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Abstract: Zirconia (ZrO2) has strong Lewis acid sites which have an affinity for the strongly electronegative phosphonate group of organo-phosphates. To investigate whether this affinity can be used for selective preconcentration, the retention of methyl, ethyl, and propylphosphonic acid (MPA, EPA and PPA) and inorganic anion matrix components on ZrO2 was investigated. Only organo-phosphates and sulfate exhibited retention on zirconia. After preconcentration, the retained species were eluted from ZrO2 by 0.75mM Na2CO3, and separated by a Dionex Ionpac AS11 anion-exchange column (250mm×2mm I.D.) and a Hypercarb RPLC column (50mm×4.6mm I.D., 3μm) in series followed by suppressed conductivity detection. Calibration curve showed a linear response for MPA, EPA and PPA in the range of 0.01μM to 1μM (R 2 >0.9989). Detection limits after preconcentration of a 10mL sample were 0.16, 0.19 and 0.16μg/L for MPA, EPA, and PPA, respectively. [Copyright &y& Elsevier]

Abstract: A simple, rapid, and low-cost coulometric method for direct detection of glyphosate and aminomethylphosphonic acid (AMPA) in water samples using anion-exchange chromatography and coulometric detection with copper electrode is presented. Under optimized conditions, the limits of detection (LODs) (S/N =3) were 0.038μgml−1 for glyphosate and 0.24μgml−1 for AMPA, without any preconcentration method. The calibration curves were linear and presented an excellent correlation coefficient. The method was successfully applied to the determination of glyphosate and AMPA in water samples without any kind of extraction, clean-up, or preconcentration step. No interferent was found in the water, like this, the recovery was, practically, 100%. [Copyright &y& Elsevier]

Abstract: Mass transfer from the first-dimension to the second-dimension column in comprehensive two-dimensional gas chromatography (GC×GC) is normally quantitative, which means that the peak areas produced by GC×GC are equal to those obtained in one-dimensional gas chromatography (GC). In view of this relationship, it was investigated whether normal GC calibration could be used to quantify compounds analysed by GC×GC. Quantification of polycyclic aromatic hydrocarbons (PAHs) in sediment was used as a test case. It was concluded that more laborious and time-consuming GC×GC area calibration can be replaced by GC area calibration if separation quality and quantity prerequisites are met. [Copyright &y& Elsevier]

Abstract: A new RP-HPLC method for the separation and quantification of the most common genetic variants of bovine milk proteins is described. A reversed-phase analytical column C8 (Zorbax 300SB-C8 RP, 3.5μm, 300Å, 150×4.6 I.D.) was used. All the most common casein (CN) and whey protein genetic variants, including β-CNI, were detected and separated simultaneously in less then 40min, with the exception of αS1-CNB and CNC variants. Purified protein genetic variants were employed in calibration and showed different absorbances at 214nm. The procedure was developed using 40 raw individual milk samples of cows belonging to four different breeds and certified skim milk powder BCR-063R. Method validation consisted in testing linearity, repeatability, reproducibility and accuracy. A linear relationship (R 2 >0.99) between the concentrations of proteins and peak areas was observed over the concentration range, with low detection limits. Repeatability and reproducibility were satisfactory for both retention times and peak areas. The RSD of peak areas ranged from 0.92 to 4.32% within analytical day and from 0.85 to 9.52% across analytical days. The recoveries, calculated using mixtures of samples previously quantified, ranged from 98.1 to 103.7%. [Copyright &y& Elsevier]

Abstract: The quantitativeness of an evaporative light-scattering detector (ELSD) for supercritical fluid chromatography (SFC) was evaluated by using an equimass mixture of uniform poly(ethylene glycol) (PEG) oligomers. Uniform oligomers, in which all molecules have an identical molecular mass, are useful for the accurate calibration of detectors. We calibrated the SFC-ELSD system for various concentrations and molecular masses by using an equimass mixture of PEG oligomers. ELSD not only showed a good linear response to the injected concentration over a wide concentration range, from 10−4 to 10−1 g/mL, but also showed a strong dependence on the molecular mass of the solute. By using chromatograms of the equimass mixture of uniform oligomers to calibrate SFC-ELSD, it was possible to determine exact values of not only the average mass but also the molecular-mass distribution for a PEG 1540 sample. The average molecular mass was shifted to a higher value by several percentage points after calibration of the ELSD. [Copyright &y& Elsevier]

Abstract: A corona-charged aerosol detector (CAD) was developed to improve the sensitivity, reproducibility and quantitativeness of detection as compared to evaporative light-scattering detector (ELSD) for liquid chromatography. Our laboratory used the corona CAD as a detector for supercritical fluid chromatography (SFC) and evaluated its performance compared to the ELSD by using a certified reference material of poly(ethylene glycol) (PEG) and a well-defined equimass mixture of uniform PEG oligomers. The corona CAD was able to detect a 10 times more dilute solution of uniform oligomers compared to the ELSD. Although the original data of molecular mass by ELSD was 4.6% smaller than the certified value of PEG 1000, molecular mass distribution obtained by corona CAD was virtually almost the same as the certified value without any calibrations. [Copyright &y& Elsevier]

Abstract: This paper reports the properties and advantages of the three-way calibration models based on parallel factor analysis (PARAFAC) in the simultaneous determination of malachite green (MG) and its metabolite (leucomalachite green, LMG) in trout. A recently method proposed by community reference laboratory AFSSA-LERMVD (Fougères, France) has been used. The method is based on liquid chromatography–triple quadrupole mass spectrometry (LC–MS/MS) in multiple reaction monitoring (MRM) mode. The validation of the method has been carried out taking into account the Decision 2002/657/EC. The figures of merit for PARAFAC and univariate calibration models of six non-consecutive days analyzed during a month were evaluated. With the samples of the first 3 days, calibration models were built and the fish fortification samples of the other days were predicted. Decision limits (CCα, α =0.01), detection capabilities (CCβ, β =0.05) and mean relative errors in absolute value (in calibration and with test samples) obtained with PARAFAC calibrations were more homogeneous than the ones obtained with the univariate calibrations, especially in LMG. These figures of merit were in the range of 0.2–0.83μgkg−1 (CCβ) and 0.2–0.49μgkg−1 (CCα), whereas mean relative errors in absolute value were in the range of 1.1–7.4% in calibration and 3–12% in test samples for MG and LMG with PARAFAC calibrations. The PARAFAC calibrations allow detecting the test samples which are not similar to the calibration samples and in this way their wrong quantification is avoided. [Copyright &y& Elsevier]

Abstract: This paper presents a detailed study on the calibration of a thermal desorption–gas chromatography–mass spectrometry (TD–GC–MS)-based methodology for quantification of volatile organic compounds (VOCs) in gaseous and liquid samples. For the first time, it is documented to what extent three widely encountered problems affect precise and accurate quantification, and solutions to improve calibration are proposed. The first issue deals with the limited precision in MS quantification, as exemplified by high relative standard deviations (up to 40%, n =5) on response factors of a set of 69 selected VOCs in a volatility range from 16Pa to 85kPa at 298K. The addition of [2H8]toluene as an internal standard, in gaseous or liquid phase, improves this imprecision by a factor of 5. Second, the matrix in which the standard is dissolved is shown to be highly important towards calibration. Quantification of gaseous VOCs loaded on a sorbent tube using response factors obtained with liquid standards results in systematic deviations of 40–80%. Relative response factors determined by the analysis of sorbent tubes loaded with both analytes and [2H8]toluene from liquid phase are shown to offer a reliable alternative for quantification of airborne VOCs, without need for expensive and often hardly available gaseous standards. Third, a strategy is proposed involving the determination of a relative response factor being representative for a group of analytes with similar functionalities and electron impact fragmentation patterns. This group method approach indicates to be useful (RSD≈10%) for quantifying analytes belonging to that class but having no standards available. [Copyright &y& Elsevier]

Abstract: Fast GC–MS with narrow-bore columns combined with effective sample preparation technique (QuEChERS method) was used for evaluation of various calibration approaches in pesticide residues analysis. In order to compare the performance of analyte protectants (APs) with matrix-matched standards calibration curves of selected pesticides were searched in terms of linearity of responses, repeatability of measurements and reached limit of quantifications utilizing the following calibration standards in the concentration range 1–500ngmL−1(the equivalent sample concentration 1–500μgkg−1): in neat solvent (acetonitrile) with/without addition of APs, matrix-matched standards with/without addition of APs. For APs results are in a good agreement with matrix-matched standards. To evaluate errors of determination of concentration synthetic samples at concentration level of pesticides 50ngmL−1 (50μgkg−1) were analyzed and quantified using the above given standards. For less troublesome pesticides very good estimation of concentration was obtained utilizing APs, while for more troublesome pesticides such as methidathion, malathion, phosalone and deltamethrin significant overestimation reaching up to 80% occurred. According to presented results APs can be advantegously used for “easy” pesticides determination. For “difficult” pesticides an alternative calibration approach is required for samples potentially violating MRLs. An example of real sample measurement is shown. In this paper also the use of internal standards (triphenylphosphate (TPP) and heptachlor (HEPT)) for peak areas normalization is discussed in terms of repeatability of measurements and quantitative data obtained. TPP normalization provided slightly better results than the use of absolute peak areas measurements on the contrary to HEPT. [Copyright &y& Elsevier]

Abstract: A new calibration method for high-performance liquid chromatography was validated. The method was called single-injection calibration approach (SICA) because it allowed to obtain a complete calibration curve by means of a single injection of a standard solution containing several non-volatile and semi-volatile organic compounds at different concentration levels. The compounds studied included carboxylic acids, polyalcohols, carbohydrates and water-soluble vitamins. This method allowed a 1–7-fold reduction in the analysis time with regard to conventional calibration methods. The method was applied to three different chromatographic detection methods: refractive index (RI) detection, diode array detection (DAD) and inductively coupled plasma atomic emission detection (ICP-AED). Good linearity was achieved (r 2 >0.999) for the three detection methods but signal correction was required for RI detection and DAD. This fact demonstrated that ICP-AES was the most universal because the signal obtained for non-volatile and semi-volatile organic compounds was not a function of the chemical nature of the compound and only depended on the mass content of carbon. The method was validated by analyzing a reference non-fat milk powder sample as well as several real food samples (three fruit juices, four wines, three candies and a multivitamin complex). [Copyright &y& Elsevier]

Abstract: In common with all gas chromatography (GC) methods, comprehensive two-dimensional gas chromatography (GC×GC) has the potential to provide both qualitative and quantitative analysis. There are fundamental differences in the way one-dimensional (1D-GC) and GC×GC results are interpreted for these parameters. Since 1D-GC produces a single measured peak in the chromatogram, there is a single retention time, and associated with this a single peak response (either area or height). Peak area and height are related by peak width. GC×GC produces a series of modulated peaks at the detector. Thus, the peak metrics of retention, area and height for one component are now not simple single values for one peak, but rather are derived from the multiple peak distribution generated by the modulation process. The peak retention is interpreted in terms of two-dimensional coordinates in a retention plane. In this study, a brief background review to quantification in GC×GC is provided. Previous reviews cover aspects of quantitative GC×GC studies up to the year 2005, including different approaches to quantification, and reports of quantitative analysis with different detectors, for different compounds classes, and in different matrices. Other studies have developed chemometric approaches based on multivariate analysis to provide quantitative reporting of individual compounds. The coverage of the earlier reviews has been updated to include material that has been presented since 2005 and includes considerations of valve-based modulation. Recently the modulation ratio (M R) concept was proposed and intended to clarify the meaning of modulation number (n M) in GC×GC, which was shown to be a rather poorly defined parameter. Based on the prior studies that introduced this concept, the role of quantitative analysis is investigated here through calculation of the peak areas and peak area ratios of selected series of modulated peaks in GC×GC. The application of isotopically labelled reference compounds for polycyclic aromatic hydrocarbon (PAH) analysis is used here to develop the quantitative metric approach. It is shown that by selecting the two or three major modulated peaks for solutes and internal standards, comparing the response ratio with the sum of all modulated peaks and also with the reference non-modulated result, quantification is statistically equivalent. Thus, adequate quantitative analysis and calibration can be accomplished by using selected major modulated peaks for each compound. This may simplify quantitative interpretation of GC×GC data. [Copyright &y& Elsevier]

Abstract: One micrometre silica particles, derivatized with C18, were electrokinetically packed into a 75-μm-i.d. capillary. The resulting column was evaluated for the separation of trimethoprim (TMP) and its impurities using pressurized capillary electrochromatography (pCEC), starting from a capillary liquid chromatographic (CLC) separation. These samples require gradient elution when separated by high performance liquid chromatography (HPLC), but with the new columns isocratic elution suffices for their separation by CLC or pCEC. Only 70,000 theoretical plates/m for impurity C were achieved using CLC mode at relative low pressure (78bar) although very small particles were utilized. When a voltage above 2kV (50V/cm) was applied, unknown peaks appeared, which was assumed due to an electrophoretic effect with the unknown peaks resolving as a result of the applied voltage. In order to minimize these unfavorable contributions, only a low voltage was applied, still leading to higher separation performances and shorter separation times than in CLC. The optimal analyzing conditions in pCEC included a pressure of 78bar, an applied voltage of 1kV, and a mobile phase consisting of 80mM sodium perchlorate (pH 3.1)/methanol (60/40, v/v). These conditions were used to separate and quantify four major impurities in TMP within 22min. The obtained calibration curves were linear (r >0.9980) in concentration ranges between 0.005 and 0.1mg/mL for impurities A and C; 0.02 and 0.10mg/mL for impurity F; and 0.01 and 0.10mg/mL for impurity H. The detection limits (S/N=3) for impurities A, C, F, and H were 0.52, 0.84, 3.18, and 2.41μg/mL, respectively. The calibration curves were successfully applied to analyze spiked bulk samples, with mean recoveries ranging from 92% to 110%. The developed method can therefore be considered simple, rapid, and repeatable. [Copyright &y& Elsevier]

Abstract: Passive sampling technology has developed very quickly in the past 15 years, and is widely used for the monitoring of pollutants in different environments. The design and quantification of passive sampling devices require an appropriate calibration method. Current calibration methods that exist for passive sampling, including equilibrium extraction, linear uptake, and kinetic calibration, are presented in this review. A number of state-of-the-art passive sampling devices that can be used for aqueous and air monitoring are introduced according to their calibration methods. [Copyright &y& Elsevier]

Abstract: The sample matrix can cause an enhancement in the observed chromatographic response for pesticide residues in a matrix extract compared with the same concentration in a matrix-free solution. The matrix increases the transfer of pesticides from hot vaporizing injectors by reducing the thermal stress for labile compounds and by masking active sites in the injector responsible for the adsorption or decomposition of polar pesticides. The use of different injector types and matrix simplification procedures can reduce matrix-induced enhancement but do not eliminate it. The most effective strategy is to use matrix-matched calibration standards or analyte protectants which equalize the response enhancement for calibration standards and sample extracts. From a practical point of view it is important that the method used to correct for matrix-induced enhancement is compatible with low system maintenance. The different approaches for correcting matrix-induced enhancement for calibration in pesticide residue analysis are discussed and compared in this review. [Copyright &y& Elsevier]

Abstract: This paper reviews calibration-, uncertainty-, and recovery-related documents from 10 consensus-based organizations. The main points from each treatise are summarized. Also included is a critique of the various approaches, as well as recommendations for a statistically sound protocol that is more compatible with chromatographic data. [Copyright &y& Elsevier]

Abstract: Chemometric techniques for calibration with three-way signals are sufficiently developed for their use in routine analysis. The advantage of the second order property (the possibility of quantifying an analyte in the presence of interferents) together with the guarantee of the uniqueness of the decomposition, what means to extract the signal corresponding only to the analyte of interest, make these calibration techniques especially useful for the quantification and identification of analytes in complex samples. This has a particular interest in the identification and quantification of banned substances or substances with a specified maximum limit. The paper describes the theory of the calibration methodology in relation to the signal order and then focuses the analysis on the three-way techniques commonly used in calibration: n-way partial least squares, multivariate curve resolution and parallel factor analysis. The figures of merit needed for the accreditation of analytical methods are analyzed from the viewpoint of n-way calibrations in chromatography. [Copyright &y& Elsevier]

Abstract: The bases of multivariate calibration are presented with special attention to some points usually not considered or underevaluated, i.e., the sampling design, the number of samples necessary to obtain a reliable regression model, the effect of noisy predictors, the significance of the parameters used to evaluate the performance ability of the regression model. [Copyright &y& Elsevier]

Abstract: Calibration is an operation whose main objective is to know the metrological status of a measurement system. Nevertheless, in analytical sciences, calibration has special connotations since it is the basis to do the quantification of the amount of one or more components (analytes) in a sample, or to obtain the value of one or more analytical parameters related with that quantity. Regarding this subject, the aim of analytical calibration is to find an empiric relationship, called measurement function, which permits subsequently to calculate the values of the amount (x-variable) of a substance in a sample, from the measured values on it of an analytical signal (y-variable). In this paper, the metrological bases of analytical calibration and quantification are established and, the different work schemes and calibration methodologies, which can be applied depending on the characteristic of the sample (analyte+matrix) to analyse, are distinguished and discussed. Likewise, the different terms and related names are clarified. A special attention has been paid to those analytical methods which use separation techniques, in relation with its effect on calibration operations and later analytical quantification. [Copyright &y& Elsevier]

Abstract: Methods were developed for the extraction from soil, identification, confirmation and quantitation by LC/MS/MS of trace levels of perfluorooctanoic acid (PFOA), perfluorononanoic acid (PFNA) and perfluorodecanoic acid (PFDA). Whereas PFOA, PFNA and PFDA all can be quantitated using the method of standard additions, PFOA also can be quantitated less laboriously using 13C4-PFOA as a matrix internal standard. The impact of extract matrices on signal varied between soils and temporally during analytical runs rendering 13C4-PFOA unsuitable as a matrix internal standard for quantitating perfluorinated carboxylic acids (PFCAs) other than PFOA, which co-elutes with 13C4-PFOA. In fact, for soil extracts, quantitation of PFCAs based on external calibrations proved about as accurate as use of matrix internal standards for target analytes that do not co-elute with the matrix internal standard. Also, 13C4-PFOA should be used carefully as a matrix internal standard for trace levels of PFOA because some 13C4-PFOA standards contain trace impurities of unlabelled PFOA. When the presence of PFCAs in soil extracts is being determined by LC/MS/MS, detection limits are best defined by statistical methods that quantify the significance of contrast between analytical signal and background noise using multiple analyses. Further, when developing a calibration of low concentrations using weighted regression, the central tendency of the calibration line is best fitted using graphical depictions of error. As the MDL for the transition-product quantitation ion is approached in LC/MS/MS, relatively weak signals of transition-product confirmation ions can be used as a rejection criterion by looking for anomalously high values of the ratio of the confirmation to the quantitation ion. [Copyright &y& Elsevier]

Abstract: The effect of the amount of matrix compounds in comprehensive two-dimensional gas chromatographic (GC×GC) separation of polycyclic aromatic hydrocarbons (PAHs) was investigated. The second dimension column was overloaded by progressively increasing the matrix amount and the separation was evaluated by inspecting peak widths, asymmetries, resolution, retention times, peak areas and volumes of the target compounds. In quantitative analysis, two different calibration methods were tested for both raw sediment extracts and cleaned extracts, i.e. for extracts in which the matrix was removed by a liquid chromatographic clean-up step. The quality of the separation was not significantly disturbed in terms of peak width, asymmetry and resolution when the amount of matrix was increased. However, the depth of the valley between PAH and preceding matrix peak increased with an increase in matrix amount and the repeatabilities of retention times, peak areas and volumes decreased. In quantitative analysis, calibration by using areas had a tendency to underestimate the trace amounts of PAH, especially for the non-cleaned samples. In case of volume calibration, the most accurate results were obtained by external calibration for both cleaned and non-cleaned samples. [Copyright &y& Elsevier]

Abstract: The main sensory defects of virgin olive oils (rancid, vinegary, winey, muddy sediment, musty and vegetable water) and one positive attribute (fruity) characteristic of three monovarietal extra virgin olive oils (Arbequina, Picual and Frantoio) have been quantified using the direct coupling headspace-mass spectrometry. The results obtained were compared with those provided by the panel test for the same samples. Taking into account that no chromatographic separation exists, multivariate calibration techniques (partial least squares, PLS, and principal components regression, PCR) were used to create the appropriate models. The best results, in terms of standard error of prediction and prediction residual error sum of squares were obtained by PLS and therefore it was used for the prediction of a new set of samples with the above-mentioned positive and negative attributes at different concentration levels. The samples were also assessed by the panel test and good correlations were obtained in all cases. In order to extend the applicability of the model with the time, a multiplicative calibration transfer was used. The benefit of this approach was found to be more marked for the negative than the positive attributes. [Copyright &y& Elsevier]

Abstract: A bi-dimensional (non-orthogonal) chromatographic system made of two columns of different length (longer along the first direction and shorter on the second) but packed with identical (or similar) materials has been employed to perform automated on-line fraction analysis and peak deconvolution of multicomponent overloaded band profiles. Multicomponent overloaded peaks, eluted from the first column, are sampled through a remotely-controlled switching valve and fractions are injected, under analytical conditions, directly into the second direction. This set-up serves a twofold purpose. First of all, it has been possible to use linear calibration curves for detector calibration (DC). Secondly, since single component peaks were resolved on the second direction, individual concentration profiles have been obtained beneath the envelop of overlapping bands. All the information necessary to operate the bi-dimensional system can be achieved by analyzing the chromatographic behavior of the mixture on the shorter column under linear conditions. Therefore, solutions of unknown concentration can be prepared and their composition reconstructed with a simple chromatographic run. Two- and three-component overloaded mixtures have been employed to validate the system. [Copyright &y& Elsevier]

Abstract: Solid-phase microextraction (SPME) was evaluated for the detection and quantification of the gas-phase dicarbonyls, glyoxal (GLY) and methylglyoxal (MGLY). Additionally, polydimethylsiloxane (PDMS), polydimethylsiloxane/divinylbenzene (PDMS/DVB), and carbowax/divinylbenzene (CW/DVB) fibers were tested to determine the optimum fiber for detection of these species. GLY and MGLY were derivatized with O-(2,3,4,5,6-pentafluorobenzyl)-hydroxylamine hydrochloride (PFBHA), extracted with SPME from headspace or bag chamber and then analyzed by GC/MS. The PDMS/DVB SPME fiber for on-fiber derivatization and subsequent sampling for gas-phase methylglyoxal provided the optimum combination of analytical reproducibility and sensitivity. Linearity of the calibration curve was achieved across a range of 11–222μg/m3 (4–75ppb). [Copyright &y& Elsevier]

Abstract: A separation and determination method for the analysis of cyanometallic complexes of Fe(II), Ni(II) and Co(III) was developed to be applied to the analysis of petroleum refinery streams (sour water). Ion-interaction chromatography was used employing an analytical column IonPac NS1 10μm and a chromatographic system ICS 2500 equipped with a membrane conductivity suppression ASRS ultra 4mm, both supplied by Dionex Corporation. The mobile phase was composed of 2mmoll−1 TBAOH, 1mmoll−1 Na2CO3, 0.1moll−1 NaCN and ACN (77:23, v/v), flowing at 0.7mlmin−1. At the optimized conditions, detection limits estimated by the calibration curve parameters and relative standard deviation were: 0.002mgCNl−1 and 3.1% for Fe(CN)6 4−; 0.003mgCNl−1 and 2.5% for Ni(CN)4 2− and 0.003mgCNl−1 and 2.8% for Co(CN)6 3−. Sour water samples without any pretreatment (except membrane filtration) from a petroleum refinery in Brazil were analyzed successfully by external calibration method. [Copyright &y& Elsevier]

Abstract: A recently introduced two-dimensional fatty acid retention index system (2D-FARI) was used as basis for prediction of equivalent chain lengths (ECL) of fatty acid methyl esters (FAME) on a BPX-70 stationary phase. Models for the relationship between 2D-FARI data and ECL values of a calibration sample with 30 common fatty acids were established by a simple multivariate regression. The models were thereafter applied on 2D-FARI data for other FAMEs and used to predict the ECLs for these compounds. The 2D-FARI values for the fatty acids in the calibration sample are given by definition. Thus, the only information necessary to calculate the ECL value for a compound run under identical conditions as the calibration sample is the 2D-FARI values for the compound, which can be acquired from literature data. The method was validated with test sets analysed with different temperature and flow programs. ECLs of various marine FAME and trans isomers of Eicosapentaenoic and Docosahexaenoic acid were predicted with root mean squared error of prediction from 0.002 to 0.012 ECL units. [Copyright &y& Elsevier]

Abstract: An ISO GUM measurement uncertainty estimation procedure was developed for a liquid-chromatographic drug quality control method—assay of simvastatin in drug formulation. In quantification of uncertainty components several practical approaches for including difficult-to-estimate uncertainty sources (such as uncertainty due to peak integration, uncertainty due to nonlinearity of the calibration curve, etc.) have been presented. Detailed analysis of contributions of the various uncertainty sources was carried out. The results were calculated based on different definitions of the measurand and it was demonstrated that unequivocal definition of the measurand is essential in order to get rigorous uncertainty estimate. Two different calibration methods – single-point (1P) and five-point (5P) – were used and the obtained uncertainties and uncertainty budgets were compared. Results calculated using 1P and 5P calibrations agree very well. The uncertainty estimate for 1P is only slightly larger than with 5P calibration. [Copyright &y& Elsevier]

Abstract: The paper presents the results of research on the calibration of permeation passive samplers equipped with polydimethylsiloxane (PDMS) membranes using the physico-chemical properties of the analytes. Strong correlations were found between the calibration constants of the samplers and the linear temperature-programmed retention indices of the analytes determined on columns coated with pure PDMS (r 2 =0.914). These correlations make it possible to estimate the calibration constants for unidentified analytes, which is impossible when using conventional procedures. This, in turn, enables the deployment of permeation passive samplers in the same way in which active samplers are deployed. The reproducibility of the calibration constants determined in different laboratories and retention indices determined using different chromatographic systems was very good, indicating that the calibration constants estimated using this approach should be reproducible as well. The approach proposed should lead to more widespread use of permeation passive samplers. [Copyright &y& Elsevier]

Abstract: The aim of this study was to identify and quantify an unknown peak in the chromatogram of a very complex mixture, a straight oil metalworking fluid (MWF). The fraction that permeated through a thin nitrile polymer membrane had less mineral oil background than the original MWF did at the retention time of the unknown peak, thus facilitating identification by total ion current (TIC) gas chromatography–mass spectrometry (GC–MS). The peak proved to be di-n-octyl disulfide (DOD) through retention time and mass spectral comparisons. Quantitation of DOD was by extracted ion chromatogram analysis of the DOD molecular ion (mass-to-charge ratio (m/z) 290), and of the m/z 71 ion for the internal standard, n-triacontane. Linear models of the area ratio (y) of these two ions versus DOD concentration showed a systematic negative bias at low concentrations, a common occurrence in analysis. The linear model of y 0.8 (from Box–Cox power transformation) versus DOD concentration showed negligible bias from the lowest measured standard of 1.51mg/L to the highest concentration tested at 75.5mg/L. The intercept did not differ statistically from zero. The concentration of DOD in the MWF was then calculated to be 0.398±0.034% (w/w) by the internal standard method, and 0.387±0.036% (w/w) by the method of standard additions. These two results were not significantly different at p ≤0.05. The Box–Cox transformation is therefore recommended when the data for standards are non-linear. [Copyright &y& Elsevier]

Abstract: Calibration of SPME by the injection of liquid standard solution is a common method for SPME quantitative analysis by gas chromatography. The feasibility of this calibration method relies on the assumption that sample transfer efficiencies are the same for both the liquid injection and the SPME fiber injection. Sample transfer efficiencies for liquid injection and SPME fiber injection were studied in this paper. The results showed that the sample transfer efficiency for liquid injection was affected by several factors, such as the dimensions of the liner, the presence of the wool, and the temperature of the injector. The sample transfer efficiency for SPME fiber injection was affected by the cross-sectional area of the space between the column and the liner, the carrier gas flow rate, and the length of the column inside the liner. An equation was proposed to estimate the sample loss rate for SPME injection. It was found that the use of a direct injection (DI) liner and program temperature vaporizing provides high sample transfer efficiencies, for both liquid injection and SPME fiber injection. When a common SPME straight liner is used, large outer diameter (o.d.) pre-column will help to achieve high sample transfer efficiency. [Copyright &y& Elsevier]

Abstract: Dimethylselenide (DMSe) and dimethyldiselenide (DMDSe) were determined in plant and water samples by capillary gas chromatography using microwave induced-plasma atomic emission spectrometry for detection. The analytes were leached from the solid samples into methanol by using an ultrasonic probe, and a portion of the extract was preconcentrated by means of a purge-and-trap system before being chromatographed. The analytes were directly purged from the water samples in the presence of 6% (v/v) methanol. Element-specific detection and quantification was carried out by monitoring the selenium (196nm) emission line. Calibration curves were obtained by plotting peak area versus concentration and the correlation coefficients for linear calibration were 0.9999 for both analytes. Detection limits of 0.8 and 1.1ngl−1 were obtained for DMSe and DMDSe, respectively, for water samples. For plant materials, the detection limits calculated for 0.5g samples were 0.3 and 0.4ngg−1 for DMSe and DMDSe, respectively. Concentration levels of DMSe ranging from 1.2 to 4.2ngg−1 were found in some of the plant materials analyzed. No DMDSe was found in any of the samples. The accuracy of the method was checked by analyzing different spiked water and plant samples. [Copyright &y& Elsevier]

Abstract: The accurate and reproducible analysis of long-chain polyunsaturated fatty acids (PUFA) is of growing importance. Especially for labeling purposes, clear guidelines are needed in order to achieve optimum accuracy. Since calibration standards cannot be used for method validation due to the instability of PUFAs, there is no direct way to check for the absence of systematic errors. In this study the sources of error that weaken the accuracy were evaluated using theoretical considerations and calibration standards with corrected composition. It was demonstrated that the key role for optimum accuracy lies in the optimization of the split injection system. Even when following the instructions outlined in the official methods of the American Oil Chemist''s Society (AOCS), systematic errors of more than 7% can arise. Clear guidelines regarding system calibration and selection of appropriate internal standards (IS) can improve precision and accuracy significantly. [Copyright &y& Elsevier]

Abstract: A calibration curve is often needed to derive from the record of the detector signal the actual concentration profile of the eluate in many studies of the thermodynamic and kinetic of adsorption by chromatography. The calibration task is complicated in the frequent cases in which the detector response is nonlinear. The simplest approach consists in preparing a series of solutions of known concentrations, in flushing them successively through the detector cell, and in recording the height of the plateau response obtained. However, this method requires relatively large amounts of the pure solutes studied. These are not always available, may be most costly, and could be applied to better uses. An alternative procedure consists of deriving this calibration curve from a series of peaks recorded upon the injection of increasingly large pulses of the studied compound. We validated this new method in HPLC with a UV detector. Questions concerning the reproducibility and accuracy of the method are discussed. [Copyright &y& Elsevier]

Abstract: A new method consisting of using band profiles to calibrate the response of a detector, without making frontal analysis measurements, is discussed. A model for the calibration-curve expression is proposed. It is convenient and reasonably accurate even when the detector response deviates slightly from linear behavior at low concentrations. [Copyright &y& Elsevier]

Abstract: A new HPLC method with fluorescence detection using pyridinium hydrobromide perbromide as a post-column derivatising agent has been developed to determine aflatoxin M1 in milk and cheese. The detection limits were 1ng/kg for milk and 5ng/kg for cheese. The calibration curve was linear from 0.001 to 0.1ng injected. The method includes a preliminary C18-SPE clean-up and the average recoveries of Aflatoxin M1 from milk and cheese, spiked at levels of 25–75ng/kg and 100–300ng/kg, respectively, were 90 and 76%; the precision (RSDr) ranged from 1.7 to 2.6% for milk and from 3.5 to 6.5% for cheese. The method is rapid, easily automatable and therefore useful for accurate and precise screening of aflatoxin M1 in milk and cheese. [Copyright &y& Elsevier]

Abstract: Restricted access material (RAM) has been used in the packing of a solid-phase extraction (SPE) column for on-line extractions under turbulent flow conditions. The bio-compatible RAM material works by the principle of size exclusion in addition to conventional reversed-phase chromatography, thereby allowing the extraction and preconcentration of small analyte molecules from biological samples such as plasma. Using small column dimensions (0.76mm×50mm) and a consequently high linear velocity, turbulent flow was achieved during online sample extractions. The improved mass-transfer rate characteristic of turbulent flow allows fast sample cleanup without decreased extraction efficiency. The novel use of the RAM column, connected upstream to a C18 monolithic column, allowed the direct injection, extraction, separation, and MS/MS detection of plasma samples spiked with rofecoxib in a span of 5min. Calibration curves obtained using this RAM turbulent flow coupled column method showed good linearity (R 2 >0.99) and reproducibility (%RSD≤7%). The lower limit of quantitation of rofecoxib in plasma samples was found to be 40ng/ml. The extraction method showed good recovery of rofecoxib from a plasma matrix with minimal signal loss and robustness after more than 200 plasma injections. [Copyright &y& Elsevier]

Abstract: A screening method for analysis of perfluoroalkylated substances (PFAS) in biota samples has been developed and validated using liver samples from polar cod (Boreogadus saida) and glaucous gull (Larus hyperboreus). The method was based on extraction of target compounds from homogenised samples into the solvent mixture used as mobile phase in high-performance liquid chromatography (HPLC), i.e. methanol/water (50:50; 2mM ammonium acetate). The extract was filtered and directly injected into a HPLC/time-of-flight mass spectrometry (TOF-MS) system. Quantification was performed using 7H-perfluoroheptanoic acid as internal standard and a calibration standard solution dissolved in sample extract for each matrix type (matrix-matched calibration standard). The method is very time and cost efficient. Except for long-chain compounds and perfluorooctane sulfonamide (which cannot be covered by this method), recoveries were between 60% and 115% and method detection limits were in the range 0.04–1.3ng/g wet weight. Blank values could be neglected with the exception of perfluorooctane sulfonate (PFOS), perfluorohexanoic acid (PFHxA) and perfluorooctanoic acid (PFOA). One of the major challenges in PFAS analysis is ionisation disturbance by co-eluting matrix in the ion source of the mass spectrometer. Both matrix and analyte specific signal enhancement and suppression was observed and quantified. Repeated extractions (n =3) gave relative standard deviations (RSD) <35% for all PFAS. Accuracy was examined by comparing the screening method to the generally applied ion pair extraction (IPE) method. PFAS concentration values of a glaucous gull liver sample deviated by less than 30% for the two methods, provided that matrix-matched calibration standards were employed in both methods. [Copyright &y& Elsevier]

Abstract: In many studies of nonlinear or preparative chromatography, chromatographic signals must be recorded for relatively concentrated solutions and the detectors, that are designed for analytical applications and are highly sensitive, must be used under such experimental conditions that their responses are often nonlinear. Then, a calibration curve is needed to derive the actual concentration profiles of the eluates from the measured detector response. It becomes necessary to derive a relationship between the concentration of the eluent and the detector signal at any given time. The simplest approach consists in preparing a series of solutions of known concentrations and in flushing them successively through the detector cell, recording the height of the plateau response obtained. However, this method requires relatively large amounts of the pure solutes being studied and these are not always available or they may be most costly, although these solutions can be recovered. We describe and validate an alternative procedure providing this calibration from a series of peaks recorded upon the injection of increasingly large pulses of the studied compound. [Copyright &y& Elsevier]

Abstract: This note describes a fundamental investigation into solid-phase microextraction (SPME) using a standard loaded into the fibre coating as a means of internal standardisation for the analysis of samples contained in vials. The loading of reproducible amounts of standards into a non-porous SPME fiber was investigated. It was found that spiking low milligram quantities of standards such as benzene, toluene, ethylbenzene, xylenes (BTEX) and/or naphthalene into a few grams of pump oil sealed in a 20mL vial provided an excellent standard generator. A single solution allowed over a hundred standard loadings with a reproducibility of <4% R.S.D. When a fiber, loaded with the standard(s) was introduced into a sample vial, extraction of analytes into the fiber and desorption of the standard(s) into the sample matrix occur simultaneously. Quantification was then based on the equilibrium distribution of the standards and the analytes between the fibre coating and the sample matrix in the vial. A comparison of equilibration profiles obtained using traditional internal standardisation and the in-fibre approach generally showed the same equilibration behaviour. The developed method was successfully used to correct for matrix effects in the BTEX analysis of a wine sample. [Copyright &y& Elsevier]

Abstract: This paper reports an improved headspace gas chromatographic (GC) technique for determination of monomer solubilities in water. The method is based on a multiple headspace extraction GC technique developed previously [X.S. Chai, Q.X. Hou, F.J. Schork, J. Appl. Polym. Sci., in press], but with the major modification in the method calibration technique. As a result, only a few iterations of headspace extraction and GC measurement are required, which avoids the “exhaustive” headspace extraction, and thus the experimental time for each analysis. For highly insoluble monomers, effort must be made to minimize adsorption in the headspace sampling channel, transportation conduit and capillary column by using higher operating temperature and a short capillary column in the headspace sampler and GC system. For highly water soluble monomers, a new calibration method is proposed. The combinations of these technique modifications results in a method that is simple, rapid and automated. While the current focus of the authors is on the determination of monomer solubility in aqueous solutions, the method should be applicable to determination of solubility of any organic in water. [Copyright &y& Elsevier]

Abstract: Applications of solid-phase microextraction (SPME) in the measurement of very hydrophobic organic compounds (VHOCs) are limited, partly due to the difficulty of calibrating SPME fibers for VHOCs. This study used a static SPME strategy with a large sample volume (1.6L) and a five-point calibration procedure to determine the distribution coefficients for a large suite of polychlorinated biphenyls (PCBs) and chlorinated pesticides between a polydimethylsiloxane (PDMS) phase (100μm thickness) coated on a glass fiber and seawater. An extraction time of 12 days was deemed adequate for equilibrium calibration from kinetic experiments. Two groups of randomly selected fibers divided into three batches (up to nine fibers in each batch) were processed separately with two gas chromatography–mass spectrometry (GC–MS) systems. Matrix effects arising from losses of the analytes to glass container walls and stirring bars were corrected. Relative standard deviations within the same batch were generally smaller than those for the entire group. Furthermore, K f V f (K f and V f are the distribution coefficient of an analyte between the polymer-coated fiber and aqueous phase and the fiber volume, respectively) values determined with two GC–MS systems were statistically different. These results indicate the calibrated K f V f values were less affected by the random selection of SPME fibers than by other experimental conditions, and therefore average K f V f values may be used for the same type of commercially available SPME fibers. The relative accuracy of our calibration method was similar to that of a previous study [P. Mayer, W.H.J. Vaes, J.L.M. Hermens, Anal. Chem. 72 (2000) 459] employing different coating thickness and calibration procedure. The present study also obtained a bell-shaped relationship between log K f and log K ow (octanol–water partition coefficient) for PCB congeners with the maximum log K f corresponding to log K ow ∼6.5. This bell-shaped relationship was attributed mainly to steric effects arising from the interplay between the PDMS thickness and molecular sizes of the target analytes. [Copyright &y& Elsevier]

Abstract: Interactive liquid chromatography (iLC) for polymer analysis is usually applied to the characterisation of distributions other than molar mass. In particular, its use for the determination of chemical-composition, functionality-type and tacticity distributions has been demonstrated. The application of iLC for the determination of molar mass distributions (MMDs), however, has not yet been fully explored. An expanded version of the reversed-phase liquid chromatography model has been developed to describe and predict how the retention behaviour of polydisperse polystyrene samples changes with molar mass. The relationship between molar mass and the parameters of the model has been investigated in some detail and non-linear correlations were found. From the model and the relationships between the model parameters and molar mass, calibration curves (retention time versus molar mass) were constructed to predict changes in chromatographic selectivity across a given molar mass range. These calibration curves were compared to experimentally obtained curves and, in most cases, excellent agreement was found. The dramatic enhancement in selectivity that can be obtained with iLC in comparison to size-exclusion chromatography (SEC) was illustrated by measuring matrix-assisted laser desorption ionisation (MALDI) MS spectra of fractions collected during a gradient-LC separation. In the low-molar mass range, essentially monodisperse fractions were obtained. Calibration curves, predicted by the model and validated experimentally using narrow-dispersity standards and MALDI-MS spectra of fractions, were used to determine the molar mass distribution of some narrowly distributed polystyrene samples. Molar mass distributions for such standards were found to be somewhat lower than the values reported by the manufacturers. The results also deviated from those obtained by MALDI-MS. [Copyright &y& Elsevier]

Abstract: The aim of this study was to evaluate the applicability of ECHO technique in pesticide residue analysis using LC/MS/MS instruments with atmospheric pressure chemical (APCI) and electrospray (ESI) ionization. The technique is based on simultaneous injections of reference standards and samples in one run. First and second injections are made ahead and behind a precolumn, respectively, thus resulting in a short difference of retention times between standard and sample peak. The obtained couple of peaks were applied to the easy detection of pesticides and simultaneous estimation of the residue content in real samples in a single run. If residues were not observed, the second sample peak did not occur and the ECHO peaks were used to demonstrate instrument performance in each run and for each analyte. Another advantage of ECHO technique is its potential to compensate matrix effects. The occurrence and compensation of matrix effects using APCI was tested with four matrix types (water containing, acidic, dry and sugar containing) and 22 pesticides. The same matrix types but 58 pesticides were used tests with electrospray ionization. Most often matrix effects had been observed with lemon. The percentage of pesticides showing significant matrix effects did not differ between APCI and ESI. But these effects caused signal enhancement in APCI measurements and signal suppression, when ESI was used. The ECHO technique was able to compensate many matrix effects in measurements with both types of ion sources. [Copyright &y& Elsevier]

A commercially available evaporative light-scattering detection (ELSD) system was adapted for micro and capillary LC. Therefore the various parameters involved in the droplet formation during the nebulization step in the ELSD system were studied. It was shown that the velocity term in the Nukiyama Tanasawa equation remains constant, leading to droplets of the same order of magnitude for narrow bore and capillary columns. Consequently, the ELSD modification was performed by decreasing the internal diameter of the effluent capillary tube in the nebulizer nozzle and by keeping its external diameter constant. Next, response curves for a conventional and the developed micro and capillary LC were compared as to investigate why a linear ELSD response is often obtained when used in micro or capillary LC. By splitting the flow rate post column, we showed that the nebulization process was not at the origin of the phenomenon. For ceramide III and tripalmitin, the response curves were found to be non-linear. However the curvature was less significant when the columns internal diameter decreased. Calculated particle size profiles for micro or capillary LC suggest that the particle entering the detection chamber are bigger than under conventional LC conditions. Last, triethylamine and formic acid were used to increase the response of the detector. The response enhancement, expected from previous studies, was established for the two lipids involved in this study. [Copyright &y& Elsevier]

A microwave-assisted desorption method was developed and coupled with solid-phase microextraction and GC–MS for the analysis of pyrethroid residues in strawberries. In the first step, pyrethroid analytes were desorbed from the whole fruits in an aqueous acetonitrile solution at 50% under microwave assistance, so preventing these compounds to be captured with strong matrix effects by endogenous constituents. Then, the 100 μm poly(dimethylsiloxane)-coated fibre was exposed for 30 min in the obtained extracting solution. Calibration curves, realised from blank strawberries spiked at different concentrations with standards, showed a linear range between 1 μg/kg and 250 μg/kg with r2>0.992 and variation coefficients below 15%. Limits of detection and quantitation were found lower than 14 μg/kg and 40 μg/kg, respectively. Observed analysis results by using this method and relative to field incurred strawberry samples were also compared to those obtained by two accredited trading laboratories using traditional methods. [Copyright &y& Elsevier]

A HPTLC–densitometric method, based on an external standard approach, was developed in order to obtain a novel procedure for routine analysis of secoisolariciresinol diglucoside (SDG) in flaxseed with a minimum of sample pre-treatment. Optimization of TLC conditions for the densitometric scanning was reached by eluting HPTLC silica gel plates in a horizontal developing chamber. Quantitation of SDG was performed in single beam reflectance mode by using a computer-controlled densitometric scanner and applying a five-point calibration in the 1.00–10.00μg/spot range. As no sample preparation was required, the proposed HPTLC–densitometric procedure demonstrated to be reliable, yet using an external standard approach. The proposed method is precise, reproducible and accurate and can be employed profitably in place of HPLC for the determination of SDG in complex matrices. [Copyright &y& Elsevier]

The determination of the interdetector volume (IDV) in size-exclusion chromatography (SEC) of polymers with dual multiangle light-scattering/concentration detection based on the s-detection is demonstrated. By s-detection the determination is meant of local molecular mass from a known relation between molecular mass and radius of gyration, obtained from the angle dependence of the intensity of scattered light. IDV is found basing on the stipulation of equal slope of local calibrations found by the dual light-scattering/concentration detection, referred to as the w- and s-detection. As the s-detection uses just the light-scattering detector and, therefore, is independent of the value of IDV, this quantity can be found by searching for its value that gives the local calibration dependence obtained by the dual detection closest to that determined independently by the s-detection. [Copyright &y& Elsevier]

Univariate data accumulated for the purpose of calibration of chromatographic and spectroscopic methods often exhibit slight but definite curvature. In this paper the performance of a non-linear calibration equation with the capacity to account empirically for the curvature, y=a+bxm, (m≠1) is compared with the commonly used linear equation, y=a+bx, as well as the quadratic equation, y=a+bx+cx2. All equations were applied to high quality HPLC calibration data using unweighted least squares. Parameter estimates and their standard errors were calculated for each equation. Standard errors and 95% prediction intervals in analyte concentrations were estimated with the aid of the fitted equations and their respective covariance matrices. Results indicate that the non-linear and quadratic equations each provide a better fit than the linear equation to the data considered here, as judged by the Akaikes information criterion (AIC), the adjusted coefficient of multiple determination, the magnitude and scatter of residuals, standard errors in estimated analyte concentrations and lack of fit analysis of variance (ANOVA). While the difference between the equations y=a+bx+cx2 and y=a+bxm as judged by the same criteria is more marginal, this work suggests that the non-linear calibration equation should be considered when a curve is required to be fitted to low noise calibration data which exhibit slight curvature. [Copyright &y& Elsevier]

Short-term solid-phase microextraction (SPME) was performed to test a recently proposed semi-empirical model for the prediction of concentrations of analyte in water samples from the fibre-extracted mass without further calibration. The mass uptake rates obtained for benzene, toluene, ethylbenzene and p-xylene (BTEX) differ considerably from the before published, showing that interfibre comparability is a serious issue. The relative prediction errors are between −55% for benzene and +82% for p-dichlorobenzene under optimal conditions, i.e. they are by an order of magnitude higher than originally published. A sensitivity analysis shows the dominant influence of the estimated thickness of the diffusional boundary layer around the fibre on the concentration predicted. Empirical modification of the model equation for this parameter yields satisfactory results under the conditions tested for both BTEX and the selected chlorobenzenes. [Copyright &y& Elsevier]

Inorganic and organic anions can be separated on an ordinary silica C18 column using a mobile phase containing tetrabutylammonium hydroxide (TBAH) and an aminosulfonic acid zwitterion reagent (MOPS). The pH of this eluent is close to 7 and the background conductivity is about 50 μS, which is low enough to permit anion analyte detection by direct conductivity. Linear calibration curves were obtained for the six anions studied and detection limits ranged from 0.075 to 0.15 mg/l (ppm) for the five inorganic anions. The method was applied to the determination of water-soluble anions in aerosol samples at concentrations as low as 0.3 mg/l. [Copyright &y& Elsevier]

Multiple solid-phase microextraction (SPME) is an useful technique for the direct quantification of solid samples removing any matrix effect. The volatile organic compounds formed in the extrusion–coating process of multilayer packaging materials have already been quantified by multiple HS-SPME coupled to gas chromatography (GC)–mass spectrometry (MS) using volatile organic compound (VOC) solutions in hexadecane for calibration. In this article, water is proposed as solvent to prepare the calibration solutions because it provides a shorter calibration time, better linearity, better reproducibility, and lower detection limits than hexadecane. Besides, the extraction of VOCs from aqueous solutions is exhaustive and avoids the extrapolations needed to calculate the total peak areas, as they can be calculated as the sum of the individual areas of each extraction. Finally, it is checked whether the two solvents provide the same mean values for the total peak areas. [Copyright &y& Elsevier]