Discussion on homogeneity assessment of reference materials based on uncertainty comparison method

Accreditation and Quality Assurance - An uncertainty comparison method is proposed to assess the homogeneity of reference materials. The method compares “standard uncertainty associated with...     

Applications of laser ablation and electrothermal vaporization as sample introduction techniques for ICP-MS

The field of applications of ICP-MS can be further increased by the use of special sample introduction techniques such as laser ablation (LA) and electrothermal vaporization (ETV). In both cases a tandem source for mass spectrometry is formed by the sample introduction device and the ICP. The first source is specifically designed for the volatilization of a sample and it can be used to introduce selectively only certain parts of a sample into the ICP-MS, based either on local distribution (LA) or volatility (ETV). Applications of LA-ICP-MS are the determination of distribution patterns of minor constituents in solid samples such as ceramics, alloys or hard biological structures. Homogeneity testing in the first two types of samples or determination of distribution patterns of trace elements in the latter can be carried out rapidly with high spatial resolution on a multielement basis. The possibility of on-line separation between fractions of different volatility in a sample with ETV-ICP-MS is demonstrated for volcanic eruption products and other samples.     

Preliminary assessment of homogeneity of new candidate agricultural/food reference materials

Summary In an effort to bring to the analytical community new natural matrix reference materials (RM's) for elemental composition quality control, ten new agricultural/food candidate reference materials prepared previously, were characterized for homogeneity. The materials include bovine muscle powder, whole egg powder, corn bran, microcrystalline cellulose, wheat gluten, whole milk powder, corn starch and three wheat flours representing a wide range of matrix types and elemental composition. Characterization was for up to some twenty five major, minor, trace and ultratrace elements of nutritional, toxicological and environmental significance. Solid sampling electrothermal atomic absorption spectrometry was applied to the determination of copper and lead in sub-milligram subsamples. A variety of other analytical techniques via an interlaboratory cooperative effort was used to determine many other elements in 100–2000 mg subsample sizes. Good material homogeneity was observed for virtually all materials and analytes with the exception of Cr and Pb in limited instances. The materials were thus deemed suitable for detailed characterization of elemental concentration to lead to recommended values and to bring these biological products to reference material status.     

Parameter optimization of electrothermal vaporization inductively coupled plasma mass spectrometry for oligoelement determination in standard reference materials

The operational parameters of the graphite furnace for electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS), i.e. the internal carrier gas flow rate, the total carrier gas flow rate, the sample pretreatment temperature and the volatilization temperature, are optimized for oligoelement determinations (⁷⁵As, ⁹Be, ¹¹²Cd, ⁵⁰Cr, ⁶⁵Cu, ¹⁰³Rh, ¹²³Sb). The volatilization temperatures of As and Cr are compared to those obtained by graphite furnace atomic absorption spectrometry (GFAAS). Several modifiers Mg(NO₃)₂, Pd(NO₃)₂, Mg(NO₃)₂/Pd(NO₃)₂, Ni(NO₃)₂, KI, (NH₄)H₂PO₄ have been tested using the concentrations recommended for GFAAS. The concentration of Mg(NO₃)₂ alone and in combination with NaCl has been varied to find the optimal modifier conditions. ETV-ICP-MS signal enhancements by a factor of 10 to 130 respective to those of conventional nebulization have been obtained. The optimized parameters are evaluated by analyzing the water standard reference NIST 1643c and the aqua regia solution of the lake sediment reference material BCR 280.     

Parameter optimization of electrothermal vaporization inductively coupled plasma mass spectrometry for oligoelement determination in standard reference materials

The operational parameters of the graphite furnace for electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS), i.e. the internal carrier gas flow rate, the total carrier gas flow rate, the sample pretreatment temperature and the volatilization temperature, are optimized for oligoelement determinations (⁷⁵As, ⁹Be, ¹¹²Cd, ⁵⁰Cr, ⁶⁵Cu, ¹⁰³Rh, ¹²³Sb). The volatilization temperatures of As and Cr are compared to those obtained by graphite furnace atomic absorption spectrometry (GFAAS). Several modifiers Mg(NO₃)₂, Pd(NO₃)₂, Mg(NO₃)₂/Pd(NO₃)₂, Ni(NO₃)₂, KI, (NH₄)H₂PO₄ have been tested using the concentrations recommended for GFAAS. The concentration of Mg(NO₃)₂ alone and in combination with NaCl has been varied to find the optimal modifier conditions. ETV-ICP-MS signal enhancements by a factor of 10 to 130 respective to those of conventional nebulization have been obtained. The optimized parameters are evaluated by analyzing the water standard reference NIST 1643c and the aqua regia solution of the lake sediment reference material BCR 280.     

Application of the ANOVA-PCA method to stability studies of reference materials

Near infrared spectroscopy (NIRS) is an analytical technique that can be very useful for stability studies in particular because of its non destructive analytical capability. However, the spectral interpretation and treatment of this kind of multivariate data remains difficult without the use of chemometrics. In this article, a recent chemometrics method, analysis of variance - principal component analysis (ANOVA-PCA), was used for NIRS stability studies of sunflower and bread wheat external reference materials (ERM). It provided a practical tool for the study of the significance of various storage conditions according to an experimental design. Thus, the effect of the temperature, the nature of the atmosphere in the packaging and the storage duration were tested. ANOVA-PCA highlighted the influence of temperature and storage duration on the stability of the sunflower materials. For the bread wheat materials, the storage conditions did not have a significant effect on stability. Consequently, by applying ANOVA-PCA to near infrared spectral data, the sunflower materials were found to be considered stable for the time length of the study, i.e. 18 months stored in a cold room, while the bread wheat materials were found to be considered stable for the time length of the study, i.e. 12 months under the same conditions.     

Direct determination of Se and As in solid certified reference materials using electrothermal vaporization inductively coupled plasma mass spectrometry

Firstly, electrothermal vaporization inductively coupled plasma mass spectrometry (ETV-ICP-MS) has been successfully applied for the direct determination of Se in solid certified reference materials (CRMs) of both biological and environmental origin. Secondly, in order to exploit one of the most important advantages of ICP-MS (the multi-element capability), Se and As have been determined simultaneously in two CRMs (“Sea lettuce” and “Estuarine sediment”), the potential of ETV-ICP-MS for accurate mono-element determination of As in solid CRMs having been proven in a previous study. For both the mono-element and the multi-element determinations, several parameters (ashing and vaporization temperature, carrier gas flow rate) have been studied systematically in order to obtain optimized conditions. Both the linearity of the mass response curves and the signal profiles (signal intensity as a function of time) are examined. Absolute detection limits, based on the 3s criterion, are found to be approximately 1 pg. Using single standard addition for calibration and Sb as an internal standard, accurate results are obtained for both the mono-element determinations of Se (mean relative difference with the certified value of 6.5%) and the multi-element determinations of Se and As (mean relative difference with the certified values of, respectively, 6.6% for Se and 7.2% for As).     

Hollow-fiber liquid-phase microextraction prior to low-temperature electrothermal vaporization ICP-MS for trace element analysis in environmental and biological samples

A new method of hollow-fiber liquid-phase microextraction (HF-LPME) prior to electrothermal vaporization (ETV) inductively coupled plasma mass spectrometry (ICP-MS) determination of trace Cu, Zn, Pd, Cd, Hg, Pb and Bi, based on gaseous compounds introduction into the plasma as their diethyldithiocarbmate (DDTC) chelates, was developed. The use of the reagent DDTC as chemical modifier could not only enhance the analytical signals, but also decrease the vaporization temperature. At a temperature of 1300 degrees C, trace Cu, Zn, Pd, Cd, Hg, Pb and Bi can be vaporized completely into the ICP. The factors affecting the formation of the chelates and their vaporization behaviors were investigated in detail, and the microextraction conditions were optimized. Under the optimized conditions, the detection limits of the proposed method were 12.4, 28.7, 7.9, 4.5, 3.3, 4.8 and 1.6 pg ml(-1) for Cu, Zn, Pd, Cd, Hg, Pb and Bi, respectively. Enrichment factors of 305, 284, 24, 29, 20, 73 and 43 could be achieved within 15 min of extraction time, and the relative standard deviations (RSDs) for the seven determinations of 0.5 ng ml(-1) of target analytes were 8.8, 6.9, 7.1, 9.4, 10.2, 6.1 and 10.8%, respectively. The newly developed method has been applied to the determination of trace Cu, Zn, Pd, Cd, Hg, Pb and Bi in environmental water and human serum samples, and the recoveries for the spiked samples were in the range of 88-116%. In order to validate this method, two certified reference materials, GBW08501 peach leaves and GBW(E)080040 seawater, were analyzed, and the determined values were in good agreement with the certified values.     

Electron probeX-ray microanalysis for the assessment of homogeneity of candidate reference materials at the nanogram level

A new approach to the assessment of homogeneity for powder samples of candidate reference materials with the help of electron probeX-ray micro-analysis (EPMA) is proposed. It is based on the utilisation of the Kolmogorov—Smirnov statistics coupled with the Akaike Information Criterion in the processing of the quantitative EPMA data. The evaluation of three IAEA candidate reference materials with the described approach is discussed.     

Characterization of the mineral fraction in botanical reference materials and its influence on homogeneity and analytical results

Summary The NIST natural-product leaf Standard Reference Materials have been widely used in developing reliable methods of analysis. A small amount of mineral matter present in these materials was separated by flotation, and characterized qualitatively by microscopy and quantiatively by neutron activation analysis. Several elements are concentrated in the mineral fraction, which can lead to analytical error through incomplete dissolution or sampling statistics. Two new candidate materials prepared by the Office of Standard Reference Materials, SRM 1515 Apple Leaves and SRM 1547 Peach Leaves, have been processed with an air-jet mill, resulting in a very finely ground leaf material, with particle size less than 200 mesh. A cyclone classifier in the process discriminates against coarse grit, so that the content of minerals in the ground material is less than in the first-generation materials. Better homogeneity was in fact observed, even down to 100 mg sample size. One must use caution, however, to assure that any inhomogeneity found in real samples are appropriately considered and dealt with.     

Determination of the rare earth elements La, Eu, and Yb using solidified floating organic drop microextraction and electrothermal vaporization ICP-MS

We have developed a method for the determination of trace levels of the rare earth elements La, Eu, and Yb in biological and environmental samples. It is based on solidified floating organic drop microextraction using 1-(2-pyridylazo)-2-naphthol (PAN) as a chelator, followed by electrothermal vaporization (ETV) and quantification by inductively coupled plasma mass spectrometry. PAN also acts as a modifier in ETV. The effects of pH, amount of PAN, extraction time, stirring rate, volume of sample solution, and temperature program were examined. Under optimized conditions, the detection limits are 2.1, 0.65 and 0.91 pg mL^?1 for the elements La, Eu and Yb, respectively. The relative standard deviations are <6.0 % (c?=?0.1 ng mL^?1, n?=?9). When applied to the analysis of (spiked) natural water samples, the recoveries range from 92 to 105 %. The accuracy was validated with certified reference materials (combined sample of branch and leaf of shrub: GBW 07603 and human hair: GBW 07601), and the results were in good agreement with the certified values.

Determination of the rare earth elements La,Eu, and Yb using solidified floating organic drop microextraction and electrothermal vaporization ICP-MS

We have developed a method for the determination of trace levels of the rare earth elements La, Eu, and Yb in biological and environmental samples. It is based on solidified floating organic drop microextraction using 1-(2-pyridylazo)-2-naphthol (PAN) as a chelator, followed by electrothermal vaporization (ETV) and quantification by inductively coupled plasma mass spectrometry. PAN also acts as a modifier in ETV. The effects of pH, amount of PAN, extraction time, stirring rate, volume of sample solution, and temperature program were examined. Under optimized conditions, the detection limits are 2.1, 0.65 and 0.91 pg mL⁻¹ for the elements La, Eu and Yb, respectively. The relative standard deviations are <6.0 % (c = 0.1 ng mL⁻¹, n = 9). When applied to the analysis of (spiked) natural water samples, the recoveries range from 92 to 105 %. The accuracy was validated with certified reference materials (combined sample of branch and leaf of shrub: GBW 07603 and human hair: GBW 07601), and the results were in good agreement with the certified values.

►Solidified floating organic drop microextraction was combined with ETV for ICP-MS. ►PAN acted as both a chelating agent and a chemical modifier. ►The method was used for analysis of rare earth elements in real samples. ►The method has the merits of low detection limit, good precision and accuracy.

     

Development of bovine muscle, liver and urine reference materials for zeranol – preparation, homogeneity and stability

 Samples of bovine muscle, liver and urine, zeranol-free (RM 508, RM 509 and RM 510, respectively) and zeranol-containing (RM 511, RM 512 and RM 513, respectively) were prepared and tested as candidate reference materials. Preliminary studies on achievement of target zeranol content, intercomparison of analytical methods (HPLC-RIA and GC-MS) and effects of lyophilisation and irradiation on zeranol content are described. The preparation of the materials and testing for homogeneity and stability of zeranol in the materials are discussed. The coefficients of variation for zeranol determinations for between-vial homogeneity (4.0%, 4.4% and 4.6% for muscle, liver and urine, respectively) are similar to those for the analytical method (4.0%, 7.3% and 6.8% for muscle, liver and urine, respectively) indicating that the materials are homogeneous. Stability data over a 12-month storage period at temperatures ranging from −18 °C to +37 °C indicate that the materials are sufficiently stable for use as reference materials. This paper is dedicated to the memory of Dr Hilary Stevenson (The Queen’s University of Belfast/DANI) who, together with Mr. W. Graham, facilitated the irradiation of the lyophilised materials.--> Received: 18 June 1996/Revised: 19 August 1996/Accepted: 23 August 1996     

Development of bovine muscle, liver and urine reference materials for zeranol – preparation, homogeneity and stability

 Samples of bovine muscle, liver and urine, zeranol-free (RM 508, RM 509 and RM 510, respectively) and zeranol-containing (RM 511, RM 512 and RM 513, respectively) were prepared and tested as candidate reference materials. Preliminary studies on achievement of target zeranol content, intercomparison of analytical methods (HPLC-RIA and GC-MS) and effects of lyophilisation and irradiation on zeranol content are described. The preparation of the materials and testing for homogeneity and stability of zeranol in the materials are discussed. The coefficients of variation for zeranol determinations for between-vial homogeneity (4.0%, 4.4% and 4.6% for muscle, liver and urine, respectively) are similar to those for the analytical method (4.0%, 7.3% and 6.8% for muscle, liver and urine, respectively) indicating that the materials are homogeneous. Stability data over a 12-month storage period at temperatures ranging from −18 °C to +37 °C indicate that the materials are sufficiently stable for use as reference materials. This paper is dedicated to the memory of Dr Hilary Stevenson (The Queen’s University of Belfast/DANI) who, together with Mr. W. Graham, facilitated the irradiation of the lyophilised materials.--> Received: 18 June 1996/Revised: 19 August 1996/Accepted: 23 August 1996     

Preparation and testing for homogeneity of spiked strawberry and cabbage candidate reference materials

Strawberry and cabbage candidate reference materials (RMs) have been prepared by our laboratory during 1992–1993. Fresh materials have been homogenized and spiked with known concentrations of pesticides representing compounds commonly used for strawberries and cabbages. The candidate RMs have been then freeze-dried, homogenized, bottled under a stream of nitrogen and stored frozen at –20°C. Homogeneity and stability of the candidate RMs have been studied during the storage period. Homogeneity of the candidate RMs for 5 g samples has been within 10%. Homogeneity and stability studies have been carried out using the conventional multiresidue method of Luke et al. [1]. Instrumental analysis have been performed by gas chromatography mass spectrometry (GC-MS) employing selected ion monitoring (SIM).     

Determination of aluminum and silicon in biological materials by inductively coupled plasma atomic emission spectrometry with electrothermal vaporization

An atomic emission spectrometric method is described for the determination of trace elements in microvolume samples especially of biological materials. Based upon the arrangement of a commercial electrothermal vaporizer and a 40-MHz inductively coupled plasma, the direct determination of aluminum and silicon in human body fluids such as urine and serum and aluminum in hemodialysis solution is performed. The instrumental system involves vaporizing the sample from a modified graphite electrode followed by atomization and excitation of the vapors in the ICP discharge. Compromise experimental conditions are reported and calibration functions compared. Limits of detection in 5-μl samples were 8 pg Al and 2.5 ng Si, and after preconcentration of Al with a poly(acrylamidoxime) resin, the detection limit was 1 pg Al. Recovery of 5 μg $\text{Si}\text{ml}$ and 10 ng $\text{Al}\text{ml}$ from aqueous and synthetic standards was 80–85% and 96–103%, respectively.     

Preparation and homogeneity characterization of ten agricultural/food reference materials for elemental composition

Summary Ten agricultural/food reference materials (RM): bovine muscle powder, corn starch, hard red spring wheat flour, soft winter wheat flour, whole milk powder, wheat gluten, corn bran, Durum wheat flour, whole egg powder and microcystalline cellulose, were prepared by milling, irradiation, sieving, blending and packaging procedures. Homogeneity tests for 14 elements in randomly selected units were performed by the initiating laboratories by application of various analytical methods. Al, Ca, Cu, Fe, K, Mg, Mn, Na, Sr and Zn were determined by acid digestion flame atomic absorption spectrometry, and Cd, Co, Ni and Pb using acid digestion graphite furnace atomic absorption spectrometry after separation/preconcentration of the analytes by co-precipitation. In addition, the extensive set of analytical results obtained from the interlaboratory cooperative characterization campaign was assessed to provide homogeneity estimates for other elements. Measures of homogeneity were estimated from the within-laboratory precision from the more precise laboratories. All materials exhibited acceptable homogeneity for virtually all 29 elements (Al, As, B, Ba, Br, Ca, Cd, Cl, Co, Cr, Cu, Fe, Hg, I, K, Mg, Mn, Mo, N, Na, Ni, P, Pb, Rb, S, Se, Sr, V, Zn) for which best estimate concentration values were available, an essential pre-requisite in establishing reference values for these materials. Sixty-two percent of all homogeneity coefficients of variation (CV) were below 5%, with Br, Ca, Cl, Mg, Na, P, Zn and especially K and N exhibiting very high homogeneity CV less than 1% in some cases.Contribution no. 92–148 from Centre for Land and Biological Resources Research     

Application of the Kelvin equation to vaporization of silver and gold in electrothermal atomic absorption spectrometry

In order to obtain additional insight into the release mechanism of the metals in electrothermal atomization atomic absorption spectrometry, a quantitative relation between the heat of vaporization and the size of the released particles is proposed on the basis of the Kelvin equation. The applicability of the equation for the investigation of silver and gold vaporization is demonstrated and the limits in which the model is valid are determined.According to the present considerations the activation energy could be equal to the heat of vaporization of the silver and gold droplets. An explanation of the observed dependence of activation energy on analyte mass is given. The proposed relation provides a possibility for definition and evaluation of an “effective” radius/size of the droplets on the basis of their heat of vaporization. A correlation between the mass of the injected sample and the “effective” radius of the droplets, obtained at higher temperature is found. The minimum and maximum “effective” radii of the droplets, following the proposed equation are calculated for Ag on pyrolytic graphite coated electrographite (PGC) and Au on PGC, uncoated electrographite (EG) and glassy carbon (GC) tubes. The results obtained are indirect evidence for the island structure of precursor metal layer and for the existence of silver and gold microdroplets on the graphite support.