Evaluation and validation of detailed and simplified models of the uncertainty of unified [Formula: see text] measurements in aqueous solutions.

The use of the unified pH concept, [Formula: see text] , applicable to aqueous and non-aqueous solutions, which allows interpreting and comparison of the acidity of different types of solutions, requires reliable and objective determination. The [Formula: see text] can be determined by a single differential potentiometry measurement referenced to an aqueous reference buffer or by a ladder of differential potentiometric measurements that allows minimisation of inconsistencies of various determinations. This work describes and assesses bottom-up evaluations of the uncertainty of these measurements, where uncertainty components are combined by the Monte Carlo Method (MCM) or Taylor Series Approximation (TSM). The MCM allows a detailed simulation of the measurements, including an iterative process involving in minimising ladder deviations. On the other hand, the TSM requires the approximate determination of minimisation uncertainty. The uncertainty evaluation was successfully applied to measuring aqueous buffers with pH of 2.00, 4.00, 7.00, and 10.00, with a standard uncertainty of 0.01. The reference and estimated values from both approaches are metrologically compatible for a 95% confidence level even when a negligible contribution of liquid junction potential uncertainty is assumed. The MCM estimated pH values with an expanded uncertainty, for the 95% confidence level, between 0.26 and 0.51, depending on the pH value and ladder inconsistencies. The minimisation uncertainty is negligible or responsible for up to 87% of the measurement uncertainty. The TSM quantified measurement uncertainties on average only 0.05 units larger than the MCM estimated ones. Additional experimental tests should be performed to test these uncertainty models for analysis performed in other laboratories and on non-aqueous solutions.

Seawater pH measurements with a combination glass electrode and high ionic strength TRIS-TRIS HCl reference buffers - An uncertainty evaluation approach.

The comparison of pH measurements in seawater collected at different locations or occasions, is meaningful if the same measurand (i.e. the quantity intended to be measured) is determined, if adequate measurement procedures are used, including the selection of calibrators, and if the measurement uncertainty is known. Depending on the purpose of this evaluation, the measurement uncertainty should be smaller than a defined target value. The measured pH should have a sound physical-chemical meaning to allow the adequate assessment of its impacts. In the present procedure TRIS-TRIS HCl solutions, of different molality ratios, prepared in artificial seawater with reference values estimated by primary measurements, were used to obtain proper calibrators for the pH meter used for the analysis of seawater samples. This work presents the uncertainty evaluation of pH measurements in seawater, performed by potentiometry using a combination glass electrode, from the interpolation uncertainty evaluated by the Least Squares Regression Model and by Monte Carlo Simulations of measured potentials and reference values. The uncertainty evaluation was critically assessed. The developed algorithms were implemented in a user-friendly MS-Excel file available as Electronic Supplementary Material. Seawater pH was measured with an expanded uncertainty of 0.019 enabling discriminating differences of pH of two samples larger than 0.029.

Metrological challenges for measurements of key climatological observables: Oceanic salinity and pH, and atmospheric humidity. Part 1: Overview.

Water in its three ambient phases plays the central thermodynamic role in the terrestrial climate system. Clouds control Earth's radiation balance, atmospheric water vapour is the strongest "greenhouse" gas, and non-equilibrium relative humidity at the air-sea interface drives evaporation and latent heat export from the ocean. On climatic time scales, melting ice caps and regional deviations of the hydrological cycle result in changes of seawater salinity, which in turn may modify the global circulation of the oceans and their ability to store heat and to buffer anthropogenically produced carbon dioxide. In this paper, together with three companion articles, we examine the climatologically relevant quantities ocean salinity, seawater pH and atmospheric relative humidity, noting fundamental deficiencies in the definitions of those key observables, and their lack of secure foundation on the International System of Units, the SI. The metrological histories of those three quantities are reviewed, problems with their current definitions and measurement practices are analysed, and options for future improvements are discussed in conjunction with the recent seawater standard TEOS-10. It is concluded that the International Bureau of Weights and Measures, BIPM, in cooperation with the International Association for the Properties of Water and Steam, IAPWS, along with other international organisations and institutions, can make significant contributions by developing and recommending state-of-the-art solutions for these long standing metrological problems in climatology.

Validation of atmospheric aerosols parallel sampling in a multifold device.

In this work, particulate matter was collected using an active sampling system consisting of a PM10 (<10 µm) inlet coupled to a multifold device containing six channels, connected to a vacuum pump. Each channel was equipped with a filter holder fitted with adequately chosen filters. The system was fixed on a metallic structure, which was placed on the roof of the laboratory building, at the Faculty of Sciences, in Lisbon. Sampling took place under flow-controlled conditions. Aerosols were extracted from the filters with water, in defined conditions, and the water-soluble fraction was quantified by ion chromatography (IC) for the determination of inorganic anions (Cl(-), NO3 (-) and SO4 (2-)). Equivalent sampling through the various channels was validated. Validation was based on the metrological compatibility of the content results for the various filters. Ion masses are metrologically equivalent when their absolute difference is smaller than the respective expanded uncertainty. When this condition is verified, the studied multifold device produces equivalent samples.

Designing valid and optimised standard addition calibrations: Application to the determination of anions in seawater.

A strategy for designing valid standard addition calibrations and for optimising their uncertainty is presented. The design of calibrations involves the development of models of the sensitivity and precision of the instrumental signal, in a wide range of analyte concentration (or any other studied quantity), and the definition of sample dilution and standard addition procedures that allow fulfilling the assumptions of the linear unweighted regression model in, typically, a smaller range of standard addition calibrations. Calibrators are prepared by diluting the sample and adding analyte with negligible uncertainty to fit in a concentration range where signals are homoscedastic. The minimisation of the uncertainty is supported on detailed measurement uncertainty models function of the calibrators preparation procedure and of analytical instrumentation performance. The number of collected signals replicates is defined by balancing their impact on the estimated expanded uncertainty, the resources needed and the target (maximum) uncertainty for the intended use of measurements. The calibration design strategy was successfully applied to the determination of the mass concentration (mg L(-1)) of Cl(-), Br(-), NO3(-) and SO4(-2) in seawater by ion chromatography. A target expanded uncertainty of 20% was defined for the determination of Cl(-), NO3(-) and SO4(-2), or 40% for the determination of the smaller mass concentration of Br(-). The developed measurement model produced reliable predictions of the measurement uncertainty from approximate concentration of the analyte in the sample, before its accurate quantification, thus proving optimisation is effective. Predictions are more prone to the variability of the measurement uncertainty estimation if based on low number of calibrators signals. The reported relative expanded uncertainty ranged from 7.1% to 49%.

Carotenoid stability in fruits, vegetables and working standards - effect of storage temperature and time.

The effects of freezing and storage temperature on the mass fraction of α- and ß-carotene, ß-cryptoxanthin, lutein, lycopene and zeaxanthin in minimally processed fresh food products, were evaluated after sample preparation, extraction and saponification (only when strictly necessary). Effects of freezing and long-term stability were studied at two temperatures, -20 and -70°C, using high performance liquid chromatography (reversed phase columns, UV-Vis diode array detector) at time points during storage; measurement uncertainty was included in the evaluation. Stability of working standard solutions was also examined. Freezing did not affect the carotenoid mass fraction under the conditions studied. Carotenoids in orange, cherry, peach, apple, and kale were stable (except α-carotene and zeaxanthin in peach) for 13, 9.7, 5.7, 2.5 and 7.5months, respectively. For these food sample matrices, no significant difference between the freezing/storage at -20 and -70°C was observed. Standard solutions (0.05-5µg/mL) were stable for at least 6months at -70°C, except lycopene which at 0.05µg/mL was apparently stable only for six weeks.

Weighted calibration with reduced number of signals by weighing factor modelling: application to the identification of explosives by ion chromatography.

The linear weighted regression model (LW) can be used to calibrate analytical instrumentation in a range of quantities (e.g. concentration or mass) wider than possible by the linear unweighted regression model, LuW (i.e. the least squares regression model), since this model can be applied when signals are not equally precise through the calibration range. If precision of signals varies within the calibration range, the regression line should be defined taking into account that more precise signals are more reliable and should count more to define regression parameters. Nevertheless, the LW requires the determination of the variation of signals precision through the calibration range. Typically, this information is collected experimentally for each calibration, requiring a large number of replicate collection of signals of calibrators. This work proposes reducing the number of signals needed to perform LW calibrations by developing models of weighing factors robust to daily variations of instrument sensibility. These models were applied to the determination of the ionic composition of the water soluble fraction of explosives. The adequacy of the developed models was tested through the analysis of control standards, certified reference materials and the ion balance of anions and cations in aqueous extracts of explosives, considering the measurement uncertainty estimated by detailed metrological models. The high success rate of the comparisons between estimated and known quantity values of reference solutions, considering results uncertainty, proves the validity of developed metrological models. The relative expanded measurement uncertainty of single determinations ranged from 1.93% to 35.7% for calibrations performed along 4 months.

Evaluation of the performance of the determination of anions in the water soluble fraction of atmospheric aerosols.

The knowledge of the mass of particulate matter in air, its chemical composition and emission sources is of relevance for taking decisions concerning air quality management in urban areas. The interpretation of these data is a function of the quality of the measurement results expressed by their uncertainties. This study aimed at developing models of the performance of the determination of anions in the water-soluble fraction of atmospheric aerosols, capable of determining, separately, the contribution of aerosols sampling, extraction of water-soluble fraction of atmospheric aerosols and quantification, by ion chromatography, of anions in the extract. The sampling procedure was assessed from the dispersion of results of duplicate parallel sampling after subtracting the analytical component of this dispersion. These models are used to evaluate the adequacy of the measurement procedure for the determination of urban aerosol composition and to support strategies for reducing measurement uncertainty or cost of analysis. The method performance was studied for the following ranges considering extract dilution up to five times: 0.23-8 µg m(-3) for chloride and nitrate, and 0.093-3.25 µg m(-3) for sulphate. Measurements are fit for the analysis of urban aerosols since the relative expanded measurement uncertainty is smaller than a maximum value of 40%. The percentage contribution of the uncertainty components varies with the analyte and its mass concentration, the major components being 24-93% for the extraction, 43-59% for sampling, 0.2-28% for the interpolation of the sample signal in the calibration curve and 4-8% for air volume measurement. The typical composition of analysed air is: (1.12±0.26) µg m(-3), (1.02±0.30) µg m(-3) and (0.76±0.22) µg m(-3) of chloride, nitrate and sulphate in the water soluble fraction of aerosol, respectively, for a confidence level of approximately 95% considering a coverage factor of 2.

Optimization of the determination of chemical oxygen demand in wastewaters.

Chemical oxygen demand (COD) is one of the most relevant chemical parameters for the management of wastewater treatment facilities including the control of the quality of an effluent. The adequacy of decisions based on COD values relies on the quality of the measurements. Cost effective management of the minor sources of uncertainty can be applied to the analytical procedure without affecting measurement quality. This work presents a detailed assessment of the determination of COD values in wastewaters, according to ISO6060:1989 standard, which can support reduction of both measurement uncertainty and cost of analysis. This assessment includes the definition of the measurement traceability chain and the validation of the measurement procedure supported on sound and objective criteria. Detailed models of the measurement performance, including uncertainty, developed from the Differential Approach, were successfully validated by proficiency tests. The assumption of the measurement function linearity of the uncertainty propagation law was tested through the comparison with the numerical Kragten method. The gathered information supported the definition of strategies for measurement uncertainty or cost reduction. The developed models are available as electronic supplementary material, in an MS-Excel file, to be updated with the user's data.

Role of physical activity and diet in incidence of hypertension: a population-based study in Portuguese adults.

BACKGROUND/OBJECTIVES: To evaluate longitudinally the role of physical activity (PA) (type and intensity) and diet (measured using DASH score, nutrients and food intake) in hypertension incidence. SUBJECTS/METHODS: As part of the EPIPorto study, 549 participants (≥40 years), resident in Porto, Portugal, at risk of developing hypertension, were evaluated. Blood pressure (BP) measurements were obtained twice (1999-2003 and 2005-2008), with a median interval of 3.8 years. Hypertension was defined as systolic BP ≥140 mm Hg and/or diastolic BP ≥90 mm Hg, and/or if the subjects were under anti-hypertensive therapy. Validated questionnaires were used to assess usual PA and dietary intake during the previous year. Poisson regression was used to calculate the incident rate ratios (IRRs) and respective 95% confidence intervals (95% CIs). RESULTS: In our population, the crude incidence rate (95% confidence interval (CI)) per 100 person-years of hypertension was 6.23 (5.26-7.20). After adjustment for several confounders, an inverse, though not significant, association was found across increasing tertiles of leisure-time PA and IRR (95% CI): 1 (reference); 0.77 (0.51-1.16); and 0.74 (0.48-1.11). No significant associations between the DASH score and hypertension incidence were observed. However, potassium intake (mg/1000 kcal) was shown to be inversely associated with hypertension development (upper tertile: >1863.0 for women and >1657.2 for men) vs first tertile (IRR=0.65 (0.44-0.96), P for trend=0.025). Additionally, in multivariate analysis, a significantly inverse association between the consumption of fruits/vegetables/pulses and hypertension incidence was found (upper vs first tertile: IRR=0.61 (0.40-0.93), P for trend=0.024). CONCLUSIONS: In Portuguese adults, after adjustment for several confounders, the development of hypertension was inversely associated with potassium and fruits/vegetables/pulses intake. A dose-response is inherent to these inverse associations.

Critical assessment of three high performance liquid chromatography analytical methods for food carotenoid quantification.

Three sets of extraction/saponification/HPLC conditions for food carotenoid quantification were technically and economically compared. Samples were analysed for carotenoids alpha-carotene, beta-carotene, beta-cryptoxanthin, lutein, lycopene, and zeaxanthin. All methods demonstrated good performance in the analysis of a composite food standard reference material for the analytes they are applicable to. Methods using two serial connected C(18) columns and a mobile phase based on acetonitrile, achieved a better carotenoid separation than the method using a mobile phase based on methanol and one C(18)-column. Carotenoids from leafy green vegetable matrices appeared to be better extracted with a mixture of methanol and tetrahydrofuran than with tetrahydrofuran alone. Costs of carotenoid determination in foods were lower for the method with mobile phase based on methanol. However for some food matrices and in the case of E-Z isomer separations, this was not technically satisfactory. Food extraction with methanol and tetrahydrofuran with direct evaporation of these solvents, and saponification (when needed) using pyrogallol as antioxidant, combined with a HPLC system using a slight gradient mobile phase based on acetonitrile and a stationary phase composed by two serial connected C(18) columns was the most technically and economically favourable method.

Detailed uncertainty budget for major and minor ions in stock combined calibration standards--influence of impurities in chemicals.

A general methodology for a systematic evaluation of the uncertainty was derived for each particular ion in stock combined calibration standards in which concentrations of different ions extend over up to five orders of magnitude resulting in detailed uncertainty budgets with the aim of recognising the major contributions to combined uncertainties. This work confirmed that it is justifiable that the mass fraction of impurities in other chemicals is taken into account when calculating the mass concentration of an ion in combined calibration standard solution similarly to what is already common practice in accounting for the purity of chemical. It was proven that impurities in chemicals which are sources of major ions have significant effect on uncertainty budget of minor ions; already if the major ion exceeds 25 times the minor ion's concentration. For several ions it was confirmed that mass fraction of the impurities was the major source of uncertainty.

Ionic composition of seawaters and derived saline solutions determined by ion chromatography and its relation to other water quality parameters.

Ion chromatography (IC) presents new possibilities for assessing information about environmental samples, namely waters of various compositions, ranging from high-purity water to highly saline ones. Constant proportion between major ions present in seawater, has been assumed in the past, from which the first practical equation relating chlorinity and salinity has been developed, being later substituted by a practical salinity scale, derived from conductivity measurements relative to a standard seawater, according to internationally accepted recommended procedures. Seawaters are characterized by salinity values around 35 while derived saline solutions may present considerable changes in ionic composition, conductivity, hence on salinity. Natural and anthropogenic phenomena may introduce new issues requiring clarification for which qualitative and quantitative information from additional sources is useful, e.g. ionic composition from IC. The different ranges of concentration of major and minor species present in seawater and derived saline solutions are a challenge for the optimization of a practical methodology for composition assessment in two single IC runs, one for anions and another one for cations, which has been attained in this work. Composition of saline solutions determined by IC was critically assessed in terms of anion-cation balance and further related to conductivity and salinity measurements aiming to evaluate the quality/completeness of ion chromatographic analyses performed at preselected conditions and to search for other meaningful relations for efficient recognition/distinction between saline solutions of different types.

[Factors associated with physical activity in the Portuguese population]. / Fatores associados à atividade física na população portuguesa.

OBJECTIVE: To evaluate the association between demographic, social and behavior characteristics and different types of physical activity. METHODS: A total of 37.692 subjects of a representative sample of the Portuguese population were studied as part of the National Health Survey in 1998 and 1999. Most were females (53.1%) aged>or=20 years. Daily physical activity was self-reported based on a questionnaire and discriminated in different types: total physical activity, leisure-time and exercise. Each type of physical activity was dichotomized into low (light/moderate) and high intensity (heavy/very heavy). Odds ratios (OR) and 95% confidence intervals were estimated using unconditional logistic regression. RESULTS: In both men and women, a significant inverse association between age and different types of physical activity and between obesity and leisure time physical activity and exercise was seen. A positive association was found between education (or=12 years) and leisure-time PA (OR 1; 1.58; 2.39 in females and 1; 1.44; 2.08 in males) and exercise (OR 1; 3.50; 9.77 in females and 1; 3.42; 7.61 in males) and an inverse association with total physical activity (OR 1; 0.65; 0.20 in females and 1; 0.47; 0.09 in males) was found. Regardless of age, single subjects were frequently more active. Alcohol consumption in both men and women and tobacco consumption in males were negatively associated with leisure-time physical activity. CONCLUSIONS: Youngest, normal weighted, single, non-drinkers subjects and non-smokers males were more likely to be physically active. In both men and women, there was a clear effect of education on the type of physical activity.

Fatores associados à atividade física na população portuguesa / Factors associated with physical activity in the Portuguese population

OBJETIVO: Avaliar como as características demográficas, sociais e comportamentais se associam a diferentes tipos de atividade física. MÉTODOS: Avaliaram-se 37.692 indivíduos de amostra representativa da população portuguesa, no âmbito do Inquérito Nacional de Saúde, 1998-99. A maioria era constituída por mulheres (53,1 por cento) e idade 20 anos. A avaliação da atividade física diária foi baseada em questionário e classificada como: total, de lazer e exercício. Cada tipo foi dicotomizado em baixa intensidade (atividades leves/moderadas) e alta intensidade (atividades pesadas/muito pesadas). Calcularam-se odds ratios (OR) e respectivos intervalos de confiança de 95 por cento por regressão logística não condicional. RESULTADOS: Em ambos os sexos, verificou-se associação inversa significativa entre idade e diferentes tipos de atividade física, e entre a obesidade e a atividade de lazer e exercício. A escolaridade (<4; 5-11; 12 anos) associou-se positivamente com a atividade física de lazer (OR 1; 1,58; 2,39 nas mulheres e OR 1; 1,44; 2,08 nos homens) e com o exercício (OR 1; 3,50; 9,77 nas mulheres e OR 1; 3,42; 7,61 nos homens) e de forma inversa com a AF total (OR 1; 0,65; 0,20 nas mulheres e 1; 0,47; 0,09 nos homens). Independentemente da idade, os solteiros eram frequentemente mais ativos. A atividade de lazer associou-se negativamente ao consumo de bebidas alcoólicas para ambos os sexos e nos homens, ao consumo de tabaco. CONCLUSÕES: Os jovens, normoponderais, solteiros, não-bebedores e os homens não-fumadores apresentaram maior probabilidade de serem fisicamente ativos. Em ambos os sexos, observou-se um efeito diferencial da escolaridade segundo os tipos de atividade física.

OBJECTIVE: To evaluate the association between demographic, social and behavior characteristics and different types of physical activity. METHODS: A total of 37.692 subjects of a representative sample of the Portuguese population were studied as part of the National Health Survey in 1998 and 1999. Most were females (53.1 percent) aged >20 years. Daily physical activity was self-reported based on a questionnaire and discriminated in different types: total physical activity, leisure-time and exercise. Each type of physical activity was dichotomized into low (light/moderate) and high intensity (heavy/very heavy). Odds ratios (OR) and 95 percent confidence intervals were estimated using unconditional logistic regression. RESULTS: In both men and women, a significant inverse association between age and different types of physical activity and between obesity and leisure time physical activity and exercise was seen. A positive association was found between education (<4; 5-11; 12 years) and leisure-time PA (OR 1; 1.58; 2.39 in females and 1; 1.44; 2.08 in males) and exercise (OR 1; 3.50; 9.77 in females and 1; 3.42; 7.61 in males) and an inverse association with total physical activity (OR 1; 0.65; 0.20 in females and 1; 0.47; 0.09 in males) was found. Regardless of age, single subjects were frequently more active. Alcohol consumption in both men and women and tobacco consumption in males were negatively associated with leisure-time physical activity. CONCLUSIONS: Youngest, normal weighted, single, non-drinkers subjects and non-smokers males were more likely to be physically active. In both men and women, there was a clear effect of education on the type of physical activity.

Uncertainty estimation and in-house method validation of HPLC analysis of carotenoids for food composition data production.

The method for separation and quantitative determination of the main carotenoids in food by high-performance liquid chromatography (HPLC) was in-house validated. Tomato (Lycopersicon esculentum M.) as food matrix was used to demonstrate its linearity, repeatability, intermediate precision, detection and quantification limits, sensitivity and bias. In addition, stability of carotenoids was studied in function of temperature and time. Method accuracy was quantified through measurement uncertainties estimate based on this validation study. Furthermore, a study was conducted to evaluate variability coming from location in an experimental field composed by 12 subfields. The use of two metal free reverse phase columns and an organic mobile phase based on acetonitrile, methanol and dichloromethane enabled the separation of the six target compounds (trans-α-carotene, trans-ß-carotene, ß-cryptoxanthin, all-lycopene, lutein, zeaxanthin) within a 30min run; detection at 450nm and external calibration allowed the quantification of the analytes. Carotenoids concentration and measurement uncertainty, in mg/100g, in tomato varieties "lido" and "for salad" were, respectively, 1.0±0.14 and 0.39±0.056 for trans-ß-carotene, 8±2.0 and 2.3±0.57 for all-lycopene and 0.10±0.017 and 0.08±0.015 for lutein; trans-α-carotene, ß-cryptoxanthin and zeaxanthin were not detected in both varieties (detection limits, in µg/100g, 0.81, 0.57 and 0.77, respectively). For ß-carotene and lutein, uncertainty associated with the entire process including small-scale within-region variation was statistically different, at a significance level of 5%, from measurement uncertainty (which includes sampling in the laboratory).

Effect of condensation phenomena on potentiometric measurements.

Results of potentiometric analysis, namely those of pH measurements, depend on temperature control of the experimental setup, as it is expressed in the analytical law, the Nernst equation, starting from the primary level, where reference values are conventionally assigned to standard solutions, through the whole traceability chain, down to the service laboratory. Fundamental studies of pH standards, based on the measurement of the potential of an electrochemical cell without transference, known as Harned cell, containing a platinum-hydrogen electrode and a silver-silver chloride reference electrode, refer condensation phenomena on the portions of the cell walls which are not immersed in the thermostatic bath, as one of the major sources of error in the assessment of both the silver-silver chloride electrode standard potential and on pH values. In this work such effect, which is bound to happen due to significant temperature differences between the ambient air temperature and the water bath, has been quantified, presenting an original contribution to the improvement of the quality of potentiometric analysis results. This was possible due to the availability of a climatic cabin "WALKIN" with a temperature control of +/-0.01 degrees C, which permitted that temperature gradients were built between the thermostat water bath (controlled to +/-0.005 degrees C) where cells filled to about 2/3 full were immersed up to 90% of their height, and the surrounding environment.

Worst case uncertainty estimates for routine instrumental analysis.

A methodology for the worst case measurement uncertainty estimation for analytical methods which include an instrumental quantification step, adequate for routine determinations, is presented. Although the methodology presented should be based on a careful evaluation of the analytical method, the resulting daily calculations are very simple. The methodology is based on the estimation of the maximum value for the different sources of uncertainty and requires the definition of limiting values for certain analytical parameters. The simplification of the instrumental quantification uncertainty estimation involves the use of the standard deviation obtained from control charts relating to the concentrations estimated from the calibration curves for control standards at the highest calibration level. Three levels of simplification are suggested, as alternatives to the detailed approach, which can be selected according to the proximity of the sample results to decision limits. These approaches were applied to the determination of pesticide residues in apples (CEN, EN 12393), for which the most simplified approach showed a relative expanded uncertainty of 37.2% for a confidence level of approximately 95%.

Estimation of precision and efficiency mass transfer steps for the determination of pesticides in vegetables aiming at the expression of results with reliable uncertainty.

A 'bottom-up' approach for the expression of results obtained from analytical methods that include analytical steps with recovery inherently different from 100% [mass transfer steps (MTS): extraction, evaporation, clean-up procedures, digestion, etc.] is presented. The estimation of the combination of all MTS uncertainty involves the comparison of the experimental dispersion of replicated analyses of spiked samples with the estimation of the uncertainty obtained for the combination of all uncertainty sources except MTS ones ('incomplete' estimation). The estimation of MTS uncertainty by difference is performed after evaluating the statistical difference between the 'incomplete' estimation and the experimental dispersion (F-test). When the two estimations are statistically equivalent, the MTS uncertainty is considered to be negligible in relation to the other sources budget. The assumption of constancy of MTS performance within the analytical range is tested through single analyses at several concentration levels and is evaluated by the inclusion of the expected values at the intervals resulting from the combination of the MTS uncertainty estimation performed at one concentration level and the 'incomplete' estimation. The developed methodology can also be useful for method optimisation and validation and for the detection of small trends in results. The determination of pesticides in sweet peppers by GC-NPD was used to explore the above concepts.

Effect of different experimental parameters on the potentiometric evaluation of blood electrolytes using K+ as a test cation.

Potentiometric evaluation of ionic concentrations in physiological media has been reported to be significantly affected by the albumin containing matrix. Previous studies have attempted to clarify the origin of different patterns of variation of E versus albumin concentration at a constant 0.01 mol dm-3 KCl, depending on the experimental methodologies. This paper reports on measurements of K+ in albumin (BSA) containing KCl solutions, which have been pursued following different methodologies concerning the sequence of measurements, i.e., starting either from the most concentrated solution (100 g dm-3) (A) or from the most dilute solution (20 g dm-3) (B), and solution preparation, (a) dilution of an initial 100 g dm-3 albumin solution by successive addition of 0.01 mol dm-3 KCl, (b) concentration of an initial 20 g dm-3 albumin solution by addition of solid substance, (c) independent preparation of each albumin solution (20, 40, 60, 80 and 100 g dm-3) or (d) preparation of each albumin solution from direct dilution of a 100 g dm-3 stock solution with 0.01 mol dm-3 KCl. Convenient calculations were made and showed a significant contribution of albumin to the liquid-liquid junction potential. The variation of potential with albumin concentration is smaller for hypertonic than for isotonic bridge solutions, both for the dilution series (A,a and A,d) and for the concentration series (B,c and B,d). When the method for increasing the concentration is performed by addition of a solid substance to an initially diluted albumin (B,b), the slope of the E versus albumin concentration plot is larger for hypertonic than for isotonic reference electrolytes. This latter finding is in agreement with Payne's results when ultrafiltration was used to concentrate a protein solution. These observations are a clear indication that the method of preparation of concentrated protein solutions may significantly affect the results.