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Development and validation of a method to determine the boron isotopic composition of crop plants
(2011)
We present a comprehensive chemical and mass spectrometric method to determine boron isotopic compositions of plant tissue. The method including dry ashing, a three-step ion chromatographic boron–matrix separation, and 11B/10B isotope ratio determinations using the Cs2BO2+ graphite technique has been validated using certified reference and quality control materials. The developed method is capable to determine δ11B values in plant tissue down to boron concentrations of 1 mg/kg with an expanded uncertainty of ≤1.7‰ (k = 2). The determined δ11B values reveal an enormous isotopic range of boron in plant tissues covering three-quarters of the natural terrestrial occurring variation in the boron isotopic composition. As the local environment and anthropogenic activity mainly control the boron intake of plants, the boron isotopic composition of plants can be used for food provenance studies.
Boron isotope studies on technical materials and geological and environmental samples are one hot topic in nowadays isotope research. Provenance studies of artefacts, food and other products might become another one, as several recent studies demonstrate. Typically boron isotope determinations have been carried out by TIMS. To open up this research field to a broader scientific community we developed two analytical procedures for boron isotope determinations based on single collector SF-ICPMS combined with a sample preparation procedure consisting of dry-ashing and a three step ion chromatographic boron-matrix separation. The developed procedures consist of one low resolution (LR) and one medium resolution (MR) procedure. The repeatability for the δ11B determination in three independently measured aliquots lies between 0.2 and 0.8‰ for the LR procedure and between 0.3 and 1.5‰ for the MR procedure. The expanded uncertainties with a coverage factor of k=2 range between 1.4 and 1.6‰ for the LR procedure and between 2.9 and 3.2‰ for the MR procedure. The accuracy, expressed as average deviation from the reference values, is 0.43‰ for the LR procedure and 0.33‰ for the MR procedure. To test the practicability of the procedures the matrix tolerance has been investigated as well and was found to be up to 2 mg kg-1 of alkaline and alkaline earth elements in the final measurement solution containing 100 µg kg-1 boron. Thus a highly efficient matrix separation for SF-ICPMS boron isotope determinations is required, similar to TIMS.
The quantitative analysis of toxic metals in plastics is very important for different sectors of industry and daily life. Many routine procedures have been established based on X-ray fluorescence or inductively coupled plasma atomic emission spectrometry. However, all of them require suitable reference materials to calibrate or validate. These reference materials ideally are being certified by reference procedures. The development of such reference procedures for sulfur and the four toxic elements cadmium, chromium, mercury and lead in plastics is described here. The procedures are based on double isotope dilution mass spectrometry including analyte–matrix separation. The applied mass spectrometric techniques are thermal ionization mass spectrometry as well as inductively coupled plasma mass spectrometry. Memory effects of mercury and dissolution of chromium(III) oxide have been considered especially. The expanded uncertainties have been improved from the percent range down to the per mill range during the development of the procedure from the early analysis of BCR-680/681 to the recent analysis of CCQM-P106. With the fully developed procedures expanded uncertainties (k = 2) between 0.1 and 0.4% for cadmium, chromium, lead and sulfur and around 1% for mercury can be achieved. The so developed procedures have been successfully applied to the certification of reference materials as well as to intercomparisons organized by CCQM.
An ICP-MS equipped with collision cell, sector field and multi-collector technology has been applied to developing analytical procedures for Fe in terms of isotope ratio measurements and isotope dilution analysis. Therefore a detailed study on the reduction of molecular interferences by a variety of collision gases (He, Ar, Kr, H2, D2 and N2) was performed. The efficiency of the reduction has been checked by high resolution mass scans. The argon based interferences disturbing the determination of 54Fe and 56Fe could be completely removed when using Faraday detectors. The interferences on mass 57 could be reduced to a level which enables accurate measurements. Also, the 40Ar12C interference, which disturbs the chromium correction, could be completely removed. Reproducibilities of 0.01% could be reached for 54Fe/56Fe isotope ratios and 0.02% for 57Fe/56Fe isotope ratios. Under these measurement conditions a sensitivity of 1.2 V per 50 µg kg1 Fe is available. This corresponds to 1.5 × 109 cps per mg kg1. Based on these results highly accurate procedures for Fe isotope ratio determinations were set up, reaching a performance level superior to that of other ICP-MS instruments. Consequently, IDMS procedures were developed on this basis in order to perform certification measurements at the highest metrological level. The first application was the contribution to the Fe reference value by IMEP-12, an interlaboratory comparison with approximately 350 participants worldwide. The analysed Fe mass content is 0.2150 mg kg1 with its expanded uncertainty (k= 2) of 0.0012 mg kg1(0.56%), which is ten times less than the uncertainty of the second certification laboratory. Another application was the certification campaign of the two reference materials Tea Leaves and Polish Herbs. The results obtained with the ICP-MS procedure, 542 ± 11 mg kg1 and 543 ± 12 mg kg1, were compared with the results analysed by TIMS, 541 ± 12 mg kg1 and 541 ± 13 mg kg1, and agree very well within the stated uncertainties (k= 2). The results presented demonstrate quite well the suitability of the developed IDMS procedures for certification or reference measurements.
Isotope reference materials are essential to enable reliable and comparable isotope data. This article reviews the work in this field within the past years. The focus is on all stable elements, except for classical stable isotopes (H, C, N, O, S) and for radioactive elements. Currently available isotope reference materials are listed. The limitations of synthetic isotope mixtures being used to characterize these materials are discussed, as well as the limitations of the isotope reference materials, such as uncertainty and homogeneity. The needs for present research on isotope variations are being considered and are compared to the limitations of current isotope reference materials. This disagreement between both can only be solved by providing isotope reference materials defining a δ-scale for each element of interest. Such materials should be provided with additional data on isotope abundances whenever possible. As an outlook a possible outline for a new program on isotope reference materials is discussed.
The variation of isotope abundance ratios is increasingly used to unravell natural and technical questions. In the past the investigation and interpretation of such variations was the field of a limited number of experts. With new upcoming techniques and research topics in the last decades, such as provenance and authenticity of food, the number of published isotope data strongly increased. The development of inductively coupled plasma mass spectrometers (ICPMS) from an instrument for simple quantitative analysis to highly sophisticated isotope abundance ratio machines influenced this process significantly. While in former times only experts in mass spectrometry were able to produce reproducible isotope data, nowadays many laboratories, never been in touch with mass spectrometry before, produce isotope data with an ICPMS. Especially for such user isotope reference materials (IRM) are indispensible to enable a reliable method validation. The fast development and the broad availability of ICPMS also lead to an expansion of the classical research areas and new elements are under investigation. Here all users require IRM to correct for mass fractionation or mass discrimination or at least to enable isotope data related to a common accepted basis. Despite this growing interest suitable IRM are still lacking for a number of isotope systems such as magnesium.
In this study, an analytical procedure for multi-element screening of 40 elements in milk and feed samples was developed. Three different digestion and two different calibration methods were tested for the best suitability. The analytical procedure for the quantification of minor and trace elements is based on sector field ICP-MS (SF-ICP-MS). The method validation revealed good agreement between the determined elemental mass fractions and the certified values of two milk and three feed reference materials. Milk samples of dairy cows at different feeding regimes were collected from two different farms. The results of our study showed significant differences of nine elemental mass fractions (Li, P, Mn, Co, Cu, Rb, Sr, Br, I) between milk from the two locations. Further, a correlation between the elemental mass fractions of milk and the ingested feed and water was observed.
Isotope ratio mass spectrometry (IRMS) is an advanced method to investigate carbon (C) and nitrogen (N) in organic samples. In particular, the N content, its isotope signature and the C/N ratio reveal important facts of nutrient cycling, niche separation and ecological food webs. In this study, the characteristics of N turnover of wood decomposing microorganisms were investigated.
The growth of the white rot causing basidiomycete Trametes versicolor is enhanced after addition of ammonia or urea, whereas the brown rot causing Oligoporus placenta is not accelerated. In addition, an interaction of each fungus with atmospheric N2 assimilating (diazotrophic) bacteria was investigated. Cultivation experiments with a gas mixture of 15N2/O2 and subsequent IRMS analysis of dry biomass of the diazotrophs Azotobacter croococcum, Beijerinckia acida and Novosphingobium nitrogenifigens revealed that they assimilated up to 12% of their N from N2. The experiments reflected N availability as a prerequisite for efficient growth of decomposing basidiomycetes and diazotrophs. Fungal–bacterial co-cultivation experiments showed that depending on the growth characteristics and bacterial N2 assimilation activity N is transferred from certain bacteria into fungal biomass. Thus, the experiments gave a first indication of an interaction between wood decomposing basidiomycetes and diazotrophs, which is a novel pathway of fungal N acquisition.