Electrothermal vaporization-inductively coupled plasma-mass spectrometry: A versatile tool for tackling challenging samples: A critical review

In this review, the literature on the subject of electrothermal vaporization-inductively coupled plasma-mass spectrometry (ETV-ICP-MS) published during the last decade is reviewed with a double purpose: an evaluation of the possibilities of this technique for dealing with very challenging analytical applications on the one hand, and the establishment of a reference guide for method development in ETV-ICP-MS on the other. First, a brief introduction, pointing out the milestones in the development of the technique will provide the reader with a better understanding of the present situation of ETV-ICP-MS and its future perspective. After a section on the basic processes occurring in the furnace and during analyte transport, a guide for method development for challenging analytical applications is proposed, based on the existing literature. Next, the latest contributions in the main application areas of the field are reviewed, with special attention to the most challenging ones: i.e. speciation, “thermal” resolution, enabling complex matrixes to be analyzed and spectral overlap to be avoided, and the direct analysis of slurries and solid samples. Finally, the advantages obtained by coupling an ETV unit to newer types of ICP-MS instrumentation, equipped with collision/reaction cells, time-of-flight (TOF) or sector field (SF) spectrometers, are also discussed.     

Recent developments in electrochemical flow detections—A review: Part I. Flow analysis and capillary electrophoresis

Recent years have provided numerous new examples of applying flow-through electrochemical detectors in chemical analysis. This review, based on about 250 original research papers cited from the current analytical literature, presents their application in flow analysis and capillary electrophoretic methods. Example applications are also given for arrays of electrochemical sensors in flow analysis and electrochemical detection in microfluidic systems. Potentiometric detection with ion-selective electrodes predominates in flow analysis carried out mostly in a flow-injection system, while amperometric and conductivity detections are most commonly employed in capillary electrophoresis.     

Determination of amoxicillin in pharmaceuticals using sequential injection analysis (SIA): Evaluation of the presence of interferents using multivariate curve resolution

In this study, we report a method for determining amoxicillin in pharmaceuticals using sequential injection analysis (SIA) with a diode-array spectrophotometric detector. Before determining the amoxicillin, we use multivariate curve resolution with alternating least squares (MCR-ALS) to investigate whether any interferents are present. With a suitable analytical sequence, we can use SIA to generate a pH gradient and, for each sample, obtain a matrix of data that have been analysed by several chemometric techniques based on multivariate analysis: principal components analysis (PCA), simple-to-use interactive self-modeling mixture analysis (SIMPLISMA) and MCR-ALS. In this way we obtain the concentration profiles and spectra of the species in the sample.We studied six pharmaceuticals containing amoxicillin. Two of these pharmaceuticals contained no interferents, one contained an interferent but amoxicillin had a selective spectral area with respect to it, and the other three contained interferents. For the first three samples, we set up a system of univariate calibration, which determines amoxicillin quickly (it can analyse 20 samples/h) using inexpensive instrumentation and reactives.     

Development of the continuously variable volume reactor for flow injection analysis: Part 1. Design, capabilities and testing

A new apparatus for mixing sample and reagent in flow injection analysis (FIA) is described. The continuously variable volume reactor (CVVR) replaces the conventional mixing coil in a flow injection (FI) manifold to provide mixing and dilution. A linear actuator motor allows control of the chamber volume via LabVIEW software. The chamber volume can be incremented in steps of 1 μl over the range 68-1704 μl. In addition, the chamber has an integral variable-speed stirring unit that is also under computer control. Experiments were performed to evaluate the dispersion characteristics of this new device, evaluate the volume reproducibility, and understand the mixing characteristics. Use of the chamber is shown in the determination of iron(II) in pond water, and in NIST SRM 1643d with excellent results and a detection limit of 3.7 μg/l iron(II). Advantages of the CVVR and future research activities using the device are discussed.     

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.     

Recent development in derivative ultraviolet/visible absorption spectrophotometry: 2004-2008: A review

Several techniques have been proposed for treatment of spectrophotometric data, with the objective of extracting a largest amount of analytical information from spectra composed of unresolved bands. Undoubtedly, a major success was achieved by derivative treatment of the absorbance curves-plotting of the first or a higher order mathematical derivative of absorbance against wavelength (dA/dλ). Derivative spectrophotometry has been applied to many chemical systems, such as pharmaceuticals, foods, cosmetics, and environmental samples.The instrumental development and analytical applications of derivative UV-vis regions absorption spectrophotometry produced in the last 5 years (since 2004) are reviewed.     

Combination of flow injection with capillary electrophoresis: 8. Miniaturized capillary electrophoresis system with flow injection sample introduction and electrogenerated chemiluminescence detection

The combined flow injection (FI)-capillary electrophoresis (CE) system was further exploited by coupling to an electrogenerated chemiluminescence (ECL) detection system. A low-cost miniaturized CE system was developed on a chip platform to provide easy interface both with FI sample introduction and with ECL detection. A falling-drop interface was employed to perform FI split-flow sample introduction while achieving electrical isolation from the CE high voltage. A plexiglas reservoir at the capillary outlet served as both the reaction and detection cell for the ECL reaction, with Ru(bpy)₃²⁺ reagent continuously flowing through the cell. An optical fiber was positioned within the reservoir close to the capillary outlet for transferring the ECL emission to the PMT. The relative positions of the capillary outlet, working electrode and optical fiber as well as reagent renewal flow-rate were optimized to achieve both good sensitivity and separation efficiency under non-interrupted sampling conditions, involving large numbers of samples. An on-column joint often used in other works for isolating the ECL detection system from the CE separation voltage was not found necessary. The performance of the system was illustrated by the baseline separation of proline, valine and phenylalanine with a high throughput of 50 h⁻¹ and plate height of 14 μm for proline under 147 V cm⁻¹ field strength. Detection limits (3σ) were 1.2, 50 and 25 μM and peak height precisions were 1.4, 5.4 and 4.3% R.S.D. (n=9) for proline, valine and phenylalanine, respectively.     

Continuous wave free precession: Practical analytical tool for low-resolution nuclear magnetic resonance measurements

The use of continuous wave free precession (CWFP) as a practical analytical tool for quantitative determinations in low-resolution nuclear magnetic resonance (LRNMR) is examined. The requirements of this technique are shown to be no more demanding than those prevailing in free-induction decay or spin-echo measurements. It is shown that the substantial gain in signal to noise ratio for a given acquisition time permitted by CWFP, can be exploited with advantage in practically any application of LRNMR. This applies not only to homogeneous low viscosity liquid samples but also to multi-component systems where differences in relaxation times of each component permit a separation of the individual contributions. As an example, the use of CWFP for fast quantitative determination of oil and moisture in various seeds is presented.     

Determination of competitive adsorption isotherms applying the nonlinear frequency response method: Part I. Theoretical analysis

In this work adsorption equilibria of binary mixtures are quantified analyzing the nonlinear frequency response of a chromatographic column. Local partial derivatives of an isotherm model can be estimated for certain steady-states from the low frequency asymptotes of the corresponding frequency response functions (FRFs). The required FRFs correspond to two different compounds and the type of the imposed inlet concentration changes, e.g. periodical inlet concentration changes of only one compound or of both of them. For an accurate determination of isotherm parameters, it is necessary to approach as close as possible the low frequency asymptotic behaviour of these functions. Based on principles valid for the FRFs corresponding to the adsorption of a single solute, frequencies needed to reach the low frequency asymptotes of the functions of interest for estimating competitive isotherms are defined in this paper. The relation between the accuracy of the isotherm parameters determined and numbers and types of periodical inlet concentration changes and steady-states analyzed is also evaluated.     

Aqueous two-phase extraction of nickel dimethylglyoximato complex and its application to spectrophotometric determination of nickel in stainless steel

A polyethylene glycol (PEG)-based aqueous two-phase system has been established for the extraction of Ni-dimethylglyoximato complex. Appropriate amounts of PEG solution and solid (NH₄)₂SO₄ were added to the Ni-dimethylglyoximato complex which had been formed in the presence of sodium tartrate and K₂S₂O₈ at pH 12 in a separatory funnel and shaken vigorously for about 1 min. The mixture was allowed to stand for 10 min and then the absorbance of the extracted complex in the upper PEG-rich phase was measured at 470 nm. Beer's law was obeyed over the range of 0.26-2.1 ppm Ni. The proposed extraction method has been applied to the determination of Ni in steel. A steel sample was decomposed with an appropriate acid mixture. An aliquot of the sample solution was taken, treated with H₃PO₄ and most of the iron and copper were removed by hydroxide precipitation using solid BaCO₃ to control the pH of the sample solution in advance of the extraction of Ni. The analytical results obtained for Ni in steel certified reference material JSS 650-10 (The Japan Iron and Steel Federation), BCS 323 (Bureau of Analysed Samples Ltd.) and NIST SRM 361 and 362 (National Institute of Standards and Technology) were in good agreement with certified values.     

Exploiting sequential injection analysis with lab-on-valve and miniaturized potentiometric detection: Epinephrine determination in pharmaceutical products

Miniaturized potentiometric units were constructed, evaluated and incorporated in a SIA-LOV manifold in order to the control of pharmaceutical analysis. The method validation was done with ephinefrine determinations in commercial pharmaceutical products. The optimization procedures were directed at potential versus epinephrine concentration. This approach was achieved by selecting 60 cm of reactor and a flow rate of 11 μL s⁻¹ and injecting 78 μL of epinephrine standard solutions in a 1.0 × 10⁻³ mol L⁻¹ IO₄⁻ solution. A linear range was found for epinephrine concentrations between 2.0 × 10⁻⁴ and 2.5 × 10⁻³ mol L^−l with a slope of 35528 mV L mol^−l and r² = 0.997. Under these conditions the analytical results for the commercial pharmaceutical formulations of 0.908 and 0.454 mg mL⁻¹, respectively, with a R.S.D. of 0.34 for both, was obtained. Through down scaling periodate-selective electrode it was possible to benefit from the recognized advantages of the lab-on-valve sequential-injection based systems, namely regarding equipment portability, reduced consumption of the sample and the reagents and the reduction of effluent waste. Furthermore, the new periodate electrode configuration is easy to achieve in common laboratories and enables the implementation of low volume detection cell where the electrical noise, that is frequently presented in potentiometric based procedures, is significantly reduced.     

Fundamentals and applications of needle trap devices: A critical review

The needle trap device (NTD) is an extraction trap that contains a sorbent inside a small needle, through which fluid can be actively drawn into and out of by a gas-tight syringe or pump, or analytes can be introduced passively to the trap by diffusion. The needle trap (NT) is a potentially solventless sampling technique/sample preparation and introduction device. Both fluid-borne analytes and particles can be trapped inside the needle and then adsorbed analytes are desorbed in an inlet of analytical instrument and introduced for identification and quantification. The fluid may be either gaseous or liquid. The objectives of this critical review are to summarize the theory of the sampling process for both active and passive time-average extraction modes in addition to outlining the evolution of the technology and main applications.     

Possible mechanism for nitric oxide and oxidative stress induced pathophysiological variance in acute myocardial infarction development: A study by a flow injection-chemiluminescence method

The acute myocardial infarction (AMI) model was established through rabbits, and this kind of model was used to investigate the possible mechanism for the AMI mediated damage, induced by NO release and oxidative stress. The biomedical parameters nitric oxide (NO), total antioxidant capacity (TAC) variation in vivo and the enzymatic activity of nitric oxide synthase (NOS) and superoxide dismutase (SOD), which are considered as the major markers for pathophysiological variation, were detected. The results obtained gave evidence that AMI can lead to the NO excess release and compensation by excess cellular respiration, and both of them can result in oxidative stress and further generation of reactive oxygen species (ROS). The latter can bring a series of damages to the organism, including decrease of the TAC value, and NOS and SOD activity.     

Analysis of polyethoxylated surfactants in microemulsion-oil-water systems: Part II

Commercial ethoxylated surfactants are always a mixture of oligomers with different ethylene oxide number (EON). The different oligomers were separated by high performance liquid chromatography (HPLC) on an amino column using a mixture of polar and nonpolar mobile phases. Surfactant oil-water-systems were studied according to the unidimensional scan technique. The surfactant content in the oil, microemulsion and water phases was determined by UV spectroscopy and HPLC. The partitioning of the surfactant oligomers in the oil and water phases of a Winsor III system was determined. The effect of different salts on the surfactant partitioning is discussed.     

Determination of competitive adsorption isotherms applying the nonlinear frequency response method: Part II. Experimental demonstration

This paper demonstrates an experimental application of the nonlinear frequency response (FR) method extension to determine adsorption isotherms of binary mixtures. This method, based on the analysis of the response of a chromatographic column subjected to the sinusoidal inlet concentration changes, is shown to be an alternative for isotherm determination. The critical issue related to the successful application of the method is to reach experimentally the low frequency asymptotic behaviour of the corresponding frequency response functions (FRFs). Although, there are different possibilities to perform periodical inlet concentration changes, in this paper only simultaneous changes for both components were considered. The adsorption of phenol and 2-phenylethanol on octadecyl silica was analyzed experimentally using a mixture of methanol and water as a solvent. Parameters of competitive isotherms were also estimated for comparison using the classical perturbation method. Despite certain differences between competitive isotherms estimated with the two methods that were found, the obtained results show the potential of the nonlinear FR method for measuring competitive isotherms.     

A flow injection procedure based on solenoid micro-pumps for spectrophotometric determination of free glycerol in biodiesel

A flow system designed with solenoid micro-pumps is proposed for fast and greener spectrophotometric determination of free glycerol in biodiesel. Glycerol was extracted from samples without using organic solvents. The determination involves glycerol oxidation by periodate, yielding formaldehyde followed by formation of the colored (3,5-diacetil-1,4-dihidrolutidine) product upon reaction with acetylacetone. The coefficient of variation, sampling rate and detection limit were estimated as 1.5% (20.0 mg L⁻¹ glycerol, n = 10), 34 h⁻¹, and 1.0 mg L⁻¹ (99.7% confidence level), respectively. A linear response was observed from 5 to 50 mg L⁻¹, with reagent consumption estimated as 345 μg of KIO₄ and 15 mg of acetylacetone per determination. The procedure was successfully applied to the analysis of biodiesel samples and the results agreed with the batch reference method at the 95% confidence level.     

Boiling enthalpy from correlations: Results for 83 families of fluids

Ten analytical models were used to calculate the enthalpy of vaporization of fluids at the boiling temperature. The correlations considered were six specific expressions valid only at that temperature, and four general correlations valid for any temperature. Most of these models require as inputs the critical properties and the acentric factor, but one of the specific models requires only the molecular weight (and, obviously, the boiling temperature). One of the models is a correlation requiring a molecular Lennard-Jones parameter and the acentric factor as inputs. Results for 1591 polar and non-polar fluids, grouped into 83 families, are compared with the values given by the DIPPR project.     

Optimisation of a flow injection system with electrochemical detection using the desirability function: Application to the determination of hydroquinone in cosmetics

A procedure is proposed in which the determination of hydroquinone using a flow injection system with electrochemical detection is described. Size and coefficient of variation of the signal are optimised by a desirability function and a central composite design. The robustness of the optimum reached in the optimisation step is evaluated by means a Plackett-Burman design. The optimised FIA system is able to determine hydroquinone with a minimum detectable net concentration of 10 μg l⁻¹ with a false positive probability of 0.05 and a false negative probability less than 0.05. In samples of bleaching cream, the proposed procedure has a recovery of 102.2% with standard deviation of 4.4% and a relative error of 6.2%.     

Crystallization in ultra-thin polymer films: Morphogenesis and thermodynamical aspects

We present a computer model for polymer crystallization in ultra-thin films where chains are considered as dynamical units. In our model chains can change their internal state of order by cooperative motions to improve thermodynamic stability. The interplay between reorganization, enthalpic interactions and the morphology of crystals enables us to explain many properties of growth, morphogenesis and melting of polymer lamellae. We emphasize the relation between the thermodynamic stability of non-equilibrium crystals and morphological features which are beyond the average thickness of the lamellae. In particular, we show that melting of polymers is preceded by reorganization processes and the stability of polymer crystals is not necessarily related to the structure formed at the crystallization point. The simulations allow for the determination of some non-equilibrium properties such as the internal energy and the non-equilibrium heat capacity. We show that multiple-peak melting endotherms result from morphological transformations. The results of our computer simulations are compared with AFM observations in ultra-thin polyethyleneoxide films.