Proficiency testing schemes: solutions for homogeneity control

The organisation of the homogeneity control of samples prepared for proficiency testing schemes (PTS) is a challenge for the PTS organisers and engenders logistical and financial problems. Samples used in PTS need to be homogeneous in order to be sure that if a laboratory has a result different from the other laboratories, its error can be attributed to its analysis method and not to its sample. The present paper describes the solutions found by the Bureau Interprofessionnel dEtudes Analytiques (Bipea) in order to answer the homogeneity control requirement. The methodologies of homogeneity control are adapted to specific characteristics of the products proposed in Bipeas PTS. The first methodology of control, the preparation approach, concerns specific PTS where the samples are formulated and where the assigned values are known and chosen. The second methodology, the target approach, concerns mainly the field of the environment and implies the choice of homogeneity targets. The third methodology, the global approach, concerns the agricultural and agro-food fields and involves comparison of near-IR spectra samples. Each methodology of control has its advantages and disadvantage and is suited to specific products prepared with specific preparation procedures. It has to be stressed that the solutions developed answer quality requirements but are not a substitute for the use of adapted and detailed preparation procedures with validated and regularly checked devices and for the a posteriori control of the results of the laboratories.     

Evaluation of 10-years French NRL proficiency tests for lead, cadmium and mercury analysis in foodstuff of animal origin

The French National Reference Laboratory (NRL) for heavy metals in foodstuffs of animal origin at the French Food Safety Agency (AFSSA) regularly organises proficiency tests schemes (PTS) focused on the quantification of the three elements lead (Pb), cadmium (Cd) and mercury (Hg), for the 23 departmental laboratories of the network of laboratories, under its responsibility. The NRL prepares test materials to promote comparability of data and support harmonisation of official analytical procedures in the case of routine analysis. When target values for the analytes are set, the PTS also provide an objective assessment and improvement of the analytical trueness in the daily work of the laboratories.The long-term performance of the network was examined using results of 10 years of PTS in various matrices from different foodstuffs of animal origin (fish, meat and milk products), with 3 different official methods, using Flame- and Electro-Thermal-AAS techniques. In all cases, participants were requested to carry out 3 independent measurements of Pb, Cd and Hg. Moreover, the assessment of the PTS was based on the Z-score approach. From 1998 to 2008, this evaluation applied to 690 results obtained in 41 PTS indicated a frequency distribution of the Z-score value for the three elements Pb, Cd and Hg of 89.2%, 94.5% and 97.7% of acceptable results, respectively. The results of this balance sheet indicated the homogeneity and ability of the laboratories network to provide comparable results of similar quality. Comparison with results obtained by a primary method were presented as complementary information to determine the interest to organize, in the future, PTS based on a certified value.     

Near-infrared absorption spectroscopy of toluene-sulfonate-diacetylene crystals

The near-infrared (NIR) absorption of toluene-sulfonate-diacetylene crystals (PTS) has been investigated during thermal polymerization. It is shown by comparative NIR and IR spectroscopy of deuterated and of protonated PTS at various temperatures, that these absorptions are vibronic in nature. They can be assigned unambiguously to distinct parts of the molecule. No low-lying electronic state has been found.     

Validation and transferability study of a method based on near-infrared hyperspectral imaging for the detection and quantification of ergot bodies in cereals

In recent years, near-infrared (NIR) hyperspectral imaging has proved its suitability for quality and safety control in the cereal sector by allowing spectroscopic images to be collected at single-kernel level, which is of great interest to cereal control laboratories. Contaminants in cereals include, inter alia, impurities such as straw, grains from other crops, and insects, as well as undesirable substances such as ergot (sclerotium of Claviceps purpurea). For the cereal sector, the presence of ergot creates a high toxicity risk for animals and humans because of its alkaloid content. A study was undertaken, in which a complete procedure for detecting ergot bodies in cereals was developed, based on their NIR spectral characteristics. These were used to build relevant decision rules based on chemometric tools and on the morphological information obtained from the NIR images. The study sought to transfer this procedure from a pilot online NIR hyperspectral imaging system at laboratory level to a NIR hyperspectral imaging system at industrial level and to validate the latter. All the analyses performed showed that the results obtained using both NIR hyperspectral imaging cameras were quite stable and repeatable. In addition, a correlation higher than 0.94 was obtained between the predicted values obtained by NIR hyperspectral imaging and those supplied by the stereo-microscopic method which is the reference method. The validation of the transferred protocol on blind samples showed that the method could identify and quantify ergot contamination, demonstrating the transferability of the method. These results were obtained on samples with an ergot concentration of 0.02 % which is less than the EC limit for cereals (intervention grains) destined for humans fixed at 0.05 %.

Measuring metal homogeneity in a matrix via the measurement of the ratio metal to matrix oxide using ICP-MS

A simple method has been developed that allows a fast determination of the homogeneity of an element M in an alloy, even for minor components. This is done by measuring a ratio of ion currents I, I ^M/I ^M’O, whereby M’ is the matrix element, by inductively coupled plasma mass spectrometry (ICP-MS). The method can be used to determine the homogeneity of one component in a binary alloy and allows to estimate the sample size necessary to minimise uncertainty contributions due to inhomogeneity in the analysis of such an alloy. In this work the homogeneity of a niobium/0.1% zirconium alloy was determined on 1 mg samples. Accurate weighings of these small samples are not required, as the method is based on the measurement of the niobium/zirconium amount ratio in the dissolved samples. As this ratio is fairly large, the Zr/NbO amount ratio was measured instead to decrease the magnitude of the measured ratio. This ratio was found to be sufficiently stable over time for homogeneity testing. In this particular case the Zr/NbO ratio in the samples was found to vary by 0.049 relative for a 1 mg samples size.     

Chemometric methods for extracting information from temperature-dependent near-infrared spectra

Temperature-dependent near-infrared(NIR) spectroscopy is a new technique for measuring the NIR spectra of a sample at different temperatures. Taking the advantage of the temperature effect, the technique has shown its potential in both quantitative and qualitative analysis. The technique has been proved to be powerful in determination of the analytes in complex samples,particularly in studying the functions of water in aqueous systems due to the significant effect of temperature on the NIR spectra of water. Because of the complicated interactions in the samples and the overlapping of the broad peaks in NIR spectra, it is difficult to extract the temperature-dependent information from the spectra. Chemometric methods, therefore, have been developed for improving the spectral resolution and extracting the temperature-induced spectral information. In this review, recent advances in the studies of chemometric methods and the applications in resolution, quantitative and structural analysis of temperature-dependent NIR spectra were summarized.     

Biodiesel classification by base stock type (vegetable oil) using near infrared spectroscopy data

The use of biofuels, such as bioethanol or biodiesel, has rapidly increased in the last few years. Near infrared (near-IR, NIR, or NIRS) spectroscopy (>4000 cm⁻¹) has previously been reported as a cheap and fast alternative for biodiesel quality control when compared with infrared, Raman, or nuclear magnetic resonance (NMR) methods; in addition, NIR can easily be done in real time (on-line). In this proof-of-principle paper, we attempt to find a correlation between the near infrared spectrum of a biodiesel sample and its base stock. This correlation is used to classify fuel samples into 10 groups according to their origin (vegetable oil): sunflower, coconut, palm, soy/soya, cottonseed, castor, Jatropha, etc. Principal component analysis (PCA) is used for outlier detection and dimensionality reduction of the NIR spectral data. Four different multivariate data analysis techniques are used to solve the classification problem, including regularized discriminant analysis (RDA), partial least squares method/projection on latent structures (PLS-DA), K-nearest neighbors (KNN) technique, and support vector machines (SVMs). Classifying biodiesel by feedstock (base stock) type can be successfully solved with modern machine learning techniques and NIR spectroscopy data. KNN and SVM methods were found to be highly effective for biodiesel classification by feedstock oil type. A classification error (E) of less than 5% can be reached using an SVM-based approach. If computational time is an important consideration, the KNN technique (E = 6.2%) can be recommended for practical (industrial) implementation. Comparison with gasoline and motor oil data shows the relative simplicity of this methodology for biodiesel classification.     

Rapid quality assessment of Gentianae Macrophyllae Radix based on near-infrared spectroscopy and capillary electrophoresis

The aim of this study was to establish a rapid quality assessment method for Gentianae Macrophyllae Radix (RGM) using near-infrared (NIR) spectra combined with chemometric analysis. The NIR spectra were acquired using an integrating sphere diffuse reflectance module, using air as the reference. Capillary electrophoresis (CE) analyses were performed on a model P/ACE MDQ Plus system. Partial least squares-discriminant analysis qualitative model was developed to distinguish different species of RGM samples, and the prediction accuracy for all samples was 91%. The CE response values at each retention time were predicted by building a partial least squares regression (PLSR) calibration model with the CE data set as the Y matrix and the NIR spectra data set as the X matrix. The converted CE fingerprints basically match the real ones, and the six main peaks can be accurately predicted. Transforming NIR spectra fingerprints into the form of CE fingerprints increases its interpretability and more intuitively demonstrates the components that cause diversity among samples of different species and origins. Loganic acid, gentiopicroside, and roburic acid were considered quality indicators of RGM and calibration models were built using PLSR algorithm. The developed models gave root mean square error of prediction of 0.2592% for loganic acid, 0.5341% for gentiopicroside, and 0.0846% for roburic acid. The overall results demonstrate that the rapid quality assessment system can be used for quality control of RGM.     

Rapid determination of hydrogen peroxide in whitening patches for teeth using a new portable near-infrared spectrometer

A nondestructive determination method of hydrogen peroxide in whitening patches for teeth was developed by using a new portable near-infrared (NIR) spectrometer. Development of the portable NIR spectrometer was based on microchip technologies with photodiode arrays. By using the portable NIR spectrometer, the new determination method is very rapid; it requires less than 1 s. The conventional method for the determination of hydrogen peroxide, redox titration, requires about 2 h of analysis, including the sample extraction time from a sample matrix. The conventional method also uses hazardous and harmful solvents and, furthermore, its samples cannot be used after titration. To find the peak due to the O–H bond vibration of hydrogen peroxide under the existence of water which shows huge absorption O–H absorption around 1450 nm, the NIR spectra of a hydrogen peroxide aqueous solution were investigated. A clear variation of absorption based on the concentration of the hydrogen peroxide due to the O–H bond vibration was found in the standard deviation plot around 1400 nm. In this study, two kinds of whitening patch products, A and B, were used for samples. A partial least squares (PLS) regression was used for calibration and validation in the 1100 to 1720 nm spectral range. For validation results, the standard error of prediction (SEP) was 0.38% for Patch A and 0.37% for Patch B. This study shows the feasibility of using the portable NIR spectrometer with photodiode arrays for the rapid and safe determination of hydrogen peroxide in whitening patches.     

罗丹明B纯度标准物质的研制

以商购罗丹明B为原料,进行重结晶提纯,制备罗丹明B纯度标准物质。采用红外光谱(IR)、高分辨质谱和核磁共振谱(NMR)进行定性分析。样品分装400瓶后,依据国家《一级标准物质研制技术规范》,采用液相色谱-二极管阵列法对随机抽取的15瓶样品进行均匀性检验,经F检验表明,在95%的置信区间范围内,样品均匀性良好。在室温下,经过30个月稳定性考察,结果表明样品稳定性良好。标准物质经国内8家具有分析资质的实验室进行协同定值,定值结果为99.0%,扩展不确定度为1.0%(k=2)。该标准物质达到了国家标准物质的技术要求,可用于有关罗丹明B的分析方法校正和质量控制。     

Measuring metal homogeneity in a matrix via the measurement of the ratio metal to matrix oxide using ICP-MS

A simple method has been developed that allows a fast determination of the homogeneity of an element M in an alloy, even for minor components. This is done by measuring a ratio of ion currents I, I ^M/I ^M’O, whereby M’ is the matrix element, by inductively coupled plasma mass spectrometry (ICP-MS). The method can be used to determine the homogeneity of one component in a binary alloy and allows to estimate the sample size necessary to minimise uncertainty contributions due to inhomogeneity in the analysis of such an alloy. In this work the homogeneity of a niobium/0.1% zirconium alloy was determined on 1 mg samples. Accurate weighings of these small samples are not required, as the method is based on the measurement of the niobium/zirconium amount ratio in the dissolved samples. As this ratio is fairly large, the Zr/NbO amount ratio was measured instead to decrease the magnitude of the measured ratio. This ratio was found to be sufficiently stable over time for homogeneity testing. In this particular case the Zr/NbO ratio in the samples was found to vary by 0.049 relative for a 1 mg samples size. Received: 30 November 1998 / Revised: 22 March 1999 / Accepted: 27 March 1999     

A new approach to determine the homogeneity in hyperspectral imaging considering the particle size

Determining homogeneity of a mixture using hyperspectral-imaging (HSI) techniques is at this moment an interesting research subject in pharmaceutical industry. A new methodology based in the macropixel analysis technique for the homogeneity assessment in powder blend mixtures has been developed. The appropriate selection of the macropixel size for evaluation of the homogeneity is a topic under discussion. We propose that the macropixel size selection would be based in a statistical criterion called “representative sample size” criterion. Moreover, it has been demonstrated that the macropixel size should be a function of the particle size and, thus, the particle size has been included in the formula to calculate the macropixel size. The proposed equations allow to accurately calculate the homogeneity index (H% Poole). By means of using simulated chemical images, a relationship between the H% Poole index and the controlled homogeneity was established. The method has been applied to determine the homogeneity of binary and ternary powder blends mixtures of different pharmaceuticals discriminating between homogeneous and non-homogeneous samples.     

One-PCR-tube approach for in situ DNA isolation and detection

Traditional real-time polymerase chain reaction (PCR) requires a purified DNA sample for PCR amplification and detection. This requires PCR tests be conducted in clean laboratories, and limits its applications for field tests. This work developed a method that can carry out DNA purification, amplification and detection in a single PCR tube. The polypropylene PCR tube was first treated with chromic acid and peptide nucleic acids (PNA) as DNA-capturer were immobilized on the internal surface of the tube. Cauliflower mosaic virus 35S (CaMV-35S) promoter in the crude extract was hybridized with the PNA on the tube surface, and the inhibitors, interfering agents and irrelevant DNA in the crude extract were effectively removed by rinsing with buffer solutions. The tube that has captured the target DNA can be used for the following real-time PCR (RT-PCR). By using this approach, the detection of less than 2500 copies of 35S plasmids in a complex sample could be completed within 3 hours. Chocolate samples were tested for real sample analysis, and 35S plasmids in genetically modified chocolate samples have been successfully identified with this method in situ. The novel One-PCR-tube method is competitive for commercial kits with the same time and simpler operation procedure. This method may be widely used for identifying food that contains modified DNA and specific pathogens in the field.     

Determination of polycyclic aromatic hydrocarbons in environmental samples by high performance thin-layer chromatography and fluorescence scanning densitometry

High performance thin-layer chromatography (HPTLC) with fluorescence scanning densitometry was used for the quantitative determination of polycyclic aromatic hydrocarbons in the soluble organic fraction of air particulate samples. A method using normalized emission response ratios was developed to determine sample identity and to test for peak homogeneity. To preserve the high sample throughput of HPTLC, the two-point calibration method was used for quantitation. The principal advantages of HPTLC as a screening technique for environmental samples are its low cost, methodological simplicity, high sample throughput, and the ability to analyze crude samples with a minimum amount of sample cleanup.     

蜂王浆冻干粉中甲硝唑残留标准样品制备的研究

建立了蜂王浆冻干粉中甲硝唑残留标准样品的研制和定值方法。对日常检测阳性样品进行匀浆、冷冻干燥和均匀化加工处理,真空包装,得到1批400袋蜂王浆冻干粉样本。F检验法和t检验法表明在95%置信区间内,样本均匀性、短期稳定性和长期稳定性均达到标准样品要求。制备的甲硝唑标准样品采用液相色谱-串联质谱法,与国内外8家实验室进行协同定值。采用格拉布斯和柯克伦检验对定值结果进行异常值检验,并对结果进行不确定度评估。     

Evaluation of a Brazilian ion chromatography interlaboratory study

This paper describes a Brazilian interlaboratory program study on anion measurement in synthetic water. The program described is promoted regularly since 2007 and recommended the use of ion chromatography as analytical technique for all participant laboratories. Two samples (X and Y) with different anion (fluoride, chloride, nitrite-N, nitrate-N, sulfate and phosphate-P) concentration levels were twice distributed in 2011. Each sample on each round had the homogeneity, and the stability tested for a period of 15 days. Upon ensuring the homogeneity and stability, the samples were distributed to 39 laboratories located around the country. The aim of this study was to verify the laboratories’ precision and to establish the measurement comparability among Brazilian laboratories that routinely use ion chromatography for water sample analysis. It was also possible to identify the most frequent sources of systematic and random errors for each measured anion, related to the ion chromatography technique. Some specific metrological issues related to the geographical region are discussed.     

Interlaboratory comparison study for the determination of halogenated hydrocarbons in water

Sixty laboratories of five different countries participated in a large-scale interlaboratory comparison test for the determination of halogenated hydrocarbons in water. Participants used their in-house method with 44 laboratories applying head space GC ECD analysis and 5 using liquid/liquid extraction. A set of two artificially produced samples was prepared; the halogenated hydrocarbons investigated were trichloroethylene, tetrachloroethylene, 1,1,1-trichloroethane, trichloromethane, tetrachloromethane, 1,1-dichloroethylene, dichloromethane, dibromochloromethane, bromodichloromethane, 1,2-dichloroethane and tribromomethane. The procedure of sample preparation, storage and distribution was monitored by an extensive quality assurance system including homogeneity tests, stability tests, and trend analysis of the submitted data. The analytical results submitted by the participants exhibited RSD values of up to 35% and outlier rates of up to 19%. The percentage of false positive and false negative results was at the highest 12% for selected substances. Recovery rates varying from 86% to 106% proved the correctness of the analytical results submitted by the participants and showed that the procedure developed in this study for sample preparation and distribution is well suited for the performance of large-scale interlaboratory comparison tests of halogenated hydrocarbons in water.     

Transferability study of a near-infrared microscopic method for the detection of banned meat and bone meal in feedingstuffs

Near-infrared microscopy (NIRM) has been proved to be a powerful tool for the detection of banned meat and bone meal (MBM) in feed. The identification of MBM traces and its ability to differentiate animal from vegetable feed ingredients is based on the evaluation of near-infrared spectra obtained from individual particles present in the sample. This evaluation is supported by appropriate decision rules for the absorbances at specific wavelengths. Here we show that the method and the corresponding decision rules can be successfully transferred from the laboratory which constructed the decision rules to two independent laboratories that were not involved in the calibration process of the method. The analytical results from blind feed samples containing MBM (positive samples) and feed samples without MBM (negative samples) revealed a very good agreement between the three laboratories, thus demonstrating the transferability of the method.     

Development of a univariate calibration model for pharmaceutical analysis based on NIR spectra

Near-infrared spectroscopy (NIRS) has been widely used in the pharmaceutical field because of its ability to provide quality information about drugs in near-real time. In practice, however, the NIRS technique requires construction of multivariate models in order to correct collinearity and the typically poor selectivity of NIR spectra. In this work, a new methodology for constructing simple NIR calibration models has been developed, based on the spectrum for the target analyte (usually the active principle ingredient, API), which is compared with that of the sample in order to calculate a correlation coefficient. To this end, calibration samples are prepared spanning an adequate concentration range for the API and their spectra are recorded. The model thus obtained by relating the correlation coefficient to the sample concentration is subjected to least-squares regression. The API concentration in validation samples is predicted by interpolating their correlation coefficients in the straight calibration line previously obtained. The proposed method affords quantitation of API in pharmaceuticals undergoing physical changes during their production process (e.g. granulates, and coated and non-coated tablets). The results obtained with the proposed methodology, based on correlation coefficients, were compared with the predictions of PLS1 calibration models, with which a different model is required for each type of sample. Error values lower than 1-2% were obtained in the analysis of three types of sample using the same model; these errors are similar to those obtained by applying three PLS models for granules, and non-coated and coated samples. Based on the outcome, our methodology is a straightforward choice for constructing calibration models affording expeditious prediction of new samples with varying physical properties. This makes it an effective alternative to multivariate calibration, which requires use of a different model for each type of sample, depending on its physical presentation.     

Cesium sorption from concentrated acidic tank wastes using ammonium molybdophosphate-polyacrylonitrile composite sorbents

The radiological examination of water requires a rapid screening that permits the determination of gross alpha-activity in order to decide if further radiological analyses are necessary. The usual method of gross alpha-determination includes sample evaporation to dryness on a disk and counting using a ZnS(Ag) scintillation detector or a proportional counter. The determination of gross alpha-activity in sea water using this method has a number of drawbacks such as high mass attenuation, lack of homogeneity in the precipitate, and very small volume of sample. In this work, coprecipitation is proposed to enable a 500 ml sample of water to be analyzed. The minimum detectable activity (MDA) achieved was 0.004 Bq^.l^–1 for a counting time of only 4 hours. Weight variation, self-absorption factor, efficiency and blanks were studied. In order to test reproducibility of the method, several Spanish coastal sea water samples were analyzed by two different laboratories.