Determination of sucrose in sugar-cane juice and molasses by flow-injection spectrophotometry

An automated procedure is proposed for the spectrophotometric determination of sucrose in sugar-cane juice and nonlasses. The previously diluted and filtered sample is introduced into a flow-injection analyzer designed with two merging streams, producing two samples zones. One zone is transported directly towards the merging-stream confluence; the other zone reaches this site after flowing through a heated coil in which partial and reproducible sucrose inversion is attained under controlled conditions of acidity and temperature. At the confluence point, a buffered periodate stream is added to oxidize the sugar. The consumption of periodate, which mainly reflects the fructose content, is measured spectrophotometrically as a transient lowering of the iodine concentration, produced by reaction of periodate with iodide. The two processed zones proceed sequentially to the flow cell and two peaks are recorded. The sucrose content in the sample is proportional to the difference in peak heights. System dimensioning and the effects of temperature, pH, reagent concentrations, flow rates and the presence of other reducing sugars in the sample are discussed. With the proposed system, about 30 sugar-cane juice samples can be analyzed per hour, and sample clarification is not required. Precise results (r.s.d. <1%), in agreement with those obtained by h.p.l.c., are achieved for sucrose contents of 11–14% (w/v) in cane juice. Modifications of the system for analysis of molasses (16–52% w/w sucrose) are described.     

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.     

Scaling behavior in on‐chip field‐amplified sample stacking

Field amplified sample stacking (FASS) uses differential electrophoretic velocity of analyte ions in the high‐conductivity background electrolyte zone and low conductivity sample zone for increasing the analyte concentration. The stacking rate of analyte ions in FASS is limited by molecular diffusion and convective dispersion due to nonuniform electroosmotic flow (EOF). We present a theoretical scaling analysis of stacking dynamics in FASS and its validation with a large set of on‐chip sample stacking experiments and numerical simulations. Through scaling analysis, we have identified two stacking regimes that are relevant for on‐chip FASS, depending upon whether the broadening of the stacked peak is dominated by axial diffusion or convective dispersion. We show that these two regimes are characterized by distinct length and time scales, based on which we obtain simplified nondimensional relations for the temporal growth of peak concentration and width in FASS. We first verify the theoretical scaling behavior in diffusion‐ and convection‐dominated regimes using numerical simulations. Thereafter, we show that the experimental data of temporal growth of peak concentration and width at varying electric fields, conductivity gradients, and EOF exhibit the theoretically predicted scaling behavior. The scaling behavior described in this work provides insights into the effect of varying experimental parameters, such as electric field, conductivity gradient, electroosmotic mobility, and electrophoretic mobility of the analyte on the dynamics of on‐chip FASS.     

用矩分析法研究流动注射分析中化学反应对试样分散的影响

根据串联釜模型和统计矩分析法，通过实验证实了化学反应对分散度的影响；在同一流路中有化学反应的峰宽和二阶矩均比无化学反应的增大。并证明了二阶矩与１／４峰高处峰宽的平方成线性关系。同时给出了一个计算响应曲线统计矩的简易方法。     

Studies on peak width measurement-based FIA acid-base determinations

Summary Studies on Peak Width Measurement-Based FIA Acid-Base Determinations Peak width measurement is employed to determine strong acid or base concentrations by injection into an aqueous indicator flowstream. Investigated parameters include concentration and pK of the indicator, dimensions and nature of the dispersion tube, signal level employed for peak width measurement, sample volume, carrier flow rate, viscosity of the carrier stream and the diffusion coefficient of the analyte.     

流动注射分析中样品的分散与载流流速关系的研究

根据串联釜模型，流动注射分析响应曲线的峰高与峰面积、１／４峰高处的峰宽两个因素有关，这两个因素与载流流速的关系分别可以用不同的ｅｘｐ（－ｑ）多项式表示（ｑ是无量纲流速），由于分散度与峰高成反比，分散度随流速的变化关系由此能够很好地得到解释。     

Dispersion-convolution model for simulating peaks in a flow injection system

A dispersion-convolution model is proposed for simulating peak shapes in a single-line flow injection system. It is based on the assumption that an injected sample plug is expanded due to a "bulk" dispersion mechanism along the length coordinate, and that after traveling over a distance or a period of time, the sample zone will develop into a Gaussian-like distribution. This spatial pattern is further transformed to a temporal coordinate by a convolution process, and finally a temporal peak image is generated. The feasibility of the proposed model has been examined by experiments with various coil lengths, sample sizes and pumping rates. An empirical dispersion coefficient (D*) can be estimated by using the observed peak position, height and area (tp*, h* and At*) from a recorder. An empirical temporal shift (Phi*) can be further approximated by Phi*=D*/u2, which becomes an important parameter in the restoration of experimental peaks. Also, the dispersion coefficient can be expressed as a second-order polynomial function of the pumping rate Q, for which D*(Q)=delta0+delta1Q+delta2Q2. The optimal dispersion occurs at a pumping rate of Qopt=sqrt[delta0/delta2]. This explains the interesting "Nike-swoosh" relationship between the peak height and pumping rate. The excellent coherence of theoretical and experimental peak shapes confirms that the temporal distortion effect is the dominating reason to explain the peak asymmetry in flow injection analysis.     

Theoretical and experimental aspects of the response of stripping voltammetry in flow injection systems

The coupling of stripping voltammetry with flow injection systems offers significant advantages over conventional stripping methods. Equations solved for conventional stripping voltammetry, with steady-state deposition current, are not applicable to flow injection systems. Theoretical equations for the peak current under flow injection conditions are derived. As the total charge passed during the deposition step, ∫i dt, is independent of the shape of the sample plug, the stripping peak current is independent of the degree of dispersion. As a result, precise control of the dispersion or deposition period may not be required. The peak current is predicted to be directly proportional to the sample volume. Experimental results are incorporated to support the theoretical conclusions. The effects of experimental variables such as flow rate, length of tubing, deposition period, or sample volume are presented using cadmium ion as test species.     

Reduction of injection variance in flow-injection analysis

In order to achieve maximum sensitivity in flow-injection analysis, sample dispersion must be kept to a minimum. This dispersion process, however, is not well understood. Studies of the dispersion process have concentrated on dispersion within the flow manifold while dispersion due to the injection process has been largely ignored. Here sample injection loops packed with inert glass beads and a Serpentine II (distorted) empty loop were constructed and compared to traditional empty sample loops. Digitization of the response curves and subsequent calculation of the statistical moments were used to compare the contribution of each sample loop type to the total system dispersion. Both packed and Serpentine II sample loops were shown to decrease dispersion and increase throughput in flow-injection systems. Plots of peak variance vs. injection volume show variance increasing 1.67 times faster with traditional open sample loops compared to packed loops. When combined with other peak width minimization techniques, this method should further lower concentration limits of detection.     

Recording the real sample distribution and concentration/time functions in flow injection analysis

A flow-injection system with zone sampling is proposed which enables images representing the real sample distribution at any time after injection to be obtained, and the concentration/time function to be monitored at any point of the analytical path. These images provide information on the mixing conditions of the system and the interaction between the injected sample and the carrier stream in real situations. It is intended to complement theoretical studies of dispersion in flow injection analysis.     

Discrimination between peak spreading in capillary zone electrophoresis of proteins due to interaction with the capillary wall and due to protein microheterogeneity

Our study attempts to find an approach to distinguishing between the contribution to peak spreading in capillary zone electrophoresis (CZE) due to protein microheterogeneity and that due to interaction with the capillary wall, by analyzing correlations between observed peak spreading and peak asymmetry. The peak asymmetry was measured as ln[(tm-t1)/(t2-tm)] where tm, t1, and t2 are migration times at the mode of the peak and at the intersection of the peak width at half-height with the ascending and descending limbs, respectively. Two isoforms of recombinant green fluorescent protein (GFP-1 and GFP-2, 27 kDa molecular mass), glucose-6-phosphate dehydrogenase (GPD, 104 kDa), and the naturally fluorescent protein R-phycoerythrin (PHYCO, 240 kDa) were subjected to CZE in polyacrylamide-coated fused-silica capillaries of 50 and 100 microns diameters under varying conditions of protein concentration, field strength, and the initial zone length. Under conditions such that contributions to peak spreading from axial diffusion, thermal effects, and electrophoretic dispersion are negligible, the analysis of the interrelations between peak width and peak asymmetry was found to allow a conclusion as to the cause of peak spreading in CZE of protein. It appears that the peak width of GFP-2 originates mostly in protein microheterogeneity while that of GFP-1 is due to protein-capillary wall interactions. For PHYCO, both microheterogeneity and protein-capillary wall interactions contribute to peak spreading. GPD exhibits relatively little microheterogeneity or interaction with capillary walls. Thus, its peak width appears to be mostly affected by an extracolumn source of spreading such as the initial zone length.     

Flow injection analysis : Part IX. A New Approach to Continuous Flow Titrations

Studies of dispersion patterns in nonsegmented streams, flowing through narrow open tubes, show that it is possible to obtain highly reproducible concentration gradients within a sample zone injected into the moving stream. By varying the geometry of the flow path, low, medium and high dispersion patterns can be achieved; the high dispersion pattern forms the basis for a new approach to continuous flow titrimetry. In this type of titration, discrete samples are passed through a gradient device and are then mixed with a continuously flowing stream of titrant of fixed concentration. The new technique has been tested for potentiometric as well as spectrophotometric end-point indication. A simple one-channel system allows titrations to be performed automatically in less than 1 min.     

Flow injection analysis of DPPH radical based on electron spin resonance

In order to construct a rapid and selective determination system of free radicals, we developed an FIA system using an electron spin resonance (ESR) spectrophotometer (flow injection spin analysis) equipped with a flow-through flat cell. In the present investigation, 1,1-diphenyl-2-picrylhydrazyl (DPPH) was used as a model free radical. Using a single line flow system, 0.5 mM DPPH was repetitively injected. When the magnetic field was fixed at 335.3 mT, the largest change in the ESR signal was observed and obtained peak height was proportional to the concentration of DPPH radical. A double line flow system was constructed in which a carrier stream containing 0.15 mM DPPH was fed into the flat cell after confluence with a sample stream. When ascorbic acid was injected as a typical DPPH radical scavenger, a negative peak appeared in proportional to the concentration. Lower detection limit of ascorbic acid was 0.01 mM (S/N=4), sampling frequency was 13 samples per h, and a satisfactory reproducibility (CV=3.2%, 0.1 mM, n=5) was obtained. The present system was also applied to estimate the DPPH radical-scavenging activity of other substances and food samples.     

Using flow injection analysis to time-resolve rhythmic and pulsatile signals

Continuous monitoring can be used to detect rhythms, an important aspect of biology. But peaks of concentration are broadened by dispersion so that they overlap their neighbours and obscure high frequency chemoperiodicities. In this study, flow injection was found experimentally to be useful in resolving these. A rhythmically varying pattern of permanganate concentration was measured spectrophotometrically. The rhythm (frequency 0.08 Hz) was observable at a dispersion coefficient of 3.0 but not at 3.9 (when only a single peak was recorded). It was again observable using the same high dispersion manifold but positioned after an injection valve that subsampled the stream at intervals. A design based on this work is proposed for an automated instrument that outputs a time series of concentration measures.     

The principles and theory of high-speed titrations by flow injection analysis

The highly reproducible concentration gradients formed between an injected sample zone and the carrier stream in flow injection analysis are exploited for titrations based on measuring the time span between points of identical gradient dispersion. Comparison of the experimental data with theoretical models, has made it possible to locate the position and physical dimension of a single mixing stage, which at medium dispersion of the sample zone allows the entire titration cycle, including sampling and washout periods, to be completed within less than 30 s. The capability of this high-speed titration technique is demonstrated for acid—base, compleximetric and iodimetric titration procedures.     

Determination of sulphuric acid in process effluent streams using sequential injection titration

Sulphuric acid in process effluent streams from an electrorefining copper plant was analysed with a sequential injection (SI) titration system using sodium hydroxide as titrant. In the proposed SI titration system a base titrant, acid analyte and base titrant zone were injected sequentially into a distilled water carrier stream in a holding coil and swept by flow reversal through a reaction coil to the detector. The base zones contained bromothymol blue as indicator and the endpoint was monitored spectrophotometrically at 620 nm. The influence of carrier stream flow rate, acid and base zone volumes and titrant concentration on the linear range of the method was studied to obtain an optimum. A linear relationship between peak width and logarithm of the acid concentration was obtained in the range 0.006-0.178 mol l(-1) of H(2)SO(4) for a NaOH concentration of 0.002 mol l(-1). The results obtained for the SI titration of process samples were in good agreement with a standard potentiometric method with an RSD<0.75% and a sample frequency of 23 samples h(-1).     

Oscillatory control of sample dispersion in a continuous flow system

A new strategy for the instrumental control of sample dispersion in continuous flow systems is presented. The method is based on shaking a loosely held straight reactor while the sample travels through the flow injection manifold. This external disturbance yields a sample transport more similar to the plug flow type because of the changes promoted on the flow pattern. Up to a three-fold increase in peak height, a comparable reduction in peak width and a more Gaussian peak profile are observed when the signals obtained with the shaken reactor are compared with those obtained with the same reactor but static. Improvements in the analytical performance as a function of different operational variables are shown for systems with or without a chemical reaction. Analytical implications and possible uses are discussed since this strategy allows the control of dispersion by simply selecting the frequency and amplitude of oscillation.     

Stopped-flow injection analysis. The influence of diffusion on dispersion of normal and reverse flow injection

A relationship is derived to enable the comparison of the dispersion heights of normal and reverse flow injection analysis (FIA). A single channel flow system is employed in the absence of a chemical reaction. The stopped-flow injection method is used to probe the influence of molecular diffusion on the overall dispersion of normal and reverse FIA, which appeared to demonstrate fundamentally different diffusion behaviors. Small discrepancies are observed between the dispersion heights, which are enhanced by the stopped-flow period, especially when unmatched matrix ionic compositions of the indicator and counter solutions were involved. For these conditions, the diffusion flux rate is enhanced considerably, displaying a peak, in addition to the transient, for both methods. The influence of diffusion on the dispersion characteristics of normal and reverse FIA is discussed theoretically. Diffusion in the proposed model is postulated to oppose dispersion by convection. The latter initiates concentration gradients in the injection zone and propagates it with flow time over the dispersion zone profile. The diffusion flux then reacts in order to confine the indicator dispersion for normal FIA and to enhance it for reverse FIA. This model is consistent with the experimental results and accounts for most of the phenomena encountered. Probably owing to the influence of secondary flow phenomena, the use of coiled tubes has suppressed the effects of diffusion on the overall dispersion behavior.Part of the experimental work was performed at IMI Institute for Research and Development, Haifa, Israel.     

Influence of channel position on sample confinement in two-dimensional planar microfluidic devices

We report enhanced sample confinement on microfluidic devices using a combination of electrokinetic flow from adjacent control channels and electric field shaping with an array of channels perpendicular to the sample stream. The basic device design consisted of a single first dimension (1D) channel, intersecting an array of 32 or 96 parallel second dimension (2D) channels. To minimize sample dispersion and leakage into the parallel channels as the sample traversed the sample transfer region, control channels were placed to the left and right of the 1D and waste channels. The electrokinetic flow from the control channels confined the sample stream and acted as a buffer between the sample stream and the 2D channels. To further enhance sample confinement, the electric field was shaped parallel to the sample stream by placing the channel array in close proximity to the sample transfer region. Using COMSOL Multiphysics, initial work focused on simulating the electric fields and fluid flows in various device geometries, and the results guided device design. Following the design phase, we fabricated devices with 40, 80, and 120 microm wide control channels and evaluated the sample stream width as a function of the electric field strength ratio in the control and 1D channels (E(C)/E(1D)). For the 32 channel design, the 40 and 80 microm wide control channels produced the most effective sample confinement with stream widths as narrow as 75 microm, and for the 96 channel design, all three control channel widths generated comparable sample stream widths. Comparison of the 32 and 96 channel designs showed sample confinement scaled easily with the length of the sample transfer region.     

Focusing particles by induced charge electrokinetic flow in a microchannel

A novel method of sheathless particle focusing by induced charge electrokinetic flow in a microchannel is presented in this paper. By placing a pair of metal plates on the opposite walls of the channel and applying an electrical field, particle focusing is achieved due to the two pairs of vortex that constrain the flow of the particle solution. As an example, the trajectories of particles under different electrical fields with only one metal plate on one side channel wall were numerically simulated and experimentally validated. Other flow focusing effects, such as the focused width ratio (focused width/channel width) and length ratio (focused length/half‐length of metal plate) of the sample solution, were also numerically studied. The results show that the particle firstly passes through the gaps between the upstream vortices and the channel walls. Afterwards, the particle is focused to pass through the gap between the two downstream vortices that determine the focused particle position. Numerical simulations show that the focused particle stream becomes thin with the increases in the applied electrical field and the length of the metal plates. As regards to the focused length ratio of the focused stream, however, it slightly increases with the increase in the applied electrical field and almost keeps constant with the increase in the length of the metal plate. The size of the focused sample solution, therefore, can be easily adjusted by controlling the applied electrical field and the sizes of the metal plates.