Review of recent developments in quantitative analysis and microchemical imaging of plants by LIBS and XRF.

A portable energy dispersive X-ray fluorescence spectrometer was evaluated in the in situ analysis of fresh sugar cane leaves for real time plant nutrition diagnosis. Fresh leaf fragments (n = 10 sugar cane varieties; 20 fragments per leaf; 2 measurement sites per fragment) were irradiated and the averaged data from X-ray characteristic emission lines intensities (for K, Ca, S and Si Kα lines) were in close agreement with mass fraction data obtained by a validated comparative method. The linear correlation coefficients (r) ranged from 0.9575 for Ca to 0.9851 for Si. The obtained limits of detection were at least two-fold lower than the critical nutrient levels. Manganese can also be properly determined, but validation still requires more robust calibration models. The proposed method is a straightforward approach towards the fast evaluation of the nutritional profile of plants avoiding time-consuming steps, which involve drying, grinding, weighing, and acid digestion.Keywords: in situ analysis, portable X-ray fluorescence spectrometry, plant mineral analysis, sugar cane, silicon IntroductionPlant nutrition diagnosis is a well-established and reliable way for guiding the proper fertilizer recommendation contributing to the healthy growing of crops of economic importance.1 The analytical protocol most frequently applied for the determination of macro-and micronutrients in plant materials often requires an a priori test sample decomposition, usually performed by microwave-assisted acid sample dissolution, followed by determination using inductively coupled plasma optical emission spectrometry (ICP OES). 2Some studies have focused on the use of the direct solid sampling analysis based on X-ray fluorescence spectrometry (XRF), [3][4][5] laser-induced breakdown spectroscopy (LIBS) [6][7][8] or laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) 9-11 for assessing the mineral nutrition profile of plants. The most common way of sample presentation by using the aforementioned direct analysis methods involves the interrogation of a previously ground plant material in a pellet form.6,9 A promising trend of research is the direct analysis of the dried plant leaf without the need for grinding or pelletizing steps.12 On this subject, both LIBS and energy dispersive XRF (EDXRF) have been used.The increasing advances in the field of portable instrumentation has expanded the application of this emerging technology. Such a fast development is due to improvements in digital electronics and in the development of miniaturized sensors and other crucial components. These novel mobile equipment offer a plethora of advantages to the analyst such as real time and onsite determinations of organic and inorganic analytes in a myriad of sample matrices. [13][14][15] In case of the use of portable LIBS and EDXRF systems, the most common Guerra et al. 1087 Vol. 29, No. 5, 2018 in situ applications involve the determination of potentially toxic metals in soil, [16][17][18][19] and analysis of samples as diverse as those o...

Microsampling strategies were evaluated for the direct analysis of dried sugar cane leaves by energy-dispersive X-ray fluorescence spectrometry (EDXRF) and laserinduced breakdown spectroscopy (LIBS). The analysis by EDXRF was carried out by irradiating each leaf fragment in its middle portion with a collimated 5 mm X-ray spot size during 50 s, allowing the determination of P, K, Ca, S, Fe, Mn and Si. EDXRF was also useful to conclude that 15 leaf fragments (37.5 % of the recommended sampling area) were enough for attaining a representative analytical response from the whole diagnostic leaf. Regarding LIBS, that employs a substantial smaller ablation area (i.e., 750 µm laser spot size), sampling strategies were defined by taking into account the microchemical distribution of P, Ca, Mg, Fe, Mn, B and Si in 9 mm x 9 mm leaf fragment area. The proposed sampling protocol relied on the interrogation (rastering) of 3 equally spaced sampling lines in each leaf fragment with 48 accumulated laser pulses per line (Nd:YAG at 1064 nm, 5 ns, 10 Hz, 50 J cm -2 ) perpendicularly to the leaf midrib. This strategy enabled the simultaneous determination of P, K, Ca, Mg, Fe, Cu, Mn, Zn, B and Si by LIBS. Cross-validation between LIBS and EDXRF for P, K, Ca, Fe, Mn and Si predicted mass fractions presented high linear correlation coefficients of up to 0.9778 (selecting 15 leaf fragments per diagnostic leaf from 10 different sugar cane varieties). Results provide insights into a novel and promising strategy for direct and fast plant nutrition diagnosis, fostering further studies for in situ analysis of fresh leaves, strengthening the implementation of Precision Agriculture and Green Chemistry concepts. 3 % (for 15 fragments, 37.5 %diagnostic leaf area). The same trend was also observed for S, as the relative error diminished from 10 % (5 fragments, 12.5 % diagnostic leaf area) to 1.6 % (15 fragments, 37.5 % diagnostic leaf area). For the remaining evaluated elements, sampling 15 leaf fragments was appropriate for this intended purpose.Another alternative to evaluate the sampling representativeness was based on the construction of microchemical maps of P, Ca, Mg, Fe, Mn, B and Si in 9 mm x 9 mm leaf fragment area by LIBS for assessing the spatial distribution of the analytes. The selected number of accumulated laser pulses (i.e., 5 consecutive pulses) was defined as the threshold to perforate the leaf fragment at the experimental conditions set herein (50 J cm -2 , 750 µm spot size and 10 Hz). In addition, the microchemical maps were built