Foliar analysis via portable X-ray fluorescence spectrometry: experimental considerations

Abstract

Portable X-ray fluorescence spectrometry (pXRF) is a well-known and accepted method for earth-materials analysis. It is also promising for analysis of plant tissues. However, some factors must be assessed in order to obtain a protocol for plants mainly for in-field measurements of intact and fresh leaves. This work defines an experimental approach for two distinct plant leaves (Spinach and Post Oak) considering water content (94% and 52% w/w, respectively) and anatomy. Using a commercial pXRF system equipped with a Rh X-ray tube, some conditions for in-situ and ex-situ analysis were assessed to obtain the concentrations of macro (P, K, Ca, Mg, and S) and micronutrients (Fe, Zn, Cu, and Mn). For in-situ applications, the penetration of X-rays through Pinus wood was evaluated. In this case, the goal was to propose a simple way to scan fresh and intact plant leaves in the field, using the wood as a background stage. Thus, the minimum wood thickness to avoid any influence of soil surface was investigated. Afterward, the following factors were assessed: i) the effect of water content; ii) the number of overlapped and pressed leaves on pXRF results; iii) the effect of adaxial or abaxial leaf surface. For ex-situ applications, the thickness of oven-dried and ground (< 0.6 mm) plant material packed into pXRF sample cup was tested. After pXRF analysis, the sum of all elements in the wood corresponded to only 0.91% having no significant influence on elemental composition of the leaves. In a qualitative manner, intact and fresh leaves can be placed on a 1-cm thick Pinus wood stage for analysis. Comparing fresh and dry leaves, water did have an effect on reported elemental concentrations, mainly for spinach leaves. For Post Oak leaves there was also an effect caused by water content but less than spinach, suggesting that for leaves with low water content (e.g., <50%) the effect of water content may be small. For both studied plant leaves there was no effect of adaxial or abaxial surface. For ex-situ analysis of oven-dried and ground plant material, the thickness in the sample cup must be strongly considered. This work suggests at least 1 cm thickness is required for accurate readings; thickness >1 cm is desirable. pXRF analysis of fresh and intact plant leaves in the field is possible in a qualitative manner.