On-line analysis of volatile fatty acids in anaerobic treatment processes

In this paper, an on-line spectrofluorimetric system is proposed for a simple, rapid and accurate measurement of volatile fatty acids (VFA) in anaerobic treatment processes. The determination method is based on the derivatization of VFA with N-(1-naphthyl)ethylenediamine (EDAN) followed by a spectrofluorimetric detection of the corresponding amide. The analytical procedure is automated with a flow analysis technique, coupling multisyringe (MSFIA) and multi-pumping (MPFS) methods. Operative conditions have been investigated with a special attention paid to the activation and amidation steps and to the liquid-liquid extraction of the derivatized final product. Fluorescence intensities (λ_em = 335 nm, λ_ex = 395 nm) were found to be proportional to the concentration of VFA, expressed as acetic equivalent, in the range 19-1000 mg L⁻¹, with a detection limit (3σ) of 5.1 mg L⁻¹. Our results showed a good selectivity for VFA as compared to other organic and inorganic compounds usually found in sewage sludges. Validation of the on-line system developed has been assessed by application of the procedure to aqueous samples originating from sewage sludge treatment plants. The results were in good agreement with ion chromatography measurements.     

In‐syringe‐assisted dispersive liquid–liquid microextraction coupled to gas chromatography with mass spectrometry for the determination of six phthalates in water samples

A fully automated method for the determination of six phthalates in environmental water samples is described. It is based in the novel sample preparation concept of in‐syringe dispersive liquid–liquid microextraction, coupled as a front end to GC–MS, enabling the integration of the extraction steps and sample injection in an instrumental setup that is easy to operate. Dispersion was achieved by aspiration of the organic (extractant and disperser) and the aqueous phase into the syringe very rapidly. The denser‐than‐water organic droplets released in the extraction step, were accumulated at the head of the syringe, where the sedimented fraction was transferred to a rotary micro‐volume injection valve where finally was introduced by an air stream into the injector of the GC through a stainless‐steel tubing used as interface. Factors affecting the microextraction efficiency were optimized using multivariate optimization. Figures of merit of the proposed method were evaluated under optimal conditions, achieving a detection limit in the range of 0.03–0.10 μg/L, while the RSD% value was below 5% (n = 5). A good linearity (0.9956 ≥ r² ≥ 0.9844) and a broad linear working range (0.5–120 μg/L) were obtained. The method exhibited enrichment factors and recoveries, ranging from 14.11–16.39 and 88–102%, respectively.     

Improved spectrophotometric determination of paraquat in drinking waters exploiting a Multisyringe liquid core waveguide system

A novel Multisyringe flow injection analysis (MSFIA) system combined with a 200 cm long pathlength liquid core waveguide (LCW) has been developed enabling for the first time the sensitive spectrophotometric determination at μg L⁻¹ levels of the herbicide paraquat (Pq²⁺) in drinking waters. The proposed system is a simple, economic and fast alternative for obtaining the first evidence of paraquat pollution prior the use of more complex instrumental techniques.The proposed methodology is based on the production of a blue free radical by reaction of Pq²⁺ with ascorbic acid (partially oxidized with potassium iodate) in basic medium. Limits of detection and quantification as low as 0.7 and 2.3 μg L⁻¹, were obtained respectively. The working range is linear up to a concentration of 250 μg L⁻¹ of Pq²⁺. The injection throughput of the proposed method is 34 h⁻¹. The results obtained with the LCW are compared with those using a conventional 1 cm flow cell. The automation of standard addition procedures has been studied and implemented for samples causing matrix effects. Finally the proposed system has been applied to the determination of paraquat in drinking water samples.     

Multisyringe Chromatography (MSC): An Effective and Low Cost Tool for Water-Soluble Vitamin Separation

The advent of monolithic columns has facilitated the combination of chromatographic techniques with nonseparation flow techniques by virtue of the low overpressure introduced by them. Thus far, these combinations have provided excellent results derived from the high selectivity of liquid chromatography and ease of sample handling in nonseparation flow techniques. The joint use of multisyringe flow injection analysis and monolithic columns has provided multisyringe chromatography (MSC).

Simultaneous spectrophotometric determination of ascorbic acid, nicotinic acid, nicotinamide, pyridoxine hydrochloride, thiamine hydrochloride, and riboflavin in commercial drinks using a MSC method, is described in this contribution as an inexpensive alternative to classical HPLC. The MSC method provides baseline separation of all these compounds with resolutions values greater than 2, except for B1 and B6 with resolution of 1,2.

A ChromolithTM Flash RP-18e (50 mm × 4.6 mm) column, a multisyringe burette provided with two 10 mL high-precision bidirectional syringes and an 8-port multiposition valve were used for a dual isocratic chromatographic separation in our study.     

Construction of a New Flow‐through Cell for Screen Printed Electrodes

In this paper a new flow‐through cell for voltammetric determinations using screen printed electrodes is described. This cell is much simpler than one with similar performances described in a previous work. This new flow‐through cell has been coupled to a multisyringe flow injection system (MSFIA) and to a multiport selection valve allowing the online calibration using the standard addition method. The uses of the MSFIA, together with the small volume of the flow cell and the reduced surface area of the solid phase electrode (SPE) have considerably reduced the volume of reagents and samples to be used. This system has allowed obtaining similar or better detection limits than those obtained using other techniques and flow analysis devices such as SIA, FIA, LOV and microfluidic channels. Graphite ink has been used for the development of screen‐printed electrodes. The determination of Cd and Pb with ASV has been made through its co‐deposition with Bi. For this, Bi(III) solution and the sample were mixed in line. Due to the creation of a new Bi film in each voltammetric cycle, very well defined and reproducible peaks corresponding to Cd and Pb have been obtained. The use of Bi is one of the most important advantages of this system, since it is a recognized substitute for Hg, and its impact on the environment is much lower due to its reduced toxicity. The fact of being an automatic system, the low cost of its components, its simplicity and ease of handling, make it a system that could be useful for monitoring tasks in fieldwork, or measurements on board.     

Multisyringe flow injection analysis hyphenated with liquid core waveguides for the development of cleaner spectroscopic analytical methods: improved determination of chloride in waters

In this work, the hyphenation of the multisyringe flow injection analysis technique with a 100-cm-long pathlength liquid core waveguide has been accomplished. The Cl⁻/Hg(SCN)₂/Fe³⁺ reaction system for the spectrophotometric determination of chloride (Cl⁻) in waters was used as chemical model. As a result, this classic analytical methodology has been improved, minimizing dramatically the consumption of reagents, in particular, that of the highly biotoxic chemical Hg(SCN)₂. The proposed method features a linear dynamic range composed of two steps between (1) 0.2–2 and (2) 2–8 mg Cl⁻ L⁻¹, thus extended applicability due to on-line sample dilution (up to 400 mg Cl⁻ L⁻¹). It also presents improved limits of detection and quantification of 0.06 and 0.20 mg Cl⁻ L⁻¹, respectively. The coefficient of variation and the injection throughput were 1.3% (n = 10, 2 mg Cl⁻ L⁻¹) and 21 h⁻¹. Furthermore, a very low consumption of reagents per Cl⁻ determination of 0.2 μg Hg(II) and 28 μg Fe³⁺ has been achieved. The method was successfully applied to the determination of Cl⁻ in different types of water samples. Finally, the proposed system is critically compared from a green analytical chemistry point of view against other flow systems for the same purpose.     

Multisyringe flow injection analysis system for automation of standard addition calibration method

In the present work, the multi-channel features of multisyringe flow injection analysis (MSFIA) were exploited for the first time to implement calibration based on standard addition method (SAM). For this, standard solutions containing different concentrations of target analyte were placed in each syringe of the multisyringe and connected to a flow network where in-line mixing of sample and standard through a merging zone approach was established prior to detection of analyte. Using this strategy, artifacts reported before in SAM using flow injection analysis were avoided as the concentration of the analyte in the resulting mixture was related to the dilution of sample and added standard within the system, and the concentration of all matrix components was kept constant during all measurements. The feasibility of the proposed MSFIA system was assessed through application to potentiometric determination of chloride ion in electroplating bath and milk samples. Results obtained for samples (n = 15) were not statistically different from those provided by titrimetric procedures, with an excellent throughput (20–31 samples h^{− 1}), comprising four-level addition of chloride ion.     

Multisyringe flow-injection system for total inorganic arsenic determination by hydride generation-atomic fluorescence spectrometry

A software-controlled time-based multisyringe flow-injection system for total inorganic arsenic determination by hydride generation atomic fluorescence spectrometry (HGAFS) has been developed. By using a multisyringe burette coupled with one multiport selection valve, the time-based injection provides precise known volumes of sample, a reducing sodium tetrahydroborate solution and a pre-reducing solution which are dispensed into a gas-liquid separation cell. An argon flow delivers the arsine into the flame of an atomic fluorescence spectrometer. A hydrogen flow has been used to support the flame.Linear calibration graphs for arsenic concentrations between 0.25 and 12 μg l⁻¹ were obtained. The detection limit of the proposed technique (3σ_b/S) was 0.07 μg l⁻¹. A sample throughput of 36 samples/h (108 injections) has been achieved. The proposed technique has been validated by means of reference solid and water materials with good agreement with the certified values. This method was compared with those reported in previous sequential injection analysis (SIA) and flow-injection analysis (FIA) systems. The proposed method offers a number of advantages in front the usual AFS applications, which are mainly a higher sampling frequency and a significant reduction in reagent consumption.     

Antimony determination and speciation by multisyringe flow injection analysis with hydride generation-atomic fluorescence detection

A new analytical procedure for determination of inorganic antimony and speciation of antimony(III) and antimony(V) is presented. For this purpose, a software-controlled time-based multisyringe flow injection system, which contains a multisyringe burette provided with a multi-port selection valve, was developed. Hydride generation-atomic fluorescence spectrometry was used as a detection technique. A 0.3% (w/v) reducing sodium tetrahydroborate solution, hydrochloric acid (2 M), an antimony solution and a pre-reducing solution of 10% (w/v) KI and 0.3% (w/v) ascorbic acid are dispensed simultaneously into a gas-liquid separation cell with further propulsion of the reaction product into the flame of an atomic fluorescence spectrometer using argon flow. A hydrogen flow was employed to support the flame.The linear range and the detection limit (3s_b/S) of the proposed technique were 0.2-5.6 μg l⁻¹ and 0.08 μg l⁻¹, respectively. A sample throughput of 18 samples per hour (corresponding to 80 injections per hour) was achieved. The relative standard deviation for 18 independent measurements was 4.6%. This technique was validated by means of reference solid and water materials with good agreement with the certified values. Satisfactory results for speciation of Sb(III) and Sb(V) by means of the developed technique were obtained.     

Speciation analysis of inorganic arsenic by a multisyringe flow injection system with hydride generation-atomic fluorescence spectrometric detection

In this study, a new technique by hydride generation-atomic fluorescence spectrometry (HG-AFS) for determination and speciation of inorganic arsenic using multisyringe flow injection analysis (MSFIA) is reported. The hydride (arsine) was generated by injecting precise known volumes of sample, a reducing sodium tetrahydroborate solution (0.2%), hydrochloric acid (6 M) and a pre-reducing solution (potassium iodide 10% and ascorbic acid 0.2%) to the system using a multisyringe burette coupled with one multi-port selection valve. This solution is used to pre-reduce As(V) to As(III), when the task is to speciate As(III) and As(V). As(V) is determined by the difference between total inorganic arsenic and As(III). The reagents are dispensed into a gas-liquid separation cell. An argon flow delivers the arsine into the flame of an atomic fluorescence spectrometer. A hydrogen flow has been used to support the flame. Nitrogen has been employed as a drier gas (Fig. 1).Several variables such as sample and reagents volumes, flow rates and reagent concentrations were investigated in detail. A linear calibration graph was obtained for arsenic determination between 0.1 and 3 μg l⁻¹. The detection limit of the proposed technique (3σ_b/S) was 0.05 μg l⁻¹. The relative standard deviation (R.S.D.) of As at 1 μg l⁻¹ was 4.4 % (n = 15). A sample throughput of 10 samples per hour was achieved. This technique was validated by means of reference solid and water materials with good agreement with the certified values. Satisfactory results for speciation of As(III) and As(V) by means of the developed technique were obtained.