Preparation of in-house reference material of benzylpenicillin in milk and results of a Brazilian proficiency testing scheme

Two milk test materials containing benzylpenicillin and a benzylpenicillin-free material were prepared and used for the operation of a Brazilian proficiency testing (PT) scheme according to the requirements of ISO/IEC 17043. The PT scheme was designed to include laboratories in charge of confirmatory analysis as well as screening analysis. Sets of sample vials, benzylpenicillin-free and spiked items containing 0.2 g of lyophilized milk, were distributed to each participant. Of 20 participants who reported screening data, 18 accomplished satisfying results, providing evidence of the capability of Brazilian laboratories to produce reliable qualitative information. The assigned value (robust average calculated from 6 results) and its uncertainty were (8.28 ± 0.52) μg/L. Participants’ performance was evaluated using z-scores. A small number of participants were able to report quantitative benzylpenicillin results, and consequently care should be taken in the interpretation of the laboratory’s performance, considering the statistical nature of z-scores. Long-term stability testing on remaining PT items showed that a stable benzylpenicillin quality control material was produced and can be used for internal quality control or validation purposes.     

Development of a proficiency testing scheme for a limited number of participants in the field of natural water analysis

A proficiency testing (PT) scheme was developed for a limited number of analytical laboratories participating in the analysis of natural water in Israel. Three fit-for-intended-use reference materials (RMs) were prepared for a pilot PT from natural water: RM-blank and two in-house reference materials (IHRMs) with seven analytes fortified to achieve different levels of analyte concentrations. The measurands for the PT participants were the traceable spike values certified in the IHRMs, i.e., added mass concentrations of the analytes. The RM-blank and IHRMs were found to be homogeneous and stable over 11 days, the time necessary for the experimental part of the PT. The RMs were distributed to the PT participants as unknown test items similar to routine samples. The test results were evaluated using their deviations from the IHRM-certified values. Eight Israeli laboratories took part in the interlaboratory comparison. Individual laboratory performance and metrological compatibility of the PT results of the participants, as a local group of laboratories, were evaluated for every analyte.     

Control charts in multi-element analysis of human urine

A plan for internal quality control (IQC) was designed to monitor the quality of analytical results obtained from the simultaneous determination by ICP-MS of nine trace elements (As, Cd, Mo, Ni, Pb, U, V, W and Zr) in 1350 samples of human urine collected as part of an epidemiological study. Analytical performances, assessed during the method validation, were as follows: limit of quantification ranging from 0.002 μg/L (U) to 0.9 μg/L (Ni); recovery rates varying between 82 % (As) and 110 % (W); intermediate precision as relative standard deviation ranging from 2.5 % (Mo) to 14.2 % (V). Only for Zr, a significantly higher relative standard deviation was obtained. Initially, two commercially certified reference materials (CRMs) based on freeze-dried human urine and containing the elements of interest at suitable levels of concentration were used as control materials. However, due to considerations of reliability, convenience and cost, a control material based on fresh human urine was prepared in-house, evaluated by comparison with the CRMs and implemented for the IQC. The IQC strategy was as follows: at least one control material was analysed in each analytical session and one more control material was analysed after every ten urine samples. The central lines of the control charts were based on the assigned concentration values; the warning and action limits were set on a statistical basis, as two-times and three-times the value of the uncertainty of certified values or the observed standard deviation. At the end of the study, the data collected for the in-house control material were re-evaluated and confirmed its suitability for the purpose. The use of an in-house control material had important advantages: it was more representative of the test samples; its preparation did not involve additional steps, therefore, reducing the risk of contamination; the cost of implementing the IQC was also reduced. Some issues concerning the control charts for As and Mo were addressed and will be discussed.     

New calibration method for solid sampling Zeeman atomic absorption spectrometry (SS-ZAAS) for cadmium

Summary Calibration of solid sampling Zeeman AAS for cadmium was investigated. A series of certified reference materials was analysed using a CRM with a different composition as standard. For routine applications this yielded satisfactory results. A method of calibration was then applied involving addition of standard solution to lyophilized samples, followed by lyophilization and homogenization. Using an iteration procedure results were obtained which no longer depend on certified reference materials. The method was applied to the analysis of a testbatch of codfish powder prepared at CBNM. Finally, the spiked fish standards obtained in this way were also used to analyse other certified reference materials.     

Reference materials for food analysis laboratories in India

An attempt has been made to prepare a Secondary Reference Material for heavy metals in milk powder. From fresh milk obtained locally, two batches of milk powder have been prepared by pasteurising, homogenising, evaporating and spray drying. One of the batches, treated as an experimental batch, was spiked before the preparation with lead, cadmium and arsenic solutions (imported BDH standards) at a level about 1 mg/kg. After spray drying, both batches were packed under nitrogen using a polyester-aluminium/polyethylene foil as inner pouch and an outer cover of metallised polyester/polyethylene as a supporting film. Preliminary data have been obtained for the concentrations of Pb, Cd and As in both batches using acid digestion and flame AAS and UV/Vis spectrophotometry. Correspondence to: M. N. Krishnamurthy     

PT/EQA standards and guidelines: some remarks on quality and reliability of test items

Current standards and international guides on PTs/EQAs consider the achievement of the quality and reliability of PT test items as important steps. Similarity to the materials routinely analysed and sufficient homogeneity and stability of the properties of interest are the main quality characteristics required. PT test items are carriers of the property values used as reference to evaluate laboratory performances. Requirements for PT test items are practically the same as those given for reference materials. ISO-REMCO defines reference material as “Material, sufficiently homogeneous and stable with respect to one or more specified properties, which has been established to be, fit for its intended use in a measurement processes”. In the case of PT test items, they should have a degree of homogeneity and stability to be fit for the purpose of identifying deviations among laboratories. This paper reports some remarks on the main characteristics of PT test items, using ISPRA experience as RM producer.     

Traceable reference values for routine drinking water proficiency testing: first experiences

Proficiency testing (PT) results have been used to improve traceability in chemical drinking water analysis. With a generalized least-square regression the mass concentrations of As and Sb were calculated in a drinking water that had been used to prepare proficiency testing samples by a spiking procedure. From the mass concentrations in the matrix and the spiked amounts, reference values with an uncertainty budget could be calculated without the need for reference measurements. The degree to which these reference values can be regarded as traceable is discussed. The results showed slight deviations in some samples between reference values and consensus means.     

Preparation of monodisperse PMMA microspheres in nonpolar solvents by dispersion polymerization with a macromonomeric stabilizer

We discuss a dispersion polymerization procedure for preparing monodisperse and micron-sized poly(methyl methacrylate) (PMMA) particles in hexanes with methacryloxypropyl-terminated polydimethylsiloxane stabilizers. We investigate the effects of the stabilizer molecular weight, stabilizer concentration, and monomer concentration on the particle size and polydispersity. We find that a minimum molecular weight of 10 000 g/mol is necessary to synthesize colloidally stable PMMA dispersions. The particle polydispersity is minimal (=5%) for stabilizer to monomer weight ratios of 0.02 to 0.1, while PMMA particles prepared under conditions outside this range are polydisperse. The particle diameter can be varied from 0.4 to 1.5 µm by appropriate choices of stabilizer and monomer concentrations. Stable PMMA suspensions can be prepared at up to 26.3% solids. The dispersions are stable in most liquid aliphatics, and are monodisperse enough to form ordered domains at high concentration. This single-stage synthesis, requiring only commercially available materials, may be of interest to those seeking a simple way to prepare highly monodisperse non-aqueous dispersions in the micron size range.     

Production of reference materials for the detection and size determination of silica nanoparticles in tomato soup

A set of four reference materials for the detection and quantification of silica nanoparticles (NPs) in food was produced as a proof of principle exercise. Neat silica suspensions were ampouled, tested for homogeneity and stability, and characterized for total silica content as well as particle diameter by dynamic light scattering (DLS), electron microscopy (EM), gas-phase electrophoretic molecular mobility analysis (GEMMA), and field-flow fractionation coupled with an inductively coupled mass spectrometer (FFF-ICPMS). Tomato soup was prepared from ingredients free of engineered nanoparticles and was spiked at two concentration levels with the silica NP suspension. Homogeneity of these materials was found sufficient to act as reference materials and the materials are sufficiently stable to allow long-term storage and distribution at ambient temperature, providing proof of principle of the feasibility of producing liquid food reference materials for the detection of nanoparticles. The spiked soups were characterized for particle diameter by EM and FFF-ICPMS (one material only), as well as for the total silica content. Although questions regarding the trueness of the results from EM and FFF-ICPMS procedures remain, the data obtained indicate that even assigning values should eventually be feasible. The materials can therefore be regarded as the first step towards certified reference materials for silica nanoparticles in a food matrix.     

Implementation of proficiency testing schemes for a limited number of participants

A metrological background for the selection and use of proficiency testing (PT) schemes for a limited number N of laboratories-participants (less than 20–30) is discussed. The following basic scenarios are taken into account: (1) adequate matrix certified reference materials (CRM) or in-house reference materials (IHRM) with traceable property values are available for PT use as test items; (2) no appropriate matrix CRM is available, but a CRM or IHRM with traceable property values can be applied as a spike or similar; (3) only an IHRM with limited traceability is available. The discussion also considers the effect of a limited population of PT participants N _p on statistical assessment of the PT results for a given sample of N responses from this population. When N _p is finite and the sample fraction N/N _p is not negligible, a correction to the statistical parameters may be necessary. Scores suitable for laboratory performance assessment in such PT schemes are compared. Presented at the 3rd International Conference on Metrology, November 2006, Tel Aviv, Israel.     

Control charts to evaluate long-term performance in proficiency tests

As proficiency tests (PTs) provider, the European Union Reference Laboratory for Chemical Elements in Food of Animal Origin (EURL-CEFAO) yearly organizes exercises on the determination of chemical elements within the area of its mandate. The scheme is addressed to the National Reference Laboratories (NRLs) of the European Union Member States. Each PT is planned giving priority to the matrices and elements for which maximum levels are set in Commission Regulation No. 1881/2006 and following amendments as well as to those mainly considered in National Residues Control Plans. Samples to be distributed are prepared in the Laboratory’s facilities using materials purchased on the market and often adjusting the concentration levels of the analytes around their maximum levels. In the 2006–2011 period, PTs were planned as a long-term programme pursuing the general objective of providing the NRLs with a scheme that can give them the opportunity to check and improve the performance of their analytical methods and to verify the effectiveness of any corrective actions through the repetition of the same matrix. Control charts give participants the possibility of a long-term follow-up of their performance promoting the improvement of Quality Control. To this aim, Shewhart and Cusum control charts for z-scores are prepared according to ISO 13528:2005 and updated after each PT for each laboratory. z-scores are assigned using values of σ _pEURL-CEFAO that correspond to the level of performance appropriate to the demand. Cusum charts are also evaluated by EURL-CEFAO in order to identify problems that may cause a bias in the measurement method.     

Designs of experiment for proficiency testing with a limited number of participants

Metrological designs of experiment for proficiency testing (PT) with a limited number of participants are discussed. The designs are based on development of in-house reference materials (IHRMs) with traceable assigned values and fit-for-purpose uncertainties, used in a PT scheme as the measurement standards. When adequate certified reference materials (CRMs) are available, a comparative approach for IHRM development, using simultaneous analysis of IHRM and CRM test portions in pairs, is proposed for PT objectives. In the case where adequate CRMs are not available – for example, in the field of analysis of unstable aqueous systems – the function of the measurement standard for PT can be fulfilled by a synthetic IHRM prepared gravimetrically using non-adequate, non-aqueous CRMs or pure substances. The CRM or the pure substance is used as a spike for fortification of a natural water sample, while the natural water sample is used as a working IHRM for the spike determination. In this case the traceability chain is longer, since two IHRMs are added for one quantity determination, but it remains unbroken.Presented at the Second International Conference on Metrology –Trends and Applications in Calibration and Testing Laboratories, 4–6 November 2003, Eilat, Israel     

The new ISO Guide 80: Guidance for the in-house preparation of quality control materials (QCMs)

Reference materials (RMs) are widely used in measurement laboratories for a variety of purposes, and it is important to recognise that the material most appropriate for a particular application should be used. Certified reference materials (CRMs) are used for method validation, the calibration of a measurement system and all other aspects of the evaluation of the measurement system where the trueness of the measurement result is required. For other aspects, such as quality control, precision studies, the checking of the variability between operators, where the results are compared relatively, any suitable reference material can be used. ISO/REMCO, the ISO Committee on Reference Materials, has prepared ISO Guide 80, a guidance document for the in-house preparation of quality control materials (QCMs). QCMs are mostly used to monitor the performance of laboratory methods that have already been validated over time to be able to detect change or when a method goes out of statistical control. QCMs are RMs and as such have to be sufficiently homogeneous and stable for the intended use. QCMs are usually prepared in-house by laboratory staff for in-house use only, and therefore, the requirements for “in-house” QCMs are less demanding than those for a CRM. For example, transport issues are not of concern. The quality assessment of QCMs should involve homogeneity and stability assessments, and a limited characterisation of the material to provide an indication of its relevant property values and their variation, prior to use.     

Fit-for-purpose shellfish reference materials for internal and external quality control in the analysis of phycotoxins

The need for reference materials for quality control of analysis of foodstuffs has been stressed frequently. This has been particularly true in the phycotoxins field, where there is a great shortage of both pure calibration standards and reference materials. Worldwide there are very few independent bodies that produce certified reference materials for phycotoxins, the main producers currently being the National Research Council Canada and the Japanese Food Research Laboratory. Limited availability of contaminated shellfish and algae, as well as the time and knowledge necessary for the production of adequate reference materials, continuously lead to limited editions of certified reference materials and even more limited production of in-house reference materials. The restricted availability of in-house quality control materials promotes the rapid use of the limited certified reference materials, which in turn hampers the production of the suite of materials required globally for complete protection of public health. This paper outlines the various options that analysts can pursue in the use of reference materials for internal and external quality control, with a view to optimising the efforts of both reference materials users and reference materials producers. For this purpose, the logical sequence is reviewed from the discovery of a new bioactive compound in shellfish, through initial method development up to regulation for food safety purposes including accepted reference methods. Subsequently, the requirements for and efforts typically spent in the production and characterisation of laboratory reference materials, certified reference materials and other test materials used in inter-laboratory studies or proficiency testing, in the area of marine biotoxins are evaluated. Particular emphasis is put on practical advice for the preparation of in-house reference materials. The intricate link between reference material characterisation and method performance is outlined to give guidance on the appropriate in-house method validation in the rapidly developing field of phycotoxins.      

Preliminary homogeneity study of in-house reference material using neutron activation analysis and X-ray fluorescence

Summary Although many biological reference materials for quality control of trace element analysis are commercially available, there is still a need for additional local materials for special matrices. In the Latin American region a preliminary study has been commenced involving analytical strategies for the characterization of in-house reference material. A biological sample, prepared in Brazil, constitutes the first regional attempt to prepare reference material. It was analyzed by neutron activation analysis (NAA) and X-ray fluorescence (XRF) to verify its homogeneity. The determination of the trace elements and certain major elements was carried out by instrumental NAA. Trace elements such as Cd, Mn, Mo and Cu were determined using NAA with radiochemical separations to improve the sensitivity and precision. XRF was applied only to major constituents and some trace elements with concentration of more than 10 g/g. From a total of 18 elements analyzed, only Fe, Cr and Sc were not homogeneously distributed.     

Synthesis and application of polythiophene-coated Fe3O4 nanoparticles for preconcentration of ultra-trace levels of cadmium in different real samples followed by electrothermal atomic absorption spectrometry

In this research, magnetic Fe₃O₄ nanoparticles were synthesised by co-precipitation method and modified with polythiophene (PT) to produce Fe₃O₄-PT nanoparticles for preconcentration and determination of cadmium (??) ion followed by electrothermal atomic absorption spectrometry. The results of FT-IR spectroscopy, EDX analysis and SEM images show that Fe₃O₄-PT nanoparticles were synthesised successfully. Different parameters such as sample pH, amounts of adsorbent, sample volume, extraction time, type and concentration of eluent and desorption time were completely investigated and optimum conditions were selected.

Under the optimum conditions, the calibration curve was linear in the range of 0.01–0.25 µg L^?1 of cadmium (??). The relative standard deviation was 4.7% (n = 7, 0.10 µg L^?1 Cd²⁺) and limit of detection was 3.30 ng L^?1. The accuracy of the proposed method was verified by the analysis of a certified reference material and spike method. Finally, the proposed method was applied for the determination of ultra-trace levels of cadmium (??) in different water and food samples.     

Determination of Melamine in Fresh Milk with Hollow Fiber Liquid Phase Microextraction Based on Ion-Pair Mechanism Combined with High Performance Liquid Chromatography

In this work, a novel analytical method based on hollow fiber liquid phase microextraction (HF-LPME) and high performance liquid chromatography (HPLC) was developed for the analysis of melamine in fresh milk. The conditions of the HF-LPME were investigated and optimized. As a result, a supported liquid membrane containing 6-undecanone and di-2-ethylhexyl phosphoric acid (D2EHPA) was selected. The extractions were made from 25 mL aqueous donor phase (prepared from milk) with pH 5.0 to a more acidic acceptor phase (36 µL 1 M HCl) and the mass transfer was driven by the proton gradient between these phases. Other optimum conditions of the HF-LPME were 60 min extraction time at 360 rpm stirring rate and an extraction factor of 21 times (extraction efficiency 3%) was obtained. The C8 column was operated at 1 mL/min at room temperature and the UV detection wavelength was 240 nm for HPLC. The mobile phase was 10 mM sodium n-octanesulfonate (pH 3.0) mixed with acetonitrile (85:15, v/v). The relative recovery of melamine for milk samples spiked with 0.5–25 mg/kg was in the range of 89.1–120.6% with the RSDs (n = 4) of 4.0–8.5%. It was found that the proposed method provided a linear range from 0.1 to 50 mg/kg (r ²  = 0.9993), method detection limit (MDL) of 0.003 mg/kg and method quantification limit (MQL) of 0.01 mg/kg. The obtained results demonstrated that HF-LPME combined with HPLC is a simple and cheap method for the determination of melamine in fresh milk.     

On-line clean-up of pressurized liquid extracts for the determination of polychlorinated biphenyls in feedingstuffs and food matrices using gas chromatography-mass spectrometry

This paper describes a fast and simple pressurized liquid extraction method for the determination of polychlorinated biphenyls (PCBs) in feedingstuffs and food matrices. The method is based on a simultaneous extraction/clean-up step requiring a minimum of sample handling. The final analysis was performed with gas chromatography-mass spectrometry. Seven PCBs (28, 52, 101, 118, 138, 153 and 180) were analyzed, all of which are indicator congeners that, according to European legislation should be included in the analytical monitoring program. The extracted matrices were spiked feed for poultry and two certified reference materials naturally contaminated with PCBs (cod-liver oil and milk powder), which showed excellent conformity with certified data.     

Two new standard reference materials for the determination of drugs of abuse in human hair

Two new standard reference materials (SRM) for drugs of abuse in human hair have been developed. SRM 2379 consists of hair spiked with cocaine, benzoylecgonine, cocaethylene, phencyclidine, amphetamine, and methamphetamine. SRM 2380 consists of hair spiked with codeine, morphine, monoacetylmorphine, and tetrahydrocannabinol (THC). The SRMs were prepared by soaking the hair in a solution of the target analytes in water-dimethylsulfoxide. The concentration of each analyte was determined using two methods, one based upon gas chromatography/mass spectrometry (GC/MS) and one based upon liquid chromatography/mass spectrometry (LC/MS). Both methods used 0.1 M HCl for extraction of all the analytes from the hair, except for THC, which was extracted with 1 M NaOH. For isolation of the analytes from the extracts, the GC/MS-based methods used different clean-up procedures from those used for the LC/MS-based methods. The results from the two methods were in good agreement with mean differences for the analytes ranging from 4% to 16%. These materials will enable laboratories performing analyses of hair for drugs of abuse to test the accuracy of their methods.     

Liquid chromatography/time-of-flight mass spectrometry analysis of postmortem blood samples for targeted toxicological screening

A liquid chromatography/time-of-flight mass spectrometry (LC-TOF-MS) method for targeted toxicological screening in human postmortem blood samples from forensic autopsy cases has been developed, validated and compared with a previously used method using gas chromatography with nitrogen–phosphorus detection (GC-NPD). Separation was achieved within 12 min by high-resolution gradient chromatography. Ions were generated in positive and negative electrospray ionization mode and were detected in 2-GHz single mass spectrometry mode, m/z range 50–1,000. Before injection, 0.25 g blood was prepared by protein precipitation with 500 μL of a mixture of acetonitrile and ethanol containing deuterated internal standards. An in-house database comprising 240 drugs and metabolites was built by analysing solutions from certified standards or other documented reference material available. Identification was based on scoring of retention time, accurate mass measurement and isotopic pattern. Validation was performed on spiked blood samples and authentic postmortem blood samples. The thresholds defined as minimum required performance levels were for most compounds in the range from 0.01 to 0.10 μg/g. Typically, a mass error of less than 2 ppm and a precision of area measurements of less than 5 % coefficient of variation were achieved. Positive identification was confirmed at concentrations up to 500 μg/g. Most compounds were determined in positive ionization mode, but for a limited number of compounds (fewer than 4 %) negative ionization was needed and a few early-eluted compounds could not be identified owing to substantial influence of interferences from the matrix and were thus not included in the screening. A robust and valid toxicological screening by LC-TOF-MS for postmortem blood samples, covering 50 % more compounds, and with higher precision and sensitivity than the previously used screening by GC-NPD was achieved.