Theil-Sen nonparametric regression technique on univariate calibration, inverse regression and detection limits.

This paper reports the combined use of the nonparametric Theil-Sen (TS) regression technique and of the statistics of Lancaster-Quade (LQ) concerning the linear regression parameters to solve typical analytical problems, like method comparison, calculation of the uncertainty in the inverse regression, determination of the detection limit. The results of this new approach are compared to those obtained with appropriate reference methods, using simulated and real data sets. The nonparametric Theil-Sen regression technique appears a new robust tool for the problems considered because it is free from restrictive statistical constraints, avoids searching for the error nature on x and y, which may require long analysis times, and it is easy to use. The only drawback is that the intrinsic nature of the method may lead to a possible enlargement of the uncertainty interval of the discriminated concentration and to the determination of larger detection limits than those obtainable with the commonly used, less robust, regression techniques.

Overall calibration procedure via a statistically based matrix-comprehensive approach in the stir bar sorptive extraction-thermal desorption-gas chromatography-mass spectrometry analysis of pesticide residues in fruit-based soft drinks.

Stir bar sorptive extraction (SBSE)-thermal desorption (TD) procedure combined with gas chromatography mass spectrometry (GC-MS) and the statistical variance component model (VCM) is applied to the determination of semi-volatile compounds including organochlorine and organophosphorus pesticides in various synthetic and real fruit-based soft drink matrices. When the matrix effects are corrected using isotopically labelled or non labelled internal standard, but matrix/calibration run-induced deviations are still present in the measurements, the adoption of a variance component model (VCM) in the quantitative analysis of various matrices via an overall calibration curve is successful. The method produces an overall calibration straight line for any analyte accounting for the uncertainty due to all the sources of uncertainty, namely matrix-induced deviations, calibration runs performed at different times, measurement errors. Small increases in the detection limits and in uncertainty in the concentration values obtained in the inverse regression face favourably the decrease in times and costs for routine analyses.

Variation of some fermentative sulfur compounds in Italian "millesime" classic sparkling wines during aging and storage on lees.

Fermentative sulfur compounds are recognized as strongly affecting wine aroma, mainly as off-flavors, but recently also as possible positive contributors to wine quality and typicity in still wines. Nevertheless, no evidence has been provided for the influence of these molecules on sparkling wine aroma, except for peculiar volatile thiols found in French Champagne. According to the traditional method, the second fermentation, occurring in sealed bottles, is the essence of the procedure. After this fermentation, sparkling wine is aged on yeast lees for a period ranging from a few months to several years so that yeast autolysis can take place. So far, no evidence is provided for the effect of yeast contact duration on the level of sulfur compounds. Following a HS-SPME/GC-MS method, 13 sulfur compounds, that is, ethylmercaptan, dimethyl sulfide, diethyl sulfide, dimethyl disulfide, diethyl disulfide, methyl thioacetate, ethyl thioacetate, 2-mercaptoethanol, 2-(methylthio)-1-ethanol, 3-(methylthio)-1-propanol, 4-(methylthio)-1-butanol, benzothiazole, and 5-(2-hydroxyethyl)-4-methylthiazole, were quantified in several Italian sparkling wines, produced according to the traditional method in two wineries from Trentino-South Tyrol, region. Additionally, the influence of winemaking technology differences and vintage effects on the evolution of the quoted sulfur compounds was considered. This investigation was carried out by coupling the HS-SPME data with those obtained by SPE method and relevant to other volatile compounds, which are considered as winemaking markers. This work furnished the first evidence of the effect of aging and lees contact at different storage temperatures on the levels of these analytes in sparkling wines. Significant increments were observed for dimethyl sulfide, diethyl sulfide, 2-(methylthio)-1-ethanol, 3-(methylthio)-1-propanol, and 4-(methylthio)-1-butanol during aging with a different variation slope possibly due to the remarkably different storage temperatures. No clear influence of lees contact duration was found for the majority of the sulfur compounds considered.

Effect of nitrogen supplementation and Saccharomyces species on hydrogen sulfide and other volatile sulfur compounds in shiraz fermentation and wine.

A Shiraz must with low yeast assimilable nitrogen (YAN) was supplemented with two increasing concentrations of diammonium phosphate (DAP) and fermented with one Saccharomyces cerevisiae and one Saccharomyces bayanus strain, with maceration on grape skins. Hydrogen sulfide (H(2)S) was monitored throughout fermentation, and a total of 16 volatile sulfur compounds (VSCs) were quantified in the finished wines. For the S. cerevisiae yeast strain, addition of DAP to a final YAN of 250 or 400 mg/L resulted in an increased formation of H(2)S compared to nonsupplemented fermentations (100 mg/L YAN). For this yeast, DAP-supplemented fermentations also showed prolonged formation of H(2)S into the later stage of fermentation, which was associated with increased H(2)S in the final wines. The S. bayanus strain showed a different H(2)S production profile, in which production was inversely correlated to initial YAN. No correlation was found between total H(2)S produced by either yeast during fermentation and H(2)S concentration in the final wines. For both yeasts, DAP supplementation yielded higher concentrations of organic VSCs in the finished wines, including sulfides, disulfides, mercaptans, and mercaptoesters. PCA analysis indicated that nitrogen supplementation before fermentation determined a much clearer distinction between the VSC profiles of the two yeasts compared to nonsupplemented fermentations. These results raise questions concerning the widespread use of DAP in the management of low YAN fermentations with respect to the formation of reductive characters in wine.

Effectiveness of isotopically labelled and non-isotopically labelled internal standards in the gas chromatography/mass spectrometry analysis of sulfur compounds in wines: use of a statistically based matrix comprehensive approach.

The effectiveness of isotopically and non-isotopically labelled internal standards in reducing matrix-induced effects is evaluated. The question is addressed in the quantitative analysis by gas chromatography/mass spectrometry (GC/MS) of dimethyl sulphide, chosen as a typical example of volatile sulphur compounds, in wine matrices. When matrix/run effects are not cancelled out the use of a variance component model (VCM) to handle the linear calibrations obtained by regression technique is successful. The method implies the estimation of an overall calibration straight line, which properly takes into account the uncertainty due to different matrices, the calibration run and the measurement error, making the use of an isotopically labelled internal standard not necessary. The obtained results show that the benefits of lowering times and costs for routine analyses compensate for the small increase in uncertainty in the concentration values obtained in the regression analysis and the slight increase in the detection limit.

Hyphenated gas chromatography-mass spectrometry analysis of 3-mercaptohexan-1-ol and 3-mercaptohexyl acetate in wine: Comparison with results of other sampling procedures via a robust regression.

This work describes a new purge and trap gas chromatography electron impact mass spectrometry (PT-GC-EIMS) method for quantifying 3-mercaptohexan-1-ol (3-MH) and 3-mercaptohexyl acetate (3-MHA), two molecules able to characterize some wines with their tropical scents. Firstly the experimental conditions of the purge and trap extraction (sample temperature, extraction time, trap temperature, flow rate) following a multivariate approach were optimized. Then the method through the construction of the calibration curves and the establishment of the detection limits was validated. The purge and trap procedure appears faster and more sensitive than both the headspace solid phase microextraction (HS-SPME) and the solid phase extraction (SPE) procedures, reaching detection limits for the two thiols closer to their sensory thresholds. Evidence of similar performances of the three sampling methods considered was gained comparing the results relevant to same wine samples. The Theil's regression method was used for purpose of comparison.

Quantitative determination of chlorophenols in leather by pressurized liquid extraction and liquid chromatography with diode-array detection.

Pressurized liquid extraction (PLE) with acetonitrile was used for the recovery of chlorophenols (4-chloro-3-methylphenol, 4-chloro-2-methylphenol, 2,4-dichlorophenol, 2-phenylphenol, 2,4,6-trichlorophenol, 2,3,4,6-tetrachlorophenol and pentachlorophenol) present as biocides in leather. After a single cycle PLE treatment, solutions underwent pre-concentration by evaporation of the solvent under vacuum and clean-up treatment with solid-phase extraction cartridges. Quantitative analysis of the target compounds was carried out by liquid chromatography with gradient elution and UV spectrophotometric detection at variable wavelength for the various analytes in the range 190-240 nm. Instrumental detection limits and operative detection limits in the real matrices were determined according to the Hubaux-Vos approach and to the US Environmental Protection Agency procedures. The detection limits for the seven analytes ranged from 10 to 70 microg kg(-1). Linearity was very good in the explored range (10(-7)-10(-5)M) giving R(2) values from 0.995 to 1.000 for pentachlorophenol and 2,4-dichlorophenol, respectively. Repeatability was satisfactory, 2-5% for a 1 x 10(-6)M level of concentration, on five repeated measurements on the sample. Recovery yield values with the proposed procedure were determined using spiked samples. Overall recovery ranged from 88 to 97%. The method was used for routine analysis of real leather samples.

Aging effects and grape variety dependence on the content of sulfur volatiles in wine.

Thirteen sulfur compounds (boiling points from 35 to 231 degrees C), usually considered as possible off-flavoring volatiles, were quantified by a concurrent headspace-solid phase microextraction method coupled with gas chromatography-mass spectrometry (HS-SPME/GC-MS) on 80 not off-flavoring wines of four varieties (Merlot, Marzemino, and Teroldego as red wines and Chardonnay as a white one) and of five vintages produced in the North Italian Trentino region. The results of the research, the first Italian data-bank per variety on such volatiles, allow us to make a comparison with the data of other winegrowing areas, to investigate the aging effect on the considered volatiles, and, finally, to try a variety discrimination using statistical procedures. Dimethyl sulfide, 3-(methylthio)-1-propanol, diethyl sulfide, and diethyl disulfide were found to increase with time whereas 2-mercaptoethanol and ethylmercaptan showed a decreasing trend. Furthermore, the concentration of several compounds was found to be dependent on the variety. For instance, sulfide, disulfides, benzothiazole, and thioalcohols are at higher levels in Merlot wines, whereas thiols and thioacetates are more abundant in Marzemino and Teroldego wines. Chardonnay products, well apart from the other wines, are the poorest in 3-(methylthio)-1-propanol and rather rich in dimethyl disulfide and in diethyl disulfide, mostly in the aged wines. Applying the principal component analysis to the data, it was possible to demonstrate that Chardonnay and Merlot wines are well-discriminated from the Italian native varietal wines, which on their turn are only partially distinguishable among them. A contribution of these compounds to the variety characteristics of wine is reasonable.

Gas chromatography-mass spectrometry determination of 3-mercaptohexan-1-ol and 3-mercaptohexyl acetate in wine: a comparison of headspace solid phase microextraction and solid phase extraction methods.

A gas chromatography-mass spectrometry method was established using headspace solid phase microextraction (HS-SPME) as the sampling procedure to analyse 3-mercaptohexan-1-ol (3-MH) and 3-mercaptohexyl acetate (3-MHA), two molecules with a tropical fruit aroma, in wine at trace level. This method offers important advantages, as it neither requires time-consuming sample preparation nor uses dangerous organic compounds, thus making control of wine aroma easier and suitable for routine analysis. As a comparison, a solid phase extraction (SPE) method, already described elsewhere for aroma analysis, was applied to quantify these analytes, extending this exhaustive enrichment to two important thiols. Detection limits for both the approaches were close to the sensory threshold value, resulting lower for the HS-SPME procedure and suitable for requirements in the oenological field. The application of the two proposed methods to 52 wines of different varieties gave similar results.

Concurrent quantification of light and heavy sulphur volatiles in wine by headspace solid-phase microextraction coupled with gas chromatography/mass spectrometry.

A new method based on headspace solid-phase microextraction coupled with gas chromatography/mass spectrometry (HS-SPME-GC/MS) to analyse 13 light and heavy volatile sulphur compounds in the same run was established. For the successful application of the procedure, various adsorption process parameters were optimised. In particular the nature of the adsorptive phase, the temperature, the ionic strength of the sample solutions and the equilibration time were considered. The best extraction conditions, in terms of the maximum signal obtainable for each compound, were obtained with a carboxen-polydimethylsiloxane-divinylbenzene (CAR-PDMS-DVB) 2 cm long coating fibre. The choice of suitable internal standards and the matrix effect were studied and the proposed method was validated by determining linearity, precision and accuracy, evaluating the critical, detection and quantification limits. This method is fast, sensitive and precise and easy to transfer to wine quality control. Finally, the proposed method was applied to the determination of the aforementioned sulphur compounds in 32 red and white wines.

A statistical overview on univariate calibration, inverse regression, and detection limits: Application to gas chromatography/mass spectrometry technique.

The paper summarizes critically the current approaches for the calculation of the limits of detection and quantification. In the context of the description of the method based on the calibration line, the arguments concerning the underlying experimental design, the choice of the appropriate model in the univariate regression, the effects of the dispersion characteristics of the data are deeply discussed. The effects of the scedasticity of the experimental data are taken into account in the obtainment of the calibration curve and in its utilization. To gain transparency, adaptability, and tutorial effectiveness the explicit formulas relevant to the use of straight line and quadratic models are reported. An application of the described procedures to GC-MS data is reported as an illustrative example.

Electrocatalytic oxidation of NADH at single-wall carbon-nanotube-paste electrodes: kinetic considerations for use of a redox mediator in solution and dissolved in the paste.

Cyclic voltammetry has been successfully used to study the oxidation of nicotinamide adenine dinucleotide (NADH) at single-wall carbon-nanotube-paste (CNTP) electrodes modified with p-methylaminophenolsulfate (p-MAP) and 3,4-dihydroxybenzaldehyde (3,4-DHB). Diffusion-like behaviour was observed for p-MAP-modified electrodes, and a diffusion coefficient of 2.4x10(-6) cm2 s(-1) was calculated for p-MAP in the paste. The behaviour of 3,4-DHB-modified CNTP electrodes was typical of that of surface-confined mediators. p-MAP electrocatalytic activity was first checked in solution, and a rate constant of 9.2 mol(-1) L s(-1) was obtained for the reaction between NADH and the mediator. The p-MAP-modified electrode did not have significant electrocatalytic activity for electro-oxidation of NADH, probably because of the formation of a complex between NADH and the confined mediator. In contrast, the 3,4-DHB-modified electrode had very good NADH electrocatalytic activity, with a heterogeneous rate constant of approximately 20x10(2) mol(-1) L s(-1).

A comparison between the use of a redox mediator in solution and of surface modified electrodes in the electrocatalytic oxidation of nicotinamide adenine dinucleotide.

Cyclic voltammetry was successfully applied to study the oxidation of nicotinamide adenine dinucleotide (NADH) both in homogeneous and heterogeneous phase. The first case was realized with a solution containing p-methylamino-phenolsulphate (MAP) as redox mediator and the diaphorase (DI) from Clostridium kluveri as enzyme while the second one by using both a glassy carbon (GC) and a carbon nanotube paste (CNTP) electrode modified with electrodeposited films derived from 3,4-dihydroxybenzaldehyde (3,4-DHB). Such systems were successively coupled with glucose dehydrogenase (GDH) reaction to realize the redox chain present in glucose biosensors. A critical comparison of the two systems was also reported.

Non-linear and non-constant variance calibration curves in analysis of volatile organic compounds for testing of water by the purge-and-trap method coupled with gas chromatography/mass spectrometry.

Currently used operating conditions for analysing volatile organic compounds (VOCs) by purge-and-trap gas chromatography/mass spectrometry (GC/MS) produced non-linear calibration curves with non-uniform variance. Second-order polynomial models therefore had to be used in weighted regression analysis of measurements of replicates spiked at various concentrations. A transparent procedure based on a reported method of very low computational complexity allowed calculation of the parameters of second-order models, confidence bands of regression lines, prediction bands, and confidence intervals of discriminated analyte concentrations. Tolerance intervals were introduced for this last purpose. Critical, detection and quantification levels drawn from the calibration curves were compared with those calculated by the EPA method.

Identification and semi-quantitative determination of gum Arabic in wines by GC-MS and size exclusion chromatography.

A method for the identification and the semi-quantitative determination of the food additive gum Arabic in wines is described. Tests carried out on solutions spiked with known amounts of wine and gum Arabic polysaccharides allowed to define the suitable conditions for their quantitative precipitation and size exclusion analysis. CG-MS analyses of the different recovered fractions allowed to discriminate between gum Arabic and wine polysaccharides through the identification of glucose and mannose present only in wine polysaccharides. The proposed method was based on the wine polysaccharides free peak area obtained by size exclusion chromatography. The same cut-off time was always used both in the preparation of the calibration plot and in the analysis of the real samples. Gum Arabic was determined in a ratio of 1/10 w/w with wine polysaccharides with a detection limit of 0.074 mg ml(-1) which is lower than the lowest gum Arabic amount usually added into wines. Owing to the moderately low natural variability of the gum Arabic standards the described procedure is suitable for a semi-quantitative analysis even if its accuracy allowed a quite reliable determination of the gum Arabic amount usually added to wine.

Global migration from styrenics to liquid food simulants.

Chemical inertness of typical styrene polymers used in food packaging was investigated by "global migration" measurements. Absorption of liquid food simulants by plastics was also studied. Migrated compounds, detected and identified by gaschromatography-mass spectrometry (GC-MS), are styrene (monomer) and styrene dimers. All migration values, measured under different experimental conditions, turn out to be lower than the overall migration limit, as fixed by current legislation.

The use of cyclic voltammetric measurements for the determination of kinetic constants in enzyme-based electrocatalysis. The case of the redox mediated dihydronicotinamide adenine dinucleotide oxidation catalyzed by flavoproteins.

On the basis of simulated voltammetric curves used as testing data, practical methods to extract the bimolecular rate constant for a mediated enzymatic catalysis with an excess of the substrate are examined. The evaluation criteria are the simplicity of treatment and the agreement between the expected and the obtained values of the rate constants. The results obtained indicate that the different approaches examined are equivalent.