An improved reaction valve for flow injection analysis

Summary A new type reaction valve for flow injection analysis has been developed. In a FIA system with this valve the injected sample is once passed into a loop of the valve, thereafter the valve is rotated to the next position. Therefore, the sample is retained in the loop in a stop-state for a long time and the next sample is introduced into another loop in the next position. When the valve is rotated for one round, the stopped sample in the fist loop is again passed into the detector flow cell. Thus, an analysis, which needs a long reaction time, can be performed with this valve with high precision.

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Verbessertes Reaktionsventil für die Flie\injektionsanalyse

     

Flow injection and solvent extraction with intelligent segment separation. Determination of quaternary ammonium ions by ion-pairing

Organic solvent segments are introduced into a premixed stream of the sample and an ion-pairing dye by a 3-way valve at regular intervals. The major mechanism of "extraction" in such a system appears to be the adsorption of the ion-pair on conduit walls and subsequent elution by the organic solvent. Following passage through the conduit, the immiscible organic segment is isolated in the loop of a 6-port valve; loop filling is sensed by a pair of conductivity sensors located respectively at the fill and drain ports of the loop and appropriate logic circuitry. The isolated segment is then injected into a purely organic carrier and detected by a suitable optical detector. Results are reported for a number of parametric studies that characterize this system. With immobilized adsorbents on conduit walls for preconcentration, a limit of detection of < 10(-7)M tetrabutylammonium ion can be obtained with bromothymol blue as the ion-pairing agent.     

Flow-reversal flow injection analysis-II Determination of glucose with a double-pump system

A new type of flow-injection procedure is proposed in which the samples are reversely pumped to the detector. In this procedure the injected samples are pumped into the reaction loop of a 6-way valve, then the valve is rotated to reverse the flow and the sample/reagent plug is pumped to the detector by another pump. The dispersion of the sample zone is low and the consumption of the reagent is very small. Therefore, its analytical potential for analysis with expensive reagents or long reaction times is high. The procedure has been applied to the determination of glucose in serum with an enzyme kit.     

HPLC--residue analysis of the herbicide pyridate in cereals

For many reasons residue analysis of plant protecting chemicals is becoming of increasing importance. The demand of modern trace analysis techniques is to detect significantly and sensitively values down to levels in the ppb range or even less. Aggravating circumstances are the complex multicomponent sample matrices out of whom the residue analysis has to be performed; theoretically thousands of compounds could be present and interferences with substances of interest are quite likely. One way to get around these difficulties is to employ multidimensional HPLC (MD-HPLC). A method using this invaluable analytical tool tracing Pyridate and its main metabolites in various plant extracts is presented. Based on a brief discussion about requirements of modern trace analysis HPLC in connection with column switching we designed an automatizable setup combining a weak anionexchanger (dimethylamine phase) with a reversed phase system handling relatively large aqueous sample volumes. Depending on specifically described sample pretreatment schemes detection limits down to 30 ppb are routinely obtained. At this level acceptable on-line UV-spectra can be obtained via inserting a spectrophotometric detector in a loop of a 6-port valve. The usefulness of the method described has been demonstrated by analysing several hundred samples.     

Continuous sample monitoring by flow reversal methodology

A flow reversal methodology based on a single change of the flow direction was used for the continuous monitoring of samples injected into an unsegmented flow-system. The photometric detector was located in the loop of the injection valve and the flow was reversed once each whole plug emerged. Sample dispersion in the carrier stream was continuously monitored in systems with and without chemical reaction. The influence of various physico-chemical variables was studied and some analytical possibilities are discussed.     

Determination of ultratrace impurity elements in high purity niobium materials by on-line matrix separation and direct injection/inductively coupled plasma mass spectrometry

The determination of 52 impurity elements in niobium materials (niobium metal, niobium oxide (V), and niobium pentaethoxide) was performed by inductively coupled plasma mass spectrometry (ICP-MS) with on-line anion exchange matrix separation as well as direct nebulization. Niobium material samples were decomposed with a mixture of hydrofluoric acid and nitric acid to prepare 10% niobium solutions. In the on-line anion exchange matrix separation/ICP-MS, the niobium and hydrofluoric acid concentrations in sample solution were adjusted to 5% and ca. 8 M, respectively. The solution was then injected into the carrier stream from the sample loop of injection valve to pass through an anion exchange resin column. In the anion exchange separation, niobium in the fluoro-complex form was adsorbed on the resin, while impurity elements were eluted. The eluted elements were introduced into ICP-MS for the determination of 25 impurity elements. On the other hand, 27 impurity elements could not be separated well from niobium matrix under the above anion exchange conditions, and then the sample solution with the niobium concentration of max. 0.2% containing internal standard elements was injected from the sample loop of injection valve directly to introduce into ICP-MS. As a result, 52 impurity elements in three kinds of niobium materials could be determined at the ng g⁻¹ level.     

Removal of humic acid and surfactant interferences in trace metal determinations by differential-pulse anodic stripping voltammetry with use of adsorption and chelate ion-exchanger columns in a flow-injection system

A flow-injection differential-pulse anodic stripping voltammetry (d.p.a.s.v.) method is modified so that interferences from humic acids or surfactants are eliminated. The injected, slightly acidic sample is passed through a silica anion-exchanger column to remove compoundswith a strong tendency to adsorb to the electrode. The sample then passes to a chelate ion-exchange column containing immobilized 8-quinolinol. The metal ions are retained and later eluted with acid into the voltammetric cell. The results show that the interferences from up to 500 mg 1^–1 humic acid or at least 50 mg 1^-1 Triton X-100 can be removed and that the metal ion can be determined in a range similar to that for normal d.p.a.s.v. methods. The complete cycle time for a determination was 12 min.     

Determination of arsenic in geological materials by electrothermal atomic-absorption spectrometry after hydride generation

Rock and soil samples are decomposed with HClO₄—HNO₃; after further treatment, arsine is generated and absorbed in a dilute silver nitrate solution. Aliquots of this solution are injected into a carbon rod atomizer. Down to 1 ppm As in samples can be determined and there are no significant interferences, even from chromium in soils. Good results were obtained for geochemical reference samples.     

High performance liquid chromatography determination of doxorubicin and daunorubicin in plasma using UV detection and column switching

A method for the determination of doxorubicin and daunorubicin in plasma is described. The plasma is injected directly into a loop column and then washed with water. After switching the injection valve, the sample is separated on a phenyl column using detection at 254 nm. The detection limit is 10 ng/mL, the coefficient of variation is 7% for 100 ng/mL of doxorubicin and 4% for 200 ng/mL of daunorubicin.     

Direct measurement of iron in serum by electrothermal atomic absorption spectrometry

A simple and rapid method is described for the determination of iron in serum by atomic absorption spectrometry with a graphite furnace atomiser. The serum is diluted 40 times with water, and injected into the graphite tube. Optimal conditions are established, and interferences from proteins and salts eliminated. Since the procedure requires no sample pretreatment such as protein precipitation or wet digestion, contamination and losses by co-precipitation are excluded. The method can determine any species of iron in serum.     

Multidimensional liquid chromatography system with an innovative solvent evaporation interface

An orthogonal two-dimensional liquid chromatographic (2D-LC) system was developed by using a vacuum-evaporation loop-type valve interface. Normal-phase liquid chromatography (NPLC) with a bonded CN phase column was used as the first dimension, and reversed-phase liquid chromatography (RPLC) with a C(18) column was used as the second dimension. All the solvents in the loop of the interface were evaporated at 90 degrees C under vacuum conditions, leaving the analytes on the inner wall of the loop. The mobile phase of the second dimension dissolved the analytes in the loop and injected them onto the secondary column, allowing an on-line solvent exchange of a selected fraction from the first dimension to the second dimension. The chromatographic resolution of analytes on the two dimensions was maintained at their optimal condition. Sample loss due to evaporation in the interface was observed that depended on the boiling point of the compound. Separation of sixteen polycyclic aromatic hydrocarbon mixtures and a traditional Chinese medicine Angelica dahurica was demonstrated.     

Automated stopped-in-dual-loop flow analysis system for catalytic determination of vanadium in drinking water

An automated stopped-in-dual-loop flow analysis (SIDL-FA) system is proposed for the determination of vanadium in drinking water. The chemistry is based on the vanadium-catalyzed oxidation reaction of p-anisidine by bromate in the presence of Tiron as an activator to produce a dye (λ_max = 510 nm). A SIDL-FA system basically consists of a selection valve, three pumps (one is for delivering of standard/sample, and others are for reagents), two six-way injection valves, a spectrophotometric detector and a data acquisition device. A 100-μL coiled loop around a heated device is fitted onto each six-way injection valve. A well-mixed solution containing reagents and standard/sample is loaded into the first loop on a six-way valve, and then the same solution is loaded into the second loop on another six-way valve. The solutions are isolated by switching these two six-way valves, so that the catalytic reaction can be promoted. The net waste can be zero in this stage, because all pumps are turned off. Then each resulting solution is dispensed to the detector with suitable time lag. A touchscreen controller is developed to automatically carry out the original SIDL-FA protocol. The proposed SIDL-FA method allows vanadium to be quantified in the range of 0.1-2 μg L⁻¹ and is applied to the determination of vanadium in drinking water samples.     

Development of an on-chip injector for microchip-based flow analyses using laminar flow

A new on-chip injector for microchip-based flow analyses has been designed and characterized. The microchip design utilizes separate laminar flow streams of buffer and sample that are brought into parallel contact for a distance of 300 microm. The buffer flow stream is first routed through a conventional 6-port injection valve fitted with a 5 microm i.d. sample loop. When the 6-port valve is actuated from load to inject for a given time, the on-chip buffer flow stream is constricted and the sample flow stream is pressurized into the buffer flow channel. Once the valve returns to the load state the separate laminar flow streams resume. Fluorescence detection was used to characterize the injector and it was found that 50 injections of a 100 microM fluorescein sample led to an average peak height of 174.32 +/- 2.05 AFU (RSD 1.18%) and average peak skew of 1.37 +/- 0.06. The injector was also interfaced with amperometric detection. Injections of catechol solutions ranging in concentration from 500 nM to 100 microM resulted in a linear response (sensitivity = 2.49 pA microM(-1), r(2) = 0.998) and a limit of detection of 155 nM (S/N = 3). Compared to an off-chip injection scheme, plug dilution, band broadening, and peak asymmetry are much reduced. Finally, the injection and subsequent lysis of an erythrocyte sample was demonstrated, with an injected plug of erythrocytes being lysed 5.72 +/- 0.15 s after injection into a flow stream containing sodium dodecyl sulfate (n = 10). The new injection scheme does not require complex valving mechanisms or high pressures and enables reproducible injections from a continuous sample flow stream in a manner where changes in analyte concentration can be monitored with high temporal resolution.     

A timed solenoid injector for flow analysis

Samples can be reproducibly injected into flow-streams by timed switching of a sample stream with a miniature solenoid valve and timer circuit. The device is simpler to assemble and use than the standard rotary valve and a direct comparison under the same operating conditions shows that the solenoid valve is an adequate replacement for the rotary valve.     

Rapid separation of inorganic anions by capillary ion chromatography using monolithic silica columns modified with dilauryldimethylammonium ion.

The rapid separation of inorganic anions was determined by capillary ion chromatography using monolithic silica capillary columns modified with dilauryldimethylammonium bromide. The stability of the modified stationary phase was satisfactory owing to a strong hydrophobic interaction between the lauryl groups of the reagent, even if the eluent did not contain dilauryldimethylammonium ion. Bromide in seawater samples could be determined by the present system. The repeatability of a retention time of bromide for six successive measurements was around 1.8% when a 500 mM sodium chloride aqueous solution was used as the eluent. Seawater samples were directly injected onto the prepared column without any interference of matrix ions, because an aqueous solution of high-concentration sodium chloride could be used as the eluent. Bromide in seawater samples could be determined within 2 min.     

A Simple injection valve for flow injection analysis

A novel sample introduction valve, based on a modified glass syringe, is described for flow injection analysis. Sample volumes of 1.4 μl or more can be injected manually or automatically, by driving with a 12-V solenoid. The valve has advantages of freedom from maintenance and long lifetime.     

Flow-injection determination of ammonia in kjeldahl digests by gas diffusion and conductometry

A flow injection/conductometric method is proposed for determing ammonia in solutions obtained from Kjeldahl digestion. The method is based on diffusion of ammonia through a PTFE membrane from an alkaline (NaOH/EDTA) medium to a deionized water stream. The change in conductance of the deionized water stream is proportional to the ammonia concentration present in the digest. The effects of flow parameters, temperature and potential interferences are reported. Approximately 100 samples can be injected per hour; the precision is about 1%. Results for total nitrogen in vegetable tissues, animal feeds and fertilizers are in good agreement with those obtained by the usual distillation/titration method.     

Simultaneous determination of orthophosphate and total phosphates (inorganic phosphates plus purine nucleotides) using a bioamperometric flow-injection system made up by a 16-way switching valve

Orthophosphate and total phosphates (inorganic phosphates plus purine nucleotides) can be determined simultaneously in a novel flow-injection system made up by a 16-way switching valve with two sample loops, acid phosphatase (AcP) immobilized reactor and a delay coil needed to separate two peaks corresponding to two sample portions injected simultaneously. An orthophosphate enzyme electrode with a hybrid membrane of trienzyme film and poly(1,2-diaminobenzene) film was used to selectively detect both the endogenous orthophosphate and orthophosphate generated enzymatically into the AcP immobilized reactor, without any interferences from electroactive species, such as ascorbate and urate. Because two sample portions passed through the flow line with different residence time, two peaks were obtained. The first peak corresponded selectively to orthophosphate and the second peak to the total of inorganic phosphates and purine nucleotides. The maximum currents of both peaks were linearly related to the concentration of orthophosphate and total phosphates (as orthophosphate) in the range 5×10⁻⁷-8×10⁻⁴ M, respectively; 30 samples per hour can be processed with an R.S.D. <2.5%.     

On-line coupling of microporous membrane liquid–liquid extraction and gas chromatography in the analysis of organic pollutants in water

A method for the determination of hydrophobic pollutants in surface waters was developed. The pretreatment was done with microporous membrane liquid–liquid extraction, the extract was eluted to a sample loop in the large-volume injector valve of the gas chromatograph and the extract was injected on-line to the gas chromatograph. The method was optimised using standard compounds and the linearity, the limits of detection and quantification of the method were studied. The method allowed the determination of hydrophobic pesticides and PAHs at the ng L^–1 level. The RSD values for the repeatability of the method varied from 4.2% to 25.6%, being on average 9.5%. Surface water samples from Finnish lakes and rivers were analysed.