ARTICLE pubs.acs.org/ac
Development and Validation of a Method to Determine the Boron Isotopic Composition of Crop Plants Martin Rosner,*,† Wolfgang Pritzkow,† Jochen Vogl,† and Susanne Voerkelius‡ †
Department I. Analytical Chemistry; Reference Materials, BAM Federal Institute for Materials Research and Testing, Unter den Eichen 87, 12205 Berlin, Germany ‡ Hydroisotop GmbH, Woelkestrasse 9, 83501 Schweitenkirchen, Germany ABSTRACT: We present a comprehensive chemical and mass spectrometric method to determine boron isotopic compositions of plant tissue. The method including dry ashing, a threestep ion chromatographic boronmatrix separation, and 11 B/10B isotope ratio determinations using the Cs2BO2þ 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 e1.7% (k = 2). The determined δ11B values reveal an enormous isotopic range of boron in plant tissues covering threequarters 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.
B
oron (B) has been a key element in certain industries (e.g., glass, nuclear, semiconductor) and academic research areas (e.g., chemistry, environmental, geo-, and biosciences) for several decades. Apart from classic inorganic applications interest has grown in using B isotopes as a tracer in biological systems because B uptake and availability play an essential role for the embryonic development and organogenesis of plants and animals. Therefore, studying the availability, uptake-mechanisms, translocation, and compartmentalization of B can be used for a better understanding of B excess and deficiencies and may lead to optimized agricultural processes.1 Unlike some well-established stable and radiogenic isotope systems (H, C, N, O, S, Sr, Pb)2 the use of B isotopes in biological matrixes like crop plants is almost unexplored.3 For metabolic studies, 10B tracer experiments have already been used to investigate uptake mechanisms of B and its translocation and compartmentalization within plants.1,4 In contrast to 10B tracer studies, natural variations in the B isotopic composition have been used mainly in geo- and environmental sciences to study mixing and fractionation processes as well as sources of anthropogenic contamination.5 Due to the essential role of B as a nutrient and its large natural isotopic variation (90%),5 the B isotopic composition of plants can provide novel insights into their physiology and provenance. The large natural isotopic variation of B originates mainly from low-temperature mass-dependent isotope fractionation in terrestrial earth surface reservoirs. The first attempts to use B isotopes for biogeochemical plantsoil interactions and provenance studies were on coffee r 2011 American Chemical Society
beans;6,7 however, a fully validated method for determination of B isotopic compositions of plant materials is still lacking. The main purpose of this study is to provide an analytically validated method for B isotope ratio determinations of plant tissue. The method presented combines decomposition by dry ashing with ion chromatographic Bmatrix separation and mass spectrometric 11B/10B isotope ratio measurements. The whole procedure is validated by a series of comparison measurements for each individual preparation step, with all measurements being traceable to reference materials (RM). The individual steps of chemical preparation and mass spectrometry have been tested for B loss and isotope fractionation; their individual maximum uncertainties have been used to calculate a full uncertainty budget for the δ11B values of the investigated samples.
’ EXPERIMENTAL SECTION An outline of the general workflow with the individual procedures applied is shown in Figure 1. Due to the lack of plant tissue RM with certified B isotopic compositions, we established a series of comparison measurements for the individual procedural steps. The validation ensures that neither significant B loss nor isotope fractionation and problematic contamination were introduced during sample preparation and mass spectrometry. In the following sections, the experimental Received: October 28, 2010 Accepted: February 22, 2011 Published: March 14, 2011 2562
dx.doi.org/10.1021/ac102836h | Anal. Chem. 2011, 83, 2562–2568
Analytical Chemistry
ARTICLE
Figure 1. Scheme of the workflow for B isotope determinations in plant tissue with a description of the validation procedure.
details are described and the results of the validation as well as the δ11B values for plant tissue samples are discussed. Samples, Reagents, and Materials. The crop plants investigated in this study are bell pepper, wheat, and pear samples of different origins. Corn bran (RM 8430) and peach leaves (RM 1547) RM with certified B mass fractions were obtained from the National Institute of Standards and Technology (NIST). Additionally, a white cabbage RM (BCR-679) from the Institute for Reference Materials and Measurements (IRMM) and a maize proficiency test sample (IPE 126 Maize) from Wageningen Evaluating Programmes for Analytical Laboratories (WEPAL) with reported information B mass fractions were used. For the investigation of potential isotope fractionation and contamination during sample preparation and mass spectrometry we used certified boric acid RM from NIST and BAM Federal Institute for Materials Research and Testing (SRM 951/952 and ERMAE101) as well as an in-house B concentration standard (Merck CertiPUR; 1000 μg/g). For the same purpose, three natural water RM (IAEA B 13) from the International Atomic Energy Agency (IAEA), with published B isotope compositions and B concentrations, were used.8 For wet chemical sample preparations ultrapure water (
