Doehlert Design Research Articles

Multivariate optimization of a spectrophotometric method for copper determination in Brazilian sugar-cane spirits using the Doehlert design

This present work reports a multivariate optimization of a spectrophotometric method for copper determination in sugar-cane spirits, exploring the reaction of Cu(II) with phenylfluorone (PF) in the presence of cetylpyridinium chloride (CPC) and Triton X-100. The optimization strategy was based on the application of a two-level full factorial design for the screening of significant variables followed by a Doehlert design to determine the optimum conditions. The highest sensitivity was achieved when the pH was 8.0 and the PF and TX-100 concentrations were 6.2×10−5 and 1.0×10−2molL−1, respectively. The optimized method presented a limit of detection of 3.4μgL−1 and a limit of quantification of 11μgL−1 and was applied in the determination of Cu in 17 samples. The obtained results were compared with those obtained by ICP OES and no statistical difference between the methods was observed.

Use of the response surface methodology for optimizing the action of mashing enzymes on wort reducing sugars of the Madjeru sorghum cultivar

A three factor Doehlert design was used to develop a statistical model to optimize the action of three commercial mashing enzymes (Hitempase 2XL, Bioglucanase TX and Brewers protease) on reducing sugars content of the worts of unmalted and malted Madjeru sorghum. The response surface methodology revealed that increasing amounts of Hitempase considerably increased reducing sugars content during mashing of unmalted and malted Madjeru sorghum grist to about 105.39 g/L and 132.25 g/L respectively. The milling process contributed to about 22 g/L and 54 g/L for the unmalted and malted mash types respectively. Increasing amounts of Bioglucanase was virtually insignificant, while for Brewers protease, reducing sugar yields rather decreased to nil for both the unmalted and malted mash types. Optimization of the concerted actions of the three enzymes for reducing sugars content of unmalted Madjeru sorghum mash gave a combination of 1995 U, 89.31 BGU and 28.86 mg for Hitempase, Bioglucanase and Brewers Protease respectively. This gave a maximal reducing sugars content of 108.78 g/L. This combination was 3000 U, 0 BGU and 49.69 mg for malted Madjeru sorghum mash, giving a maximal reducing sugars yield of 153.15 g/L. Key words: Response surface methodology, optimization, mashing enzymes,Madjeru, reducing sugars.

Multivariate optimization of the determination of zinc in diesel oil employing a novel extraction strategy based on emulsion breaking

This paper describes the extraction/pre-concentration of Zn from diesel oil and its determination by Flame Atomic Absorption Spectrometry (FAAS), proposed as a novel approach for these kinds of analyses and the multivariate optimization of the proposed procedure. The extraction of Zn is based on the emulsification of an aqueous solution containing Triton X-114 and HNO 3 with diesel oil samples followed by breaking of the emulsion by heating. The aqueous phase obtained after the emulsion breaking was collected and used for Zn quantification by FAAS. The methodology was optimized using a Doehlert design and the system variables were the concentrations of surfactant and HNO 3 in the solution employed in the emulsification and the temperature used in the emulsion breaking. The ratio between absorbance and the time required to break the emulsions was taken as response. Two sets of experiments, using different emulsifier agents, were run: the first one using Triton X-100 and the second one using Triton X-114. At optimized conditions, the emulsions were prepared by mixing 10 mL of diesel oil with 2 mL of a solution containing 5% w/v of Triton X-114 and 15% v/v of HNO 3 and broken by heating at 80 °C. The proposed analytical procedure was applied in the analysis of six real samples of diesel oil and a recovery test was carried out by spiking the samples with known amounts of Zn (25 and 50 μg L −1), added as organometallic oiled standard. Recovery percentages achieved in this test were between 92 and 109%.

Multiple response optimization for Cu, Fe and Pb determination in naphtha by graphite furnace atomic absorption spectrometry with sample injection as detergent emulsion

The present paper reports the optimization for Cu, Fe and Pb determination in naphtha by graphite furnace atomic absorption spectrometry (GF AAS) employing a strategy based on the injection of the samples as detergent emulsions. The method was optimized in relation to the experimental conditions for the emulsion formation and taking into account that the three analytes (Cu, Fe and Pb) should be measured in the same emulsion. The optimization was performed in a multivariate way by employing a three-variable Doehlert design and a multiple response strategy. For this purpose, the individual responses of the three analytes were combined, yielding a global response that was employed as a dependent variable. The three factors related to the optimization process were: the concentration of HNO 3, the concentration of the emulsifier agent (Triton X-100 or Triton X-114) in aqueous solution used to emulsify the sample and the volume of solution. At optimum conditions, it was possible to obtain satisfactory results with an emulsion formed by mixing 4 mL of the samples with 1 mL of a 4.7% w/v Triton X-100 solution prepared in 10% v/v HNO 3 medium. The resulting emulsion was stable for 250 min, at least, and provided enough sensitivity to determine the three analytes in the five samples tested. A recovery test was performed to evaluate the accuracy of the optimized procedure and recovery rates, in the range of 88–105%; 94–118% and 95–120%, were verified for Cu, Fe and Pb, respectively.

Biomonitoring of lead in Antarctic lichens using laser ablation inductively coupled plasma mass spectrometry

A method for direct determination of Pb by LA-ICP-MS in lichen samples collected in a heavily anthropogenically impacted area of Maritime Antarctica was developed. The lichen samples were cryogenically ground and digested with nitric acid and hydrogen peroxide. Lead determination by solution-based ICP-MS was validated by the use of 2 certified reference materials. Once the Pb concentrations were measured with adequate accuracy and precision, a method for its direct determination by LA-ICP-MS was developed. The laser ablation parameters were optimized by the use of a Doehlert design matrix. The final optimized conditions were: laser energy (60%), spot size (150 μm) and repetition rate (10 Hz). Lead concentration of the analyzed lichen samples were plotted against the intensity of the lead isotopes (208Pb, 207Pb and 206Pb) determined by LA-ICP-MS. Satisfactory linear correlation factors were obtained when 13C was used as internal standard. Lead distribution profiles in lichen thalli cross-sections were also obtained. By these results, LA-ICP-MS emerges as a potential analytical tool for Pb concentration estimation in lichen samples. Minimum amount of sample required, bioimaging capability, high analytical throughput, and minimization of waste generation are the major analytical features of this approach. The significant differences between the Pb concentration in the lichen samples from the control site (1.12 ± 0.05 mg kg−1) and from the impacted points (mean = 5.03 ± 0.57 mg kg−1) permitted qualify these organisms as good biomonitors. Elemental bioimaging of lichens demonstrated that the medulla region of lichen thalli is consistently the main atmospheric lead bioaccumulating lichen tissue.

Optimization of microwave cooking of courgette in terms of nutrient preservation and energy consumption

The main objective of this work was to establish the kinetics of spoilage of nutritional and organoleptic criteria of frozen courgette when using microwave and stir-frying cooking but also to evaluate the effect of these cooking methods on these criteria. An optimization of the microwave parameters was also achieved to get the best quality of the courgette after cooking with the lowest energy consumption. The raw and cooked courgettes were analyzed for the total phenols and vitamin C contents, texture, dry matter content and final surface temperature. A remarkable decrease in vitamin C and phenols contents from the first minutes of cooking was noticed for both cooking methods applied. Moreover, these micronutrients seemed to be better preserved at higher power levels with microwave cooking. In addition, a same texture was reached in microwave cooking in a shorter time compared to the conventional process. A two factors Doehlert design was used to find the optimal settings which ensured a cooked product with good nutritional values and energy saving. Adequate levels for the two factors (microwave power and cooking time), were found thanks to the previously established kinetics. The calculated responses given by the quadratic models are quite close to the experimental values. This study will be used in the future for the optimisation of microwave cooking of a mixture of vegetables.

Minimisation of instrumental noise in the acquisition of FT-NIR spectra of bread wheat using experimental design and signal processing techniques

Spectral resolution (R) and number of repeated scans (S) have a significant effect on the S/N ratio of Fourier transform-near infrared (FT-NIR) spectra, but the optimal values of these two parameters have to be determined empirically for a specific problem, considering separately both the nature of the analysed matrix and the specific instrumental setup. To achieve this aim, the instrumental noise of replicated FT-NIR spectra of wheat samples was modelled as a function of R and S by means of the Doehlert design. The noise amounts in correspondence to different experimental conditions were estimated by analysing the variance signals derived from replicate measurements with two different signal processing tools, Savitzky-Golay (SG) filtering and fast wavelet transform (FWT), in order to separate the "pure" instrumental noise from other variability sources, which are essentially connected to sample inhomogeneity. Results confirmed that R and S values leading to minimum instrumental noise can vary considerably depending on the type of analysed food matrix and on the different instrumental setups, and helped in the selection of the optimal measuring conditions for the subsequent acquisition of a wide spectral dataset.

Preconcentration/Cleanup Studies of Tin from Environmental Water Samples by Oxidized Multiwall Carbon Nanotubes Packed Column and its Determination by ETAAS

The use of multiwall carbon nanotubes (MWCNT) as an efficient solid extractor in preconcentration/cleanup studies for tin determination in water samples by electrothermal atomic absorption spectrometry (ETAAS) is proposed. In the proposed method, tin adsorption onto MWCNT was carried out by percolating the solution previously buffered (pH 4.79 with 0.24 mol L−1 acetic acid/acetate buffer) at 4.0-mL min−1 flow rate, followed by elution with 1.0 mL of 2.7 mol L−1 HNO3. Factors such as sample pH, preconcentration/cleanup flow rate, type and concentration of eluent, and buffer concentration were appraised and optimized from chemometric tools based on fractional factorial design and Doehlert design. A limit of detection of 0.73 μg L−1 and precision (n = 8) assessed as relative standard deviation of 8.6% and 7.0% for tin concentration of 8.0 and 43.0 μg L−1, respectively, were achieved. Foreign metallic ions (Ni2+, Pb2+, Co2+, Zn2+, Cd2+, Mn2+, and Fe3+) were checked as potential interferents, and no interference was observed up to an analyte/interference ratio of 1:10 (m/v). Direct tin determination by ETAAS in water samples containing high salt amount is drastically affected by background signal. However, previous cleanup of sample by MWCNT has promoted a significant improvement and makes the method useful for tin monitoring in water samples (mineral, lake, mine, and natural waters) by ETAAS. Quantitative recovery values ranging from 91.5% to 103.0% attested the applicability of the proposed preconcentration/cleanup for tin determination in water samples.

Preparation and characterization of PLGA nanoparticles containing demethylfruticulin A from Salvia corrugata Vahl. (Lamiaceae)

Demethylfruticulin A (SCO1) is the major diterpenoid component of the exudate produced by the trichomes of Salvia corrugata leaves. The activity of this new compound against Gram-positive pathogens [1] and the apoptosis effect on mammalian cell lines [2] were investigated in previous works. The aim of this study was to evaluate the possibility of incorporating this natural product into PLGA nanoparticles in order to improve its solubility in the aqueous medium and to favour the interaction with bacterial membrane. Nanoparticles were prepared by an emulsion-diffusion-evaporation method, using Poloxamer 407 as a stabilizer. A Doehlert design for two variables was used to study the influence of the stirring time and the final volume of water on the size (D) and the zeta potential (Z) of the nanoparticles. The obtained nanoparticles showed interesting features: small size (D=207±8nm), a negative zeta potential (Z=–14.0±0.4 mV), an increasable drug loading (3% w/w) and high encapsulation efficiency (95% w/w). They also showed a good stability after redispersion in water for 30 days. These preliminary results indicate that PLGA nanoparticles can be designed as a carrier for delivery of SCO1, possibly allowing this new compound to exceed the intrinsic resistance of some bacteria to several antibiotics and to carry out its potential antitumour activity.

Thick and Dense Anodic Oxide Layers Formed on Aluminum in Sulphuric Acid Bath

Thick and dense oxide layers were obtained on aluminium in sulphuric acid electrolyte. For this purpose, the methodology of experimental design was used. A three-variables Doehlert design (bath temperature, anodic current density, sulphuric acid concentration), was achieved. In order to maximize the growth rate and the density of the anodic oxide layer, optimum path study was conducted. Under the determined optimal anodizing conditions (5.7°C, 3 A·dm −2 , C sul =140 g·L −1 ), the estimated response values were 0.86 μm·min −1 and 3.12 g·cm −2 for growth rate and density, respectively. The morphology of optimum layer was examined by scanning electron microscopy (SEM). The compactness of the optimum anodic layer can be correlated with its morphology revealed by SEM observations.

Factorial and Doehlert Design Used as Optimization Procedures for the Direct Determination of Lead in Whole Blood Samples by Graphite Furnace Atomic Absorption Spectrometry

In this work, the direct determination of lead in whole blood samples by graphite furnace atomic absorption spectrometry is proposed. The samples were diluted 1:9 with HNO3 and Triton® X-100, both 0.2% v/v. Multivariate optimization was made to evaluate the pyrolysis and atomization temperatures and the use of a chemical modifier. The method allowed lead determination with a curve ranging from 0 to 6.0 μg dL−1. Recovery studies presented results from 88 to 109%. The LOD and the characteristic mass were 0.21 μg dL−1 and 7.4 pg, respectively. Intra- and inter-assay studies showed respective coefficients of variation of 4.2 and 8.9%.

Degradation of Prototype Pesticides Submitted to Conventional Water Treatment Conditions: The Influence of Major Parameters

The behavior of several pesticides in aqueous solution, namely bifenthrin, amethrin (pyrethroid insecticides), endosulfan and endosulfan sulfate (organochlorine pesticides), disulfoton, methyl pyrimiphos, and phorate (organophosphorus pesticides), submitted to the conditions typically employed in water treatment stations was investigated. Continuous pesticide depletion was monitored by solid-phase microextraction sampling followed by gas chromatography–mass spectrometry analysis. The influence of major parameters (sodium hypochloride concentration, solution pH, and exposure time to ultraviolet (UV) light) was, thus, adequately established via two complementary approaches: factorial (23, three variables—two levels) and Doehlert designs. Hence, the sodium hypochloride concentration and the solution pH produced distinct effects depending on the pesticide evaluated (for instance, acidic and basic media caused increasing rates of degradation for the organophosphorus/pyrethroid and organochlorine pesticides, respectively). Conversely, higher rates of degradation were achieved for all of the pesticides investigated when increased exposure times to UV radiation were employed. Finally, the exposure time to UV radiation that lead to complete degradation of disulfoton and endosulfan sulfate (organophosphorus and organochlorine pesticides, respectively) in aqueous media under ordinary conditions employed in water treatment stations was established; disulfoton and endosulfan sulfate were completely degraded after 10 and 40 h, respectively.

Optimization of a centrifugation and ultrasound-assisted procedure for the determination of trace and major elements in marine invertebrates by ICP OES

A three factor Doehlert design was applied to optimize the extraction of elements in marine invertebrate samples (oyster, mussel and clam) using centrifugation with diluted acids. The factors were: concentration of acid mixture (HCl + HNO 3 + H 3CCOOH; 1–3 mol L − 1 ), centrifugation time (10–40 min), and rotation velocity (6000–10,000 rpm). In order to evaluate the ultrasound effect on the extraction of trace metals, a portion of the certified oyster tissue sample (NIST 1556b; 0.250 g) was sonicated for 15 min and then the solution was centrifuged at the optimized conditions. The optimized procedure was then applied to nine marine invertebrate samples (oysters, clams and mussels) collected at Todos os Santos Bay, Bahia, Brazil. The optimized condition of the developed method was: 20 min of centrifugation at 9000 rpm using the acid mixture 1.0 mol L − 1 . The effect of sonication prior centrifugation was very variable, but it promoted a significant increase in the extraction (12 to 44%) of some trace elements. Moreover, the combination of the centrifugation with sonication offered other advantages, such as improvement of the precision and accuracy, both demonstrated using the oyster tissue reference sample. The results of major and trace elements in marine invertebrate samples showed that elements such us Cd and Cu in oyster samples and Zn in mussel and clam samples were above recommendations, suggesting that its consumption may impose health risks.

Microwave-assisted extraction at atmospheric pressure coupled to different clean-up methods for the determination of organophosphorus pesticides in olive and avocado oil

An effective extraction method was devised for the determination of organophosphorus pesticides (OPPs) in olive and avocado oil samples, using atmospheric pressure microwave-assisted liquid-liquid extraction (APMAE) and solid-phase extraction or low-temperature precipitation as clean-up step. A simple glass system equipped with an air-cooled condenser was designed as an extraction vessel. The pesticides were partitioned between acetonitrile and oil solution in hexane. Analytical determinations were carried out by gas chromatography-flame photometric detection and gas chromatography-tandem mass spectrometry, using a triple quadrupole mass analyzer, for confirmation purposes. Several factors influencing the extraction efficiency were investigated and optimized through fractional factorial design and Doehlert design. Under optimal conditions the recovery of pesticides from oil at 0.025 microg g(-1) ranged from 71% to 103%, except for fenthion in avocado oil, with RSDs < or = 13% (n=5). The LOQ for the entire method ranged from 0.004 to 0.015 microg g(-1). Finally, the proposed method was successfully applied to the extraction and determination of the selected pesticides in 20 commercially packed extra virgin olive oils and four commercially packed avocado oils produced in Chile. Detectable residues of different OPPs were observed in 85% of samples.

Combination of Fractional Factorial and Doehlert Experimental Designs in Biodiesel Production: Ethanolysis ofRaphanus sativusL. var.oleiferusStokes Oil Catalyzed by Sodium Ethoxide

Fodder radish—Raphanus sativus L. var. oleiferus Stokes—oil has been regarded as an interesting option to produce biodiesel, because the oil cannot be used for human consumption, the seeds have high oil content, and the cost of production is low. Furthermore, the plant has been used for green fertilization during the interval between harvests of other crops, due to its rapid development as well as its great ability to recycle nutrients. The content of free fatty acids in the crude oil is less than 0.5%, which makes it appropriate for basic catalyzed synthesis. However, basic catalyzed synthesis is very sensitive to the presence of water in the reaction environment. This study proposes the optimization of biodiesel synthesis using sodium ethoxide (sodium ethylate) as a catalyst, with the purpose of minimizing water formation during reaction, increase efficiency, and thus carry out transesterification in a single step. Ethanol was used instead of the methylic route, aiming at the production of an entirely renewable and environmentally preferable fuel. The experiments were proposed and carried out using a combination of fractional factorial design and Doehlert design, in order to allow an extensive study of the process variables with a minimum of experiments. Very intense levels of agitation and high temperatures proved to be inadequate to reach an effective reaction. At optimum conditions the ester content reached approximately 97.9%, which along with several other physical chemistry assays confirm the good quality of the product and that the synthesis of fodder radish crude oil can be performed in a single step efficiently.

Cyclodextrin-MEEKC for the analysis of oxybutynin and its impurities

The development of a cyclodextrin-MEEKC method for the analysis of oxybutynin and five related impurities is described. Experimental design strategies were applied in order to reach baseline separation of the compounds in a short analysis time. Mixture design made it possible to find the best composition for the microemulsion acting as pseudostationary phase, which was constituted by 89.1% 10 mM borate buffer pH 9.2, 1.7% n-heptane, 9.2% SDS/ n-butanol in 1:2 ratio. The addition of (2-hydroxypropyl)-β-cyclodextrin to the background electrolyte was found to improve analysis performance. A Doehlert design, for the factors cyclodextrin concentration and voltage, was carried out and Derringer desirability function led to the identification of 18 mM and 29 kV as the optimal values. Applying the optimum conditions, separation of all the compounds, including the enantiomers of impurity 1, was obtained in less than 12 min. The method was validated according to ICH guidelines for drug assay and determination of impurities and was applied to oxybutynin tablet analysis.

Application of Doehlert Design in the Optimization of Microwave-Assisted Extraction for Determination of Zinc and Copper in Cereal Samples Using FAAS

The present work describes the development of a microwave-assisted extraction procedure for the determination of zinc and copper in cereal samples employing flame atomic absorption spectrometry (FAAS). A Doehlert design was carried out in order to determine the optimum conditions for the procedure through response surface study. Three variables (irradiation power, time, and temperature) were regarded as factors in the optimization study. The working conditions were established as a compromise between optimum values found for each analyte. These values were 110.0 °C, 176.0 W, and 16.0 min, for temperature, irradiation power, and time, respectively. The accuracy of the optimized procedure was evaluated by analysis of certified reference materials. The procedure proposed was applied for the determination of copper and zinc in cereal samples.

Multivariate optimization and validation of an analytical methodology by RP-HPLC for the determination of losartan potassium in capsules

This paper describes the optimization and validation of an analytical methodology for the determination of losartan potassium in capsules by HPLC using 2 5-1 fractional factorial and Doehlert designs. This multivariate approach allows a considerable improvement in chromatographic performance using fewer experiments, without additional cost for columns or other equipment. The HPLC method utilized potassium phosphate buffer (pH 6.2; 58 mmol L −1)–acetonitrile (65:35, v/v) as the mobile phase, pumped at a flow rate of 1.0 mL min −1. An octylsilane column (100 mm × 4.6 mm i.d., 5 μm) maintained at 35 °C was used as the stationary phase. UV detection was performed at 254 nm. The method was validated according to the ICH guidelines, showing accuracy, precision (intra-day relative standard deviation (R.S.D.) and inter-day R.S.D values <2.0%), selectivity, robustness and linearity ( r = 0.9998) over a concentration range from 30 to 70 mg L −1 of losartan potassium. The limits of detection and quantification were 0.114 and 0.420 mg L −1, respectively. The validated method may be used to quantify losartan potassium in capsules and to determine the stability of this drug.

Determination of Phthalates and Adipate in Physiological Saline Solutions by Solid-Phase Microextraction and Gas Chromatography

This study describes an analytical methodology developed for determination of phthalate esters (di-n-butyl, butylbenzyl and di-2-ethylhexyl phthalate) and adipate (di-n-ethylhexyl adipate) by solid-phase microextraction (SPME) and gas chromatography in normal saline solutions and, in bi-distilled water used to reconstitute medicines. An 85-microm-polyacrylate SPME fiber was selected and used in all analyses. The parameters affecting extraction efficiency were simultaneously optimized, firstly by means of a two-level full factorial design including a center point and subsequently through a Doehlert design for two-variables. Satisfactory detection limits in the range of 0.18 to 0.75 microg L(-1) and excellent precision, with relative standard deviation values being lower than 15.1% (n = 6), were obtained. Relative sensitivities of between 72.3 and 124.3% were obtained.

Optimisation of the chemical generation of singlet oxygen (1O2, 1Δg) from the hydrogen peroxide-lanthanumiii catalytic system using an improved NIR spectrometer

A near-IR chemiluminescence spectrometer designed to study chemical sources of singlet oxygen ((1)O(2), (1)Delta(g)), was built by coupling a reactor compartment to a nitrogen-cooled Ge diode through a bundle of optical fibres. This device was used to optimise the generation of (1)O(2) from the hydrogen peroxide-lanthanum(iii) catalytic system. The reaction kinetics were studied with a 2(3)3(3)//12 screening experimental design comprising twelve experiments. The influence of six factors was examined: the nature of the lanthanum salt (hydroxide, oxide or nitrate) and its concentration (0.05 or 0.1 mol L(-1)), the pH value (5, 7 or 9), the concentration of H(2)O(2) (0.5, 1 or 2 mol L(-1)), the temperature (20 or 30 degrees C) and the concentration of EDTA (0 or 5 mmol L(-1)). Two responses were measured: the rate of H(2)O(2) disproportionation and the intensity of the luminescence of (1)O(2) at 1270 nm. The essential factor is the nature of the lanthanum salt since La(NO(3))(3) induces the disproportionation of H(2)O(2) about 60 x faster than La(2)O(3) or La(OH)(3). Other influencing factors are the pH value, the concentration of H(2)O(2), the temperature and the concentration of the lanthanum salt whereas the concentration of EDTA has no effect on the reaction. The catalytic activity of La(NO(3))(3) was then investigated in further detail by studying the influence of two factors (pH and [H(2)O(2)]) thanks to a Doehlert design.