Isotachophoresis separations of enantiomers on a planar chip with coupled separation channels.

The use of a poly(methylmethacrylate) chip, provided with a pair of on-line coupled separation channels and on-column conductivity detectors, to isotachophoresis (ITP) separations of optical isomers was investigated. Single-column ITP, ITP in the tandem-coupled columns, and concentration-cascade ITP in the tandem-coupled columns were employed in this investigation using tryptophan enantiomers as model analytes. Although providing a high production rate (about 2 pmol of a pure tryptophan enantiomer separated per second), single-column ITP was found suitable only to the analysis of samples containing the enantiomers at close concentrations. A 94-mm separation path in ITP with the tandem-coupled separation channels made possible a complete resolution of a 1.5 nmol amount of the racemic mixture of the enantiomers. However, this led only to a moderate extension of the concentration range within which the enantiomers could be simultaneously quantified. The best results in this respect were achieved by using a concentration-cascade of the leading anions in the tandem-coupled separation channels. Here, a high production rate, favored in the first separation channel, was followed by the ITP migration of the enantiomers in the second channel under the electrolyte conditions enhancing their detectabilities. In dependence on the migration configuration of the enantiomers, this technique made possible their simultaneous determinations when their ratios in the loaded sample were 35:1 or less (D-tryptophan a major constituent) and 70:1 or less (L-tryptophan a major constituent).

Conductivity detection and quantitation of isotachophoretic analytes on a planar chip with on-line coupled separation channels.

A poly(methylmethacrylate) chip, provided with two separation channels in the column-coupling (CC) arrangement and on-column conductivity detection sensors and intended, mainly, to isotachophoresis (ITP) and ITP-capillary zone electrophoresis (CZE) separations was developed recently. The present work was aimed at assessing its performance relevant to the detection and quantitation of the ITP analytes. Hydrodynamic (HDF) and electroosmotic (EOF) flows of the solution in the separation compartment of the CC chip were suppressed and electrophoresis was a dominant transport process in the ITP separations with model analytes carried out in this context. When the surfaces of the detection electrodes of the conductivity sensors on the chip were appropriately cleaned qualitative indices of the test analytes [relative step heights (RSHs)], provided by a particular detection sensor, agreed within 1% (expressed via RSDs of the RSH values). Their long-term reproducibilities for one sensor, as estimated from 70 ITP runs repeated in 5 days, were 2% or less. Sensor-to-sensor and chip-to-chip fluctuations of the RSH values for the test analytes were 2.5% or less. In addition, experimentally obtained RSH values agreed well with those predicted by the calculations based on the ITP steady-state model. Reproducibilities of the migration velocities attainable on the CC chips with suppressed EOF and HDF, assessed from the migration time measurements of the ITP boundary between well-defined positions on the separation channels of the chips (140 repeated runs on three chips), ranged from 1.4 to 3.3% for the migration times in the range of 100-200 s. Within-day repeatabilities of the time-based zone lengths for the test analytes characterized 2% RSDs, while their day-to-day repeatabilities were less than 5%. Chip-to-chip reproducibilities of the zone lengths, assessed from the data obtained on three chips for 100 ITP runs, were 5-8%.

Capillary zone electrophoresis of orotic acid in urine with on-line isotachophoresis sample pretreatment and diode array detection.

Potentialities of capillary zone electrophoresis with on-line isotachophoresis sample pretreatment and diode array detection (ITP-CZE-DAD) to the separation, detection and identification of trace analytes present in biological matrices were investigated. Urine represented a multicomponent, variable and high ionic strength matrix while orotic acid was chosen as a model analyte of a practical clinical relevance in this investigation. Using the ITP-CZE combination in the column-coupling configuration of the separation system ITP provided an enhanced sample load capacity to the separation system (a 30 microl sample injection volume), concentrated the analyte and served as an on-line sample clean up technique. On the other hand, CZE performed a final separation of the analyte from matrix constituents present in the ITP pretreated sample and provided favorable conditions for its detection and identification by DAD. Using current correction and smoothing procedures analytically relevant DAD spectra of orotic acid could be obtained also in instances when this was injected in a model sample at a 2 x 10(-7) mol/l concentration (an estimated limit of determination of orotic acid at a 218 nm detection wavelength). ITP-CZE separations of urine samples (based on differences in acid-base properties and host-guest complexations of the analyte and matrix anionic constituents) led to significant sample clean ups. Consequently, DAD spectra of orotic acid matching its reference spectrum, could be acquired also in instances when the acid was present in urine matrices (loaded in 30 microl injection volumes of 20-fold diluted urine samples) at 4-6 x 10(-7) mol/l concentrations. Here, residual trace matrix interferents prevented a closer approach to the above value attainable for model samples. Although this work was focused only on one analyte and urine matrix it implies very promising potentialities of the ITP-CZE-DAD combination in the identification and quantitation of trace analytes present in biological matrices, in general.

Isotachophoresis and isotachophoresis--zone electrophoresis separations of inorganic anions present in water samples on a planar chip with column-coupling separation channels and conductivity detection.

The use of a poly(methylmethacrylate) chip, provided with two separation channels in the column-coupling (CC) arrangement and on-column conductivity detection sensors, to electrophoretic separations of a group of inorganic anions (chloride, nitrate, sulfate, nitrite, fluoride and phosphate) that need to be monitored in various environmental matrices was studied. The electrophoretic methods employed in this study included isotachophoresis (ITP) and capillary zone electrophoresis (CZE) with on-line coupled ITP sample pretreatment (ITP-CZE). Hydrodynamic and electroosmotic flows of the solution in the separation compartment of the CC chip were suppressed and electrophoresis was a dominant transport process in the separations performed by these methods. ITP separations on the chip provided rapid resolutions of sub-nmol amounts of the complete group of the studied anions and made possible rapid separations and reproducible quantitations of macroconstituents currently present in water samples (chloride, nitrate and sulfate). However, concentration limits of detection attainable under the employed ITP separating conditions (2-3 x 10(-5) mol/l) were not sufficient for the detection of typical anionic microconstituents in water samples (nitrite, fluoride and phosphate). On the other hand, these anions could be detected at 5-7 x 10(-7) mol/l concentrations by the conductivity detector in the CZE stage of the ITP-CZE combination on the CC chip. A sample clean-up performed in the ITP stage of the combination effectively complemented such a detection sensitivity and nitrite, fluoride and phosphate could be reproducibly quantified also in samples containing the macroconstituents at 10(4) higher concentrations. ITP-CZE analyses of tap, mineral and river water samples showed that the CC chip offers means for rapid and reproducible procedures to the determination of these anions in water (4-6 min analysis times under our working conditions). Here, the ITP sample pretreatment concentrated the analytes and removed nanomol amounts of the macroconstituents from the separation compartment of the chip within 3-4 min. Both the ITP and ITP-CZE procedures required no or only minimum manipulations with water samples before their analyses on the chip. For example, tap water samples were analyzed directly while a short degassing of mineral water (to prevent bubble formation during the separation) and filtration of river water samples (to remove particulates and colloids) were the only operations needed in this respect.

Determination of organic acids and inorganic anions in wine by isotachophoresis on a planar chip.

Isotachophoretic (ITP) separation and determination of a group of 13 organic and inorganic acids, currently present in wines, on a poly(methyl methacrylate) chip provided with on-column conductivity detection was a subject of a detailed study performed in this work. Experiments with the ITP electrolyte systems proposed to the separation of anionic constituents present in wine revealed that their separation at a low pH (2.9) provides the best results in terms of the resolution. Using a 94 mm long separation channel of the chip, the acids could be resolved within 10-15 min also in instances when their concentrations corresponded to those at which they typically occur in wines. A procedure suitable to the ITP determination of organic acids responsible for some important organoleptic characteristics of wines (tartaric, lactic, malic and citric acids) was developed. Concentrations of 2-10 mg/l of these acids represented their limits of quantitation for a 0.9 microl volume sample loop on the chip. A maximum sample load on the chip, under the preferred separating conditions, was set by the resolution of malate and citrate. A complete resolution of these constituents in wine samples was reached when their molar concentration ratio was 20:1 or less. ITP analyses of a large series of model and wine samples on the chip showed that qualitative indices [RSH (relative step height) values] of the acids, based on the response of the conductivity detector, reproduced with RSD better than 2% while reproducibilities of the determination of the acids of our interest characterized RSD values better than 3.5%.

Discrete spacers for photometric characterization of humic acids separated by capillary isotachophoresis.

A group of twenty discrete spacers suitable for photometric characterization of humic acids (HAs) after their isotachophoretic (ITP) separation at pH 10 was found. The spacers, inorganic and organic acids and amino acids of suitable acid-base and migration properties exhibiting no light absorption in the UV region of the light spectrum, made possible to perform this characterization in a sensitive spike mode of the ITP analysis. Using this approach a complex mixture of humic constituents present in a test HA preparation was separated into 22 fractions migrating in the interzonal boundary layers formed by the zones of discrete spacers and 21 fractions mixed with the zones of the spacers. A photometric monitoring of the fractions in the ITP stack at a 405 nm detection wavelength provided an adequate selectivity and sensitivity into the characterization. Relative sizes of the detected fractions of the test HA preparation ranged from 0.2-0.3 to 27.5% (based on the response of the phototometric detector at 405 nm). The fractions representing ca. 0.2-0.3% of the total peak area could be still quantified when 800 ng of the test preparation was loaded onto the ITP column. A typical repeatability of the total area of the detection signal corresponding to humic constituents in the ITP stack was ca. 2.5%. Repeatabilities of the peak areas of the fractions of the humic constituents defined by the spacers ranged from 2 to 6% for the fractions representing 1% or more of the total area and from 8 to 12% for those representing less than 1%. No marks of aggregations of the humic constituents were detected and reproducible ITP profiles (fingerprints) of the studied humic preparation were achieved under the developed working conditions.

Conductivity detection cell for capillary zone electrophoresis with a solution mediated contact of the separated constituents with the detection electrodes.

A contact conductivity detection cell for capillary zone electrophoresis (CZE) with an electrolyte solution mediated contact of the separated constituents with the detection electrodes (ESMC cell) was developed in this work. This new approach to the conductivity sensing in CZE is intended to eliminate detection disturbances due to electrode reactions and adsorption of the separated constituents when these are coming into direct contact with the detection electrodes. An optimum detection performance of the cell was achieved when the carrier electrolyte solution mediated the electric contact of the detection electrodes with the separated constituents. Different compositions of the mediator and carrier electrolyte solutions led to large drifts of the detection signals. Isotachophoresis experiments performed in this context with the ESMC cell revealed that origins of these drifts are in transport processes (diffusion and electromigration) between the detection compartment and the detection electrodes in the cell. These processes affected, to some extent, other analytically relevant performance parameters of the ESMC cell of the present construction as well [e.g., concentration limits of detection (LODs), a contribution of the cell to the band broadening]. For example, the ESMC cell gave, under optimum operating conditions, 3-4 times higher concentration LODs for the test analytes than a current on-column conductivity cell employed under identical working conditions. On the other hand, these LOD values (25-150 nmol/l) were still 20-25 times lower than those estimated from reference experiments for a contactless conductivity detector. CZE experiments with iodide, carried out under working conditions leading to electrochemical reactions of this anion on the detection electrodes of current conductivity cells, did not occur in the ESMC cell. In addition, this cell, contrary to a reference contact conductivity cell, required no special care (e.g., cleaning of the surfaces of the detection electrodes by chemical or electrochemical means) to maintain its reliable long-term performance. Anionic CZE analyses of tap and mineral water samples monitored by the conductivity detector provided with the ESMC cell demonstrated a practical applicability and certain limitations of this detection approach in the analysis of ionic constituents present in high ionic strength sample matrices.

Separation of multicomponent mixtures of 2,4-dinitrophenyl labelled amino acids and their enantiomers by capillary zone electrophoresis.

The use of capillary zone electrophoresis (CZE) for the separation of a group of 33 2,4-dinitrophenyl labeled amino acids (DNP-AA), including DNP-AA racemates, DNP-L-AA enantiomers and achiral DNP-AAs, was investigated. Alpha-, beta- and gamma-cyclodextrins (CDs) and their derivatives (hydroxypropyl derivatives of alpha-, beta- and gamma-CDs, polymeric beta-CD and 6A-methylamino-beta-cyclodextrin (MA-beta-CD)) served as complexing agents and chiral selectors in this investigation. Although native alpha- and gamma-CDs and their derivatives influenced the effective mobilities of the studied DNP-AAs in different ways, they generally failed to resolve enantiomers of the individual DNP-AAs. On the other hand, beta-CD and all of its derivatives were found to be effective in this respect. Of these, the best results were achieved with a positively charged MA-beta-CD and this chiral selector resolved enantiomers of ten DNP-AA racemates available for this study. However, a complete resolution of these enantiomers in one CZE run required that the effect of the chiral selector be complemented by complexing effects of polyvinyl pyrrolidone (PVP) or gamma-CD. Complexing and chiral recognition capabilities of MA-beta-CD combined with complexing effects of gamma-CD and PVP provided separating conditions suitable for the CZE separations of multicomponent mixtures of DNP-AAs with preserved resolutions of the enantiomers. For example, a mixture consisting of 43 DNP-AA constituents was resolved using an MA-beta-CD/gamma-CD combination with three peak overlaps.

Capillary electrophoresis separations on a planar chip with the column-coupling configuration of the separation channels

Some basic aspects of capillary electrophoresis (CE) separations on a poly(methyl methacrylate) chip provided with two separation channels in the column-coupling (CC) configuration and on-column conductivity detectors were studied. The CE methods employed in this study included isotachophoresis (ITP), capillary zone electrophoresis (CZE), and CZE with on-line ITP sample pretreatment (ITP-CZE). Hydrodynamic and electroosmotic flows of the solution in the separation compartment of the chip were suppressed, and electrophoresis was a dominant transport process in the separations performed by these methods. Very reproducible migration velocities of the separated constituents were typical under such transport conditions, and consequently, test analytes could be quantified by various ITP techniques with 1-2% RSD. The CC configuration of the separation channels provides means for an effective combination of an enhanced load capacity of the separation system with high detection sensitivities for the analytes in concentration-cascade ITP separations. In this way, for example, succinate, acetate, and benzoate could be separated also in instances when they were present in the loaded sample (1.2 microL) at 1 mmol/L concentrations while their limits of detection ranged from 8 to 12 micromol/L concentrations. A well-defined ITP concentration of the analyte(s) combined with an in-column sample cleanup (via an electrophoretically driven removal of the matrix constituents from the separation compartment) can be integrated into the separations performed on the CC chip. These sample pretreatment capabilities were investigated in ITP-CZE separations of model samples in which nitrite, phosphate, and fluoride (each at a 10 micromol/L concentration) accompanied matrix constituents (sulfate and chloride) at considerably higher concentrations. Here, both the concentration of the analytes and cleanup of the sample were included in the ITP separation in the first separation channel while the second separation channel served for the CZE separation of the ITP pretreated sample and the detection of the analytes.

Separations of enantiomers by preparative capillary isotachophoresis.

The use of capillary isotachophoresis (ITP), operating in a discontinuous fractionation mode, for preparative separations of enantiomers of chiral compounds was studied. The ITP separations were carried out in the column-coupling configuration of the separation unit provided with the preseparation column of a 1.0 mm ID and the trapping column of a 0.8 mm ID. Such a configuration of the CE separation unit offers several working regimes suitable to preparative separations of enantiomers. 2,4-Dinitrophenyl-DL-norleucine (DNP-Norleu) was employed as a model analyte in our experiments with beta-cyclodextrin serving in the electrolyte solutions as a chiral selector. The preparative separations lasting about 20 min were evaluated by ITP and (more often) by capillary zone electrophoresis (CZE). It was found that one preparative run provided up to 14 microg of pure DNP-Norleu enantiomers. This corresponded to a 75 times higher production rate of ITP relative to a maximum value of this parameter as estimated for preparative CZE runs in cylindrical capillaries (0.5 pmol/s). About 75% of the DNP-Norleu enantiomers loaded into the preparative equipment could be recovered in pure enantiomer fractions. Contiguous natures of the zones in the ITP stack and adsorption losses of the enantiomers in the isolation step were found to set practical limits for a further enhancement of the recovery rates in the isolation of pure enantiomers.

Capillary zone electrophoresis separations of enantiomers present in complex ionic matrices with on-line isotachophoretic sample pretreatment.

Analytical capabilities of capillary zone electrophoresis (CZE) with on-line coupled capillary isotachophoresis (ITP) sample pretreatment in the column-coupling capillary electrophoresis equipment to separate and determine enantiomers present in multicomponent ionic matrices were studied. Tryptophan was used as a model analyte in the ITP-capillary zone electrophoresis experiments performed in this context while a 90-component model mixture of UV-light absorbing organic anions and urine served as multicomponent sample matrices. Various working modes in which the on-line coupled capillary isotachophoresis-capillary zone electrophoresis combination in the column-coupling separation system can operate were employed in the anionic regime of the separation with direct injections of the samples. Advantages and limitations of these working modes in the separations of enantiomers present in model and urine matrices were assessed. Experiments with model mixtures of tryptophan enantiomers revealed that the two were resolved in the capillary zone electrophoresis stage with the aid of alpha-cyclodextrin also when their concentration ratio in the sample was 1:200 while the concentration of L(-)-tryptophan was 25 nmol/l. The limits of detection for the enantiomers were at approximately 10 nmol/l (approximately 1.5 ng/ml) concentrations for a 220 nm detection wavelength of the UV detector employed in the capillary zone electrophoresis stage and for a 30 microliters sample load. A high sample load capacity of the on-line coupled capillary isotachophoresis stage was effective in separating the samples corresponding to 3-6 microliters volumes of undiluted urine. The results from the runs with urine samples showed that only the capillary isotachophoresis-capillary zone electrophoresis combination with a post-column on-line coupled capillary isotachophoresis sample clean-up (responsible for a removal of more than 99% of the sample anionic constituents migrating in the on-line coupled capillary isotachophoresis stack and detectable in the capillary zone electrophoresis stage) provided a universal alternative for the detection and quantitation of the model analyte (L(-)-tryptophan).

Capillary electrophoresis of inorganic anions.

This review deals with the separation mechanisms applied to the separation of inorganic anions by capillary electrophoresis (CE) techniques. It covers various CE techniques that are suitable for the separation and/or determination of inorganic anions in various matrices, including capillary zone electrophoresis, micellar electrokinetic chromatography, electrochromatography and capillary isotachophoresis. Detection and sample preparation techniques used in CE separations are also reviewed. An extensive part of this review deals with applications of CE techniques in various fields (environmental, food and plant materials, biological and biomedical, technical materials and industrial processes). Attention is paid to speciations of anions of arsenic, selenium, chromium, phosphorus, sulfur and halogen elements by CE.

Capillary zone electrophoresis of nitrophenols with off-line isotachophoretic sample pretreatment.

Preparative capillary isotachophoresis (ITP) operating in a discontinuous fractionation mode was studied as a sample pretreatment technique for capillary zone electrophoresis (CZE) trace analysis of a group of ten nitrophenols. Different separation mechanisms employed by these capillary electrophoresis techniques made possible a group isolation of the studied analytes by ITP while their CZE resolutions based on differences in the interactions with polyvinylpyrrolidone (PVP) provided suitable conditions for a final analytical evaluation. Experiments with model and practical water samples revealed high and reproducible recovery rates of the ITP pretreatment of nitrophenols when sample loadability limits of the preparative ITP equipment were met. A main disadvantage of the studied ITP-CZE combination was an inefficient use of the pretreated sample as only 0.5% of the fraction containing nitrophenols was used in the final CZE step. Despite these limitations associated with the CZE sample injection, the concentration limit of detection (LOD) for nitrophenols in practical water samples could be reduced to 2-8 ppb concentrations (photometric absorbance detector operating at a 254 nm wavelength) when 200 microL sample volumes were taken for the ITP pretreatment. This was accompanied by an efficient sample clean-up as no other trace ionic constituents originating from the samples were detected on CZE.

Electroosmotic flow suppressing additives for capillary zone electrophoresis in a hydrodynamically closed separation system.

Electroosmotic flow in a hydrodynamically closed capillary zone electrophoresis (CZE) separation compartment must be minimized to achieve high efficiency CZE separations. A group of eight potential electroosmotic flow suppressors was investigated in this context for the separations in fluorinated ethylene-propylene capillary tubes. The suppressors included water soluble methylhydroxyethyl derivatives of cellulose, polyvinylalcohol, polyvinylpyrrolidones and polyethyleneglycols of different molecular masses and Triton X-100. Methylhydroxyethylcellulose derivatives and polyvinylalcohol were found to provide the highest separation efficiencies for a group of model anions when the electroosmotic flow suppressors were used as the carrier electrolyte additives. Using a methylhydroxyethylcellulose coated separation compartment very significant improvements in the separation efficiencies were achieved for polyvinylpyrrolidones and polyethyleneglycols applied in the carrier electrolyte solutions. For example, polyvinylpyrrolidone K 90 applied in this way gave for some of the model analytes the plate height values approaching those estimated in the calculations as theoretical limits for our experimental conditions (H approximately 3.5 microns). CZE experiments with albumin and gamma-globulin showed that the use of methylhydroxyethylcellulose derivative in the carrier electrolyte solution at pH = 9.2 was effective in eliminating potential disturbances in the separation efficiencies of the analytes due to adsorption of the proteins.

Capillary zone electrophoresis of inorganic anions with conductivity detection.

A carrier electrolyte system for capillary zone electrophoresis (CZE) resolving chloride, bromide, iodide, fluoride, nitrite, nitrate, sulfate, and phosphate in a hydrodynamically closed separation compartment is described. The carrier electrolyte combines the effects of pH, polyvinylpyrrolidone (PVP) and the counterionic constituent on the effective mobilities of the anions. In 300 microns ID capillary tubes made of fluorinated ethylene-propylene copolymer (FEP), and with detection of analytes with the aid of an alternating current conductivity detector, detection limits in the range of 3-10 ppb could be achieved for 200 nL sample volumes. The separation efficiencies were in the range of 1.5-2.5 x 10(5) theoretical plates per meter for an adequate sample load. The reproducibility was evaluated for two concentration levels. For concentrations close to the limits of quantitation (50-120 ppb), the RSD values ranged from 1.5-12.6%, with the highest scatter for fluoride and phosphate. The RSD values were in the range of 0.4-1.5% for 300-1200 ppb concentrations of the anions. Typical analysis times were 2-5 min, depending on the anion species. A series of water samples (drinking, river, rain) was used to assess the practical applicability of the CZE method. The method is a suitable alternative for the determination of both anionic macro- and microconstituents in water with a good overall selectivity.

Determination of synthetic dyes in food products by capillary zone electrophoresis in a hydrodynamically closed separation compartment.

The application of capillary zone electrophoresis (CZE) in a hydrodynamically closed separation system to determine synthetic food colorants added to food products was investigated. The CZE separations were carried out in a 300-micron-i.d. capillary tube made of fluorinated ethylene-propylene copolymer. The inner diameter of the capillary tube made it possible to enhance sample loads (100-nL injection volumes) so that 10-300 ppb limits of detection (LOD) values could be achieved for the studied dyes by a photometric absorbance detector operating at a 254-nm detection wave-length. With the exception of erythrosine (which exhibited a residual adsorption), very good reproducibilities of the determination were typical for 4- and 32-ppm concentrations of the dyes. This rapid CZE procedure (migration times of the resolved analytes were between 2.5 and 10.5 min) provided good selectivities in the determination of the dyes in various food matrices (soft drink concentrates, liqueurs, and chewing gums). Simple sample preparation steps were effective for the sample matrices used in the investigation.

A comparison of some metabolitesin skeletal muscle of different halothane sensitivity withmeat quality traits of pigs using a capillary isotachophoresis technique.

Isotachophoretic (ITP) measurements were made on biopsy samples from live muscle M. longissimus lumborum and carcasses of pigs. Two experiments were performed. In Experiment 1, the methodological aspects of ITP measurements from skeletal muscle, were studied. In Experiment 2, ITP measurements on muscle and carcass samples were made. The muscle and carcass samples were obtained by shot or spring-loaded biopsy from the Longissimus lumborum muscle of 30 Belgium Landrace × Duroc pigs, of which 10 were halothane-positive. The pigs were slaughtered by electro-stunning and manguination at approximately 105 kg body l. w. The potential meat quality in live pigs and after slaughter using small biopsy samples of M. longissimus lumborum was also determined. The experimental results show that ITP (mainly inosinemonophosphate and lactate) and meat quality data (water-holding capacity test, pH and R value) can differentiate halo thane-positive from halothane-negative pigs. Out results confirmed previous results which showed that the water-holding capacity test defined as fluid volume', pH and R value measurements on biopsy samples can also predict the potential meat quality in live pigs.

Isotachophoretic determination of short-chain fatty acids in drinking water after solid-phase extraction with a carbonaceous sorbent.

A macroporous carbon sorbent, packed into disposable columns (Separcol-Carb), was investigated for the off-line preconcentration of short-chain fatty acids from drinking water in conjunction with their determination by capillary isotachophoresis (ITP). Of the acids investigated (C1-C9), butyric acid and higher homologues could be enriched into a high degree from samples of drinking water. Their detection limits from the ITP conductivity detector were in the low parts per 10(9) range when an amount equivalent to 8 ml of the sample was taken for analysis. The lowest homologues (C1-C3) were not adsorbed sufficiently to achieve their reasonable enrichment by the sorbent under the working conditions employed (acidification of the sample to pH 2.0). Acetone and diethyl ether were employed for the elution of the adsorbed analytes. The latter was more convenient in the analysis of practical samples as it co-eluted a considerably smaller number of the adsorbed anionic constituents. Octadecyl-bonded silica, evaluated in parallel, was found to be of only very limited utility for the same purpose.

Isotachophoretic determination of chromium(VI) at low parts per billion concentrations.

Factors important in the isotachophoretic determination of CrVI at low ppb concentrations were studied. To increase the selectivity of the analysis, the use of photometric detection at a 405 wavelength was preferred. Losses of CrVI due to adsorption were found to be the main potential source of analytical errors at the concentration levels of interest. The presence of sulphate in the sample solution at ca. 10(-4) M concentration eliminated the losses due to adsorption on the walls of the sample handling glassware. In analysis with a low pH of the leading electrolyte, adsorption of CrVI on the walls of the separation compartment also played a role; addition of naphthalene-1,3,6-trisulphonate to the sample solution considerably reduced this disturbance. When the precautions concerning adsorption were taken, the detection limits for CrVI were in the range 4-5 ppb (depending on the pH of the leading electrolyte) for a 30-microliters sample volume. The calibration graphs were linear over the concentration range 10(-7)-5 X 10(-6) M with good correlation coefficients. The reproducibilities of the determinations within this concentration range were 2-3% or better. The practical utility of capillary isotachophoresis in the trace determination of CrVI in drinking water and wastewater samples seems promising.

Photometric detection at 405 nm in trace analysis by capillary isotachophoresis.

The possibilities of photometric detection at 405 nm were investigated for trace isotachophoretic analysis of various ionic constituents. Increased selectivity of the detection enabled, for example, the detection and determination of nitrophenols at 10(-8) M in a 25-microliter sample. 5-Nitrofurylacrylic acid, a wine stabilizer, could be detected with confidence at ca. 0.1 ppm in wine without any sample pretreatment. Approximately 250-fmol amounts of amino acids, labelled with 2,4-dinitrophenyl, were detectable with the technique employed. This is almost three decades lower in comparison to what is achievable with an high resolution universal detector. The adsorption of the analytes in the sample handling devices as well as in the separation compartment were problems in the analysis at this concentration level. The detection limit given above was achieved when these phenomena were suppressed by the addition of naphthalene-1,3,6-trisulphonate or pyrophosphate to the sample solution.