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2 X‐Ray Fluorescence for Multi‐elemental Analysis of Vegetation Samples

       Eva Marguí1 and Ignasi Queralt2

       1 Department of Chemistry, University of Girona, Girona, Spain

       2 Institute of Environmental Assessment and Water Research, IDAEA‐CSIC, Barcelona, Spain

2.1 Introduction

      Presently, the development of analytical methods to determine elemental composition of vegetation samples is of significance in many different fields. For instance, vegetal species are commonly used in environmental studies as bioindicators due to their ability to accumulate metal elements. Vegetables are also an essential part of human diet and thus, the determination of multi‐elemental composition of vegetal foodstuff is also relevant for safety and nutritional purposes. Finally, it is also important to note the considerable number of studies published comparing edible plant growth and development under different fertilizers or farming regimes through laboratory‐controlled and field experiments.

An analytical method's fitness for purpose for the analysis of vegetation samples in the aforementioned fields is related to the fulfillment of some “desired” characteristics (Figure 2.1). Additionally, in some applications such as the analysis of mass‐limited vegetation samples (i.e. biofilms), or the study of element distribution within the vegetation tissue, more specific analytical characteristics such as microanalytical or imaging capabilities can be also of special interest.

In fact, there exists not one single analytical technique which combines all these features. Multi‐elemental atomic spectrometry techniques such as inductively coupled plasma‐optical emission spectrometry (ICP‐OES) or inductively coupled plasma mass spectrometry (ICP‐MS) are usually employed to measure major, minor and trace elements in vegetal samples [1, 2]. The main drawback of these techniques in the field of vegetation analysis is the need to bring the solid sample into the liquid state by means of a decomposition procedure. Vegetal matrices seem to be very similar to other biological ones which can be decomposed easily using mixtures of strong acids (i.e. nitric acid) and oxidant agents (i.e. hydrogen peroxide). However, there is a clear difference between these types of matrices, namely a higher silicon content (up to 10%) in the case of plant material. This factor complicates the dissolution step and sometimes the addition of stronger acids such as hydrofluoric acid is mandatory to enable a total destruction of the matrix [3]. In such cases, the use of alternative analytical methods that obviate matrix destruction is tempting. Therefore, the development of other methods for direct and multi‐elemental analysis of vegetal samples has been increased in the last years [4]. In this field, it is interesting and needed to highlight the potential of X‐ray fluorescence (XRF) spectrometry.