Gabriel Gustinelli Arantes de Carvalho

Optimization and validation of a LIBS method for the determination of macro and micronutrients in sugar cane leaves.

Laser induced breakdown spectrometry (LIBS) was applied for the determination of macro (P, K, Ca, Mg) and micronutrients (B, Cu, Fe, Mn and Zn) in sugar cane leaves, which is one of the most economically important crops in Brazil. Operational conditions were previously optimized by a neuro-genetic approach, by using a laser Nd:YAG at 1064 nm with 110 mJ per pulse focused on a pellet surface prepared with ground plant samples. Emission intensities were measured after 2.0 μs delay time, with 4.5 μs integration time gate and 25 accumulated laser pulses. Measurements of LIBS spectra were based on triplicate and each replicate consisted of an average of ten spectra collected in different sites (craters) of the pellet. Quantitative determinations were carried out by using univariate calibration and chemometric methods, such as PLSR and iPLS. The calibration models were obtained by using 26 laboratory samples and the validation was carried out by using 15 test samples. For comparative purpose, these samples were also microwave-assisted digested and further analyzed by ICP OES. In general, most results obtained by LIBS did not differ significantly from ICP OES data by applying a t-test at 95% confidence level. Both LIBS multivariate and univariate calibration methods produced similar results, except for Fe where better results were achieved by the multivariate approach. Repeatability precision varied from 0.7 to 15% and 1.3 to 20% from measurements obtained by multivariate and univariate calibration, respectively. It is demonstrated that LIBS is a powerful tool for analysis of pellets of plant materials for determination of macro and micronutrients by choosing calibration and validation samples with similar matrix composition.

Evaluation of laser induced breakdown spectrometry for the determination of macro and micronutrients in pharmaceutical tablets

The aim of this work is to demonstrate the feasibility of laser induced breakdown spectrometry (LIBS) for the determination of macro and micronutrients in multielement tablets. The experimental setup was designed by using a laser Q-switch (Nd:YAG, 10 Hz, λ = 1064 nm) and the emission signals were collected by lenses into an optical fiber coupled to an echelle spectrometer equipped with a high-resolution intensified charge coupled device (ICCD). Tablets were cryogenically ground and thereafter pelletized before LIBS analysis. Calibration curves were made by employing samples and mixtures of commercial multielement tablets with binders at different ratios. Best results were achieved by using the following experimental conditions: 29 J cm−2 laser fluence, 165 mm lens to sample distance (f = 200 mm), 2.0 μs delay time, 5.0 μs integration time and 5 accumulated laser pulses. In general, the results obtained by the proposed LIBS procedure were in agreement with those obtained by ICP OES from the corresponding acid digests and coefficients variation of LIBS measurements varied from 2 to 16%. The metrological figures of merit indicate that LIBS fits for the intended purposes, and can be recommended for the analysis of multielement tablets and similar matrices aiming the determination of Ca, Cu, Fe, Mg, Mn, P and Zn.

Evaluation of grinding methods for pellets preparation aiming at the analysis of plant materials by laser induced breakdown spectrometry

It has been demonstrated that laser induced breakdown spectrometry (LIBS) can be used as an alternative method for the determination of macro (P, K, Ca, Mg) and micronutrients (B, Fe, Cu, Mn, Zn) in pellets of plant materials. However, information is required regarding the sample preparation for plant analysis by LIBS. In this work, methods involving cryogenic grinding and planetary ball milling were evaluated for leaves comminution before pellets preparation. The particle sizes were associated to chemical sample properties such as fiber and cellulose contents, as well as to pellets porosity and density. The pellets were ablated at 30 different sites by applying 25 laser pulses per site (Nd:YAG@1064 nm, 5 ns, 10 Hz, 25J cm(-2)). The plasma emission collected by lenses was directed through an optical fiber towards a high resolution echelle spectrometer equipped with an ICCD. Delay time and integration time gate were fixed at 2.0 and 4.5 μs, respectively. Experiments carried out with pellets of sugarcane, orange tree and soy leaves showed a significant effect of the plant species for choosing the most appropriate grinding conditions. By using ball milling with agate materials, 20 min grinding for orange tree and soy, and 60 min for sugarcane leaves led to particle size distributions generally lower than 75 μm. Cryogenic grinding yielded similar particle size distributions after 10 min for orange tree, 20 min for soy and 30 min for sugarcane leaves. There was up to 50% emission signal enhancement on LIBS measurements for most elements by improving particle size distribution and consequently the pellet porosity.

Comparison of analytical performance of benchtop and handheld energy dispersive X-ray fluorescence systems for the direct analysis of plant materials

The analytical performances of benchtop and handheld energy dispersive X-ray fluorescence spectrometry (EDXRF) systems were evaluated aiming at the direct and simultaneous determination of P, K, Ca, S, Fe, Mn, and Si in plant materials. Pressed pellets of comminuted leaves from 23 varieties of sugar cane were used as laboratory samples. Both systems presented similar figures of merit, and were able to provide useful data for plant nutrition diagnosis. Linear correlation between the elemental mass fractions in the test samples and characteristic X-ray intensities was obtained for all analytes with both types of equipment. Correlation coefficients from 0.9601 to 0.9918 and from 0.9094 to 0.9948 were attained for benchtop and handheld EDXRF, respectively. The coefficient of variation of measurements carried out in 3 different test samples was also appropriate, being lower than 13% for all analytes. Limits of detection were comparable for both systems (20 mg kg−1 for Fe and Mn, and approximately 0.1 g kg−1 for P, K, Ca, S and Si) and permit the evaluation of the mineral nutrition status of sugar cane crop taking into account the critical levels of these elements. The handheld system is a cost-effective and appealing option for those who intend to carry out faster in situ and laboratory analysis with equivalent performance of the benchtop equipment.

Total and inorganic mercury determination in fish tissue by flow injection cold vapour atomic fluorescence spectrometry.

A simple and reliable method for Hg determination in fish samples has been developed. Lyophilised fish tissue samples were extracted in a 25% (w/v) tetramethylammonium hydroxide (TMAH) solution; the extracts were then analysed by FI-CVAFS. This method can be used to determine total and inorganic Hg, using the same FI manifold. For total Hg determination, a 0.1% (w/v) KMnO4 solution was added to the FI manifold at the sample zone, followed by the addition of a 0.5% (w/v) SnCl2 solution, whereas inorganic Hg was determined by adding a 0.1% (w/v) L-cysteine solution followed by a 1.0% (w/v) SnCl2 solution to the FI system. The organic fraction was determined as the difference between total and inorganic Hg. Sample preparation, reagent consumption and parameters that can influence the FI-CVAFS performance were also evaluated. The limit of detection for this method is 3.7 ng g−1 for total Hg and 4.3 ng g−1 for inorganic Hg. The relative standard deviation for a 1.0 µg L−1 CH3Hg standard solution (n = 20) was 1.1%, and 1.3% for a 1.0 µg L–1 Hg2+ standard solution (n = 20). Accuracy was assessed by the analysis of Certified Reference Material (dogfish: DORM-2, NRCC). Recoveries of 99.1% for total Hg and 93.9% inorganic Hg were obtained. Mercury losses were not observed when sample solutions were re-analysed after a seven day period of storage at 4°C.

Direct analysis of plant leaves by EDXRF and LIBS: microsampling strategies and cross-validation

Microsampling strategies were evaluated for the direct analysis of dried sugar cane leaves by energy-dispersive X-ray fluorescence spectrometry (EDXRF) and laser-induced breakdown spectroscopy (LIBS). The analysis by EDXRF was carried out by irradiating each leaf fragment in its middle portion with a collimated 5 mm X-ray spot size for 50 s, allowing the determination of P, K, Ca, S, Fe, Mn and Si. EDXRF was also useful to conclude that 15 leaf fragments (37.5% of the recommended sampling area) were enough for attaining a representative analytical response from the whole diagnostic leaf. Regarding LIBS, which employs a substantial smaller ablation area (i.e., 750 µm laser spot size), sampling strategies were defined by taking into account the microchemical distribution of P, Ca, Mg, Fe, Mn, B and Si in a 9 mm × 9 mm leaf fragment area. The proposed sampling protocol relied on the interrogation (rastering) of 3 equally spaced sampling lines in each leaf fragment with 48 accumulated laser pulses per line (Nd:YAG at 1064 nm, 5 ns, 10 Hz, 50 J cm−2) perpendicular to the leaf midrib. This strategy enabled the simultaneous determination of P, K, Ca, Mg, Fe, Cu, Mn, Zn, B and Si by LIBS. Cross-validation between LIBS and EDXRF for P, K, Ca, Fe, Mn and Si predicted mass fractions presented high linear correlation coefficients of up to 0.9778 (selecting 15 leaf fragments per diagnostic leaf from 10 different sugar cane varieties). The results provide insights into a novel and promising strategy for direct and fast plant nutrition diagnosis, fostering further studies for in situ analysis of fresh leaves, strengthening the implementation of Precision Agriculture and Green Chemistry concepts.

Evaluation of calcium alginate beads for Ce, La and Nd preconcentration from groundwater prior to ICP OES analysis

Analytical methods for the determination of rare earth elements (REE) in natural waters by plasma spectrochemical techniques often require sample preparation procedures for analytes preconcentration as well as for removing matrix constituents, that may interfere on the analytical measurements. In the present work, calcium alginate (CA) beads were used for the first time aiming at Ce, La and Nd preconcentration from groundwater samples for further determination by inductively coupled plasma optical emission spectrometry (ICP OES). Test samples were analyzed in batch mode by transferring a 40mL test portion (pH=5±0.2) into a 50mL polyethylene flask containing 125mg CA beads. After 15min contact, the analytes were quantitatively extracted from the loaded CA beads with 2.0mL of 1.0molL-1 HCl solution for further determination by ICP OES, using Ce (II) 456.236, La (II) 379.478 and Nd (II) 430.358nm emission lines. The proposed approach is a reliable alternative for REE single-stage preconcentration from aqueous samples, as it provided accurate results based on the addition and recovery analysis of groundwater. The results obtained by the proposed method were also compared with those from reference method based on inductively coupled plasma mass spectrometry (ICP-MS) and no significant differences were observed after applying the Students t-test at 95% confidence level.

Determination of europium isotope ratios in natural waters by MC-ICP-MS

An analytical protocol for the accurate and precise determination of the europium isotope ratio in low level natural waters is presented for the first time by using multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Parameters affecting the separation of Eu from the matrix by employing a single-step column separation with di(2-ethylhexyl) orthophosphoric acid (HDEHP) resin were investigated in detail. Matrix elements (e.g., alkali, alkali-earth and transition metals) were efficiently removed and Eu was quantitatively separated from other rare earth elements (REEs) by sequential elution using gradient HCl concentrations (from 0.25 to 0.75 mol L−1). Fractionation of Eu isotopes in the column was not observed, since quantitative europium recovery (101.2 ± 0.5% mean and 1 SD, n = 3) was obtained (data based on 150 mL SLRS-6 river water CRM test portions spiked with 1.0 ng mL−1 Eu standard solution). A combined standard-sample bracketing with internal normalization method was employed for the mass bias correction, wherein Gd was added to both the sample and standard as an internal standard. The developed method was validated by processing 0.02 ng mL−1 Eu spiked river water CRM SLRS-6 (wherein Eu was removed first by using the above-mentioned column separation method) using the proposed method, and satisfying results (δ151/153Eu value of 0.01 ± 0.02‰; 1 SD) relative to Eu standard solution were obtained. No significant differences were found in the δ151/153Eu values in river water SLRS-6, drinking water AQUA-1 and collected groundwater samples from Brazil, which ranged from −0.07 ± 0.02 to 0.12 ± 0.24 (mean ± 1 SD, n = 5).

Calcium alginate microparticles for rare earth elements preconcentration prior to ICP-MS measurements in fresh water

This work presents a method for the single-stage preconcentration and simultaneous determination of 14 rare earth elements (REEs), La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, in fresh waters using calcium alginate (CA) microparticles as a substrate for further determination by single quadrupole inductively coupled plasma mass spectrometry (ICP-MS). On-line analyte preconcentration was achieved using a flow-injection system incorporating a column filled with CA microparticles (<180 μm size; 300 mg). The proposed procedure was validated for 100 mL test portions, providing an enrichment factor of 100. The method was applied for the analysis of tap water and river water test samples, and validation was carried out with the analysis of the corresponding test samples spiked with 10 μg kg−1 REEs standard solution (La–Lu), wherein recoveries ranging from 90 to 102% were obtained. Coefficients of variation of REEs determinations were smaller than 4.0% (inter-column reproducibility, n = 3). The limit of detection of the method ranged from 0.01 to 0.03 ng kg−1 (achieved with a 100-fold pre-concentration). The procedure (sample loading, rinsing and elution) takes ca. 30 min per replicate. The proposed approach using CA columns is a reliable and low cost alternative for single-stage REEs preconcentration prior to ICP-MS measurements, and can be recommended for the analysis of river water and tap water samples. CA column re-usability was confirmed up to 50 adsorption–desorption cycles.

Recent advances in LIBS and XRF for the analysis of plants

The ability to provide a fast and multielemental analytical response directly from a solid sample makes both laser-induced breakdown spectroscopy (LIBS) and X-ray fluorescence spectrometry (XRF) very versatile tools for plant nutrition diagnosis. This review focuses on the main developments and advances in LIBS and XRF in the analysis of plant materials over the last ten years. Fundamental aspects and instrumentation are given for both techniques. The developments in the quantitative analysis of plant leaves are discussed, with special emphasis on the key aspects and challenges concerning field sampling protocols, sample preparation, and calibration strategies. Microchemical imaging applications by LIBS and XRF (including synchrotron radiation) are also presented in a broader selection of plant compartments (e.g., leaves, roots, stems, and seeds). Challenges, expectations and complementarities of LIBS and XRF towards plant nutrition diagnosis are thoroughly discussed.