Ricardo S. Honorato

Standard additions in flow injection analysis based on merging zones and gradient exploitation: application to copper determination in spirits

A novel strategy for implementing the standard addition method in flow analysis is described. It requires only one standard solution and provides several addition levels using a single injection. Data acquisition and treatment are performed by means of a microcomputer running a software in C-language. A mathematical model to account for the changes in sample matrix due to differences in dispersion associated with the different addition levels is derived. It corrects also deviations related to the asymmetry of the merging zones process. Model systems are critically examined for potassium determination in synthetic samples by flame photometry, to which ethanol or glycerol are added to promote different matrix effects. As application, the determination of copper in spirits by flame atomic absorption spectrometry was selected. The proposed system handles about 60 samples per hour and yields precise results (R.S.D. usually < 5%) regardless of the ethanol content of the assayed samples. Comparison with an alternative procedure involving sample evaporation and manual water dilution revealed systematic deviations usually < 3%.

An automated flow-injection titrator for spectrophotometric determinations of total acidity in wines, using a single standard solution and gradient calibration

An automated flow-injection titrator to perform spectrophotometric determinations of acidity in wine samples is described. The proposed titrator exploits concentration gradients generated in the flow-injection system and, in contrast to the automated flow titrators previously proposed, it does not require analytical curves based on several standard solutions of tartaric acid. Only one single standard solution, the proper titrant, is directly used for gradient calibration and calculating of the analyte concentrations. A simple laboratory-made photometer that uses an LED (Light-Emitting Diode) as light source and a phototransistor as detector was built in to this titrator. Yellow LED was usually used for spectrophotometric measurements because m-cresol purple (7.4 < pH < 9.0 yellow–purple transition) was selected as acid–base indicator. The system is fully controlled by microcomputer and the software for control and for acquisition and treatment data was written in C language. A good agreement between results yielded by the proposed titrator and those produced by a reference method of the Association of Official Analytical Chemistry (AOAC) was obtained and no statistical difference between results at the 95% probability level was confirmed by applying the paired t-test. An overall relative accuracy of about 0.9% and a mean relative standard deviation lower than 1.5% were obtained with nine replicate titrations. The proposed titrator allows 72 determinations per hour to be carried out, consuming just 200 µl of the sample and 1.5 ml of the titrant.

Gunshot residues: screening analysis by laser-induced breakdown spectroscopy

Laser induced breakdown spectroscopy (LIBS) is further developed for the detection of gunshot residue (GSR) on the hands of a suspected shooter. To develop the procedure, 114 samples were obtained by pressing a small piece of adhesive tape against the dorsal region of the hands of non-shooters, of shooters just after firing a weapon, and of shooters after washing their hands with soap and water. The tapes were analyzed directly in a time-resolved echelle based LIBS system, assembled in the laboratory, and 20 single laser pulses, spread uniformly on the tape surface, were shown to be enough to ensure the detection of true positives. The spectra obtained by LIBS from the tape present a signature with emission lines that are attributed to barium and lead. After the spectral data was submitted to a pattern recognition method SIMCA (Soft Independent Modeling of Class Analogy), shooters and non-shooters were corrected classified. The method based on LIBS and SIMCA has been demonstrated to be non-destructive of crime evidence and enabled discrimination between the samples collected from non-shooter volunteers and shooter volunteers before and after washing their hands.

Spectrophotometric cocaine determination in a biphasic medium employing flow-batch sequential injection analysis.

Quantifying cocaine in apprehended samples is important to the Brazilian Federal Police because the concentration can indicate the origin of the drug and consequently the traffic route. In addition to the other risks of using this drug is the variability in cocaine concentration, which makes large doses lethal. Gas Chromatography with a Flame Ionization Detector (GC-FID) and a Mass Selective Detector (GC-MS) are the techniques usually employed, but these systems are not available in all police laboratories, due to the relatively high cost. In the present work, a flow-system procedure for the spectrophotometric determination of cocaine using cobalt thiocyanate as a complexing reagent was developed. In this reaction, two phases are formed: the superior (pink) contains an excess of cobalt thiocyanate solution and the lower layer (blue) contains the complex cocaine-cobalt thiocyanate. Samples and reagent are inserted through a sequential-injection valve between two air bubbles inside a reaction chamber. An optic fiber sensor connected to the chamber recorded the absorbance at 630 nm signal. The detection and quantification limits were 29.4 mg L(-1) and 98 mg L(-1), respectively. Relative standard deviation was 4.9% for solutions containing 400 mg L(-1) (n=10), with stable baselines. The analytical throughput was 12 determinations per hour.

Prior assay as an approach to flow titrations. Spectrophotometric determination of iron in alloys and ores

In titrimetric procedures, the number of data points can be reduced when the analyte concentration is roughly known. This aspect is emphasised in automated titrations where a foresight of the analyte concentration can be fast and easily achieved by processing known amounts of sample and titrant, and evaluating the titrant excess. This prior assay permits a previous definition of the different titrant amounts to be further added. The feasibility of this strategy is demonstrated in designing a flow-batch titrator for spectrophotometric determination of iron in steels, iron ores and similar. The method involves titration of ferrous ions by permanganate performed inside a 400ml reaction chamber assembled in the manifold. The proposed system handles about 20 samples per hour, requires 94mg KMnO4 per determination and yields precise results (R.S.D. ca. 1.5%, nD6) for 1.0‐1010 3 mol l 1 Fe in the injectate. Accuracy was assessed by running a standard reference material and different samples already analysed by a manual titrimetric procedure.

Classification of Brazilian and foreign gasolines adulterated with alcohol using infrared spectroscopy

The smuggling of products across the border regions of many countries is a practice to be fought. Brazilian authorities are increasingly worried about the illicit trade of fuels along the frontiers of the country. In order to confirm this as a crime, the Federal Police must have a means of identifying the origin of the fuel. This work describes the development of a rapid and nondestructive methodology to classify gasoline as to its origin (Brazil, Venezuela and Peru), using infrared spectroscopy and multivariate classification. Partial Least Squares Discriminant Analysis (PLS-DA) and Soft Independent Modeling Class Analogy (SIMCA) models were built. Direct standardization (DS) was employed aiming to standardize the spectra obtained in different laboratories of the border units of the Federal Police. Two approaches were considered in this work: (1) local and (2) global classification models. When using Approach 1, the PLS-DA achieved 100% correct classification, and the deviation of the predicted values for the secondary instrument considerably decreased after performing DS. In this case, SIMCA models were not efficient in the classification, even after standardization. Using a global model (Approach 2), both PLS-DA and SIMCA techniques were effective after performing DS. Considering that real situations may involve questioned samples from other nations (such as Peru), the SIMCA method developed according to Approach 2 is a more adequate, since the sample will be classified neither as Brazil nor Venezuelan. This methodology could be applied to other forensic problems involving the chemical classification of a product, provided that a specific modeling is performed.

An improved leaping detector for flow analysis applied to iron speciation in drugs

A low inner volume (ca. 64 ml) probe was built up in an injector-commutator in order to behave as a photometric leaping detector in flow analysis. It comprises a bicolour light-emitting diode (BLED), as a source of pulsed radiation in the red and green visible region, and two phototransistors as transducers. Sample injection, detector relocation, analytical signal recording, data treatment and definition of the spectral working range were computer-controlled. The feasibility of the system was initially demonstrated in the flow-injection speciation of iron, and the overall standard deviation of results was estimated as ± 1.6 and ± 1.4% for 1.6–4.0 mg l−1 Fe(II) or total iron after eightfold processing of synthetic samples. The system was further applied to drug analysis: the mean deviations of results for typical samples were estimated as ± 5.2 and ± 3.3%, and the relative standard deviation as ± 1.6 and ± 1.3% for Fe(II) and total iron, respectively. Results were compared with those obtained by a conventional spectrophotometric procedure and no statistic differences at the 95% confidence level were found. In relation to an earlier system with multi-site detection, the proposed system is more stable, presenting low drift with a relative standard deviation of 0.026% and 0.039% for measurements (n=120 during 4 h of observation) with green and red emission. It is also faster with a sampling rate of 133 h−1 and carryover problems are not found. The possibility of compensating the Schlieren noise by dual-wavelength spectrophotometry is discussed.

Single standard calibration and data processing in flow injection titration based on concentration gradients.

This paper describes use of gradients of concentration generated in flow injection (FI) systems to perform determinations based on points where the concentration of titrant and analyte are at stoichiometric ratio. Two procedures were developed. In one procedure the titrant is injected in a FI manifold and merges with the sample which is continuously pumped towards the detector. In the other procedure the sample is injected and merged with the titrant which is continuously pumped. Both techniques make use of concentration gradients of the sample or titrant generated in FI manifolds that contain a mixing chamber. This gradient is calibrated employing only one standard solution (usually the titrant) in order to convert any detector signal, obtained in the elapsed time after injection, to instantaneous concentration values. The flow system is microcomputer controlled and data are treated to locate points where the concentration of titrant and analyte are at the stoichiometric ratio. These points are found in abrupt changes of the signal × concentration curves obtained in the presence of the reaction. The method has been evaluated for determination of Fe(II) and acetic acid by spectrophotometric and conductimetric detection, respectively. Results show a mean relative standard deviation lower than 1%, an average accuracy of 1% and a high sampling processing capability (40 to 60 samples per hour).

Automated flow-rate meter for flow-analysis systems

An automated flow-rate meter easy to construct and operate is described. This is contralled by a micracomputer connected to an analog/digitai converter interface and by a contrai software. It uses two acrylic flow cells, each comprising a bicolor light-emitting diode anda phototransistor piaced on opposite sides. The performance of this flow-rate meter was evaiuated by appiying it to three flow manifoids: one monosegmented flow titration system by binary sampling; one conventionai monosegmented flow system, and one singie-iine FIA system. The FIA system flow rate was assessed by means of a Vanderslices expression. The time range of measurement of the flow rates was of 3.5-19.5 seconds and relative errars of 0.2-2.6% as well as relative stanCorrespondence to: Mario Cesar U. Araujo.

Authenticity assessment of banknotes using portable near infrared spectrometer and chemometrics

Spectra recorded using a portable near infrared (NIR) spectrometer, Soft Independent Modeling of Class Analogy (SIMCA) and Linear Discriminant Analysis (LDA) associated to Successive Projections Algorithm (SPA) models were applied to identify counterfeit and authentic Brazilian Real (R