Adhering Particles - Methods of Correction
4.2 Materials and Methods
calculate a correction term for adhering material. We will present three methods of calculating a correction term and show the effect of these correction terms on the trace element concentrations of plant matter.
4.2 Materials and Methods
4.2.1 Study Sample Set
The sample set consist of plants and corresponding soil samples from different re-search projects between 2011 and 2015. In all projects a broad element range was measured focusing on trace elements. The plants in these studies comprised forage and energy crops (for biogas production), perennial and catch crops (Table 4.1). In this study the element data of a total of 1040 plant samples of 19 species and at least 10 samples for each species is presented.
The plant and soil samples were retrieved from open arable field trials and from (open) pot trials filled with differently soils. Of the 1040 plant samples 789 plants were from open field (20 locations in southern Lower Saxony and Bavaria, Germany), and 251 from open pot trials (a total of 13 different locations in Lower Saxony). The pots were located close to our department. None of the plant samples were rinsed. The soil types in the field ranged between silt, silt loam and sandy loam with a pH of 5.8 to 7.0 (measured in 0.01 mol/L CaCl2).
Table 4.1:List of plants comprised in the used data set.
Plant Species Scientific name Nr. of samples Nr. of locations
Amaranth Amaranthusspp. 255 12
Barley Hordeum vulgareL. 11 2
Beet (leaves) Beta vulgarisL. 18 7
Buckwheat Fagopyrum esculentum 47 16
Catnip Nepeta catariaL. 14 11
Cup Plant Silphium perfoliatumL. 13 8
Faba Bean Vicia fabaL. 59 11
Maize Zea maysL. 151 23
Mustard Sinapsis albaL. 15 12
Oat Avena sativaL. 24 2
Pea Pisum sativumL. 14 3
Quinoa Chenopodium quinoa 50 16
Rye Secale cerealeL. 102 20
Ryegrass Lolium perenneL. 28 4
Sorghum Sorghum bicolorL. 13 3
Sunflower Helianthus annuusL. 52 14
Triticale TriticosecaleWittm. 92 19
Vetch Vicia villosaL. 55 6
Wheat Triticum aestivumL. 27 15
32 Chapter 4. Alteration of Trace Element Concentrations in Plants by Adhering Particles - Methods of Correction
4.2.2 Sampling, Sample Preparation and Analysis
The plant samples were cut at ca. 10 cm above soil surface (except from some samples, see section 4.5.4) at the stage of lactic ripeness or end of flowering. The roots are not included in the analysis. About 500 g of soil samples taken in the upper part of the soil (up to 30 cm depth) and are air dried and sieved to < 2 mm in grain size. All samples, soils and plants, were dried at 105◦C. A minimum of 100 g of the soils and ca. 500 g of plants were ground in an agate ball mill to avoid metal contamination.
Aliquots of 150 mg of soil respectively 700 mg of plant powder were fully digested with a mixture of ultra pure concentrated HNO3, HClO4and HF in closed ultra clean PTFE vessels (PicoTrace, Bovenden, Acid sample digestion system (DAS 30)). In the soil samples a small amount of HCl is added. Soil samples were diluted to 100 ml the plant samples to 50 ml for measurement (see Supplement 1.1. for further remarks on the digestion method). In the resulting clear sample solutions 47 elements could be quantified by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) and Mass Spectrometry (ICP-MS).
To ensure clean handling and quality of analysis for every 30 samples one to two blank solutions and one or two international reference standards were digested and analyzed together with the samples. For plant samples the following international reference materials were used: GBW 10052 (green tea, Institute of Geophysical and Geochemical Exploration, China), BCR-129 (hay powder, Community Bureau of Ref-erence, Luxembourg), WEPAL-IPE-126 (maize plant, Wageningen University, Nether-lands), WEPAL-IPE-168 (sunflower, Wageningen University, Netherlands). For soils the in-house geochemical standard TW-45 (Wissenbach slate, Harz Mountains, Ger-many) was used. Results were only accepted if concentrations were above the limit of detection (3-fold standard deviation of the blank concentrations for each analysis batch) and if the corresponding reference samples showed concentrations within the certified values. Average precisions for the elements were better than 5% for all main elements and most of the trace elements; for plants the ultra-trace elements Cr, Co, Mo, Sn, and the heavy REE showed average precision between 10% - 20%. All con-centration data are given inmg/kgdry weight (d.w.).
4.3 Theory
All correction terms are based on the following simplified model: The Plant samples collected from open field trials consist of the pure plant material (Plant) and a minor content of deposited solid material from different sources, from now on called adher-ing particles (AP).
With a digestion method ensuring complete and total dissolution of the sample the an-alytically determined element concentration,PlantSample, contains both components, PlantandAP. Hence the concentrations ofPlantcan be calculated by subtractingAP fromPlantSample(Eq. 4.1).
The most important sources of adhering particles are airborne dust, soil particles from wind or rain splash. In this study the adhering particles are assumed to be mainly soil particles. Especially the lower parts of plants will be strongly affected by splashed soil particles, but this holds also for the higher parts of the plants even when less affected.
Hence for the calculation of the correction term we use AP = Soil. We are aware that a fractionation of soil particles may occur during transport through the air or that
4.4. Calculation 33 collected atmospheric dust could locally be a better proxy for adhering material, but to simplify our calculations we take into account only data of the soils corresponding to the plants. Air dust collection and analysis would be extremely laborious and time-consuming. The content of adhering particles can be approximated by elements with good analytical precision and accuracy and with negligible uptake by plants, hence concentrations approach zero, but high concentrations in the adhering particles. We call these elements "indicator elements" (Elind). In order to estimate the content of adhering soil (xin Eq. 4.1) to the plant samples we propose three different methods:
Method 1: Using one pre-defined elementElindfor calculating the content (x) ofAP inPlantSample From Elements fulfilling all requirements of Elind one is chosen to calculatexin Eq. 4.4 (Ti, Al, Zr, Sc...). For example using Ti asElindthe content ofAP is calculated throughx = PlantSample[Ti]/Soil[Ti]and thisxis then used in Eq. 4.2.
Negative concentrations should be set to zero.
Method 2: Subtracting the smallest possible content ofAP(smallestx) For each sample the element with the smallestxof all ratios
x = PlantSample[El]/Soil[El] of each sample is taken as Elind, hence every sample is corrected based on a different Elind. With this method only the smallest possible content ofAPis subtracted fromPlantSample(Eq. 4.2). Typical indicator elements are Ti, Al, Th, Cs, Zr etc., if they can be measured with high sensitivity and reproducibility.
With this method there are no negative concentrations.
Method 3: Using the median of several elements with a very smallxfor calculating the content ofAP In order to reduce the uncertainty of the content of adhering par-ticles based on only one element as in method 1 and 2 an average of severalxofElind elements can be calculated.
With∆x being the absolute error of x we suggest to take the median of the x of all elements which valuesx−∆x are smaller thanxsmallest+∆xsmallest. The value of the median ¯xis then used asxin Eq. 4.2. Negative concentrations should be set to zero.
Because statistically thexof all elements, which error overlaps the error of the element with smallestx, are indistinguishable. We also suggest to set all elements contributing to ¯xto zero, because these small values should not be interpreted.
4.4 Calculation
The easiest way to calculate a subtraction of concentrations while taking the math-ematical constraints of concentration into account is to use vectors (Aitchison, 1986;
Aitchison, 2003; Buccianti et al., 2006; Pawlowsky-Glahn et al., 2015). The composi-tion of each sample can be written as a vector with the concentracomposi-tion of each element as a vector component: PlantSample,PlantandAP. For easier reading the vector ar-row is omitted in the following. For example the concentration of Ca in the adhering particles is written as AP[Ca], for the concentrations of the five elements Ca, Fe, Mg, S and P the notation isAP[Ca,Fe,Mg,S,P]. The notation without any[ ], only AP, is short version forAP[Al,As, ...,Zr].
34 Chapter 4. Alteration of Trace Element Concentrations in Plants by Adhering Particles - Methods of Correction
With a variablexfor the content ofAPinPlantSample, 0<x <1, the composition of PlantSamplecan be expressed as the sum ofPlantandAP:
PlantSample= Plant∗(1−x) +AP∗x (4.1) Resolving equation 4.1 forPlantresults in equation 4.2:
Plant= PlantSample−x∗AP
1−x (4.2)
of which the composition of APand the content of the adhering particles, expressed asx, are unknown and assumptions have to be made.
Assuming that the concentration of one of theElindinPlantis zero,Plant[Elind] = 0, equation 4.2 can be used for solvingx, the content ofAP:
Plant[Elind] = PlantSample[Elind]−x∗AP[Elind]
1−x =0 (4.3)
x= PlantSample[Elind]
AP[Elind] (4.4)
Note that for the calculation of x only elements of the group of Elind are allowed.
Otherwise the correction term will lead to unrealistic element concentrations inPlant.
The absolute error ofx,∆x, is calculated through the general formula for error propa-gation for random and uncorrelated errors ofPlantSample,with ∆PlantSampleas the absolute error range for analyzed plant values, and AP, ∆AP as the absolute error range of the adhering particles:
∆x= ∂x
∂PlantSample ∗∆PlantSample+ ∂x
∂AP ∗∆AP
= 1
AP ∗∆PlantSample− PlantSample
AP2 ∗∆AP
(4.5)
To determine the absolute error∆PlantSamplewe use the relative standard deviation of more than 30 measurements of the international reference sample GBW10052 as rel-ative errorδPlantSample: ∆PlantSample= δPlantSample∗PlantSample. To simplify the error calculation∆Plant for equation 4.6 we suggest to use in the error-equation as∆x¯the standard deviation of ¯x.
For most results we used Method 3 explained in theory part. We assume that the major source of adhering particles are particles of the soil on which the plants grew (AP=Soil). Hence based on equation 4.2 the correction equation is
Plant= PlantSample−x¯∗Soil
1−x¯ (4.6)
If an element has a low concentration inSoil the term ¯x∗Soilbecomes close to zero.
With ¯x∗Soil ≈ 0 and the termPlantSample/(1−x¯)always> PlantSamplethe term
PlantSample−x¯∗Soil
1−x¯ may result for main elements in plants intoPlant> PlantSample.
For all calculations and graphs we used the free software R (R Core Team, 2017) and the package ggplot (Wickham, 2016).