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%.     

Effect of citrate impurities on the reference pH value of potassium dihydrogen buffer solution

Solutions of potassium dihydrogen citrate, KH₂C₆H₅O₇, exhibit reference pH values close to those of the primary standard pH buffer solutions of tartrate and phthate. Nevertheless, as opposed to all other recommended primary buffer standards, the citrate reagent is not commercially available as a standard reference material (SRM).In this work, experiments have been performed on solid reagent from different sources, which revealed the presence of the various citrate species in variable amounts. This may arise from the sensitivity of the system to the purification strategy, owing to the proximity of the acidity constants of the triprotic citric acid, H₃C₆H₅O₇.An experimental procedure has been developed which leads to a satisfactory preparation of the citrate reagent, to an extent that its solutions, independently of batch on supplier, exhibit reproducible pH values, in agreement with the conventionally assigned pH (S) = 3.776 at 25 °C for 0.05 m KH₂C₆H₅O₇.Comparison of expected and experimental values is used as a quantitative test for the quality of reagent, eventually indicating the need for purification according to recommendations.     

Acidity function p(a(H) gamma(Cl)) as a step to pH assessment

The conventional assignment of pH reference buffer standards, pH(S), is achieved by means of a series of procedures that follow from measurement of Harned cell potentials for an electrolyte solution which is the buffer solution of interest. An intermediate step is assessment of the acidity function p( a(H) gamma(Cl))(0), the extrapolated value of a linear representation of the dependence of p( a(H) gamma(Cl)) on m(Cl) for at least three different molalities, m(Cl), of added alkali chloride (0.005; 0.010; 0.015 mol kg(-1) KCl). This experimental value can be compared with a theoretically expected value calculated from the dissociation constants of the buffer species. Whereas these calculations always give negative slopes for diprotic and triprotic acids and zero slope for monoprotic acids, experimental values with negative or positive slopes can be obtained for well fitting straight lines obtained for buffer solutions with ionic strengths from 0.0025 to 0.144 mol kg(-1). Such disagreement between theoretically and experimentally obtained values introduce an extra source of uncertainty in the establishment of pH(S) and on its traceability chain. In this work examples are presented and discussed for which the discrepancy between expected and experimental values leads to different intercept p( a(H) gamma(Cl))(0).     

Enzyme‐Mediated Preparation of Chiral 1,3‐Dioxane Odorants

The enantiomerically enriched diastereoisomers of the chiral 1,3‐dioxane odorants Floropal^® ( 1 ) and Magnolan^® ( 2 ) were prepared by an enzyme‐mediated approach. Their olfactory properties were evaluated to investigate differences in the odor perception for the stereoisomers.     

Polymer bound pyrrole compounds,VII: Xanthobilirubinic acid esters and amides from an insoluble,polystyrene-supported precursor

Summary A series of xanthobilirubinic acid (XBR) esters (i.e.,1c to1p) and amides (2a–2c) have been prepared by a procedure involving detachment of the chromophore previously ester-bound to a functionalized, insoluble polystyrene (1b). Detachment is achieved by treatment with the respective alcohol in aqueous alkali or with the amines, yielding directly the corresponding esters or amides. With primary, short-chained alcohols the ester yields are high (60% forn-C₄H₉-OH to 99% for C₂H₅-OH), but decrease rapidly with chain length (40% forn-C₁₀H₂₁-OH and 0% forn-C₁₆H₃₃-OH). The same trends are observed with the amines. These results are interpreted in terms of an (increasingly) unfavourable entropic interaction between the polymer matrix to which the chromophore is bound and the (growing) alcohol chain. The impairment by the long chained nucleophile to reach the transesterification center could also contribute, and, for the most lipophylic alcohols, their low solubility in aqueous alkali is an additional drawback. With secondary and tertiary alcohols, no ester is obtained, in agreement with a B_AC2 mechanism involving a tetrahedral intermediate.

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Polymergebundene Pyrrolverbindungen, 7. Mitt.: Darstellung von Xanthobilirubinsäureestern und-amiden, ausgehend von einer unlöslichen polystyrolgebundenen Vorstufe

Zusammenfassung Unlösliche, durch eine Estergruppe an Polystyrol gebundene Xanthobilirubinsäurederivate liefern mit Alkoholen die monomeren Ester (1c–1p), mit Aminen die monomeren Amide (2a–2c). Die Ausbeuten für primäre kurzkettige Alkohole sind hoch (n-C₄H₉-OH: 60%, C₂H₅-OH: 99%), mit steigender Kettenlänge sinken sie rasch (n-C₁₀H₂₁-OH: 40%,n-C₁₆H₃₃-OH: 0%). Denselben Trend beobachtet man bei Aminen. Die Ergebnisse werden durch eine zunehmend ungünstige entropische Wechselwirkung zwischen Chromophor an der Matrix und Alkohol interpretiert. Durch die steigende Kettenlänge des Alkohols könnte auch aus räumlichen Gründen die Annäherung des Nucleophils an das Reaktionszentrum erschwert werden. Die geringe Löslichkeit der höheren lipophilen Alkohole in wäßrigem Alkali wirkt sich ebenfalls ungünstig auf den Reaktionsverlauf aus. Für sek. und tert. Alkohole wird keine Reaktion erhalten, wie es für einen B_AC2-Mechanismus mit tetraedrischem Zwischenprodukt zu erwarten ist.

     

Numerical simulation of polymerization in interdigital multilamination micromixers

Free radical polymerization in microfluidic devices modeled with the help of numerical simulations is discussed. The simulation method used allows the simultaneous solvation of partial differential equations resulting from the hydrodynamics, thermal and mass transfer (convection, diffusion and chemical reaction). Three microfluidic devices are modeled, two interdigital multilamination micromixers respectively with a large and short focusing section, and a simple T-junction followed by a microtube reactor together considered as a bilamination micromixer with a large focusing section. The simulations show that in spite of the heat released by the polymerization reaction, the thermal transfer in such microfluidic devices is high enough to ensure isothermal conditions. Moreover, for low radial Peclet number, microfluidic devices with a large focusing section can achieve better control over the polymerization than a laboratory scale reactor as the polydispersity index obtained is very close to the theoretical limiting value. As the characteristic dimension of the microfluidic device increases, i.e. for high radial Peclet number, the reactive medium cannot be fully homogenized by the diffusion transport before leaving the system resulting in a high polydispersity index and a loss in the control of the polymerization.     

The response of metal clusters toq- andL-dependent external fields

We have calculated the static polarizability and mean excitation energy of metal clusters submitted toq-andL-dependent external fields ofj _L (qr)Y _L0(Ω) type. Use has been made of an Extended Random-Phase Approximation which includes exchange and correlation effects within a local model, and of the spherical jellium model to describe the neutralizing positive background.     

Application of dried spot cards as a rapid sample treatment method for determining hydroxytyrosol metabolites in human urine samples. Comparison with microelution solid-phase extraction

Two different rapid sample pretreatment strategies, dried spot cards, and microelution solid-phase extraction plates (μSPE), with ultra-high performance liquid chromatography coupled to tandem mass spectrometry (UPLC-MS/MS) have been developed and validated for the determination of hydroxytyrosol and its metabolites in spiked human urine samples. Hydroxytyrosol, hydroxytyrosol-3′-O-glucuronide, hydroxytyrosol-4′-O-glucuronide, hydroxytyrosol-3-O-sulphate, and homovanillic alcohol-4′-O-glucuronide were used as the target compounds. Using the FTA DMPK-A dried urine spot card under optimum conditions, with 5 μL of preconcentrated urine volume and 100 μL of methanol/water (50/50, v/v) as the elution solvent, the extraction recovery (%R) of the compounds studied was higher than 80 %, and the matrix effect (%ME) was less than 8 %. The stability of these cards and punching at the centre or side of the card were also studied, obtaining an excellent stability after 7 days of storage and complete homogeneity across the surface of the dried drop. The different μSPE parameters that affect the efficiency were also studied, and under optimum conditions, the %R and the %ME were higher than 70 % and lower than 17 %, respectively. The linearity range in dried urine spot cards was 2.5-20 μM for all the metabolites, with the exception of hydroxytyrosol-3-O-sulphate and hydroxytyrosol, which were 0.3-70 μM and 2.5-50 μM respectively. With regards to μSPE, the linearity range was 0.5-5 μM for all the studied compounds, except for hydroxytyrosol-3-O-sulphate, which was 0.08-5 μM. The quantification limits (LOQs) were 0.3-2.5 μM and 0.08-0.5 μM in dried spot cards and in μSPE, respectively. The two developed methods were then applied and compared for determining hydroxytyrosol and its metabolites in human 24 h-urine samples after a sustained consumption (21 days) of a phenol-enriched virgin olive oil. The metabolites identified were hydroxytyrosol in its glucuronide and sulphate forms, homovanillic alcohol in its glucuronide and sulphate forms, homovanillic acid sulphate and hydroxytyrosol acetate sulphate.