Search

A user-friendly MS-Excel spreadsheet is developed for evaluation of global consumer's and producer's risks in conformity assessment of chemical composition of a multicomponent material or object, when up to four component concentrations are under control. These risks are probabilities of incorrect conformity decisions related to a material batch (lot or similar) randomly drawn from a statistical population of such batches. The probabilities characterize the material quality globally, allowing the prediction of false decisions on conformity of a future batch, based on the future measurement results. The spreadsheet program evaluates risks using Monte Carlo simulations. As input data, the following need to be provided to the software: parameters of normal or lognormal distribution of actual (‘true’) values of the component concentrations (prior distribution); parameters of the distribution of measurements results at the actual value of the component concentration (likelihood function); and correlation matrices for couples of the actual components' concentrations under control and also for corresponding measurement results. The spreadsheet is validated by comparison of the risk estimates with those calculated in R programing environment by numerical integration of the relevant analytical formulae. The developed Excel file and a demonstration videos of its use are available as electronic supplementary material.

Recommendations are given concerning the terminology of concepts and methods used in spectroscopy in analytical chemistry, covering nuclear magnetic resonance spectroscopy, atomic spectroscopy, and vibrational spectroscopy. © 2021 IUPAC and De Gruyter. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. For more information, please visit: http://creativecommons.org/licenses/by-nc-nd/4.0/ 2021.

The illegal drug 3,4-methylenedioxymethylamphetamine, also known as MDMA, ecstasy or ‘E’, synthesised as the hydrochloride and then made into tablets, has organic impurities arising from manufacture that can be used to profile seized material. Knowing if two samples come from the same batch gives strategic information about the drug manufacturing trade. We report similarity measures (Pearson correlation coefficient, reported as the modified Pearson distance, and its Fisher transform) between impurity content of pairs of samples manufactured using four common reductive amination routes. Powder and tablets are compared using the n th root of GC–MS peak normalised areas of 8 or 31 impurities, ( n = 2, 3, 4, 5, 10). Overall using 31 compounds with 4th root pre-treatment gave the best discrimination. PtO 2 /H 2 , and Al/Hg reductions were completely discriminated among batches while NaBH 4 and NaBH 3 CN routes gave around 4% false assignments. Synthesis parameters were systematically altered to determine what parameters have significant effects on the overall purity of the product and on the impurity profiles. The amount ratio of methylamine and MDP2P, and the temperature control of the reaction mixture were both significant. Comparison of modified Pearson's distances ( r scaled to ∈[0–100]), the Fisher transform of r , and ROC curves are simple ways of providing initial evidence whether seized drugs originate from the same batch.

4-Substituted 2-(4-X-2,5-dimethoxyphenyl)-N-[(2-hydroxyphenyl)methyl] ethanamine (25X-NBOH, X = Cl, Br, or I) are new psychoactive substances (NPS). Identification of these NBOH analogues by gas chromatography-mass spectrometry is difficult because they are found in very low concentrations in the blotting paper in which they are sold, and because of the thermolabile nature of NBOH. In the ion source of a mass spectrometer they break down to the known NPS 2,5-dimethoxy-4-Xphenethylamine (2C-X) thereby giving no indication of the initial compound present. We identify 25XNBOH by heptafluorobutyric anhydride (HFBA) derivatization and subsequent mass spectrometric analysis. The major peak is the bis-derivative where both the amine and phenol are derivatized, with a minor peak of the mono-derivative in which only the amine is derivatized. There is a small amount of degradation of 25X-NBOH to the 2C-X amine which is then derivatized to the amide in-situ. Reaction with HFBA is an efficient and simple derivatization that works well with NBOH and NBOMe analogs and is recommended for routine forensic analysis.

Electrochemical DNA biosensors composed of a redox marker modified nucleic acid probe tethered to a solid electrode is a common experimental construct for detecting DNA and RNA targets, proteins, inorganic ions, and even small molecules. This class of biosensors generally relies on the binding-induced conformational changes in the distance of the redox marker relative to the electrode surface such that the charge transfer is altered. The conventional design is to attach the redox species to the distal end of a surface-bound nucleic acid strand. Here we show the impact of the position of the redox marker, whether on the distal or proximal end of the DNA monolayer, on the DNA interface electrochemistry. Somewhat unexpectedly, greater currents were obtained when the redox molecules were located on the distal end of the surface-bound DNA monolayer, notionally furthest away from the electrode, compared with currents when the redox species were located on the proximal end, close to the electrode. Our results suggest that a limitation in ion accessibility is the reason why smaller currents were obtained for the redox markers located at the bottom of the DNA monolayer. This understanding shows that to allow the quantification of the amount of redox labeled target DNA strand that hybridizes to probe DNA immobilized on the electrode surface, the redox species must be on the distal end of the surface-bound duplex.

Platinum electrodes have been electrochemically roughened (roughness factors up to 430) and evaluated for use as neural stimulation electrodes. The roughened electrodes show superior interfacial properties with increasing surface roughness. The roughened electrode (fR = 250) has a charge injection limit of 1.0 mC cm(-2) (400 μs pulse width), which is superior to that of titanium nitride (0.87 mC cm(-2)) but comparable to that of carbon nanotubes (1.0-1.6 mC cm(-2)). The surface roughness can also be optimized for different neural stimulation applications based on the available charge density at a particular pulse width of stimulation. The roughened platinum electrodes demonstrated good mechanical stability under harsh ultrasonication and electrochemical stability under continuous biphasic stimulation, indicating the potential of this biological interface to be safe and stable.

RATIONALE Mass spectrometric identification of compounds in chromatography can be obtained from molecular masses from soft ionization mass spectrometry techniques such as field ionization (FI) and fragmentation patterns from hard ionization techniques such as electron ionization (EI). Simultaneous detection by EI and FI mass spectrometry allows alignment of the different information from each method. METHODS We report the construction and characteristics of a combined instrument consisting of a gas chromatograph and two parallel mass spectrometry ionization sources, EI and FI. When considering both ion yield and signal-to-noise it was postulated that good-quality EI and FI mass spectra could be obtained simultaneously using a post-column splitter with a split fraction of 1:10 for EI/FI. This has been realised and we report its application for the analysis of several complex mixtures. RESULTS The differences between the full width at half maximum (FWHM) of the EI and FI chromatograms were statistically insignificant, and the retention times of the chromatograms were highly correlated (r2 =0.9999) with no detectable bias. The applicability and significance of this combined instrument and the attendant methodology are illustrated by the analysis of standard samples of 13 compounds with diverse structures, and the analysis of mixtures of fatty acids, fish oil, hydrocarbons and yeast metabolites. CONCLUSIONS This combined dual-source instrument saves time and resources, and more importantly generates equivalent chromatograms aligned in time, in EI and FI (i.e. peaks with similar shapes and identical positions). The identical FWHMs and retention times of the EI and FI chromatograms in this combined instrument enable the accurate assignment of fragment ions from EI to their corresponding molecular ions in FI. Copyright © 2014 John Wiley & Sons, Ltd.

Horse racing authorities impose a limit on the concentration of plasma 'total carbon dioxide' (TCO2), typically 36 mM with action taken above 37 mM, as measured by an electrochemical gas analyzer. It is of interest to understand the distribution of TCO2 in a 'normal' population of racehorses and determine probabilities of members of this population exceeding these current regulatory and action limits. TCO2 levels in equine plasma samples have been modelled for 12 months (2011-2012) of thoroughbred (3076 measurements) and standardbred (3788 measurements) data in Australia. The two populations have a common seasonal pattern, while the non-seasonal distributions differ. A single Gaussian distribution about the seasonal pattern explains the thoroughbred data, but there is evidence for a second Gaussian component for the standardbred horses. A Gaussian mixture model for standardbred horses gave a main component that matched the thoroughbred distribution, which was centred about 30.2 mM, and a smaller (about 20 % of the total density) Gaussian centred at 32.3 mM. The existence of a second, higher-meaned population of standardbred horses points to increased use of alkalinizing salts among a minority of trainers, whom still, however, maintain mostly legal levels of TCO2. Identification of this group can be used to direct intelligence-based testing with a view to limiting use of these products. Probabilities of exceeding limits are affected by seasonality, but the current rules remain conservative.

An accurate method for the measurement of carbon isotope ratios of steroids in human urine has been developed at the National Measurement Institute, Australia (NMIA) for the certification of a freeze-dried human urine reference material (CRM NMIA MX005). The method measures δ13C values by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) analysis following hydrolysis, solvent extraction and high performance liquid chromatography (HPLC) purification. Reference δ13C values for testosterone metabolites etiocholanolone, androsterone, and endogenous reference compounds (ERCs) 11β-hydroxyandrosterone and pregnanediol were determined, as well as information δ13C values for testosterone, epitestosterone, 11-oxoetiocholanolone, and a range of differences (Δ13C) between testosterone metabolites and ERCs. The measurement uncertainty was rigorously evaluated with expanded uncertainties for the reference δ13C values between 1.1 and 1.6 ‰ at the 95% coverage level. Copyright © 2012 John Wiley & Sons, Ltd.

Orthogonal acceleration time-of-flight (oa-TOF) mass spectrometry (MS) was coupled to gas chromatography (GC) to measure ion yields (ratio of ion counts to number of neutrals entering the ion source) and signal-to-noise (S/N) in the electron ionization (EI) mode (hard ionization) as well as in the soft ionization modes of chemical ionization (CI), electron capture negative ion chemical ionization (NICI) and field ionization (FI). Mass accuracies of the EI and FI modes were also investigated. Sixteen structurally diverse volatile organic compounds were chosen for this study. The oa-TOF mass analyzer is highly suited for FI MS and provided an opportunity to compare the sensitivity of this ionization method to the more conventional ionization methods. Compared to the widely used quadrupole mass filter, the oa-TOF platform offers significantly greater mass accuracy and therefore the possibility of determining the empirical formula of analytes. The findings of this study showed that, for the instrument used, EI generated the most ions with the exception of compounds able to form negative ions readily. Lower ion yields in the FI mode were generally observed but the chromatograms displayed greater S/N and in many cases gave spectra dominated by a molecular ion. Ion counts in CI are limited by the very small apertures required to maintain sufficiently high pressures in the ionization chamber. Mass accuracy for molecular and fragment ions was attainable at close to manufacturer's specifications, thus providing useful information on molecular ions and neutral losses. The data presented also suggests a potentially useful instrumental combination would result if EI and FI spectra could be collected simultaneously or in alternate scans during GC/MS.

Matching spectra is necessary for database searches, assessing the source of an unknown sample, structure elucidation, and classification of spectra. A direct method of matching is to compare, point by point, two digitized spectra, the outcome being a parameter that quantifies the degree of similarity or dissimilarity between the spectra. Examples studied here are correlation coefficient squared and Euclidean cosine squared, both applied to the raw spectra and first-difference values of absorbance. It is shown that spectra do not fulfill the requirements for a normal statistical interpretation of the correlation coefficient; in particular, they are not normally distributed variables. It is therefore not correct to use a Student's t-test to calculate the probability of the null hypothesis that two spectra are not correlated on the basis of a correlation coefficient between them. We have investigated the effect on the similarity indices of systematically changing the mean and standard deviation of a single Gaussian peak relative to a reference Gaussian peak, of changing one peak, and of changing many peaks, in a simulated 10-peak spectrum. Squared Euclidean cosine is least sensitive to changes and the first-difference methods are most sensitive to changes in mean and standard deviation of peaks. A shift of the center of a peak has a greater effect on the indices than increases in peak width, but a decrease in peak width does lead to significant changes in the indices. We recommend that if these indices are to be used to match spectra, appropriate windows should be chosen to avoid dilution by regions with no significant change.

An electrochemical sensor for the detection of lead ions is described which is made by modifying a gold electrode substrate with self-assembled monolayers (SAMs) of 3-mercaptopropionic acid (MPA) or thioctic acid (TA) followed by covalent attachment of a lead binding peptide, human angiotensin I. Cyclic voltammetry of MPA–angiotensin modified gold electrodes complexed with lead displayed voltammograms with prominent lead peaks at E 0′ , −0.29 V. A detection limit of 1 nM was achieved using Osteryoung square wave voltammetry. However, the electrodes were not stable over repeated electrochemical cycles due to partial electrochemical desorption of the SAM. The TA–angiotensin modified gold electrode showed greater stability and were able to be regenerated several times. Using Osteryoung square wave voltammetry for TA–angiotensin modified electrodes, lead concentrations down to 1.9 nM were detected. Although the detection limit of the TA–angiotensin modified electrode is higher than achieved with MPA–angiotensin, it is still well below Australian drinking water guidelines. Studies of interference effects on the Pb 2+ current showed Hg 2+ as a significant interferent but only at levels significantly greater than those found in natural waters.

Established in 1919 following World War I and with the optimism of the League of Nations forging a peace in which technological advancement and world cooperation had no bounds, The International Union of Pure and Applied Chemistry (IUPAC) is the body that purports to speak for all chemists. The founding aims of the Union were standardization of terminology, units of measurement and the understanding that an international field such as chemistry must have a common language and grammar. These are still of major concern. The present aims of IUPAC are not much different: “… to advance the worldwide aspects of the chemical sciences and to contribute to the application of chemistry in the service of Mankind”. As well as traditional areas of terminology and nomenclature, quality assurance and critical evaluation of published data and methods, IUPAC is firmly committed to helping chemistry in emerging nations through education and support of meetings and conferences.

Peptide-modified electrode surfaces have been shown to have excellent recognition properties for metal ions. An efficient method of screening a potential peptide for its selectivity for a given metal would involve the synthesis of the peptide directly on the electrode surface. This paper outlines a procedure in which the tripeptide Gly-Gly-His was synthesized one amino acid at a time on a gold surface modified with a self-assembled monolayer of the mixed alkanethiolates 3-mercaptopropionic acid (MPA) and 3-mercaptopropane (MP). Electrochemistry and high-resolution mass spectrometry were used to elucidate the structure of the adsorbed species and follow the synthesis. The amino acids can be attached only to MPA, but the presence of a diluting unreactive molecule of MP reduces steric crowding about the reaction center. The maximum coverage of synthesized tripeptide occurs at a ratio of MPA/MP of 1:1.

Fluorescence spectroscopy is ideally suited to the analysis of oil spills as it allows chemical information of polycyclic aromatic hydrocarbons to be acquired quickly, sensitively and selectively. Unlike infrared spectra which have detailed peak information, many fluorescence spectra have only a few broad peaks. Nine different samples of crude and diesel oils were used for testing point-to-point matching across the spectral range. Five of them were discriminated by point-to-point matching algorithms and the other four very similar samples were not. Principal components analysis (PCA) did successfully discriminate among all similar samples. PCA could also distinguish the extent of weathering of different samples, an important factor in matching environmental spills.

An uncertainty budget is presented for the results of measurements of purity of the agrochemical glyphosate using 1H and 31P quantitative nuclear magnetic resonance (QNMR) spectroscopy. The budget combines intralaboratory precision from repeated independent measurements of a batch, and other Type A and Type B effects. Consideration of correlation of uncertainties in ratios of mass determinations has been included. Expanded uncertainties of the purity for single determinations of purity by 1H and 31P QNMR are 0.66% and 0.82% (95% confidence interval, k=2), respectively.

The purities of the widely-used herbicide glyphosate (N-(phosphonomethyl)glycine), and the insecticide profenofos (O-(4-bromo-2-chlorophenyl) O-ethyl S-propyl phosphorothioate) were determined by 1 H and 31 P quantitative nuclear magnetic resonance (QNMR) spectrometry using an internal standard. QNMR does not need a standard reference of the same target analyte, in contrast to chromatographic methods, but only a compound containing the nucleus of interest. Sodium acetate and sodium phosphate of known purity were chosen as internal standards for 1 H NMR and 31 P NMR), respectively for the water soluble glyphosate and a single internal standard, trimethyl phosphate for both 1 H and 31 P NMR quantitative analysis of the organic soluble profenofos. These standards have NMR peaks that do not interfere with those of the analyte, they are chemically inert and are soluble in the deuterated solvent. The average purity of glyphosate obtained by 1 H NMR (97.07%, σ=0.68) agreed with that by 31 P NMR (96.53%, σ=0.90; ANOVA, P=0.074) for the five batches provided by the manufacturer according to the procedures for chemical registration in Australia. The standard deviations of seven independent analyses of a single batch by 1 H NMR and 31 P NMR were σ=0.24% and σ=0.33%, respectively, values which confirm the exceptional precision of the method. The purity of profenofos by 1 H NMR (94.63%, σ=0.14) also agreed with that by 31 P NMR (94.62%, σ=0.59; ANOVA, P=0.97). Uncertainty budgets for the measured purities of glyphosate and profenofos show that the uncertainty in the purity of the internal standard is a major contributor to the uncertainty of the result. NMR was also used to establish the impurity profile of both compounds, and quantify the impurities present.

The kinetics of irreversible adsorption has been modeled numerically for cases for which the rate-limiting process is adsorption through to those for which diffusion to the surface is rate limiting. Comparison with limiting analytical models shows that even for a system under diffusion control the Ilkovic diffusion model with rate proportional to the surface coverage does not describe the kinetics adequately. The best approximate analytical models are a simple first-order model for rate-limiting adsorption and the Ilkovic model for diffusion control. The models were compared to experimental quartz crystal microbalance data for the attachment of glucose oxidase to a self-assembled monolayer. Although none of the limiting analytical models could adequately describe the adsorption behavior of this system, excellent fits to the experimental data were obtained with the numerical model for rate-limiting adsorption with heat of adsorption proportional to coverage (Frumkin adsorption kinetics). Good agreement bet...

The propagation of uncertainty in peak area or peak height in HPLC with UV–VIS detection, is derived in terms of injection volume, flow rate, injected concentration, retention volume and temperature. It is shown that the sample concentration at the detector has a significant uncertainty distribution, which adds to the uncertainty of the measurement of absorbency. For measurement of peak area and peak height, the uncertainty of the injection volume has the greatest effect. To this is added temperature for peak height, and flow rate and temperature for peak area. Analytical solutions are presented for all uncertainties and are compared with data from a numerical simulation using Latin Hypercube sampling.

An electrochemical sensor for the detection of Cu2+ is reported which incorporates poly-l-aspartic acid (PLAsp) with 32–96 aspartate units as a selective ligand for the metal ion. PLAsp is covalently attached to a gold electrode modified with a monolayer of 3-mercaptopropionic acid using carbodiimide coupling via an N-hydroxysuccinimide (NHS) ester intermediate. The acid side groups and deprotonated peptide nitrogens on two aspartate moieties are thought to be primarily responsible for chelation of Cu2+, which remains bound when reduced to Cu+. A consequence of the multiple binding points that are available with a polypeptide is the low detection limit. The lowest concentration detected was 3 nM (0.2 ppb) achieved with Osteryoung square wave voltammetry. This detection limit compares favourably with that of ICP-OES and previously reported cysteine-modified electrodes. Analysis of tap and lake water samples using the PLAsp-modified electrode agreed well with ICP-OES analysis of the same samples.

Recently the bonding of enzymes to self-assembled monolayers (SAMs) of alkanethiols on gold surfaces has begun to receive attention as a method of constructing enzyme electrodes. This review highlights the features of SAMs which are applicable to enzyme immobilisation and the different immobilisation methods that can be used for constructing enzyme electrodes. An emphasis is placed on the ability of each method to produce reproducible enzyme electrodes and to give control over the molecular architecture of the sensor.

A simple method is described for the construction of biosensors based on the immobilisation of a variety of enzymes throughout a polytyramine membrane electrodeposited on a platinised glassy carbon electrode. Tyramine was polymerised from a mixed phosphate buffer/methanol solution at an `enzyme-friendly' `pH' of 6.0. The free amine groups located on each tyramine molecule were exploited to covalently attach the enzyme to the polymer chains using carbodiimide coupling. The resultant enzyme electrodes are shown to have excellent reproducibility (

The development of a sulfite biosensor based on the immobilization of sulfite oxidase (SOD) in electrodeposited polytyramine is described. Electropolymerization of polytyramine and simultaneous immobilization of SOD on platinized glassy carbon (GC/Pt) were performed in an aqueous solution of 0.1 M tyramine containing SOD. The sulfite biosensor constructed with five voltammetric sweep cycles showed a sensitive response to sulfite with a linear calibration curve in the concentration range 0.002–0.3 mM sulfite, slope 2.70 µA l mmol–1, and detection limit 0.001 mM sulfite (S/N = 3). Deposited polytyramine enhanced the selectivity of the biosensor by eliminating direct oxidation of sulfite and other interfering agents at the electrode. The biosensor was applied for the determination of sulfite in various types of wine samples, and the results obtained are in agreement with those using an AOAC method.

We report the use of the Kalman filter to detect the onset of chaos in a non-linear system for which the model is exactly known. The procedure is based on predicting the trajectories of a non-linear dynamical system and testing the innovation sequence for the predicted trajectory using the local overall method tests (LOMT). This is applied to the Rossler–Wegmann model of the Zhabotinskii reaction system. The Rossler–Wegmann model is a three dimensional set of non-linear ordinary differential equations, the solution to which gives the time-evolution of the concentrations of the major species in the Zhabotinskii reaction. With suitable parameters and starting values of concentrations, stable, oscillatory and chaotic solutions may be found. Five thousand points were generated from the model, for each of the three species in the model, by fourth order Runge–Kutta integration. For a complex oscillatory case the LOMT showed no chaos, and when parameters that lead to chaos were used the LOMT quickly revealed the mismatch between predicted and actual trajectories. It is concluded that the Kalman filter with LOMT is a quick and accurate method of diagnosing chaos, which could be used in a monitoring and on-line controlling system for a chemical process.

A flow cell with a poly(vinyl chloride) (PVC) neutral-ionophore liquid-membrane ion-selective electrode has been developed for flow injection potentiometry (FIP). The flow system was optimised and five substituted azacrown ethers: 7,16-dithenoyl-1,4,10,13-tetroxa-7,16-diazacyclooctadecane (DTODC), 7,16-di-(2-thiopheneacetyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (DTAODC), 7,16-dithenyl-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane (DTDC), 1,10-dioxa-4,7,13,16-tetraazacyclooctadecane (TC) and 4,7,13,16-tetrathenoyl-1,10-dioxa-4,7,13,16-tetraazacyclooctadecane (TTOTC) synthesised and investigated as the ionophores in Pb 2+ (DTAODC, TTOTC) and Hg 2+ (DTDC, DTODC, TC, TTOTC) selective electrodes. The short contact times between analyte and ionophore in FIP allow the measurement of a strongly complexing ion such as Hg 2+ that saturates the electrodes in batch analysis, or in continuous flow analysis that comes to a steady state. For the mercury-selective electrodes with ionophores with amide functional groups (TTOTC and DTODC) a carrier of 10 mM potassium nitrate was found to increase the speed of response and recovery to baseline. The linear calibration range for a DTAODC lead-selective electrode was pPb=2.0 to 5.0 with a slope of 32.5 mV decade −1 and for a TTOTC mercury-selective electrode, pHg=3.0 to 5.5 with a slope of 28.4 mV decade −1 . Highly reproducible measurements were obtained (RSD −1 giving a typical throughput of 40 samples h −1 for Pb 2+ and 30 samples h −1 for Hg 2+ .

The determination of free cadmium ions with solid-state cadmium ion-selective electrode can be performed in non-flow measurements in non-buffered solutions in a wide concentration range down to pCd 10. In cadmium ion buffered solutions linear Nernstian response was obtained even down to pCd 12, which is lower, that expected based on calculation of cadmium solubility from the conditional solubility product. Interferences of trace amounts of Fe(III), Cu(II) and Pb(II) commonly present in natural waters in larger concentrations than Cd(II) can be eliminated by reduction with hydroxylamine, complexation with Neocuproine and ion-exchange on anion-exchange resin in sulphate form, respectively. The developed procedure might be suitable for the determination of activity of free cadmium ions in natural water. A preliminary study on this subject is demonstrated for river water sample using stopped-flow flow-injection system.

Principal components regression (PCR) and linear discriminant analysis (LDA) have been applied to the classification of ion chromatographic detectors using information about the sample and other IC method conditions (19 attributes in total), a training set of 12 693 cases and a randomly-chosen test set of 1410 cases. Missing data was entered as a separate `unknown' code. When the value of each attribute was coded in a simple cardinal series (e.g., column=1, 2, 3, etc.), PCR correctly predicted the detector in 27% of the training set and 28% of the test set. By creating a variable (taking a value between 0 (absent) and 1 (present)) for each value of each attribute, the PCR prediction for both sets increased to 60%. LDA was more successful, predicting 69% of the detectors of each set, using a prior probability of the frequency of a given detector in the database, but this included zero hits for detectors that were poorly represented in the database. If equal prior probabilities were chosen the overall success rate dropped to 33% but now the classification of less frequently used detectors was improved. The ability of these numerically-oriented methods to classify discrete, non-numerical data, is surprisingly good and compares with induction methods, neural networks and expert systems reported previously.

Three flow injection systems with potentiometric detection have been developed for speciation of fluoride using a fluoride ion-selective electrode with a single flow-through detector. Among systems with (i) splitting of the sample zone and confluence point, (ii) flow reversal and (iii) resampling, the latter was most suitable. Its application was demonstrated for a study of the effectiveness of releasing fluoride from complexes with Al(III). Several ligands were examined as decomplexing agents such as EDTA, cyclohexane-1,2-diamine-N,N,N′,N′tetraacetic acid (DCTA), citrate and 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt (Tiron). In flow injection measurements, only Tiron was found suitable for decomposition of Al(III)–fluoride complexes. In stopped-flow measurements, it was observed that with use of citrate or DCTA about a 10 min delay was required to release fluoride from Al(III) complexes. A flow-injection system with resampling was also utilised for speciation of calcium with ion-selective electrode in ligand solutions and in natural waters.

Measurement uncertainty estimated under repeatability conditions in batch chemical analysis using calibrated instruments may be considered to be composed of contributions from two major effects: (i) precision of the analysis that encompasses sufficient variability of the measurement and (ii) the assessment of trueness by quantifying and if necessary correcting for bias. This paper considers under what conditions of measurement to assess bias, and from the results of a six-round blind-duplicated interlaboratory proficiency program for creatinine in urine shows that bias is present in each individual run with components from that batch and from the laboratory over the rounds of the program. We conclude that bias should be determined in each batch run under repeatability conditions. Measurement of laboratory bias alone is not sufficient to account for effects in each batch run.

The response characteristics of Taguchi TGS812 and TGS824 type semiconductor gas sensors were evaluated for flow injection monitoring of alcohol concentrations in liquid samples. The detection method was based on the head space analysis of vapour above aqueous samples containing ethanol. A low cost, hand-held monitor has been developed for this application in which an air carrier stream is pumped through a twin sensor cell. Different sensitivities to aqueous ethanol samples were observed at each sensor. Analysis of commercial liquor samples such as beers, wines and liqueurs showed a correlation coefficient as high as 0.9999 in comparison with gas chromatography analysis. This system is therefore applicable as a portable, hand-held flow injection monitor for process control during production and for monitoring the quality of products containing ethanol.

Gas chromatograms (GCs) of oil extracted from Eucalyptus species have been compared using principal components analysis (PCA), linear discriminant analysis (LDA), and the Tanimoto distance in which the number of similar peaks between two species are determined. Two systems were studied, a number of examples of E. camaldulensis Dehnh. (River Red Gum) and clones of that species, and samples from two trees E. woodwardii Maiden and E. torquata Luehm. and a number of their hybrids (E. torwoodii). The PCA revealed natural groupings of E. camaldulensis clones and showed, via loading plots, that eight intensities of the 180 times studied were sufficient to explain most of the variance and characterise each tree. The chemical species were identified by GC-mass spectrometry as a-pinene, 1,8-cineole, p-cymene, terpinen-4-ol/aromadendrene, cryptone, borneol, spathulenol, and torquatone. Each hybrid (E. torwood) of the trees E. woodwardii and E. torquata has a distribution of oils that lies between those of the parents. LDA was able to define two distinct groups and performed better than the Tanimoto distance in classifying the trees.

Hibbert, D.B., 1993. Generation and display of chemical structures by genetic algorithms. Chemometrics and Intelligent Laboratory Systems , 20: 35–43. A genetic algorithm (GA) is described for generating isomeric structures of a molecule given a molecular formula and information about allowed bonding of atoms. Bonds are represented by tuples of the bonded atoms. These are manipulated as the genes by a greedy procedure that attempts to maintain valid molecules. GAs that draw a molecule in two dimensions given bonding information are also demonstrated. The function to be minimised is optimum when bonded atoms are at one bond length distance from each other, and non-bonded atoms are as far apart as possible. The chromosome may be x, y coordinates or a series of angles that each atom makes with two previous atoms in a bonding list. The potential for use of GAs in three-dimensional structure determination is discussed.

Genetic algorithms (GAs) are described to optimise the rate coefficients for the hydrolysis of adenosine 5′-triphosphate by fitting a kinetic model to concentration versus time data. A simple GA is compared with one in which the genes are real variables, and to one that operates with the so-called incest prevention algorithm of Eshelman and Schaffer. The fastest convergence to a good optimum is achieved by a hybrid GA in which a steepest descent, pseudo-Newton procedure is iterated with an incest-preventing GA, each providing a starting point for the other. The best optimum was one of the results of using a pseudo-Newton routine on each of the converged population of a GA (not necessarily the best from the GA). This approach gives good results for problems having a multimodal response surface when there is no good initial guess for a traditional optimisation algorithm. After a small number of generations a GA gives good starting points for steepest ascent optimisers.

Genetic algorithms (GAs), a set of optimisation techniques, are so called after their similarity to evolutionary processes in nature. The algorithm's equivalents of genes and chromosomes are the unknown parameters of the problem and these may be mated and mutated to give better solutions. The major strengths of GAs, namely the ability to search a large parameter space with no initial guesses per se, no derivatives of the objective function and to cope with local minima, make it a candidate method for several areas of chemistry. Chemical problems that have been tackled by GAs are described and suggestions for new applications are made. Subroutines in object-oriented Pascal are given to set up a simple GA.

The concept of a fractal as a self-similar geometrical object with non-integer dimension is introduced. The measurement of fractal dimension by the box counting method is described and a program in Pascal is given. Diffusion limited aggregation (DLA), kinetics on fractals and chromatography in fractal media are used as chemical examples. A program to simulate a random DLA fractal is described.

A project has been initiated by the International Union of Pure and Applied Chemistry (IUPAC) to create a glossary of concepts and terms in chemometrics. This will be accomplished by consultation with the community through the means of a wiki – a web site that can be modified by users (see http://www.iupacterms.eigenvector.com/index.php?title=Main_Page ). Over time new terms can be added, and consensus definitions arrived at. The definitions will be published as IUPAC recommendations.

Parallel factor analysis (PARAFAC) was used to analyze data from the high throughput screening of an array of organometallic rhodium and iridium complexes as catalysts for the intramolecular hydroamination of 2-(2-phenylethynyl)aniline to give 2-phenylindole. The progress of the hydroamination reactions was monitored using UV-visible spectroscopy. The overlapped UV-visible spectra of the mixture of starting material, product and solvent in the samples taken at different times were deconvoluted using PARAFAC. Unique PARAFAC models led to close approximations of the actual UV-visible spectra of the compounds in the mixture. The performance of the catalysts was then compared by estimating the final concentration of the starting material and product using PARAFAC loadings. A library of 63 complexes generated in situ was examined in a single experiment using this methodology. The complexes were generated from combinations of seven ligands (bis(N-methyl2-imidazolyl)methane, bis(1-pyrazolyl)methane, 1,10-phenanthroline, N,N'-bis(p-tolyl)diazabutadiene, N,N'-bis(p-tolyl)1,2-dimethyldiazabutadiene, N,N'-bis(mesityl)1,2-dimethyldiazabutadiene and bis(2,4,6-trimethylphenylimino)acenapthene) and nine metal precursors ([Ir(COD)Cl](2) (COD = 1,5-cyclooctadiene), [Ir(CO)(2)Cl](n), [Ir(COE)(2)Cl](2), [IrCp*Cl(2)](2) (Cp* = 1,2,3,4,5-pentamethylcyclopentadiene), [Rh(COD)Cl](2), [Rh(CO)(2)Cl](2), [Rh(COE)(2)Cl](2), [RhCp*Cl(2)](2) and [RhCpCl(2)](2)) (Cp = cyclopentadiene)). The proposed method can be used for the fast screening of arrays of metal complexes for identifying effective catalysts, providing information that can augment traditional methods used for the analysis of catalyzed reactions.

Four comparison statistics ('similarity indices') for the identification of the source of a petroleum oil spill based on the ASTM standard test method D3414 were investigated. Namely, (1) first difference correlation coefficient squared and (2) correlation coefficient squared, (3) first difference Euclidean cosine squared and (4) Euclidean cosine squared. For numerical comparison, an FTIR spectrum is divided into three regions, described as: fingerprint (900-700 cm(-1)), generic (1350-900 cm(-1)) and supplementary (1770-1685 cm(-1)), which are the same as the three major regions recommended by the ASTM standard. For fresh oil samples, each similarity index was able to distinguish between replicate independent spectra of the same sample and between different samples. In general, the two first difference-based indices worked better than their parent indices. To provide samples to reveal relationships between weathered and fresh oils, a simple artificial weathering procedure was carried out. Euclidean cosine and correlation coefficients both worked well to maintain identification of a match in the fingerprint region and the two first difference indices were better in the generic region. Receiver operating characteristic curves (true positive rate versus false positive rate) for decisions on matching using the fingerprint region showed two samples could be matched when the difference in weathering time was up to 7 days. Beyond this time the true positive rate falls and samples cannot be reliably matched. However, artificial weathering of a fresh source sample can aid the matching of a weathered sample to its real source from a pool of very similar candidates.

The standard error of a result obtained from a straight line calibration is given by a well known ISO-endorsed expression. Its derivation and use are explained and the approach is extended for any function that is linear in the coefficients, with an example of a weighted quadratic calibration in ICPAES. When calculating the standard error of an estimate, if QC data is available it is recommended to use the repeatability of the instrumental response, rather than the standard error of the regression, in the equation.

Bias in an analytical measurement should be estimated and corrected for, but this is not always done. As an alternative to correction, there are a number of methods that increase the expanded uncertainty to take account of bias. All sensible combinations of correcting or enlarging uncertainty for bias, whether considered significant or not, were modeled by a Latin hypercube simulation of 125,000 iterations for a range of bias values. The fraction of results for which the result and its expanded uncertainty contained the true value of a simulated test measure and was used to assess the different methods. The strategy of estimating the bias and always correcting is consistently the best throughout the range of biases. For expansion of the uncertainty when the bias is considered significant is best done by SUMU(Max):U(C(test result))=ku(c)(C(test result))+ |delta(run)|, where k is the coverage factor (= 2 for 95% confidence interval), u(c) is the combined standard uncertainty of the measurement and delta(run) is the run bias.

A simple method is described to determine sulfite in beer samples using a fill and flow channel biosensor. A droplet of sample is placed into the inlet of a rectangular flow cell and begins to flow through the channel by capillarity. The flow is maintained and controlled by a porous outlet plug of defined porosity. In a rectangular flow cell, the sample solution flows through three consecutive zones: over a predictor electrode, an enzyme layer and a detector electrode. Together these three zones enable the differentiation between current due to sulfite and current due to other electroactive species in the sample. The predictor electrode is located upstream, and on the opposite channel wall to the enzyme layer and detector electrode, and is poised at the same potential (+0.65 V versus Ag/AgCl) as the detector electrode. On this electrode, the current contribution from all species in the sample solution that are oxidized at that potential is determined. The enzyme layer contains sulfite oxidase, which, in the process of oxidizing sulfite, produces hydrogen peroxide, which itself is reduced by excess sulfite. The current at the downstream detector electrode is therefore different from that at the predictor electrode as a result of the enzyme reaction and the difference of the currents, corrected for the dimensions of the electrodes, is proportional to the concentration of sulfite. The method enables a straightforward correction of the interfering current at the detector electrode and a determination of the analyte concentration. The effect of interferences from ascorbic acid, ethanol, sorbic acid and tartaric acid in the detection of sulfite is efficiently removed. The concentration of sulfite in a sample of beer measured by the biosensor is equivalent to that measured using a reference method based on the AOAC-recommended Monier-Williams method.

The slow agglomeration of gold colloids of approximate diameter 30 nm in the presence of a small concentration of L-cysteine·HCl has been followed by multiple techniques, namely particle tracking analysis (PTA), differential centrifugal sedimentation (DCS), UV-visible spectroscopy (UV), second order spectroscopy (SOS) and transmission electron microscopy (TEM). The citrate-stabilized Au nanoparticles were characterized by PTA, DCS, UV and dynamic light scattering (DLS) prior to exposure to the cysteine. Hydrodynamic forces during centrifugation can cause the disintegration of weakly held agglomerates. TEM reveals small linear agglomerates that become open linked chains of fractal-like structures after several hours of agglomeration. Second order Rayleigh scattering observed at the harmonic wavelength of 680 nm was resonantly enhanced by surface plasmon excitation of the growing agglomerates. During the initial stage of the agglomeration process, when chainlike or quasi-chainlike agglomerates were the dominant species, the SOS signal goes up by a factor of about three before reaching saturation. This study of a model nanoparticle system provides insights into the information obtained from a range of measurement techniques, with recommendations for characterisation of agglomerating nanoparticles under end-use conditions.

The simultaneous determination of Cu(2+), Cd(2+) and Pb(2+) is demonstrated at four modified gold electrodes using N-PLS calibration. Three of the electrodes were modified with the peptides Gly-Gly-His, gamma-Glu-Cys Gly and human angiotensin I which were covalently attached to thioctic acid self-assembled monolayers and the fourth electrode was modified with thioctic acid only. Voltammetry at the modified electrodes in the presence of the three metal ions revealed one peak due to the reduction of copper and another due to the overlapping peaks of cadmium and lead which made quantification using conventional methods difficult. N-PLS was used to calibrate and predict trace concentrations (100 nM to 10 microM) of mixtures of Cu(2+), Cd(2+) and Pb(2+).

An electrochemical sensor for the detection of cadmium ions is described using immobilized glutathione as a selective ligand. First, a self-assembled monolayer of 3-mercaptopropionic acid (MPA) was formed on a gold electrode. The carboxyl terminus then allowed attachment of glutathione (GSH)via carbodiimide coupling to give the MPA-GSH modified electrode. A cadmium ion forms a complex with glutathione via the free sulfhydryl group and also to the carboxyl groups. The complexed ion is reduced by linear and Osteryoung square wave voltammetry with a detection limit of 5 nM. The effect of the kinetics of accumulation of cadmium on the measured current was investigated and modeled. Increasing the temperature of accumulation and electrochemical analysis caused an increase in the voltammetric peak of approximately 4% per degrees C around room temperature. The modified electrode could be regenerated, being stable for more than 16 repeated uses and more than two weeks if used once a day. Some interference from Pb(2+) and Cu(2+) was observed but the effects of Zn(2+), Ni(2+), Cr(3+) and Ba(2+) were insignificant.

Hibbert, D.B., 1989. A Prolog program for the calculation of isotope distributions in mass spectrometry. Chemometrics and Intelligent Laboratory Systems , 6: 203–212. The isotopic distribution of the mass spectrum of a molecule is calculated by the expansion of the equivalent polynomial in abundances and masses of the isotopes of each element. In Prolog the expansion is done symbolically more efficiently than by procedural languages. Binomial coefficients are calculated from the logarithms of factorials, themselves obtained from the Gamma function. A pruning algorithm is applied to avoid continued expansion of the polynomial when the coefficients are below a predetermined fraction of the most intense peak. The spectrum may be displayed to any mass accuracy — limited by the precision and the memory of the computer. Running on an IBM-AT clone the mass spectrum of the molecular ion bovine insulin (C 254 H 377 N 65 O 75 S 6 ) was calculated in 35 s and polystyrene n = 1000 (C 8004 H 8010 ) in 2.1 s.

There is a consensus that electron impact ionization mass spectrometry is not capable of discriminating among geometrical isomers of unsaturated fatty acid methyl esters (and in general olefinic compounds). In this paper, we report the identification of all eight geometrical isomers of alpha-linolenic acid, one of the few essential omega-3 fatty acids that has attracted great attention, using low-energy electron ionization mass spectrometry. Three electron energies 70, 50, and 30 eV were studied and the mass spectrum of each isomer was obtained from the analysis of different concentrations of a standard mixture of alpha-linolenic acid methyl ester geometrical isomers to ensure the robustness of the method. Principal component analysis was employed to model the complex variation of m/z intensities across the isomers. Only using the data of 30 eV energy was complete differentiation among geometrical isomers observed. The unique cleavage pattern of the alpha-linolenic acid methyl ester isomers leading to a benzenium ion structure is discussed and general fragmentation rules are derived using the mass spectra of over 300 compounds with different kinds and levels of unsaturation. Application of the proposed method is not limited to alpha-linolenic acid. It can potentially be used to identify the geometrical isomers of any compounds with an olefinic chain.