The impact of signal pre-processing on the final interpretation of analytical outcomes - A tutorial.

The present tutorial paper is aimed to analyse and critically discuss the consequences of row pre-processing (conversion of measurement units, derivatives, and standard normal variate transform) on the evaluation of final outcomes of chemometric data analysis. An in-depth focus on pre-processing effects both on the signal shape and on misinterpretation of results - a crucial and disregarded issue in the analytical field - is presented. It is shown how this preliminary step of data processing may lead, in many cases, to draw incongruous conclusions, not actually based on real information embodied within data, but on artefacts arising from the mathematical transforms. This tutorial is not limited to a description of the problem, it also introduces strategies and tools for overcoming such unwanted effects, allowing a direct interpretation of the importance of original variables to be performed, explaining the chemical information that actually characterises samples. The dangerous implications of row pre-processing on instrumental signals is demonstrated on real datasets coming from different analytical techniques: transmission and attenuated total reflection infrared spectroscopy, cyclic voltammetry, X-ray fluorescence spectroscopy, Raman spectroscopy, and ultraviolet-visible spectroscopy. Hence, the impact of this widespread problem in most of the branches of analytical chemistry is illustrated.

A novel electroanalytical approach based on the use of a room temperature ionic liquid for the determination of olive oil acidity.

In this paper, a novel voltammetric/amperometric approach for the direct determination of free acidity (FFA, expressed as mass percentage of free oleic acid) in olive oil samples is presented. The method is based on the reduction processes occurring at a platinum microdisk electrode involving the free fatty acids present in the matrices. To overcome problems related to the low conductivity of the samples investigated, olive oils were mixed with suitable amounts of the room temperature ionic liquid (RTIL), tri-hexyl(tetradecyl)phosphonium bis (trifluoromethylsulfonyl) imide ([P14,6,6,6]+[NTf2]-), which acted as a supporting electrolyte. Conditions for a reliable quantification of the acids were preliminarily investigated by performing voltammetric and chronoamperometric measurements in RTIL solutions containing oleic acid at different concentrations. Oleic acid (OA) was chosen as a model compound as it is the main component of the FFA content in olive oils. In order to establish the effect of oxygen on the electroanalytical responses, the reduction process of OA was investigated under both deoxygenated and oxygenated conditions. It was found that, in both situations, the current arising from the electrode process of OA depended linearly on the OA concentration over a wide range varying from 0.1% to 8% OA (w/w). This range includes FFA values which can be found on all categories of commercially available oil samples, including extra-virgin, virgin and lampante oils. Voltammetric and chronoamperometric experiments were also performed in oil/RTIL samples artificially acidified (extra-virgin olive oils with known addition of oleic acid) and in natural olive oils from some commercial categories. The results obtained indicated that the electrochemical procedure developed was satisfactory in terms of both sensitivity and detection limits. The reliability of the proposed approach for the detection of FFA was finally assessed by comparison of the voltammetric/chronoamperometric values with those obtained by the official method for quantification of olive oil acidity, which is an acid/base volumetric titration.

Has your ancient stamp been regummed with synthetic glue? A FT-NIR and FT-Raman study.

The potential of FT-NIR and FT-Raman spectroscopies to characterise the gum applied on the backside of ancient stamps was investigated for the first time. This represents a very critical issue for the collectors' market, since gum conditions heavily influence stamp quotations, and fraudulent application of synthetic gum onto damaged stamp backsides to increase their desirability is a well-documented practice. Spectral data were processed by exploratory pattern recognition tools. In particular, application of principal component analysis (PCA) revealed that both of the spectroscopic techniques provide information useful to characterise stamp gum. Examination of PCA loadings and their chemical interpretation confirmed the robustness of the outcomes. Fusion of FT-NIR and FT-Raman spectral data was performed, following both a low-level and a mid-level procedure. The results were critically compared with those obtained separately for the two spectroscopic techniques.