenzoquinone as the PSII artificial electron acceptors. Evaluation is presented on the limitations imposed in the measurement of PSII electron flow to artificial quinones in chloroplasts. Limitations include the static quenching of excitation energy in the pigment bed by added quinones, the fraction of PSII centers (PSII_β) with low affinity to native and added quinones, and the loss of reducing equivalents to molecular oxygen. Such artifacts lowered the yield of steady-state electron transport in isolated chloroplasts and caused underestimation of PSII electron-transport capacity. The limitations described could explain the low PSII concentration estimates in higher plant chloroplasts (Chl/PSII = 600 ± 50) resulting from proton flash yield and/or oxygen flash-yield measurements. It is implied that quantitation of PSII by repetitive flash-yield methods requires assessment of the slow turnover of electrons by PSII_β and, in the presence of added quinones, assessment of the PSII quantum yield.     

CO disproportionation on Ni-based catalysts

Ni/SiO₂ exhibits higher activity for CO dissociation than coprecipitated Na–Mn–Ni, which is a higher oxygenate synthesis catalyst at 303 K, while Na–Mn–Ni exhibited higher CO dissociation activities at 673 K.     

Cross-linking dendrimers with allyl ether end-groups using the ring-closing metathesis reaction

A third generation Frechet-type dendrimer containing 24 allyl ether end-groups was synthesized, cross-linked using the ring-closing metathesis (RCM) reaction, and the core was removed hydrolytically without significant fragmentation. The results are analogous to those previously reported for homoallyl ether dendrimers (Wendland, M. S.; Zimmerman, S. C. J. Am. Chem. Soc. 1999, 121, 1389-1390) suggesting that the less readily available homoallyl ether dendrimers can be replaced by their allyl ethers analogues in a range of applications.     

Time-resolved gas-phase kinetic and quantum chemical studies of the reaction of silylene with nitric oxide

Time-resolved kinetic studies of the reaction of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with NO. The reaction was studied in the gas phase over the pressure range 1-100 Torr in SF6 bath gas at five temperatures in the range 299-592 K. The second-order rate constants at 10 Torr fitted the Arrhenius equation log(k/cm3 molecule(-1) s(-1)) = (-11.66 +/- 0.01) + (6.20 +/- 0.10 kJ mol(-1))/RT ln 10 The rate constants showed a variation with pressure of a factor of ca. 2 over the available range, almost independent of temperature. The data could not be fitted by RRKM calculations to a simple third body assisted association reaction alone. However, a mechanistic model with an additional (pressure independent) side channel gave a reasonable fit to the data. Ab initio calculations at the G3 level supported a mechanism in which the initial adduct, bent H2SiNO, can ring close to form cyclo-H2SiNO, which is partially collisionally stabilized. In addition, bent H2SiNO can undergo a low barrier isomerization reaction leading, via a sequence of steps, ultimately to dissociation products of which the lowest energy pair are NH2 + SiO. The rate controlling barrier for this latter pathway is only 16 kJ mol(-1) below the energy of SiH2 + NO. This is consistent with the kinetic findings. A particular outcome of this work is that, despite the pressure dependence and the effects of the secondary barrier (in the side reaction), the initial encounter of SiH2 with NO occurs at the collision rate. Thus, silylene can be as reactive with odd electron molecules as with many even electron species. Some comparisons are drawn with the reactions of CH2 + NO and SiCl2 + NO.     

Solid-phase synthesis of thalidomide and its analogues

A novel solid-phase synthesis of thalidomide and its metabolites and analogues is described. The synthetic strategy involves the coupling of hydroxymethyl polystyrene with phthalic anhydride to form the resin-linked acid. The acid is then reacted with primary amines followed by acid or base treatment to form thalidomide and its analogues with either open or closed phthalimide rings. Most of the analogues are synthesized with high yields (40.3-98.1% in three steps) and purities (92.3-98.9%).     

Type 2 intramolecular N-acylnitroso Diels-Alder reaction: scope and application to the synthesis of medium ring lactams

Heteroatom variants of the type 2 intramolecular Diels-Alder reaction provide an efficient method for the preparation of bridged bicyclic heterocycles. The type 2 variant of the intramolecular N-acylnitroso Diels-Alder reaction is an effective method for the synthesis of bridged bicyclic oxazinolactams. Structural studies of the cycloadducts have allowed for quantification of the deformations of the bridgehead functionalities and provided a strategy for the stereoselective synthesis of substituted seven- and eight-membered ring lactams. Diastereoselective cycloadditions followed by cleavage of the oxazine ring afford azepin-2-ones or azocin-2-ones.     

Trimethylsilyl-directed 1,3-dipolar cycloaddition reactions in the solid-phase synthesis of 1,2,3-triazoles

[reaction: see text] A regioselective method for the preparation of 1,5-trisubstituted 1H-1,2,3-triazoles via a 1,3-dipolar cycloaddition of 1-trimethylsilylacetylenes with organoazides is described. Immobilization of the azide on REM resin and subsequent cycloaddition afforded a 2 x 2 x 4 x 3 membered 1,5-disubstituted 1H-1,2,3-triazole library with an average purified yield of 68%.     

Probing electric fields at the ionic liquid-electrode interface using sum frequency generation spectroscopy and electrochemistry

The arrangement of ions at the platinum electrode in the room-temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate has been determined using sum frequency generation vibrational spectroscopy (SFG), electrochemical impedance spectroscopy (EIS), and the vibrational Stark effect. The results indicate that CO adsorbed on the Pt electrode has a Stark shift of 30-35 cm(-1)/V in the ionic liquid. The potential of zero charge (PZC) of the ionic liquid-Pt system is approximately -500 mV (vs Ag wire), with a capacitance of 0.12 F/m2. Further, polarization-dependent SFG experiments suggest the ions reorganize at the surface depending on the electrode charge. In combination, all these results indicate that the ions of a neat ionic liquid are organized in a Helmholtz layer at the electrified metal electrode interface.     

Capillary and microelectrophoretic separations of ligase detection reaction products produced from low-abundant point mutations in genomic DNA

Capillary gel electrophoresis (CGE) and polymer-based microelectrophoretic platforms were investigated to analyze low-abundant point mutations in certain gene fragments with high diagnostic value for colorectal cancers. The electrophoretic separations were carried out on single-stranded DNA (ssDNA) products generated from an allele-specific ligation assay (ligase detection reaction, LDR), which was used to screen for a single base mutation at codon 12 in the K-ras oncogene. The presence of the mutation generated a ssDNA fragment that was >40 base pairs (bp) in length, while the primers used for the ligation assay were <30 bp in length. Various separation matrices were investigated, with the success of the matrix assessed by its ability to resolve the ligation product from the large molar excess of unligated primers when the mutant allele was lower in copy number compared to the wild-type allele. Using CGE, LDR product models (44 and 51 bp) could be analyzed in a cross-linked polyacrylamide gel with a 1000-fold molar excess of LDR primers (25 bp) in approximately 45 min. However, when using linear polyacrylamide gels, these same fragments could not be detected due to significant electrokinetic biasing during injection. A poly(methylmethacrylate) (PMMA) microchip of 3.5 cm effective column length was used with a 4% linear polyacrylamide gel to analyze the products generated from an LDR. When the reaction contained a 100-fold molar excess of wild-type DNA compared to a G12.2D mutant allele, the 44 bp ligation product could be effectively resolved from unligated primers in under 120 s, nearly 17 times faster than the CGE format. In addition, sample cleanup was simplified using the microchip format by not requiring desalting of the LDR prior to loading.