Ana Paula Silveira Paim

Determination of mercury in rice by cold vapor atomic fluorescence spectrometry after microwave-assisted digestion

A cold vapor atomic fluorescence spectrometry method (CV-AFS) has been developed for the determination of Hg in rice samples at a few ngg(-1) concentration level. The method is based on the previous digestion of samples in a microwave oven with HNO(3) and H(2)O(2) followed by dilution with water containing KBr/KBrO(3) and hydroxylamine and reduction with SnCl(2) in HCl using external calibration. The matrix interferences and the effect of nitrogen oxide vapors have been evaluated and the method validated using a certified reference material. The limit of detection of the method was 0.9ngg(-1) with a recovery percentage of 95+/-4% at an added concentration of 5ngg(-1). The concentration level of Hg found in 24 natural rice samples from different origin ranged between 1.3 and 7.8ngg(-1).

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.

Simultaneous determination of synthetic colorants in yogurt by HPLC

This article reports on a method to determine synthetic dyes in yogurts and milk drinks. Initially a method for extraction of artificial dyes was developed to pretreat samples in order to extract most of the artificial colorants. Then, the colorants were analyzed by HPLC-PAD using gradient elutions. The method was linear in the range of 0.5-25mgL(-1) colorants (0.9991<R(2)<0.9998), with LOD of 0.18-4.58μgL(-1). The results recovery test on two levels ranged from 82% to 115%. The reference solution of the analytes remained stable for 3months. In the yogurt samples analyzed, the dye E122 (1.4-11.75mgL(-1)) was found in most samples, followed by dye E123 (3.12 and 12.56mgL(-1)). All samples had concentrations below the maximum level allowed for use in yogurt (5-30mg/100g dyes). [corrected].

Determination of total mercury in nuts at ultratrace level

Total mercury, at μg kg(-1) level, was determined in different types of nuts (cashew nut, Brazil nuts, almond, pistachio, peanut, walnut) using a direct mercury analyser after previous sample defatting and by cold vapour atomic fluorescence spectrometry. There is not enough sensitivity in the second approach to determine Hg in previously digested samples due to the strong matrix effect. Mercury levels in 25 edible nut samples from Brazil and Spain were found in the range from 0.6 to 2.7μg kg(-1) by using the pyrolysis of sample after the extraction of the nut fat. The accuracy of the proposed method was confirmed by analysing certified reference materials of Coal Fly Ash-NIST SRM 1633b, Fucus-IAEA 140 and three unpolished Rice Flour NIES-10. The observed results were in good agreement with the certified values. The recoveries of different amounts of mercury added to nut samples ranged from 94 to 101%. RSD values corresponding to three measurements varied between 2.0 and 14% and the limit of detection and quantification of the method were 0.08 and 0.3μg kg(-1), respectively.

A multicommuted flow analysis method for the photometric determination of amoxicillin in pharmaceutical formulations using a diazo coupling reaction

A multicommuted flow analysis method for the spectrophotometric determination of amoxicillin in pharmaceutical formulations is reported. The proposed method is based on the reaction of diazotised o-nitroaniline with amoxicillin in an alkaline medium, producing a yellow compound that was monitored at 435 nm. The main features of the method were: a linear relationship with amoxicillin concentration ranging from 25 to 400 mg L-1; a detection limit of 5.1 mg L-1; an analytical throughput of 50 determinations per hour; and a relative standard deviation lower than 3.9% for a typical amoxicillin sample with concentration of 100 mg L-1 (n = 10). To verify the accuracy of the method, samples were also analyzed employing an independent method, and applying the paired t-test to the results no significant difference at the 95% confidence level was observed.

Spectrophotometric Determination of Thiocyanate in Human Saliva Employing Micropumping Multicommutation Flow System

ABSTRACT A procedure based on a multicommutated approach for the determination of thiocyanate (SCN−) in human saliva is described. The method was based on the reaction of Fe(III) with SCN−, producing a red complex that was monitored at 480 nm. Under optimum experimental conditions, a linear response ranging from 0.5 to 10.0 m mol L−1 SCN− (R = 0.9991), a relative standard deviation of 1.0% (n = 10) for a sample 5 m mol L−1 SCN−, a detection limit (3σ criterion) of 50 µ mol L−1, a sampling rate of 60 determinations per hour, reagent consumption of 0.02 mg Fe(III) per determination, and a waste generation of 1.6 mL per determination were achieved.

Fluorescence probe for mercury(II) based on the aqueous synthesis of CdTe quantum dots stabilized with 2-mercaptoethanesulfonate

Manipulation of the QD surface by changing morphology and surface capping ligands, as well as adjusting nanocrystal size, plays a crucial role on the selectivity and sensitivity exhibited by QDs towards a given target analyte. In this study, a novel aqueous synthesis of CdTe QDs capped with a thiol compound containing a sulfonate (SO3−) terminal group, viz., 2-mercaptoethanesulfonate (MES), was thoroughly investigated with the aim of obtaining a fluorescent probe for chemical analysis. The results obtained with the prepared CdTe-MES QDs in the determination of various metal ions demonstrated the high efficiency of these nanomaterials for determining Hg(II) levels. Upon optimization, linear working calibration curves for Hg(II) concentrations of up to 0.5 μmol L−1 were obtained, with a determination coefficient of 0.9984. The detection limit was 0.0095 μmol L−1 and the quantification limit was 0.0324 μmol L−1. When applied to the determination of Hg(II) in tap water samples, the developed method provided analytical results similar to those obtained with a reference method. The accuracy and precision of both methods were comparatively evaluated using the Students t-test and the Fischer test, and the tabulated values showed a good agreement at a 95% confidence level.

Clean photoinduced generation of free reactive oxygen species by silica films embedded with CdTe–MTA quantum dots

CdTe quantum dots capped with mercaptopropionic acid, 3.5 nm in size, were entrapped in sol–gel films prepared with tetramethyl orthosilicate under mineral acidic catalysis in the presence of Triton X-100 as a non-ionic surfactant. The follow-up of the sol–gel process was performed in real-time both with fluorescent crystal violet as a molecular rotor and with quantum dots. Clusters of nanoparticles 500 nm in size become homogeneously distributed in the films, but preserving initial photoluminescence quantum yields (21%), and the emission spectrum had increased excited state lifetimes (65 ± 4 ns) and photostability. Films photoactivation inside a multi-pumping flow system enabled reproducible generation of reactive oxygen species as determined by chemiluminescence using the alkaline luminol reagent, thus opening future developments for clean and environmentally friendly analytical applications.

Employment of electrochemically synthesized TGA-CdSe quantum dots for Cr(3+) determination in vitamin supplements.

The fluorescence quenching of TGA-CdSe quantum dots (QDs) was used for Cr(3+) quantification in vitamin supplements. The QD was electrochemically synthesized, demonstrating high reproducibility with control of particle size, thus making it a clean method, without the presence of reducing agents. Under ideal conditions, with the fluorescence band at 551 nm (excitation 365 nm), the maximum fluorescence quenching was observed at pH 4.0, with a time of 200 s for each data acquisition. Under optimum experimental conditions, linear quenching was observed for Cr(3+) in the range of 25.0-325.0 ng L(-1) (R=0.9996, n=6), a limit of detection of 5.67 ng L(-1), and relative standard deviation of 4.43% (n=10). The recovery test for Cr(3+) quantification in vitamin supplements presented results from 82% to 98%. These Cr(3+) determination results were compared to the same vitamin supplement sample using flame atomic absortion spectrometry (FAAS) method, and no significant differences were observed at 95% confidence level.