Characterization of bile acids and fatty acids from ox bile in oil paintings by gas chromatography?mass spectrometry*1

Characterization of ox bile, traditionally used in painting, is of interest in the fields of archaeometry and conservation and restoration of works of art. Bile acids, fatty acids (F), and cholesterol found in ox bile have been identified using a derivatization method that combines the formation of ethyl esters from the carboxylic groups and the trimethylsilyl ethers from hydroxyl groups. This method of analysis is consistent with these others proposed by the authors to analyze drying oils, proteins, and diterpenic resins usually used as binders and varnishes by the painters. Bile acids from binary samples such as animal glue/ox bile, casein/ox bile and Arabic gum/ox bile have been successf…

Calibration transfer in chemiluminescence analysis

Abstract Direct standardization (DS) and piecewise direct standardization (PDS) methods are applied to multivariate standardization of chemiluminescence signals using PLS model as the calibration model. The linear concentration interval of the chemiluminescence determination was determined. This interval was located using univariate robust calibration by least median of squares (LMS) method. The linear calibration model was corroborated by conventional least-squares method. The standardization subset and window size were optimized by means of the prediction residual error sum of squares. Several instrumental sources of variability have been studied: the detection cell, instrument, batch ver…

Identification of lipid binders in paintings by gas chromatography

Abstract The influence of the presence and the type of pigments in the lipid binding media of paintings were studied by gas chromatography with flame ionization detector. The drying oils were linseed stand oil, poppy oil and sunflower oil, and the pigments studied were cadmium red, cobalt blue, tin white, lead white, chalk and plaster of Paris, commonly used in paintings. The results indicate that the stearic/palmitic ratio and the presence of pigments are quite stable during ageing. However, some differences in the oleic acid/palmitic acid ratio were found, depending on the type of pigment present in the lipid binding media. These variations are related to the drying effect of the pigments…

Evaluation and elimination of the “blank bias error” using the H-point standard addition method

Abstract The basis of the H-point standard addition method in the cases where the analyte determination requires the use of an absorbent reagent is considered. The method evaluates and eliminates the blank bias error present in such procedures by using the absorbance increment at two selected wavelengths as the analytical signal of the calibration graphs. Three different determinations were tested: the determination of proteins with the biuret method, thorium with thoron and of magnesium with Titan Yellow. These procedures are better described by the proposed method than by the traditional approach using absorbance values against reagent blank.

Standard addition method in FIA. Comparison between different assemblies

A comparison between the different proposed methods to perform the standard addition in an on-line way (flow injection analysis) is made. The different assemblies (reverse and conventional FIA) and the employed standards in every method are compared. Three analyses were carried out, two of them were analyses of samples of pH indicators (methylorange and chlorophenol red) prepared at an acid pH, while the standards were prepared at a basic pH. The third analysis was the analysis of chloride in different synthetic waters with a perfectly known composition for all the ions present in the samples. Besides a new method to perform the on-line standard addition is proposed, trying to reduce the co…

Determination of theophylline and paraxanthine in urine samples by liquid chromatography using the H-point standard additions method

Abstract The simultaneous determination of theophylline and paraxanthine in urine samples by the H-point standard additions method (HPSAM) is described. Samples are extracted with C 18 solid-phase extraction cartridges and chromatographed using a Hypersil C 18 -ODS column and a mobile phase consisting of acetonirile-phosphate buffer in the gradient elution mode. Under these conditions theophylline and paraxanthine are eluted with short retention times. Although their chromatographic peaks are overlapped and their spectra are very similar, the H-point standard additions method provides excellent results in the determination of both xanthines at therapeutic levels.

H-Point Standard Addition Method Applied to Voltammetry of Microparticles. Quantitation of Dyes in Pictorial Samples

A solid-state electrochemical application of the H-point standard addition method (HPSAM) for quantifying two electroactive compounds, A, B, that produce strongly overlapped voltammetric peaks is described. It is based on peak current measurement in square-wave voltammograms recorded for solid samples containing a reference compound R, upon additions of a A- (or B-) containing standard compound. The method allows to the determination of the mass fraction of A and B by applying the H-point standard addition method to solid state voltammetry. The quotients between the currents measured at two selected potentials and the peak current of R vary linearly with the mass ratio of the added standard…

A comparison of various calibration techniques applied to the ninhydrin-cefoxitin determination

Abstract This report deals with spectrophotometric determination of cefoxitin from intact antibiotics or from its acid induced degradation product by means of their reaction with ninhydrin in a strong sulphuric acid medium. To obtain the calibration graphs four methods were used: absorbance of reacted cefoxitin against a reagent blank as an analytical signal, first-derivative signal of absorbance against a water blank as an analytical signal, first-derivative signal of absorbance against a reagent blank as an analytical signal and the H-Point Standard Additions Method (HPSAM) by using absorbance increment values as analytical signals. The HPSAM is applied to the determination of cefoxitin i…

Screening and mapping of pigments in paintings using scanning electrochemical microscopy (SECM)

The use of the scanning electrochemical microscopy (SECM) technique for identifying and mapping of both organic and inorganic pigments in sub-microsamples from pictorial specimens is described. This methodology, inspired by the voltammetry of immobilized particles technique, permits the study of textural properties of paint layers and mapping the distribution of pigment grains upon application of different potentials to the substrate. A combination of the redox competition SECM strategy with voltammetry yields a local identification methodology for different organic and inorganic pigments in paint samples.

Voltammetric Identification of Lead(II) and (IV) in Mediaeval Glazes in Abrasion-Modified Carbon Paste and Polymer Film Electrodes. Application to the Study of Alterations in Archaeological Ceramic

Direct identification of lead(II) and (IV) in glazed ceramics by cyclic voltammetry and differential pulse voltammetry is described. Characteristic cathodic and anodic profiles are obtained for microsample coatings in Paraloid B72-film electrodes immersed in NaCl and HAc/NaAc media. The electrochemical response of PbO, PbO2 and Pb3O4, and a series of test specimens of glaze is compared with that of microsamples extracted from the glaze of blue and white decorated ceramic produced in Manises (Spain) in the 15th century which have been buried in a damp place. Under abrasive conditions, striping peaks at –0.12 and –0.25 V (vs. SCE) appear for Pb(IV) centers and, at –0.55 and –0.75 V, for Pb(II…

Electrochemistry of iron oxide pigments (earths) from pictorial microsamples attached to graphite–polyester composite electrodes

The electrochemical response of different iron pigments attached to graphite–polyester composite electrodes is described. Characteristic differential pulse voltammetric profiles were obtained for green earth, iron oxide red, Mars black, ochre yellow, Sienna raw, umber raw and Van Dyke brown. Proton-assisted reductive dissolution processes occur at the three-phase boundary of the substrate electrode–attached solid–electrolyte solution system. Microsamples extracted from polychromed sculptures, canvas paintings, wall paintings, altarpieces and panel paintings from Spain, Ethiopia and Italy from the 12th to the 20th centuries were electrochemically identified in agreement with polarized light …

Evaluation and elimination of the blank bias error using the H-point standard additions method (HPSAM) in the simultaneous spectrophotometric determination of two analytes

Abstract The aim of this report is to demonstrate the usefulness of the HPSAM in all instances where the determination of two analytes requires the use of an absorbent reagent, particularly when it does not present a constant analytical signal because of its consumption. This problem can be treated as a ternary mixture by the HPSAM method and then, the fundamentals of the method for resolving these kind of mixtures can be applied. The method can calculate the concentration of one species from spectral data at two wavelengths where the two other species present the same absorbance relationship. These wavelength pairs are easily found, and can be selected to give the most precise results. The…

H-Point Curve Isolation Method for Coupled Liquid Chromatography and UV−Visible Spectrophotometry

The H-point curve isolation method (HPCIM) for the detection of unknown interferences in chromatography is proposed. The method allows one to estimate the UV-vis spectra of interfering species in a sample as well as to test the purity of the chromatographic peaks. Besides the detection of the unknown interferences in a sample, this method allows one to calculate the concentration of an analyte in the presence of unknown compounds. To illustrate the reliability of the proposed method, samples of diuretics and amphetamines have been analyzed by normal- and reversed-phase high-performance chromatography.

Comparative Study on the Determination of Cephalexin in its Dosage Forms by Spectrophotometry and HPLC with UV-vis Detection

This paper discusses the spectrophotometric determination of cephalexin as the intact cephalexin or as its acid-induced degradation product. Cephalexin can be determined in the range 1 × 10−5−18 × 10−5M with relative standard deviations of 5-1%. The limits of quantitation and detection were 10−5 and 0.3 × 10−5M, respectively. These procedures were compared with reversed-phase HPLC determination. No interference was observed in the presence of common pharmaceutical adjuvants. The H-point standard additions method was applied in order to correct for the possible presence of the cephalexin precursor, 7-aminocephalosporanic acid; this improves the selectivity of the UV-vis spectrophotometric me…

Study by FT-IR spectroscopy of ageing of adhesives used in restoration of archaeological glass objects

Abstract A study by FT-IR spectroscopy which aims to compare the stability of five polymers, Araldit XW396/XW 397, Vitralit 7256, Loctite 350, Desmodur N 75/Viacryl SM 564/65 and Krafft silicone commonly used in the restoration of archaeological glass objects when subjected to thermal ageing, UV light ageing and cyclic ageing in SO 2 pollutant chamber has been carried out. Two series of test specimens were prepared in order to reproduce the two methods of application of polymers commonly used in restoration treatments as adhesive and consolidant or coating of glass fragments. Modifications in the chain or side chain structure of polymers, alterations of carbonyl groups in Araldit and isocya…

The generalized H-point standard-additions method to determine analytes present in two different chemical forms in unknown matrix samples. Part II. Cr(vi) determination in water samples by absorption spectrophotometry

The generalized H-point standard-additions method (GHPSAM) is used in order to obtain the total Cr(VI) and chromate concentration in water samples whose matrices are completely unknown. Moreover, a new methodology, which is a modification of the GHPSAM, is proposed for the simultaneous determination of the two major chemical forms of Cr(VI) present in the sample. The method is based on the location of spectral intervals where the behaviour of the interferent absorbance can be considered as linear. From these intervals, the analyte concentration free from bias error can be estimated. Spiked samples of dig and harbour water measured in the UV–visible spectral region have been tested to check …

Hyperbolic subtraction method: Determination of the concentration of an analyte in the presence of an unknown interferent via spectral data.

A theoretical model to determine the concentration of an analyte in the presence of unknown interferents using spectral-type data is described. The method involves absorbance measurements at three wavelengths and the calculation of specific absorbances yielding a hyperbolic relationship between absorbance ratios and analyte concentrations. The concentration of the analyte of known spectrum can be determined in the presence of an interferent or mixture of interferents of unknown concentration(s) and spectra can be determined combining data for different sets of wavelengths. Application to indigo and isatin solutions in DMSO related to the so-called Maya blue problem is reported as an illustr…

Analytical study of canvas painting collection from the Basilica de la Virgen de los Desamparados using SEM/EDX, FT-IR, GC and electrochemical techniques

A new method, based on the combined use of scanning electron microscopy-x-ray microanalysis, Fourier transform infrared spectroscopy, gas chromatography and differential pulse voltammetry, is proposed which permits a parallel investigation of organic and inorganic materials and the characterization of the artistic technique employed in canvas painting. The proposed method has led to analyze succesfully the canvas painting collection exhibited in the Basílica de la Virgen de los Desamparados (Valencia, Spain) which includes art works from 17th to 20th century.

New spectrophotometric procedure for determining cefotaxime based on derivatization with 1,2-naphthoquinone-4-sulphonate into solid-phase extraction cartridges--application to pharmaceutical and urine samples.

Cefotaxime was derivatised with 1,2-naphthoquinone-4-sulphonate (NQS), extracted into solid-phase cartridges (C18) and detected using a UV-visible detection system. Optimum conditions for this new procedure were: hydrogencarbonate-carbonate buffer, pH 10.5, 5-min reaction time at 25 degrees C and an NQS concentration of 7.1x10(-3) mol l(-1). The accuracy and the precision of the liquid-solid procedure were tested. The procedure was used to measure cefotaxime in pharmaceutical and urine samples. The results obtained were contrasted with those reported for a HPLC method for urine samples. The generalized H-point standard additions method was used to measure cefotaxime in urine samples.

Development of the H-point standard additions method for coupled liquid chromatography and UV-visible spectrophotometry

Abstract This work establishes the fundamentals of the H-point standard additions method for liquid chromatography for the simultaneous analysis of binary mixtures with overlapped chromatographic peaks. The method was compared with the deconvolution method of peak suppression and the second derivative of elution profiles. Different mixtures of diuretics were satisfactorily resolved.

Identification of inorganic pigments from paintings and polychromed sculptures immobilized into polymer film electrodes by stripping differential pulse voltammetry

Abstract Inorganic pigments in paintings and polychromed sculptures are studied by cyclic voltammetry and differential pulse stripping voltammetry using micro-sample coatings in Paraloid B72-film modified electrodes. Characteristic cathodic and anodic differential pulse profiles were obtained in the +0.4 to −1.0 V vs. SCE potential range for different cadmium, copper, lead, mercury and zinc pigments used in traditional colour palettes. Under optimized conditions, excellent reproducibility was obtained. Microsamples extracted from polychromed sculptures, wall paintings, canvas paintings, panel paintings and altarpieces from Spain, Ethiopia and Italy from the 12th to the 20th centuries have b…

Determination of the Boron/Lead Ratio in Ceramic Materials Based on Electrochemical Quartz Crystal Microbalance

A method for quantitatively determining the relationship boron/lead in ceramic materials is presented. It is based on the attachment of microsamples of ceramic frits to an electrochemical quartz crystal microbalance (EQCM). After a reductive deposition step in the � 0.25 to � 0.85 V (vs. AgCl/Ag) potential range, well-defined stripping peaks are recorded in 0.10 M NaCl plus 0.10 M mannitol. The deposition of boron and lead from suspensions of solid materials, and its subsequent oxidative dissolution can be monitorized at the EQCM. The quotient between the maximum mass gain and the corresponding charge passed enables for a direct quantitation of the boron/lead molar ratio in ceramic material…

Development of the H-point standard additions method for analyte determinations in unknown matrix

Abstract The development of the H-point standard additions method is proposed in order to obtain the unbiased analyte concentration when the matrix of the sample is completely unknown. A spectral region where the interferent behaviour can be considered linear at three wavelengths must be chosen. The method uses the analytical signal data at these three wavelengths, usually situated in the maxima region of the analyte. Two procedures are proposed in order to know and to locate this behaviour for the unknown interferent. Several binary and ternary mixtures of phenol, 4-chlorophenol and 4-chloro-3-methylphenol as representative examples have been assayed, with accurate (less than 3% relative e…

The H-point and generalized H-point standard additions methods for flow injection procedures.

This paper establishes the fundamentals of the H-point standard additions method (HPSAM) and generalized H-point standard additions method (GHPSAM) in the flow injection technique. Two kinds of analytical signals can be employed, FIA peaks and spectra. Different analytical problems have been studied: determinations of one analyte in the presence or absence of matrix effect when different blank features are present, and determination of two analytes in the presence of blank bias error affecting the development of one of them. The determination of chloride with mercury thiocyanate which presents a matrix effect, and the determinations of calcium and/or magnesium with arsenazo III are examined…

Validation of a method for the determination of boron in ceramic materials by X-ray fluorescence spectrometry

Abstract A method is proposed for boron determination in ceramic materials, boracic raw materials and ceramics frits of different compositions by wavelength dispersive X-ray fluorescence technique. The instrumental conditions were studied and the sample preparation procedures were optimized. The reference method used was the potentiometric titration of mannitol–boric complex acid and the technique was optimized for the determination of B 2 O 3 in each of the materials used. The samples for XRF determination were obtained as pellets and as glass discs. For the pellets different binders were studied, such as mannitol and cellullose for the samples of raw materials, and polyvinylpirrolidone an…

Suppression of pigment interference in the gas chromatographic analysis of proteinaceous binding media in paintings with EDTA.

A method to suppress the interference of pigments in the analysis of proteinaceous media used in paintings is presented in this paper. This method is based on the formation of metallic ion-ethylenediaminetetraacetic acid (EDTA) complexes previous to the derivatisation process, using ethyl chloroformate (ECF), to transform the amino acids in N(O,S)-ethoxycarbonyl (EOC) ethyl esters. Test specimens, containing different proteinaceous media such as albumin, porcine gelatine and casein mixed with lead white, chalk, verdigris and raw Sienna have been prepared for carrying out this study. Different pH conditions have been probed for the different pigments studied. Values of peak area ratio of ami…

Electrochemical determination of boron in minerals and ceramic materials

Abstract A method for quantitatively determining boron in minerals and ceramic materials is described. It is based on the abrasive attachment of mixtures of ZnO plus sample to PIGEs. After a reductive deposition step, square wave voltammograms recorded for those modified electrodes immersed into 0.10 M NaCl+0.25 M mannitol provide well-defined stripping peaks at −0.85 and −0.15 V vs. AgCl/Ag for the oxidation of Zn and B, respectively. The quotients between the peak areas and the peak currents for the stripping oxidation of B and Zn vary linearly with the molar ratio of B and Zn in the mixture, thus providing an electrochemical method for determining the boron content in solid samples. Resu…

Analyte estimation using the generalized H-point standard additions method and a new methodology for locating linear spectral intervals for unknown interferents

An improvement in the way to locate linear spectral intervals for unknown interferents which overlap the analyte signal in UV–visible spectrophotometry is offered as an alternative to the method previously described by Campins-Falco et al. (Anal. Chim. Acta, 302, 323 (1995)). In that report, quotients of the second-derivative spectra of the sample and analyte were used. The new method improves on the old one by using first- instead of second-derivative data, thus eliminating the possible divisions by zero (inflection points for the analyte) obtained in the previous procedure. The linear intervals are now obtained by plotting for each wavelength the differences in first-derivative data betwe…

Correction of Matrix Effect in Multielemental Quantitative Analysis by X-Ray Fluorescence Spectroscopy Using the Linear Behavior in the Analytical Range of the Substitution-Dilution Method

This paper is an analytical study of the possibility of applying the linear range of the substitution-dilution method to correct the matrix effect in quantitative analysis by X-ray fluorescence (XRF) spectroscopy. The analytical range is obtained from a series of samples prepared in the form of glass discs by substituting the unknown sample with a standard sample (substitution factor, h) including a diluent-melt. In general, the substitution-dilution method is hyperbolic in character and therefore the diluent is required to ensure linear behavior between If vs. h in the experimental range. The linear range is located between the concentrations of standard and unknown for each element analy…

Principal component analysis for the selection of variables in the application of the H-point and generalised H-point standard addition method

The present paper deals with the selection of variables for the H-point and generalised H-point standard additions methods (HPSAM and GHPSAM, respectively). Both methods are applied for the resolution of spectroscopic interfered signals in the UV-vis range. The HPSAM is a suitable method for the resolution of binary and ternary mixtures when the interferent is known. The GHPSAM is applied for the resolution of samples that contain unknown interferents. In this paper, a method based on the study of a principal components analysis (PCA) for the selection of variables for the HPSAM and GHPSAM is proposed. The PCA results show the isolation of the analyte signal from the sample signal, achieved…

Quantitation of Metal Ions in Archaeological Glass by Abrasive Stripping Square-Wave Voltammetry Using Graphite/Polyester Composite Electrodes

A method for identifying Co, Cu, Sb, Sn, Fe, Zn, and Zr pigments in glasses using square-wave voltammetry as the detection mode is described. It is based on the abrasive attachment of glass microsamples to graphite/polyester composite electrodes. Sample amounts less than 1 μg are used allowing for the study of archaeological samples. In all cases well-defined anodic stripping peaks corresponding to the oxidation of metal deposits generated at potentials ranging from −0.6 to −1.6 V (vs. SCE) are obtained. This response is in agreement with that displayed by the corresponding metal oxides. Quantitative estimates of the relative population of metal ions in glasses are obtained from peak area m…

A new flow cell design for chemiluminiscence analysis.

The present study proposes a new flow cell called a bundle cell for chemiluminescence analysis. The results obtained were compared with those achieved by manual and automated batch procedures and flow manifolds with different cells: an common quartz flow cell, a helix cell and the most used spiral cell. Figures of merit such as limit of detection, sensitivity, accuracy and precision for the Cr(III) determination were established with light emission produced by catalysed Cr(III) luminol oxidation by hydrogen peroxide in a basic aqueous solution. An improvement in sensitivity about 50% as compared with the spiral cell and even larger with respect to the other flow cells tested was observed. T…

Systematic Study of Several Causes and Effects Due to Absorbent Blanks in Determinations of One Analyte

A systematic study is performed about several factors involved in the measurements when a absorbent blank is used, as molar absorption coefficient of the formed complex, absorbance of the blank at the measured wavelength, and kind of spectrophotometer used (conventional double-beam or diode-array). It is proved that the usual methodology, measuring against reagent blank, produces less accurate measurements than those obtained against solvent blank. On the other hand, use of the H-Point standard additions method provides always equal or better results than the best ones obtained at single wavelengths by using measurements against solvent bank, at the maximum of the formed complex. To test th…

Electrochemical analysis of the alterations in copper pigments using charge transfer coefficient/peak potential diagrams. Application to microsamples of baroque wall paintings attached to polymer film electrodes

The alteration of copper pigments in art samples was studied by linear scan and cyclic voltammetry using sample-modified Elvacite 2044 film electrodes on the basis of two-dimensional diagrams of charge transfer coefficients calculated from Tafel plots of reductive dissolution processes vs. peak potential. Characteristic voltammetric peaks were obtained for pigments used in the baroque vault frescoes of the Basílica de la Virgen de los Desamparados painted by Antonio Palomino. Results obtained by voltammetric techniques were compared with those from SEM/EDX and FT-IR analysis obtaining a good agreement and leaving to an unambiguous identification of pigments used by Palomino and their altera…

ATR-FTIR and XRD quantification of solid mixtures using the asymptotic constant ratio (ACR) methods. Application to geological samples of sodium and potassium feldspars

Two asymptotic constant ratio methods applied to the quantification of individual components of solid samples using attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray diffraction (XRD) are described. The methods involve the measurement of the peak current/peak areas of selected signals (diffraction peaks in XRD and absorption bands in ATR-FTIR) relative to the signal of standard added in constant proportion to the sample and the sample enriched with the analyte following the usual standard additions methodology. The proposed method compensates the absorption effects appearing in XRD and the presence of overlapping absorption bands of interferents by me…

Analyser of chromium and/or cobalt

Abstract Two stopped-flow manifolds have been proposed for individual or simultaneous determination of chromium and cobalt in water samples. Automated procedures based on multicommutation systems have emphasised the differences of their catalytic effect in luminol–hydrogen peroxide chemiluminescence reaction. A more rapid decay of signal was observed for Co for both configurations (flow injection or continuous injection). The influence of chemical and hydrodynamic variables has been studied in order to establish the robustness of method. The analysis rate was lower 1.5 min per replicate. Chemometric tools have been employed for the resolution of their contributions. Partial least squares (P…

Electrochemical Determination of the Fe(III)/Fe(II) Ratio in Archaeological Ceramic Materials Using Carbon Paste and Composite Electrodes

A method for determining the iron(III)/iron(II) ratio in ceramic materials based on microsample attachment to graphite/polyester composite electrodes is presented. This quotient is of considerable interest for determining the nature and firing conditions of the original materials in archaeological ceramics. The electrochemistry of iron in ceramic materials in acetic/acetate and Na2EDTA aqueous solutions is described in terms of reductive dissolution processes of iron(III) materials and oxidative dissolution processes of iron(II) ones. The iron(III)/iron(II) ratio is determined from peak area measurements in linear scan voltammograms performed in 0.10 M acetic/acetate buffer (pH 4.70) and 0.…

Study of Burseraceae resins used in binding media and varnishes from artworks by gas chromatography–mass spectrometry and pyrolysis-gas chromatography–mass spectrometry

In the present work, a study attempting to characterize the Manila elemi and Mexican copal resins from the Burseraceae family, which are used as components of varnishes and binding media of artworks, has been carried out. A new GC-MS method involving the use of methyl chloroformate as derivatisation reagent has been proposed. A second method which uses pyrolysis-GC-MS and hexamethyldisilazane as derivatization reagent has also been applied. Characterization of the main components of the mono-, sesqui- and triterpenoid fractions occurring in the raw materials has been achieved. Both alpha- and beta-amyrin have been established as the major triterpenoid compounds occurring in these resins tog…

H-Point standard additions method for resolution of binary mixtures with simultaneous addition of both analytes

Abstract The basis of the H-point standard additions method, HPSAM, with simultaneous addition of both analytes is proposed for the resolution of binary mixtures. It is a modification of the previously described H-point standard additions method that permits the resolution of both species from a unique calibration set by making the simultaneous addition of the two analytes. The method uses as analytical signals the absorbances at pairs of wavelengths where each species shows the same absorbance. The required data to apply the method are the absorbance values at the previously selected wavelengths for the sample alone and spiked with both species at known concentrations. Linear relations bet…

Elimination of the unknown irrelevant matrix absorbance by using the H-Point Standard Additions Method (HPSAM)

It is demonstrated how the H-Point Standard Additions Method (HPSAM) using DeltaA as analytical signal (from data at three previously selected wavelengths) is only related with analyte concentration when unknown irrelevant matrix absorbance is present. The method is compared with the most common previously reported methods, such as derivative spectroscopy or the compensation method. The obtained results show that the proposed HPSAM leads to the same found concentration of analyte as the other reported methods, except detection limits and standard deviation for six replicates which are lower, because of the use of absorbance data (instead of first derivative data as usual). In addition, the …

Classification of Spanish Unifloral Honeys by Discriminant Analysis of Electrical Conductivity, Color, Water Content, Sugars, and pH

To ascertain the most discriminant variables for seven types of Spanish commercial unifloral honeys, stepwise discriminant analysis was performed. Fifteen parameters [pH; water content; electrical conductivity; x, y, and L, chromatic coordinates from the CIE-1931 (xyL) color space; fructose; glucose; sucrose; maltose; isomaltose; maltulose; kojibiose; and the fructose/glucose and glucose/water ratios] were considered. The studied honey types were rosemary, citrus, lavender, sunflower, eucalyptus, heather, and forest. The most discriminant variables, as selected by the multivariate program, were electrical conductivity, color (x, y, L), water content, fructose, and sucrose. All sunflower, eu…

The generalized H-point standard-additions method to determine analytes present in two different chemical forms in unknown matrix samples. Part I. General considerations

This paper shows how the generalized H-point standard-additions method (GHPSAM) can be used to obtain the total concentration or concentrations of different chemical forms of an analyte when the matrix of the sample is completely unknown. The spectral regions where the interferent behaviour can be considered as linear are found and the analyte concentration free from bias error is estimated. This paper includes the already published features of the GHPSAM and a new modification of this method which allows the simultaneous determination of two chemical forms of an analyte in a sample.

Identification by GC-FID and GC-MS of amino acids, fatty and bile acids in binding media used in works of art.

GC-FID was used as single methodology for the identification and differentiation of proteins, lipids and ox bile from binders used in artistic paintings. The samples were hydrolyzed by HCl. Subsequently, the simultaneous formation of volatile derivatives of the amino, fatty and bile acids with ethyl chloroformate was performed quickly and safely in an aqueous medium. The derivatives were separated by capillary GC and characterized by GC-MS. The ageing of drying oils was studied, identifying pelargonic acid among other degradation products. Proteinaceous and lipoid binding media were characterized by means of the quotients between the areas of the peaks for each amino or fatty acid with resp…

A new derivatization procedure for the determination of cephalexin with 1,2-naphthoquinone 4-sulphonate in pharmaceutical and urine samples using solid-phase extraction cartridges and UV–visible detection

Abstract The present report shows how to derivatize cephalexin with 1,2-naphthoquinone-4-sulphonate (NQS) into solid-phase extraction cartridges (C18) using UV–visible detection. Optimum conditions for this new procedure are: hydrogen carbonate/carbonate buffer pH=10.5, 5 min reaction time at 25°C and NQS concentration of 7.1×10−3 mol l−1. The accuracy and the precision of the method were tested. The procedure was used to measure cephalexin in pharmaceutical and urine samples. The results obtained were contrasted with those reported by UV-spectrophotometric and HPLC methods for pharmaceutical samples and with HPLC method for urine samples. The H-point standard additions method was used to m…

On-line database of voltammetric data of immobilized particles for identifying pigments and minerals in archaeometry, conservation and restoration (ELCHER database)

[EN] A web-based database of voltammograms is presented for characterizing artists' pigments and corrosion products of ceramic, stone and metal objects by means of the voltammetry of immobilized particles methodology. Description of the website and the database is provided. Voltammograms are, in most cases, accompanied by scanning electron microphotographs, X-ray spectra, infrared spectra acquired in attenuated total reflectance Fourier transform infrared spectroscopy mode (ATR-FTIR) and diffuse reflectance spectra in the UV-Vis-region. For illustrating the usefulness of the database two case studies involving identification of pigments and a case study describing deterioration of an archae…

Corrigendum to “ATR-FTIR and XRD quantification of solid mixtures using the asymptotic constant ratio (ACR) methods. Application to geological samples of sodium and potassium feldspars” [Spectrochim. Acta A 236 (2020) 118328]

Analytical study of proteinaceous binding media in works of art by gas chromatography using alkyl chloroformates as derivatising agents.

In this work, we present the results obtained in an analytical study of the different types of proteinaceous binding media most commonly used in paintings, using GC-FID as the technique of analysis and GC-MS as a confirmatory technique. The application of this methodology requires prior hydrolysis of the proteins in the binding media to obtain free amino acids and then volatile derivatives, in this case by reaction with chloroformates due to advantages of speed, safety and the aqueous medium in which the reaction occurs. The method proposed for the proteinaceous binding media study is to calculate the proportions of the different amino acids with respect to alanine. This method provided goo…

Substitution–dilution method to correct the matrix effect in multi-element quantitative analysis by X-ray fluorescence

Abstract A mathematical model based on the dilution–addition method (DAM) for multi-elemental analysis using an X-ray fluorescence technique is proposed. The conditions for sample preparation do not require both the unknown and standard samples to be similar in composition and mineralogy, and the unknown sample is replaced quantitatively by the standard sample, hence the denomination substitution–dilution method (SDM). This method makes it possible to correct the matrix effect in multi-elemental quantitative analysis by X-ray fluorescence for each analyte. The proposed model presents hyperbolic behaviour of the experimental data when the X-ray fluorescence intensities are represented versus…

Multivariate versus univariate calibration for nonlinear chemiluminescence data

Abstract Multivariate calibration is tested as an alternative to model chromium(III) concentration versus chemiluminescence registers obtained from luminol-hydrogen peroxide reaction. The multivariate calibration approaches included have been: conventional linear methods (principal component regression (PCR) and partial least squares (PLS)), nonlinear methods (nonlinear variants and variants of locally weighted regression) and linear methods combined with variable selection performed in the original or in the transformed data (stepwise multiple linear regression procedure). Both the direct and inverse univariate approaches have been also tested. The use of a double logarithmic transformatio…

Electrochemical identification of metal ions in archaeological ceramic glazes by stripping voltammetry at graphite/polyester composite electrodes

The electrochemical response of metal ions in different samples of coloured ceramic tin-lead glazes attached to graphite/polyester composite electrodes is described. In addition to the ubiquous signals for lead, reductive dissolution processes are followed by anodic stripping peaks for Co, Cu, Sb, Mn, Sn and Fe, enabling the direct identification of such elements in microsamples proceeding from archaeological glazed tiles from Valencia (Spain) workshops (16th-18th century). Additional anodic and cathodic peaks corresponding to redox processes involving metal species in solution generated during stripping processes are also used. Peak potentials, Tafel plots and shape parameters are used for…

Quantification of minerals from ATR-FTIR spectra with spectral interferences using the MRC method.

A method for quantifying the individual components of mineral samples based on attenuated total reflectance - Fourier transform infrared spectroscopy (ATR-FTIR) is described, extending the constant ratio (CR) method to analytes absorbing in a common range of wavenumbers. Absorbance values in the spectral region where the analytes absorb relative to the absorbance of an internal standard absorbing at a wavenumber where the analytes do not absorb, permits the quantification of N analytes using measurements at N fixed wavenumbers. The method was tested for mixtures of albite, orthoclase, kaolin and quartz.

Sugar profiles of Spanish unifloral honeys

The levels of various sugars (fructose, glucose, sucrose, maltose, maltulose, kojibiose, isomaltose, raffinose, erlose and melezitose) as well as the glucose/fructose and glucose/water ratios were determined in different Spanish unifloral honey types (rosemary, orange blossom, lavender, sunflower, eucalyptus, heather, honeydew). Sugars were determined by gas chromatography of the trimethylsilyloxime derivatives. There were significant differences among the honey types in relation to sugar composition. Fructose, glucose, sucrose, maltose and the glucose/water ratio were selected by discriminant analysis as the better parameters for the correct classification of the honey samples into their p…

Identification of diterpenes in canvas painting varnishes by gas chromatography-mass spectrometry with combined derivatisation.

A derivatisation method that combines the formation of ethyl esters from the carboxylic groups and trimethylsilyl ethers from hydroxyl groups of the components of diterpenic resins is presented in this paper. This methodology involves two experimental steps: (1) formation of ethyl esters using ethyl chloroformate; and (2) the esterified compounds are lead to react with trimethylsilylimidazole to form the corresponding trimethylsilyl ethers. The main advantage of the proposed method is the possibility of performing simultaneously the analysis of amino acids from proteins, fatty acids from drying oils, and diterpenic compounds from natural resins usually found in works of art. This methodolog…

Curve resolution procedure for isolating the spectra of unknown interferences from the sample spectrum in analyte determinations

The method described is based on the selection of a wavelength in the sample spectrum that is called ‘reference wavelength’, all measured wavelengths are previously assayed in order to select this reference wavelength. Several wavelengths can be used as the reference. The interferent spectrum is calculated as a function of the interferent absorbance at this wavelength, making use of the ratio between the absorbance of the analyte at the selected wavelength and at every measured wavelength. The proposed methodology can estimate the unknown interferent spectrum from the sample and pure analyte spectra. As a quality guarantee of the estimated spectrum, its shape is also estimated in spiked sam…

Spectrophotometric determination of phenols in water samples by the GHPSAM method

Abstract The generalized H-point standard-additions method (GHPSAM) is proposed in order to obtain the phenol concentration in water samples when the matrix is completely unknown. The procedure involves solid-phase extraction in BondElut PPL cartridges and data handling of the UV-visible spectrophotometry measurements. The spectral regions where the unknown interferent behaviour can be considered as linear are found and the analyte concentration free from bias error is estimated. The percentages of recovery of phenols in spiked samples were similar to those obtained by HPLC. Cresols or chlorophenols can be also determined in real samples by this method. The concentration range tested was 0.…