A flow-based procedure with solenoid micro-pumps for the spectrophotometric determination of uric acid in urine

The determination of uric acid in urine shows clinical importance, once it can be related to human organism dysfunctions, such as gout. An analytical procedure employing a multicommuted flow system was developed for the determination of uric acid in urine samples. Cu(II) ions are reduced by uric acid to Cu(I) that can be quantified by spectrophotometry in the presence of 2,2′-biquinoline 4,4′-dicarboxylic acid (BCA). The analytical response was linear between 10 and 100 μmol L^− 1 uric acid with a detection limit of 3.0 μmol L^− 1 (99.7% confidence level). Coefficient of variation of 1.2% and sampling rate of 150 determinations per hour were achieved. Per determination, 32 μg of CuSO₄ and 200 μg of BCA were consumed, generating 2.0 mL of waste. Recoveries from 91 to 112% were estimated and the results for 7 urine samples agreed with those obtained by the commercially available enzymatic kit for determination of uric acid. The procedure required 100-fold dilution of urine samples, minimizing sample consumption and interfering effects. In order to avoid the manual dilution step, on-line sample dilution was achieved by a simple system reconfiguration attaining a sampling rate of 95 h^− 1.     

Exploiting Mn(III)/EDTA complex in a flow system with solenoid micro-pumps coupled to long pathlength spectrophotometry for fast manganese determination

The formation of the Mn(III)/EDTA complex in a flow system with solenoid micro-pumps was exploited for fast manganese determination in freshwater. Manganese(II) was oxidized in a solid-phase reactor containing lead dioxide immobilized on polyester. Long pathlength spectrophotometry was exploited to increase sensitivity, aiming to reach the threshold limit established by environmental legislation. A linear response was observed from 25 to 1500 μg L^− 1, with a detection limit of 6 μg L^− 1 (99.7% confidence level). Sample throughput and coefficient of variation were 36 samples/h and 2.6% (n = 10), respectively. EDTA consumption and waste generation were estimated as 500 μg and 3 mL per determination, respectively. The amount of Pb in the residue corresponds to 250 μg per determination and a solid-phase reactor could be used for up to 1600 determinations. Adsorption in active charcoal avoided interferences caused by organic matter and the developed procedure was successfully applied for determination of manganese in freshwater samples. Results were in agreement with those attained by GFAAS at the 95% confidence level.     

A multicommuted flow system with solenoid micro-pumps for paraquat determination in natural waters

A flow system designed with solenoid micro-pumps is proposed for the determination of paraquat in natural waters. The procedure involves the reaction of paraquat with dehydroascorbic acid followed by spectrophotometric measurements. The proposed procedure minimizes the main drawbacks related to the standard chromatographic procedure and to flow analysis and manual methods with spectrophotometric detection based on the reaction with sodium dithionite, i.e. high solvent consumption and waste generation and low sampling rate for chromatography and high instability of the reagent in the spectrophotometric procedures. A home-made 10-cm optical-path flow cell was employed for improving sensitivity and detection limit. Linear response was observed for paraquat concentrations in the range 0.10–5.0 mg L⁻¹. The detection limit (99.7% confidence level), sampling rate and coefficient of variation (n = 10) were estimated as 22 μg L⁻¹, 63 measurements per hour and 1.0%, respectively. Results of determination of paraquat in natural water samples were in agreement with those achieved by the chromatographic reference procedure at the 95% confidence level.     

A clean method for flow injection spectrophotometric determination of cyclamate in table sweeteners

A flow system based on the multicommutation is proposed for fast and clean determination of cyclamate. The procedure exploits the reaction of cyclamate with nitrite in acidic medium and the spectrophotometric determination of the excess of nitrite by iodometry. The flow system was designed with a set of solenoid micro-pumps to minimize reagent consumption and waste generation. The detection limit was estimated as 30 μmol L⁻¹ (99.7% confidence level) with linear response ranging up to 3.0 mmol L⁻¹. The coefficient of variation was estimated as 1.7% for a solution containing 2.0 mmol L⁻¹ cyclamate (n = 20). About 60 samples can be analyzed per hour, consuming only 3 mg KI and 1.3 μg NaNO₂, and generating 2.0 mL of effluent per determination, thus providing an environmentally friendly alternative to previously proposed procedures. Common artificial and natural sweeteners did not interfere when present in concentrations 10-times higher than cyclamate. The procedure was successfully applied for determination of cyclamate in artificial table sweeteners with results in agreement with the reference method at the 95% confidence level.     

Automatic flow procedure for the determination of glycerol in wine using enzymatic reaction and spectrophotometry

An automatic flow procedure for the determination of glycerol in wines by employing a flow system based on multicommutation and enzymatic reaction is described. Glycerol dehydrogenase was immobilized on aminopropyl glass beads and packed into a column that was coupled to the flow system. The NADH produced by the enzymatic reaction was monitored by spectrophotometry at 340 nm and its radiation absorption presented a relationship with glycerol concentration. The system manifold comprised a set of three-way solenoid valves controlled by a microcomputer, which was furnished with electronic interfaces and runs a software that was designed to carry out on-line sample dilution, reagent addition, and data acquisition. The procedure allows the determination of glycerol in wine samples without any prior pretreatment. The procedure presented as profitable features a linear response range between 2.0 and 10.0 g l⁻¹ glycerol (R=0.998), a detection limit of 0.006 g l⁻¹ glycerol, a relative standard deviation of 1.8% (n=14) for a typical wine sample presenting 5.3 g l⁻¹ glycerol, a sampling throughput of 33 determinations per hour, and a NAD⁺ consumption of 0.8 mg per determination. The results were compared with those obtained using a reference method and no significant difference at 90% confidence level was observed.     

Determination of phenols in waters using micro-pumped multicommutation and spectrophotometric detection: an automated alternative to the standard procedure

An automated and greener spectrophotometric procedure has been developed for the determination of phenol in water at 700 nm. The method uses the reaction between phenol, sodium nitroprusside, and hydroxylamine hydrochloride in a buffered medium at pH 12.3. The flow manifold comprises four solenoid micro-pumps employed for sample and reagent introduction into the reaction coil and to transport the colored product formed to the detector. The linear dynamic range was 50–3,500 ng mL⁻¹ (R = 0.99997; n = 6) and the method provided a limit of detection (3σ) of 13 ng mL⁻¹. The sampling throughput was estimated to be 65 measurements per hour and the coefficient of variation was 0.5% (n = 10) for a 1.0 μg mL⁻¹ phenol concentration. Recoveries of 92–105% were obtained for phenol determination in spiked water samples at concentration levels from 50 to 5,000 ng mL⁻¹. The use of multicommutation reduced the reagent consumption 25-fold, the sample consumption 225-fold, and the waste generation 30-fold compared with the batch procedure. The proposed method is an environmentally friendly alternative to the official 4-aminoantipyrine method since it avoids the use of chloroform.     

An improved procedure for flow-based turbidimetric sulphate determination based on a liquid core waveguide and pulsed flows

An improved flow-based procedure is proposed for turbidimetric sulphate determination in waters. The flow system was designed with solenoid micro-pumps in order to improve mixing conditions and minimize reagent consumption as well as waste generation. Stable baselines were observed in view of the pulsed flow characteristic of the systems designed with solenoid micro-pumps, thus making the use of washing solutions unnecessary. The nucleation process was improved by stopping the flow prior to the measurement, thus avoiding the need of sulphate addition. When a 1-cm optical path flow cell was employed, linear response was achieved within 20–200 mg L⁻¹, described by the equation S = −0.0767 + 0.00438C (mg L⁻¹), r = 0.999. The detection limit was estimated as 3 mg L⁻¹ at the 99.7% confidence level and the coefficient of variation was 2.4% (n = 20). The sampling rate was estimated as 33 determinations per hour. A long pathlength (100-cm) flow cell based on a liquid core waveguide was exploited to increase sensitivity in turbidimetry. Baseline drifts were avoided by a periodical washing step with EDTA in alkaline medium. Linear response was observed within 7–16 mg L⁻¹, described by the equation S = −0.865 + 0.132C (mg L⁻¹), r = 0.999. The detection limit was estimated as 150 μg L⁻¹ at the 99.7% confidence level and the coefficient of variation was 3.0% (n = 20). The sampling rate was estimated as 25 determinations per hour. The results obtained for freshwater and rain water samples were in agreement with those achieved by batch turbidimetry at the 95% confidence level.     

A spectrophotometric flow procedure for the determination of cationic surfactants in natural waters using a solenoid micro-pump for fluid propulsion

An automatic flow-analysis procedure for spectrophotometric determination of cationic surfactants in surface water using a solenoid micro-pump for propelling solutions of reagents and sample is described. The proposed method is based on a ternary formation complex between chromazurol S, the Fe(III) ion, and the cationic surfactant. The flow network comprised four solenoid micro-pumps controlled by a microcomputer, which performed the sampling step by loading a reaction coil with sample and reagent solutions and displacing the sample zone through the analytical path. The system is simple, easy to operate, and very flexible, with sufficient sensitivity to determine cationic surfactants in water without any pre-concentration or separation step. After determining the best operational conditions, favourable features such as a linear response between 0.34 and 10.2?mg?L^?1 of surfactant (R?=?0.999), a relative standard deviation of 0.6% (n?=?11) for a sample containing 3.4?mg?L^?1 of surfactant, a detection limit of 0.035?mg?L^?1 of surfactant, and a sampling throughput of 72 determinations per hour were achieved. The system was used to determine cationic surfactant in river-water samples, and recovery values between 91 and 106% were achieved.     

Simultaneous gas chromatographic determination of methanol and free glycerol in biodiesel

Summary The methanol and free glycerol content of vegetable oil methyl esters used as diesel fuel (biodiesel) is very important in describing the quality of this fuel and is therefore limited by specifications. A previously described GLC method for the determination of free glycerol in biodiesel has been further developed and also allows the simultaneous determination of methanol. Sample preparation includes dissolving in dimethylformamide, silylation with bis-trimethylsilyltrifluoracetamide (BSTFA) and separation on a methylsilicone fluid, coated-capillary column using either FID or MS-detection. Ethanol and 1,4-butanediol were used as internal standards. Both detection systems show sufficient sensitivity for concentrations relevant to biodiesel samples. The recovery was tested using a RME-sample containing known amounts of methanol and glycerol.     

A single standard calibration module for flow analysis systems based on solenoid microdevices

Only two computer-controlled microsolenoid devices, namely two micropumps or one micropump and one microvalve, are sufficient for the construction of on-line dilution modules useful in several flow analytical systems for the calibration using single standard. Three simple constructions of such modules were tested and compared. The most promising is the one based on the concept of a microvalve controlling dilution ratio of the standard and a solenoid micropump playing a double role: solution pumping device and mixing segments homogenizer. All investigated modules were tested with paired emitter detector diode (PEDD) as photometric flow-through detector and bromothymol blue as a model analyte. The best module was implemented into more advanced flow-injection system dedicated for optical detection of alkaline phosphatase activity using UV-PEDD-based flow-through detector for the enzyme reaction product.     

A multicommuted flow system for sequential spectrophotometric determination of hydrosoluble vitamins in pharmaceutical preparations

A multicommuted flow system is proposed for spectrophotometric determination of hydrosoluble vitamins (ascorbic acid, thiamine, riboflavine and pyridoxine) in pharmaceutical preparations. The flow manifold was designed with computer-controlled three-way solenoid valves for independent handling of sample and reagent solutions and a multi-channel spectrophotometer was employed for signal measurements. Periodic re-calibration as well as the standard addition method was implemented by using a single reference solution. Linear responses (r=0.999) were obtained for 0.500-10.0 mg l⁻¹ ascorbic acid, 2.00-50.0 mg l⁻¹ thiamine, 5.00-50.0 mg l⁻¹ riboflavine and 0.500-8.00 mg l⁻¹ pyridoxine. Detection limits were estimated as 0.08 mg l⁻¹ (0.5 μmol l⁻¹) ascorbic acid, 0.8 mg l⁻¹ (2 μmol l⁻¹) thiamine, 0.2 mg l⁻¹ (0.5 μmol l⁻¹) riboflavine and 0.1 mg l⁻¹ (0.9 μmol l⁻¹) pyridoxine at 99.7% confidence level. A mean sampling rate of 60 determinations per hour was achieved and coefficients of variation of 1% (n=20) were estimated for all species. The mean reagent consumption was 25-fold lower in relation to flow-based procedures with continuous reagent addition. Average recoveries between 95.6 and 100% were obtained for commercial pharmaceutical preparations. Results agreed with those obtained by reference methods at 95% confidence level. The flow system is suitable for application in quality control processes and in dissolution studies of vitamin tablets.     

A portable and low cost equipment for flow injection chemiluminescence measurements

A compact, reliable and low cost flow injection chemiluminescence system is described. The flow system consists of a set of solenoid micro-pumps that can dispense reproductive micro-volumes of solutions. The luminometer was based on a coiled cell constructed from polyethylene tubing that was sandwiched between two large area photodiodes. The whole equipment costs about US$ 750 and weights ca. 3 kg. Equipment performance was evaluated by measuring low concentrations of hydrogen peroxide by oxidation of luminol and for the determination of ammonium, based on its inhibition of the luminescence provided by the reaction of luminol and sodium hypochlorite. Linear responses were achieved within 1.0-80 μmol L⁻¹ H₂O₂ and 0.6-60 μmol L⁻¹ NH₄⁺ with detection limits estimated as 400 nmol L⁻¹ H₂O₂ and 60 nmol L⁻¹ NH₄⁺ at the 99.7% confidence level. Coefficients of variation were 1.0 and 1.8%, estimated for 20 μmol L⁻¹ H₂O₂ and 15 μmol L⁻¹ NH₄⁺ (n = 20), respectively. Reagent consumption of 55 μg luminol, effluent volume of 950 μL per determination and sampling rate of 120 samples per hour were also achieved.     

An environmentally friendly flow system for high-sensitivity spectrophotometric determination of free chlorine in natural waters

A green and highly sensitive analytical procedure was developed for the determination of free chlorine in natural waters, based on the reaction with N,N-diethyl-p-phenylenediamine (DPD). The flow system was designed with solenoid micro-pumps in order to improve mixing conditions by pulsed flows and to minimize reagent consumption as well as waste generation. A 100-cm optical path flow cell based on a liquid core waveguide was employed to increase sensitivity. A linear response was observed within the range 10.0 to 100.0 µg L^− 1, with the detection limit, coefficient of variation and sampling rate estimated as 6.8 µg L^− 1 (99.7% confidence level), 0.9% (n = 20) and 60 determinations per hour, respectively. The consumption of the most toxic reagent (DPD) was reduced 20,000-fold and 30-fold in comparison to the batch method and flow injection with continuous reagent addition, respectively. The results for natural and tap water samples agreed with those obtained by the reference batch spectrophotometric procedure at the 95% confidence level.     

HPLC-ELSD法测定生物柴油中游离甘油含量

建立了高效液相色谱(HPLC)-蒸发光散射(ELSD)法测定生物柴油中游离甘油的方法。用水萃取生物柴油中的甘油,选用强酸型阳离子交换柱(Ultimate XB-SCX)分离甘油和杂质,流动相为乙腈-水(25:75,V:V),柱温35℃;ELSD漂移管温度30℃,载气压力360 kPa。结果表明:在7.12～307.31 mg/kg范围内,甘油峰面积和其浓度的对数具有良好的线性相关性,相关系数r为0.9929。甘油的回收率为96.3%～105.6%,RSD均小于2.0%(n=5)。最低检测限(LOD)和最低定量限(LOQ)分别为2.50 mg/kg、7.12 mg/kg。该研究为生物柴油中游离甘油的提取和检测提供了一种快速高效的方法。     

Automatic flow procedure based on multicommutation exploiting liquid-liquid extraction for spectrophotometric lead determination in plant material

A not expensive automatic flow system based on multicommutation and exploiting the liquid-liquid extraction methodology for the determination of lead in plant material is described. The spectrophotometric procedure for lead determination was based on the reaction with dithizone followed by extraction using an organic solvent. The facilities afforded by the multicommutation approach allowed the use of an air stream as carrier, thus contributing to reduce the overall waste generation. The results obtained analysing plant materials compare very well with those obtained employing inductive coupled plasma optical emission spectrometry (ICP OES) at 90% confidence level. Others profitable features such as a linear response range between 50 and 200 μg l⁻¹ Pb (r = 0.999); a sampling rate of 15 determination per hour; a relative standard deviation of 1.8% (n = 12) for a typical sample containing 163 μg l⁻¹ Pb; a detection limit of 12 μg l⁻¹; a reagent consumption of 4.5 mg dithizone; and a waste generation of 225 μl organic solvent per determination were also achieved.     

Multi-pumping flow systems: the potential of simplicity

Nowadays, the trend towards more compact, smarter and simpler devices is generally recognized as one of the most challenging aspects in the development of analytical instrumentation. Modern flow-based procedures do not escape this tendency. The level of integration and automation and the operational functionality of Multi-pumping flow systems (MPFS) would, in most of the situations, meet this requirement. The essential elements of MPFS are multiple solenoid actuated micro-pumps strategically positioned in the flow manifold, which are accountable for solutions insertion, propelling and commutation, conditioning the establishment and subsequent detection of the reaction zone. Being the only active components of the flow manifold they provide a great operational simplicity and assure a straightforward run-time control of important analytical variables. Moreover, the reduction of active components minimizes the probability of occurrence of equipment failures, malfunctions or errors. The low size and low cost of solenoid micro-pumps make them ideal tools to build up compact environmentally friendly analytical systems, which are characterized by low solutions consumptions and the minimisation of hazardous waste generation. Furthermore, the reproducible pulsed flowing stream produced by micro-pumps actuation has proven to be a valuable feature regarding sample/reagent mixing and reaction zone homogenisation.     

Liquid-liquid extraction procedure exploiting multicommutation in flow system for the determination of molybdenum in plants

A liquid-liquid extraction flow analysis procedure for the spectrophotometric determination of molybdenum in plants at μg l⁻¹ level is described. The flow network comprised a set of solenoid valves assembled to implement the multicommutation approach under microcomputer control. Radiation source (LED, 475 nm), detector (photodiode) and separation chamber were nested together with the flow cell comprising a compact unit. The consumption of reagents (potassium thiocyanate and stannous chloride) and also extracting solvent (isoamyl alcohol) were optimized to 32 mg and 200 μl per determination, respectively. Accuracy was assessed by comparing results with those obtained with ICP-OES and no significant difference at 95% confidence level was observed. Other favorable characteristics such as a linear response ranging from 25 to 150 μg l⁻¹ molybdenum (r=0.999); detection limit of 4.6 μg l⁻¹ sample throughput of 25 determinations per hour and relative standard deviation of 2.5% (n=10) were also achieved.     

A multicommuted stop-flow system employing LEDs-based photometer for the sequential determination of anionic and cationic surfactants in water

It has been developed an automatic stop-flow procedure for sequential photometric determination of anionic and cationic surfactants in a same sample of water. The flow system was based on multicommutation process that was designed employing two solenoid micro-pumps and six solenoid pinch valves, which under microcomputer control carry out fluid propelling and reagent solutions handling. A homemade photometer using a photodiode as detector and two light emitting diodes (LEDs) with emission at 470 nm (blue) and 650 nm (red) as radiation sources, which was tailored to allow the determination of anionic and cationic surfactants in waters. The procedure for anionic surfactant determination was based on the substitution reaction of methyl orange (MO) by the anionic surfactant sodium dodecylbenzene sulfonate (DBS) to form an ion-pair with the cetyl pyridine chloride (CPC). Features such as a linear response ranging from 0.35 to 10.5 mg L⁻¹ DBS (R = 0.999), a detection limit of 0.06 mg L⁻¹ DBS and a relative standard deviation of 0.6% (n = 11) were achieved. For cationic surfactant determination, the procedure was based on the ternary complex formation between cationic surfactant, Fe(III) and chromazurol S (CAS) using CPC as reference standard solution. A linear response range between 0.34 and 10.2 mg L⁻¹ CPC (R = 0.999), a detection limit of 0.05 mg L⁻¹ CPC and a relative standard deviation of 0.5% (n = 11) were obtained. In both cases, the sampling throughput was 60 determinations per hour. Reagents consumption of 7.8 μg MO, 8.2 μg CPC, 37.2 μg CAS and 21.6 μg Fe(III) per determination were achieved. Analyzing river water samples and applying t-test between the results found and those obtained using reference procedures for both surfactant types provide no significant differences at 95% confidence level.     

Development and critical comparison of greener flow procedures for nitrite determination in natural waters

Two greener procedures for flow-injection spectrophotometric determination of nitrite in natural waters were developed and critically compared. Replacement of toxic reagents, waste minimization and treatment were exploited to attend the standards of clean chemistry. The flow system was designed with solenoid micro-pumps in order to minimize reagent consumption and waste generation. The first procedure is based on the Griess diazo-coupling reaction with sulfanilamide and N-(1-naphthyl)ethylenediamine (NED) yielding an azo dye, followed by photodegradation of the low amount of waste generated based on the photo-Fenton reaction. The second procedure is based on the formation of iodine from nitrite and iodide in acid medium in order to avoid the use of toxic reagents. For Griess method, linear response was achieved up to 1.0 mg L^− 1, described by the equation A = − 0.007 + 0.460C (mg L^− 1), r = 0.999. The detection limit was estimated as 8 μg L^− 1 at the 99.7% confidence level and the coefficient of variation was 0.8% (n = 20). The sampling rate was estimated as 108 determinations per hour. The consumption of the most toxic reagent (NED) is reduced 55-fold and 20-fold in comparison to batch method and flow injection with continuous reagent addition, respectively. A colorless residue was obtained by in-line photodegradation with reduction of 87% of the total organic carbon content. The results obtained for natural water samples were in agreement with those achieved by the reference method at the 95% confidence level. For the nitrite–iodide method, linear response was observed up to 2.0 mg L^− 1, described by the equation A = − 0.024 + 0.148C (mg L^− 1), r = 0.999. The detection limit was estimated as 25 μg L^− 1 at the 99.7% confidence level and the coefficient of variation was 0.6% (n = 20). The sampling rate was estimated as 44 determinations per hour. Despite avoiding the use of toxic reagents, the nitrite–iodide method presented worst performance in terms of selectivity and sensitivity.     

流动注射光度法直接测定海水中的微量苯胺

基于苯胺与亚硝酸盐的重氮化反应及反应产物与甲萘酚的显色,借助流动分析技术,实现了海水中苯胺含量的分析测定。体系以30.9g/L的NaCl做载液、人工海水配制标准样品,对各个影响因素进行了优化。苯胺浓度在0.01～1.0mg/L范围内与相对峰高呈线性关系,线性方程ΔH(mV)=200.53ρ+1.0728(n=8,ρ为苯胺浓度mg/L),相关系数R2=0.9982。方法的检出限(3σ)为0.005mg/L,相对标准偏差(RSD)为4.8%(n=11)。考察了共存离子、不同盐度样品对分析测定的影响。用于实际海水样品的分析,回收率为95.8%～106.6%。