Sequential injection titration with spectrophotometric detection for the assay of acidity in fruit juices.

A simple sequential injection analysis (SIA) with spectrophotometric detection for an assay of acidity in fruit juice was investigated. An alkaline reagent (sodium hydroxide), a sample and an indicator (phenolphthalein) were first aspirated and stacked as adjacent zones in a holding coil. With flow reversal through a reaction coil to the detector, zone penetration occurred, leading to a neutralization reaction that caused a decrease in the color intensity of the indicator being monitored for absorbance at 552 nm. The effects of various parameters were studied. Linear calibration graphs for acidities of 0.2 - 1.0 and 0.5 - 2.5% w/v citric acid as a standard, with a relative standard deviation of 1% (acidity of 0.3 - 0.6% w/v as citric acid, n=11) and a sample throughput of 30 samples h(-1), were achieved. The developed method was validated by a standard titrimetric method for assaying the acidity of fruit juice samples.     

Confluent streams in flow injection analysis

The addition of a confluent stream increases the mean length of the sample zone and simultaneously decreases the involved concentrations. Simple equations describing these effects under ideal mixing conditions are proposed. The effects of the confluent stream on the overall sample dispersion may be more or less compensated, depending on the decrease in the post-confluence dispersion. When this compensation is almost total, a paradoxical situation occurs, in which the recorded peak height becomes practically unaffected by variations in the flow rate of the merging stream. In this situation, the peak width approaches a limiting value which is independent of the injected volume and of the flow rate of the confluent stream. Limiting values for the peak width and for the mean length of the sample zone are postulated. When the post-confluence dispersion is negligible, the effects of the confluence appear in the recorded peak. The confluent stream addition reduces the overlap between successive sample zones. The site of the confluence may be an important dispersion factor. A saturation index is proposed and practical implications are discussed.     

Determination of the total acidity in soft drinks using potentiometric sequential injection titration

A potentiometric SI titration system for the determination of total acidity in soft drinks is proposed. The concept is based on the aspiration of the acid soft drink sample between two base zones into a holding coil with the volume of the first base zone twice to that of the second one and channelled by flow reversal through a reaction coil to a potentiometric sensor. A solution of 0.1 mol l⁻¹ sodium chloride is used as ionic strength adjustment buffer in the carrier stream. The system has been applied to the analysis of some South African soft drinks having a total acidity level of about 0.2–0.3% (w/v). The method has a sample frequency of 45 samples per h with a linear range of 0.1 and 0.6% (w/v). It is easy to use, fully computerised, and gives the results that are comparable to both automated batch titration and manual titration.     

Effect of process parameters on transmembrane flux during direct osmosis

Direct osmosis is a non-thermal membrane process employed for the concentration of fruit juices at ambient temperature and atmospheric pressure, thereby maintaining the organoleptic and nutritional properties of fruit juices. In the present study, concentration of pineapple juice by direct osmosis was explored. Aqueous solution of sucrose (0–40%, w/w)–sodium chloride (0–26%, w/w) combination was investigated as an alternative osmotic agent. The sucrose–sodium chloride combination can overcome the drawback of sucrose (low flux) and sodium chloride (salt migration) as osmotic agents during direct osmosis process. The effect of the hydrodynamic conditions in the module and feed temperature (25–45 °C) on transmembrane flux was evaluated. For a range of hydrodynamic conditions studied, it was observed that transmembrane flux increases with Reynolds number. The increase in feed temperature resulted in an increase in transmembrane flux. The pineapple juice was concentrated upto a total soluble solids content of 60 °Brix at ambient temperature. The effect of direct osmosis process on physico-chemical characteristics of pineapple juice was also studied. The ascorbic acid content was well preserved in the pineapple juice concentrate by direct osmosis process.     

Continuous-flow determination of reducing sugars in serum by reaction with periodate ions,with use of a flow-through periodate-sensitive electrode

A continuous-flow method for the determination of reducing sugars in serum is described. The sample reacts with an excess of periodate in a flow system and the decrease in periodate

activity is monitored with a periodate-sensitive flow-through electrode. The recorded potential peak heights are indirectly linearly related to the reducing sugar concentration expressed as glucose, in the range 50–290 mg/100 ml. The analysis is completely automated and requires no sample pretreatment, and samples can be analyzed at the rate of 30 per hour with average errors and relative standard deviation of about 1–2%. Comparison with an enzymatic method for serum gave satisfactory results.     

An improved monosegmented continuous-flow system for sample introduction in flame atomic spectrometry

A monosegmented system for sample introduction which does not require air removal is proposed. The sample is intercalated into an unsegmented carrier stream together with air plugs. Segmentation involving the placement of a single air plug in the tailed portion of the sample is also investigated and compared with the original approach in which the sample is introduced between two air bubbles. Effects of sample volume (25–1000 μl) and length of the air plugs (5–200 cm) on the recorded peak and measurement reproducibility are discussed. Flame instability caused by the air plugs is not a problem with the larger samples. The single-line system proposed for determination of zinc in plant digests is very stable. Limited dispersion is achieved by injecting only about 100 μl of sample; a sampling rate of 400 measurements per hour at the 1% carryover level can be attained. Precise results (r.s.d. ca. 1%) in agreement with those obtained by conventional atomic absorption spectrometry (a.a.s.) are achieved. The advantages of the proposed system compared to a conventional flow-injection system for a.a.s. are emphasized. The determination of calcium in natural waters demonstrates the feasibility of monosegmentation when a reagent solution must be added. Lanthanum solution can be added by merging zones or by confluence, either before or after the sample injection. The accuracy, precision, sample consumption and system stability attained are favourable.     

Determination of sucrose by flow injection analysis with fourier transform infrared detection

A new methodology for the determination of sucrose in complex aqueous matrices by flow injection analysis with FTIR detection is presented. The methodology based on the enzymatic hydrolytic cleavage of sucrose by means of invertase to -D-glucose and -D-fructose. A special manifold consisting of two internally coupled injection valves being switched simultaneously is applied to facilitate recording FTIR spectra of the sample before and after the enzymatic reaction. The analytical readout is taken from the resulting difference spectrum obtained by subtracting the FTIR spectra of the sample before and after the reaction. The developed methodology uses a GC-IR software to continuously record the FTIR spectra of the effluent from the manifold. The proposed method gives linear results in the range of 10 to 100 mmol/l and has been successfully applied to the analysis of sucrose in synthetic mixtures as well as in real samples such as soft drinks.     

Spectrofluorimetric flow injection determination of trace amounts of periodate

A sensitive, rapid and selective procedure is proposed for the flow injection determinations of periodate by spectrofluorometric detection. The method is based on the reaction of periodate with Alizarin Navy Blue in basic solution. The reagents and manifold variables influence on the sensitivity have been investigated and the optimum conditions are established. Periodate can be determined for the range of 0.250-5.00 microg ml(-1) with a limit of detection of 0.08 microg ml(-1), and with a sample rate of 15 +/- 2 samples h(-1). The relative standard deviations for eight replicate determination of 0.500 and 5.00 microg ml(-1) was 1.3 and 1.1%, respectively. Periodate can be determined in the presence of iodate and bromate. The proposed method was used to determination of periodate in water samples.     

Asynchronous merging zones system: spectrophotometric determination of Fe(II) and Fe(III) in pharmaceutical products

An asynchronous merging zones was proposed with simultaneous introduction of the sample and of the modifier reagent, ascorbic acid in the studied case, for sequential determination of Fe(II) and Fe(III) in pharmaceutical products. The sample and reagent attained a merging point at different times owing to the use of a delay reactor in the reagent channel. By inserting a large sample and controlling the dispersion in the flow system, Fe(II) and Fe(III) were sequentially measured in the front and in the rear of the sample zone, respectively. The results obtained for Fe redox speciation in pharmaceutical products are in agreement at a 95% confidence level with those obtained with a manual spectrophotometric procedure. The analytical frequency with the proposed flow analysis system was 54 samples h(-1), i.e. 108 determinations h(-1).     

Merging zones in flow injection analysis: Part 5. Simultaneous determination of aluminium and iron in plant digests by a zone-sampling approach

A flow injection procedure is proposed for the simultaneous determination of aluminium and iron in plant material. The method is based on a flow configuration involving zone sampling and merging zones. Aluminium is determined spectrophotometrically with eriochrome cyanine R as reagent and iron by atomic absorption spectrometry. The advantages of this method over other procedures already reported are discussed. The effects of reagent composition for the aluminium determination are described in detail. The zone-sampling approach permits an easier pH control in the aluminium determinations so interferences caused by variations in sample acidity are avoided without the need for very concentrated buffers. The merging zones configuration greatly reduces the consumption of reagents. The proposed method permits the analysis of about 120 samples (240 determinations) per hour, with good precision (r.s.d. < 2%) in both the aluminium and iron channels. The results agree with those obtained by inductively-coupled argon plasma spectrometry.     

Flow injection systems with inductively-coupled argon plasma atomic emission spectrometry : Part 1. Fundamental Considerations

Flow injection systems with inductively-coupled argon plasma atomic emission spectromeiry are proposed. Effects of flow rates, injected volumes and mixing Coil lengths are investigated and conditions for the measurement of the flow injection transient signal are discussed. The peak profile measured with the spectrometer corresponds well with the estimate of the true sample zone distribution near the inlet of the spectrometer made by a zone-sampling process; thus the plasma is not a limiting factor in the proposed systems. For plant analysis, the system provides nearly zero sample dispersion and so the inherent sensitivity of the spectrometric method is preserved. The results obtained for 10 elements in the NBS Orchard Leaves reference material (SRM 1571) are in good agreement with the certified values. For determinations of calcium and magnesium in dolomitic limestones, cadmium is used as internal standard and so the merging zones configuration is employed. The proposed system provides medium sample dispersion and permits about 100 samples to be analysed per hour. Relative standard deviations of 1.34% and 1.23% were calculated for the calcium and magnesium data, respectively. The analytical results compare favorably with those obtained by normal i.c.p. spectrometry with pneumatic sample aspiration, after manual sample dilution.     

Determination of cyclodextrin content using periodate oxidation by capillary electrophoresis

A simple and general method suitable for the determination of cyclodextrin content in various matrices is described. The proposed method involves selective cleavage of C-C bonds with vicinal hydroxyl groups by means of periodate (Malaprade's reaction). The amount of produced iodate is monitored by capillary electrophoresis. Optimized electrophoretic conditions (20 mM disodium tetraborate with 1 mM tetradecyltrimethylammonium bromide, direct UV detection lambda = 200 nm) ensure complete separation of periodate and iodate ions and sufficient sensitivity towards iodate. Under optimized reaction conditions (2-fold excess of periodate, temperature 70 degrees C) reproducible quantitative results were obtained for alpha-, beta- and gamma-cyclodextrins as model samples. The proposed method was tested on a real sample of acrylamide--2'-O-allyl-beta-CD copolymer. The values of beta-cyclodextrin content were compared with those obtained by reference NMR measurement and were found to be identical.     

Application of graphene for the SPE clean‐up of organophosphorus pesticides residues from apple juices

In this paper, an effective graphene‐based SPE clean‐up procedure coupled with GC–MS was developed for the determination of organophosphorus pesticide residues in apple juices. The apple juice samples were diluted with water and could be loaded onto the cartridge directly. Several parameters affecting the extraction efficiency were investigated, including the type of elution, washing solution, and sample pH. Under the optimized conditions, excellent limits of quantitation for the target analytes were found to be 0.15–1.18 ng/mL, and the average recoveries of the analytes at two spiked levels for real‐sample analysis ranged from 69.8 to 106.2% with RSDs less than 7.3%. Furthermore, the graphene‐based cartridges exhibited superior reusability for juice sample analysis. The proposed method is sensitive, simple, and cost saving, and provides a detection platform for the monitoring of pesticide residues.     

Magnetic nanoporous carbon as an adsorbent for the extraction of phthalate esters in environmental water and aloe juice samples

In this work, magnetic nanoporous carbon with high surface area and ordered structure was synthesized using cheap commercial silica gel as template and sucrose as the carbon source. The prepared magnetic nanoporous carbon was firstly used as an adsorbent for the extraction of phthalate esters, including diethyl phthalate, diallyl phthalate, and di‐n‐propyl‐phthalate, from lake water and aloe juice samples. Several parameters that could affect the extraction efficiency were optimized. Under the optimum conditions, the limit of detection of the method (S/N = 3) was 0.10 ng/mL for water sample and 0.20 ng/mL for aloe juice sample. The linearity was observed over the concentration range of 0.50–150.0 and 1.0–200.0 ng/mL for water and aloe juice samples, respectively. The results showed that the magnetic nanoporous carbon has a high adsorptive capability toward the target phthalate esters in water and aloe juice samples.     

On-line electrolytic dissolution of alloys in flow injection analysis : Part 2: Spectrophotometric determination of molybdenum in steels

Electrolytic dissolution of steels is used for the determination of molybdenum by the flow injection spectrophotometric thiocyanate method. A direct current (200 mA) is applied for 8 s to the solid sample placed on a simple electrolytic chamber; the dissolved material serves as the injected sample zone. The best electrolyte for use as carrier stream is 1.0 M KCl/0.1 M HCl. No measurable baseline drift was observed during continous operation for 4 h. The sample throughput is 40 h^?1. Calibration is done with reference steel standards and is linear in the range o.7–2.7% (w/w) molybdenum; for a content of 2.21%, the relative standard deviation is 2.3% (n=10).     

Atomic absorption determination of elements in natural samples using a vaporization crucible with two condensation zones

It is shown that elements can be determined in natural samples and concentrates using a vaporization crucible with two condensation zones. After vaporization in the crucible, element vapors first enter the heated condensation zone, in which some elements are condensed and the residual amounts of organometallic substances decompose, and then they enter the ??cold?? condensation zone. To analyze the condensates after the fractional evaporation of the sample, an atomizer was proposed with two vaporization zones and a common analytical zone. The performance characteristics of metal determination in suspensions of organic and mineral components, concentrates, natural waters, and bottom sediments were improved.     

Zone trapping/merging zones in flow analysis: a novel approach for rapid assays involving relatively slow chemical reactions

A novel strategy for accomplishing zone trapping in flow analysis is proposed. The sample and the reagent solutions are simultaneously inserted into convergent carrier streams and the established zones merge together before reaching the detector, where the most concentrated portion of the entire sample zone is trapped. The main characteristics, potentialities and limitations of the strategy were critically evaluated in relation to an analogous flow system with zone stopping. When applied to the spectrophotometric determination of nitrite in river waters, the main figures of merit were maintained, exception made for the sampling frequency which was calculated as 189 h⁻¹, about 32% higher relatively to the analogous system with zone stopping. The sample inserted volume can be increased up to 1.0 mL without affecting sampling frequency and no problems with pump heating or malfunctions were noted after 8-h operation of the system. In contrast to zone stopping, only a small portion of the sample zone is halted with zone trapping, leading to these beneficial effects.     

Determination of Fe, Cu and Zn in sugar-cane spirits commercialized in Southeastern Brazil by TXRF

Total reflection X-ray fluorescence (TXRF) was used to determine simultaneously the concentration of Cu, Fe and Zn in seven sugar-cane spirit samples (“aguardente”), a processed alcoholic beverage made from the fermentation and distillation of sugar-cane juice. The limits of detection ranged from 35 to 8 ng^.ml^-1 for V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As and Se. Excluding Fe, Cu and Zn, the concentrations were below their limits. Two samples showed Cu concentrations higher than the value allowed by Brazilian legislation (5 μg^.ml^-1), and concerning Fe and Zn, no sample were above the limits permitted by WHO for drinking water (2 and 3 μg^.ml^-1, respectively). This revised version was published online in August 2006 with corrections to the Cover Date.     

Single reaction interface in flow analysis

The dual or multiple reaction interface concept, commonly associated to the distinct flow techniques, was replaced by a single interface concept, which do not no rely on the utilisation of a well-defined and compelling sample volume but only on mutual penetration of sample and reagent zones at a single reaction interface where both sample and reagent met together prior to detection. In the proposed approach basic principles of flow analysis, such as controlled dispersion and reaction zone formation, are not influenced by sample and reagent volumes but determined exclusively by the extension of the overlap of two adjoining quasi-infinite zones enhanced by multiple flow reversals and the pulsed nature of the flowing streams.The detector is positioned at the core of the flow manifold (not in the conventional terminal position), and repetitive flow reversals enable interface manipulations, including multi-detection of the entire reaction interface or the monitoring of the evolution of a pre-selected interface zone by using suitable reversal cycle times.The implementation of the developed approach was facilitated due to the configuration simplicity and operational versatility of multi-pumping flow systems. Its performance was evaluated by monitoring processes involving two or four-solution reaction interfaces.     

Sequential flow injection determination of iodate and periodate with spectrophotometric detection

A flow injection (FI) system is described for the sequential determination of periodate and iodate based on their reaction with iodide at pH 3.5. Two sample plugs were injected into the same carrier stream sequentially. One injection is for the iodate determination and the other for the sum of iodate and periodate determination. For iodate determination, molybdate solution buffered at pH of 3.5 was used for selective masking of periodate. The influences of reagent concentrations were studied by a univariable method and the influence of FI manifolds was studied using univariable and simplex method. Periodate and iodate can be determined in the range of 0.050-5.0 and 0.050-10 microg/ml, respectively. The 3 sigma limit of detection was 0.030 and 0.050 microg/ml for periodate and iodate, respectively. The proposed method has been applied for the sequential determinations of periodate and iodate in water samples.