On-line Dilution Research Articles

On-line Dilution for ICP-MS With a Flow Injection Recirculating Loop Manifold

A FI manifold consisting of a recirculating loop has been coupled to a plasma source mass spectrometer to provide successive on-line dilutions. Part of the loop is injected into the carrier stream followed by dilution, within the loop, of the remaining part with the carrier solution. For a manifold with a calculated dilution factor of 2.02 (based on the volume ratio), 10 successive injections gave a mean of 1.99 and a 95% confidence interval of ±0.065 for the ratio of successive peak heights. The between-run precision for a particular peak height ranged from 1.7 to 3.2% relative standard deviation (RSD). In a study of the decay of the concentration oscillations in the recirculating loop, it was found that the reciprocal of the time to achieve uniform concentrations decreased linearly with increasing flow rate and decreasing loop volume. The dilution behaviors of 19 elements were studied. Of these, nine (Ag, Ba, Cr, Cu, Ni, Pb, Sb, Tl and U) could be diluted from 100 ppb by three orders of magnitude with a precision of 5% RSD or better, six (As, Cd, Co, Th, V, Zn) could be diluted over the same range with precisions between 5 and 10% RSD, and four elements (Be, Mo, Se and Hg) displayed a systematic decrease in the dilution factor which was interpreted as retention of these elements within the loop. The influence of a wine matrix on the determination of Ce was removed by five successive dilutions with a factor of 3.04 per injection for a total dilution factor of 260. For a total loop volume of 1–2 ml, rapid damping of the concentration oscillations could be produced by the destructive interference produced by a two-line network (split and confluence) with tube lengths of 50 and 25 cm.

The use of particle monitoring in the performance optimisation of conventional clarification and filtration processes

The application of laser diffraction particle monitoring to the performance optimisation of a pilot clarifier and full-scale rapid gravity filters (RGF), operating on water supply works in Hampshire, is described. Furthermore the dosing of powdered activated carbon (PAC) into the works' clarifiers has been evaluated in terms of RGF performance. A costly proposal to install a third filter medium was subsequently abandoned when it was found that particle numbers in the filtered water were consistently below 1 × 102ml.Various combinations and doses of coagulants and flocculant aids, shown to give optimum particulates removal during intensive jar testing trials, were transferred to the pilot clarifier. Particle monitoring enabled a more accurate derivation of suitable blanket chemistry and optimum blanket heights than turbidity changes Raw water turbidities were 10–15 NTU at start-up with corresponding counts beyond the upper limit of the particle monitor. An on-line dilution system was developed to overcome this problem. Latex bead (4.33 µm) and Lycopodium spore (4–5 µm) suspensions (about 1 × 109 particles) were injected into the pilot clarifier to assess the removal efficiency of Cryptosporidium-sized particles. Reductions of about 1.7 log and 2.6 log were achieved for the beads and spores, respectively. Particle distributions of various PAC's and a bentonite were obtained in order to assess their potential effects on the coagulation process during clarification. Bentonite was also beneficial as an on-line means of checking particle monitor response and calibration.The works' filters achieved 1.5 to 2.0 log removals of 2–5 µm particles without media addition or operational changes. Combined clarification and filtration gave better particulates removal than two-stage microfiltration.

A New Approach to Dialysis in Sequential Injection Systems: Spectrophotometric Determination of L(+)-Lactate in Wines

A sequential injection analysis (SIA) system for the spectrophotometric determination of L(+)-lactate using lactate oxidase/peroxidase was developed and applied to wine analysis. It incorporates a dialysis unit between the pump and the selecting valve to provide on-line dilution and interference minimization of the sample matrix. A volume of 100 µL of carrier solution trapped in one channel of the dialysis unit, acts as donor and acceptor solution. With an assay cycle of 262 seconds, the system allows a sample throughput of 14 samples per hour and yields precise results (rsd < 2%). Peroxidase (0.0512 IU) and 0.0384 IU of lactate oxidase promote the conversion of L(+)-lactate in 160 µL of sample, allowing its determination within the 0.25 to 2.5 g/L range. A detection limit of 0.074 g/L was calculated. Results are in good agreement with those obtained with the conventional method (Boehringer UV-Kit).

Extension of the dynamic range of flame atomic absorption spectrometry using flow injection analysis with variable-volume dilution chambers

A simple and inexpensive procedure is proposed for the extension of the dynamic range of flame atomic absorption spectrometry measurements using on-line dilution. The proposed methodology is based on the use of a manifold with two coupled dilution chambers and a zone injection system. The samples are prediluted in a closed system which includes a variable-volume mixing chamber (10–120 ml) and two injection valves. The samples are injected through one of these valves, and the other is employed to take 100 μl of prediluted samples which are then passed through a new dilution chamber (volume 1–10 ml) and aspirated by the nebulizer of the instrument. A third injection valve mounted in the last part of the manifold is used for the direct injection of diluted standard solutions. Various dilution factors are obtained, ranging from 2 to 130 000 times, thus extending the analytical range of copper determination to more than 100 000 mg l −1.

On-line preconcentration and high-performance liquid chromatographic determination of polycyclic aromatic hydrocarbon-DNA adducts using copper phthalocyanine trisulfonic acid as a group-specific adsorbent

An on-line preconcentration high-performance liquid chromatography system for the determination of polycyclic aromatic hydrocarbon-DNA adducts such as trihydroxytetrahydrobenzo[ a]pyrene-deoxyguanotide in a DNA hydrolysate is described. Restricted-access copper phthalocyanine (CP) diol modified silica was used as a group-specific sorbent. For the determination of the benzo[ a]pyrene adduct, two preconcentration columns (precolumns) in a dual-precolumn setup were used. Elution of the benzo[ a]pyrene adduct from the CP precolumn was performed with 100% methanol. The analyte was re-concentrated on a C 18 precolumn applying on-line dilution with water. The C 18 precolumn can be desorbed with LC mobile phase used for the analytical column. Trihydroxytetrahydrobenzo[ a]pyrene-deoxyguanotide was determined in a DNA hydrolysate using the dual-precolumn procedure. A recovery of 95% was achieved. With fluorescence detection, a mass detection limit of 0.1 pmol injection was obtained.

Determination of highly concentrated Na, K, Mg and Ca in dialysis solution with flow injection on-line dilution and flame atomic absorption spectrometry

An on-line flow injection dilution method for flame atomic absorption spectrometry was developed. This method is based on the cascade method of dilution. The method combines the technology of merging streams with on-line sample volume reduction using stream splitting to obtain precise dilution factors: a series of split and confluence points are traversed by the injected sample zone. At each split point, the sample volume is reduced and at each confluence point dilution occurs to an extent governed by the relative flow rates in the two merging streams. Using various configurations of the manifold and by changing the flow rate of the carrier stream, dilution factors ranging from 10 to 10 000 are obtained. Normally, to achieve high dilution factors the carrier flow rate must be reduced. However, the dilution performance depends critically on the carrier flow rate; when the latter is low, the throughput is correspondingly reduced as well. In this work, it is shown that a reasonably high dilution factor is achievable when using relatively high carrier flow rates. Higher carrier flow rate provides precise dilution factors with the use of peristaltic pumps. To combine precise performance with high sample throughput, the dilution range, using this method, is recommended at 10-3000. To demonstrate the performance of this manifold, high concentrations of Na, K, Mg and Ca were measured in a dialysis solution. Using a 1000-fold dilution, the sample throughput for a fully automated analysis was 70 measurements per hour, with a precision of approximately 2% (RSD n = 11). The automated results were checked with a conventional manual dilution method and the results between the two methods were comparable and with no significant difference found at the 95% level ( t-test) for analyte concentration range from 64 000 to 450 mg l −1.

Automatic calibration in continuous flow analysis

A manifold which permits calibration graphs to be obtained automatically during continuous flow analysis is discussed. The entirely computer-controlled system is based on the use of two variable-speed peristaltic pumps. One of the pumps moves a standard solution of the analyte and the other displaces a diluent solution, both solutions being merged before reaching the detector. The flow rate delivered by the first pump is increased linearly with time, while that of the second pump is decreased linearly at the same rate. In this way, an on-line concentration gradient is obtained, the flow rate through the detector remaining constant. The signal-time profiles thus obtained can be used for calibration graphs after suitable treatment of the data. A sample solution can then be quantified by pumping it continuously through the manifold. The system is versatile and easy to handle since it is easily adapted to determinations which need a previous chemical reaction, and the same computer that controls the pumps also acquires and treats the signal-time data. The performance of the system is first discussed and then experimentally checked by using two common spectrophotometric determinations. The experiments prove that the approach, which can be extended to other detection systems, is highly reproducible and accurate. In addition, the manifold permits the on-line dilution (up to ten times) of samples which are too concentrated to be measured directly. A saving of time and reagents can be achieved since, if needed, reliable calibration can be obtained in 50 s while only 145 μl of a single standard solution is consumed.

Extending the dynamic range of flame atomic absorption spectrometry: a comparison of procedures for the determination of several elements in milk and mineral waters using on-line dilution

Three approaches based on continuous flow methodology are assessed for the purpose of extending the dynamic range of flame atomic absorption spectrometry. The determination of several elements in infant liquid and powdered milk and water samples is used to check the performances of the manifolds. Two of the systems are fully computer-controlled and permit a calibration graph to be obtained by using a single standard solution. The results confirm that continuous flow methodology is a reliable alternative to the time-consuming common dilution procedures based on glassware. Since the systems are versatile and permit a wide range of degrees of dilution to be obtained, they can be easily adapted for the automated or semi-automated analysis of other liquid samples which are too concentrated to be aspirated directly into the atomic absorption spectrometer.

A microdialysis fibre based sampler for flow injection analysis: determination of L-lactate in biofluids by an electrochemically synthesised bilayer membrane based biosensor

A microdialysis fibre based, low volume sampler is described which can be used in flow injection analysis (FIA) when an on-line dilution of the sample and/or removal of high molecular weight interferents is required. This device used in combination with a lactate amperometric biosensor based on lactate oxidase electrochemically immobilised in a bilayer membrane of poly(o-phenylendiamine) and overoxidized poly(pyrrole) permits the extension of the linear range of response up to 10 mM lactate. Combining microdialysis sampling with FIA and amperometric detection at an interference-free and fast-response biosensor, lactate determination in complex media such as serum, milk and yoghurt can be easily achieved with a high sample throughput and no sample pre-treatment.

Factorial design for the response exploration of a flow injection system

A factorial composite design was employed for the exploration of the response as a function of the flow rate and length of the dispersion coil of a flow injection system employed for on-line dilution and determination of copper by flame-AAS. For this purpose, an empirical response function (RF), which takes into account the peak height and the response time, was employed to evaluate the response. This RF was modified using the coefficient of variation in order to discriminate not only for high absorbance and adequate response time, but also for the best precision.

Automatic continuous-flow determination of sulphite in the presence of a high concentration of thiosulphate

A spectrophotometric method for the determination of sulphite in the presence of up to 200-fold of thiosulphate is proposed. The derivatizing reaction, which is based on the decoloration of p-rosaniline in neutral or slightly basic medium, is developed in an unsegmented flow system. A linear calibration range between 1 and 20 μg ml −1 was obtained, with a relative standard deviation of 1.5% and sampling frequency of 40 h −1. The method has been applied to the determination of the analyte in samples from an ammonium thiosulphate production plant, where the concentration of sulphite is the quality parameter. The concentration of sulphite and thiosulphate in these samples [0.2–1.0% and 50–60% (w/w), respectively] makes necessary a 1:1000 on-line dilution, and the use of peak-width measurements. In these working conditions the linear range was 2.7–20.0 g l −1 and the R.S.D. values ranged between 1.3 and 2.8%, depending on the constant absorbance value selected for measurement. The sampling frequency was 8 h −1.

An Efficient Flow Injection On-Line Dilution System for Flame Atomic Absorption Spectrometry Based on Micro-Zone Penetration

Flow injection microsampling dilution was combined with the zone penetration technique to produce an on-line dilution system capable of achieving 27,000-fold dilutions in flame atomic absorption spectrometric determinations with a sampling frequency of 45/h and a precision of better than 2% r.s.d. Readouts were made at the valleys between two sequentially injected micro-sample zones in the microliter range which penetrated each other. The method was applied to the determination of several grams per liter of magnesium in brine solutions without further dilutions.

Fast kinetic measurements and on-line dilution by flow injection cytometry.

An improved flow injection cytometry (FIC) system suitable for fast cellular kinetic measurements and on-line dilution is described. The instrument allows for measurements from 1.2 s (+/- 0.05 s) after the initiation of mixing, up to any time thereafter. Crucial factors in determining fast kinetic measurements, such as the displacement volume for sample introduction, rate of transport to cytometric interrogation point, and the mixing speed are evaluated and discussed. By varying the volume aspirated, this instrument can facilitate the dilution of cells and/or reagent over a range of one order of magnitude. The fast kinetic and on-line dilution capabilities were demonstrated by on-line staining of DNA in trout erythrocytes with 4',6-diamidino-2-phenylindole (DAPI). The utility of the instrument for measurement of enzyme kinetics was illustrated using human lymphocytes, measuring the glutathione S-transferase (GST) catalyzed reaction between monochlorobimane (MCB) and glutathione (GSH).

Flow injection flame atomic absorption spectrometry for slurry atomization: Determination of manganese, lead, zinc, calcium, magnesium, iron, sodium and potassium in cements

A flame atomic absorption spectrometric procedure for the rapid determination of manganese, lead, zinc, calcium, magnesium, iron, sodium and potassium in cements is described. For minor components, slurries are prepared in the 0.05–0.25% m/v range by direct suspension of the samples in a medium containing 2% v/v nitric acid, 2% hydrofluoric acid and 30% v/v glycerol. 50–200 μl aliquots are injected in a simple flow injection manifold using water as the carrier. When determining major components, the FI manifold includes an easy-to-construct variable volume chamber allowing the on-line dilution of the suspensions. The degree of dilution can be easily modified to obtain signals within the linear response range of the instrument. Aqueous standards are used for calibration. The data obtained for six cements agree with those obtained by conventional procedures based on the compl ⊂ Mv£ewete dissolution of the samples.

Calibration in flame atomic absorption spectrometry using a single standard and a gradient technique

To obtain calibration graphs, a standard solution of the analyte was pumped using an increasing flow rate while pure water was aspirated, for compensation, through a T-piece. In this way an on-line concentration gradient was obtained and a constant flow was supplied to the nebulizer. The absorbance was monitored via a computer which facilitated data acquisition and handling. In spite of the problems posed at low flow rates by the pulsations caused by the rollers of the peristaltic pump, this calibration strategy gave graphs comparable to those obtained by means of the separate measurement of several standard solutions. A calibration graph could be obtained in only 50 s with errors of less than ±3%. The mode of operation also permits the on-line dilution of moderately concentrated solutions, which cannot be measured by direct aspiration, and simplifies the standard additions method.

Investigation of carbon dioxide modified supercritical and near supercritical carrier streams for flow injection systems

Since flow injection (FI) is a dilution technique, efforts have been undertaken to minimize online dilution or dispersion. Solutes in supercritical fluids exhibit increased diffusion coefficients which have been shown to decrease dispersion of the sample zone. This work investigates the use of supercritical fluids (or CO 2 modified fluids) as carrier streams for FI. Both a non-reacting tracer and an online chemical reaction were employed to investigate the behavior of solutes in supercritical and near critical systems. Further, these results are compared to those obtained in the system studied with a conventional carrier stream. Plots of peak response vs% CO 2 modifier increase with a sharp break at moderate modifier composition (20–30%). Plots of peak variance vs% CO 2 modifier show decreased variance with increasing % modifier. The system was also optimized with regards to temperature and pressure. The optimized system displayed improved limits of detection and decreased variance relative to 0% CO 2 modifier carrier streams.

Flow injection flame atomic absorption spectrometry for slurry atomization. Determination of iron, calcium and magnesium in samples with high silica content

A study on the use of slurries in flame atomic absorption spectrometry is reported. Samples with a very high silica content are ground and then slurried in a solution containing 2% hydrochloric and 3% v/v hydrofluoric acids. The suspensions are prepared in the 0.01–1% m/v range and introduced into the flame by means of a simple flow injection manifold. Relative standard deviations for the measurements of iron, calcium and magnesium in diatomaceous earth samples are in the 1.5–2.8, 2.2–5.3 and 2.8–5.0% ranges, respectively. To avoid the use of suspensions prepared with a very low percentage of solid sample and to improve the reproducibility, an on-line dilution manifold is tried. The use of an easy-to-construct variable volume dilution chamber allows the on-line dilution of the slurries, thus permitting the determination of calcium and magnesium over a wide range of concentrations. Calibration is performed using aqueous standards. The experimental conditions, optimized for the determination of iron, calcium and magnesium in diatomaceous earth samples, can also be applied to other silica-based materials, as is shown by the analysis of several standard reference materials.

Flow-injection flame atomic absorption spectrometry for slurry atomization. Determination of calcium, magensium, iron, zinc and manganese in vegetables

A flame atomic absorption spectrometric procedure for determining the iron, manganese, calcium and magnesium contents of slurries prepared from vegetables is described. Samples were first submitted to a mild calcination step and then suspensions were prepared and introduced into an air-acetylene flame using a simple flow-injection manifold. The calibration of iron, zinc and manganese was carried out using either aqueous standards of a flow-injection sample-to-standard additions method applicable to suspensions. The calcium and magnesium contents were too high for the direct introduction of the slurries into the flame and so an on-line dilution system was used. Excellent agreement was found between the results of the slurry-based procedures and those obtained by an alternative method involving calcination and dissolution in acid. The reliability of the approach was confirmed by analysing two certified reference materials.

On-line monitoring of penicillin V during penicillin fermentations: a comparison of two different methods based on flow-injection analysis

Two different flow-injection analysis (FIA) methods used for on-line monitoring of penicillin V during fed-batch penicillin fermentations are compared. The fermentations are carried out using a high yielding strain of Penicillium chrysogenum and both analyzers are used continuously for on-line monitoring during 200 h. Both methods are based on a reaction catalyzed by β-lactamase, which converts penicillin V to penicilloic acid. In analyzer 1 the sample is injected into a buffer stream and carried to a stirred measurement cell containing an enzyme electrode. The enzyme electrode consists of β-lactamase immobilized onto a sensitive pH glass electrode with a very low response time. No predilution of the sample is required in this analyzer. In analyzer 2 the sample is injected into an enzyme reactor containing immobilized β-lactamase. The formed penicilloic acid is detected by decolorization of an iodine-starch complex. By means of a second injection valve it is possible to have the sample by-pass the enzyme reactor and instead pass a dummy reactor in which no β-lactamase is immobilized. Thereby both the concentration of penicillin and penicilloic acid can be measured. On-line dilution of the sample is ensured by a gradient technique. Experimental results from on-line monitoring of penicillin V using the two methods are presented. The large amount of precise measurements enables a very good resolution of the penicillin production; something of extreme value for verification of mathematical models describing the fermentation kinetics.

Direct FIA—AS determination of potassium and magnesium in cement samples by use of the slurries approach

A direct procedure has been developed for the flame atomic determination of potassium and magnesium in cement samples. A 50-mg sample is dispersed in 25 ml of 0.13 M nitric acid; 100 μl of this slurry is injected in a double channel FIA manifold simultaneously with 100 μl of a 10% (w/v) lanthanum solution. This procedure allows the rapid extraction of potassium and magnesium by leaching of the sample; nitric acid is not necessary if only potassium must be determined and the sample can be diluted with only distilled water. Aqueous standards are used. The manifold employed includes a well-stirred mixing chamber, which provides an adequate on-line dilution of the sample, in order to obtain emission or absorbance measurements in the dynamic range of the elements to be determined. The results obtained in the analysis of real samples agree with those found by flame atomic spectrometry after a previous alkaline fusion with lithium carbonate, and exhibit better precision. The limit of detection of the procedure is 0.007% for K 2O and 0.01% for MgO and the precision of the entire procedure corresponds to a relative standard deviation of 1%.