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A rapid and multi-element method for the analysis of major and trace elements in grass using energy-dispersive X-ray fluorescence spectroscopy

Karen Daly and Anna Fenelon

Teagasc Environmental Research Centre, Johnstown Castle, Wexford.

Introduction

Conventional methods of crop analysis typically involve strong acid or alkaline digestion followed by analysis of the filtrate by either colorimetric analysis, atomic absorption or inductively coupled plasma (ICP) analysis. In routine analytical laboratories delays are common due to high sample throughput, which can hold up the transfer of important results back to the farmer. Extending the grazing season in pasture-based agricultural systems requires rapid and reliable grass analysis to ensure grass quality throughout the growing season, so that grazing animals can met their dietary needs at all times.  Energy dispersive XRF can provide higher sample throughput with reliable results than our current methods allow.

Figure 1.  X-ray fluorescence theory.

This technique allows simultaneous analysis of all elements from (11Na to 92U) non-destructively in minutes, eliminating the time spent using different digestive reagents for different elements. Samples presented for XRF measurement are treated with an X-ray radiation source to excite inner orbital electrons within the sample, to an excited state. When electrons relax to ground state, fluorescent energy is emitted and the process results in measurable intensities and spectral lines, specific to each element. (Figure 1). This technique has been more widely used in mining and geochemistry to determine the elemental composition of rocks and minerals (and for ensuring quality control in the production of cement and other industrial materials. For environmental samples such as soils and plants, with large elemental compositions, the presence and predominance of other elements can interfere with values determined for elements of interest. These are known as matrix effects and are often overcome by calibrating using simple matrices or synthetic material spiked with a range of element concentrations.

Materials and Methods

This study used an archive of 600 grass samples with known % P, K and Mg determined using digestive methods with ICP analysis. 21 samples were selected as empirical standards and a further 50 were selected for validation and analysed by XRF to determine P, K and Mg. Three approaches to calibration in XRF were examined, namely; an empirical calibration using grass samples as standards (EMP); a theoretical or Fundamental Parameters (FP) approach using the instrument settings and finaly, an FP method with a matching library of grass samples attached (FPML).

Results and Discussion

Excellent agreement between XRF and ICP determined values for all elements was found, however, the level of agreement depended on the calibration approach used. For K, best agreement was found using the FP calibration. For Mg, agreement was good but improved with the addition of a matching library. For P, some bias was observed using the FP methods but excellent agreement with empirical calibration using grass standards (Figure 2).

Conclusions

Best agreement was found when grass samples were used as either empirical standards or matching library. XRF is a comparable alternative to conventional methods for grass analysis when samples of similar matrix type are used as empirical standards or matching library.

Figure 2. An example of bias observed in % P determination using FP method (top) which was corrected with empirical calibration (bottom).

Acknowledgements

The authors would like to thank Ms. Linda Moloney-Finn for ICP analysis, Mr. Conor Nolan for technical support and Dr. Wall and Mr. Philip Murphy for access to sample archives.

References

Daly K and Fenelon, A.  2017. A rapid and multi-element method for the analysis of major nutrients in grass (lolium perennae) using energy dispersive X-Ray fluorescence spectroscopy. Irish Journal of Agriculture and Food Research. DOI: 10.1515/ijafr-2017-0001.