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Article

Metals Contained in Various Formulations of Mineral Nitrogen Fertilizers Determined Using Portable X-ray Fluorescence

by
Aleksandra Perčin
1,
Željka Zgorelec
1,*,
Tomislav Karažija
2,
Ivica Kisić
1,
Nikolina Župan
3 and
Ivana Šestak
1
1
Department of General Agronomy, University of Zagreb Faculty of Agriculture, Svetosimunska cesta 25, 10000 Zagreb, Croatia
2
Department of Plant Nutrition, University of Zagreb Faculty of Agriculture, Svetosimunska cesta 25, 10000 Zagreb, Croatia
3
Graduate Study in Agroecology, Department of General Agronomy, University of Zagreb Faculty of Agriculture, Svetosimunska cesta 25, 10000 Zagreb, Croatia
*
Author to whom correspondence should be addressed.
Agronomy 2023, 13(9), 2282; https://doi.org/10.3390/agronomy13092282
Submission received: 30 June 2023 / Revised: 4 August 2023 / Accepted: 7 August 2023 / Published: 29 August 2023
(This article belongs to the Special Issue Sustainable Circular Agricultural Food Production Systems)
Versions Notes

Abstract

:
According to the Scopus database, over the last five years, 91 scientific papers with the keyword “pXRF” (portable X-ray fluorescence) were published in indexed journals in the domain of environmental science and agricultural science, which indicates more frequent applications of this technique in scientific research. The pXRF method is characterized by speed, precision, accuracy, and the possibility of a simultaneous analysis of a large number of elements, albeit with higher limits of detection (LODs) as a major disadvantage. The presence of metals in certain phosphate fertilizers is well established, though not to the same extent as in mineral nitrogen fertilizers. The aim of this research was to determine the metal content (As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Si, Sr, Th, U, Zn, Zr, and Y) in thirteen commercial mineral nitrogen fertilizers via the pXRF method. Six straight fertilizers (ammonium nitrate, ammonium sulphate nitrate, limestone ammonium, nitrate, and urea) and seven complex fertilizers (various NPK formulations), which are different even according to their production technology, produced in Croatia were analyzed using the handheld Vanta C (Olympus) XRF analyzer according to the loose powder method and “point and shoot” technique. Data quality control was performed by analyzing the reference fertilizer samples and certified and reference soil samples. The results revealed that the determined contents of Cd, Mn, and Th were relatively higher in the single-component fertilizers, while the contents of As, Cr, Fe, Ni, Si, Sr, Zn, Zr, Y, and U were relatively higher in the complex fertilizers. Due to the higher LODs of Co and Pb (3 mg/kg) and Mo (2 mg/kg), the pXRF method was not appropriate for the determination of these metals in the analyzed fertilizers. The quantified metal content in the analyzed fertilizers varied as follows: 2.0–8.0 mg As/kg; 11.5–31.3 mg Cd/kg; 29.8–118.5 mg Cr/kg; 7.8–26.3 mg Cu/kg; 16.5–2209 mg Fe/kg; 20.3–5290 mg Mn/kg; 6.2–27.8 mg Ni/kg; 1156–4581 mg Si/kg; 2.0–469.8 mg Sr/kg; 3.0–35.3 mg Th/kg; 2.0–82.8 mg U/kg; 1.4–166 mg Zn/kg; 9.7–15.3 mg Zr/kg; and 16.5–128.0 mg Y/kg. The results indicated that the pXRF method is particularly suitable for measurement and metal detection in complex nitrogen mineral fertilizers with higher amounts of metals, but it is not suitable for the detection and quantification of the lower amounts of As, Zr, Y, Cu, Ni, and Cr in single-component nitrogen fertilizers. Compared to all of the investigated fertilizers, the highest amounts of As, Cr, Cu, Fe, Ni, U, Zn, and Zr were quantified in the NPK 7-20-30 formulation.

1. Introduction

Currently, in order to promote sustainable agriculture practices with the goal to preserve healthy ecosystems and support the sustainable use of soil and water along with ensuring world food security, the need arises for rapid, non-destructive, and accurate analytical techniques for the composition quantification of many agricultural inputs. Some of the main inputs in the agriculture sector that can be singled out are seeds and planting material, feed, energy, fertilizers, and plant protection agents [1]. These inputs, especially fertilizers and plant protection agents, can significantly negatively affect the environment [2,3]. In 2020, the global pesticide use was 2.7 million tonnes, while the total agricultural use of inorganic fertilizers (expressed as the sum of nitrogen, phosphorus (P2O5), and potassium (K2O)) was 201 million tonnes [4], which imposes the requisite for the constant monitoring of the quality of those inputs. The amount of applied fertilizers indicates that the regular application of mineral fertilizers is a common and irreplaceable practice today.
Fertilization, primarily mineral nitrogen fertilization, can be beneficial for plant production and crop yield [5], but it can also impact soil and water quality degradation [6,7]. Along with essential nutrients, mineral fertilizers contain certain amounts of heavy metals (As, Cd, Co, Cr, Cu, Hg, Mo, Ni, Pb, and Zn) [8,9,10,11], and the long-term application of such fertilizers can cause a significant accumulation of metals in soil [12] and crops [13]. Heavy metals occur naturally in the raw materials used for fertilizer production, but they also occur in mineral fertilizers as a result of fertilizers binding with recycled industrial waste (e.g., mine tailings and steel flue dust) [14]. Permissible concentrations of heavy metals in fertilizers are subject to the special legal regulations of individual countries [15], and in Croatia, this is regulated through national legislation [16] and EU legislation [17]. The legislation also proposes methods for metal detection and quantification. Mostly, heavy metals in solid fertilizers are detected using atomic spectroscopy with previous acid digestion. For example, the contents of Cd, Cu, Cr, Co, Mn, Ni, Pb, and Zn in various solid nitrogen and phosphorus mineral fertilizers have been quantified using the AAS (atomic absorption spectrometry) method [9,11,18], but there were also other methods used such as GF-AAS (graphite furnace atomic absorption spectrometry) [19] and ICP-MS (inductively coupled plasma mass spectrometry) [10].
Lately, methods for the analysis of metals in solid mineral fertilizers, which do not include an acid preparation of the sample, are increasingly being applied. One of them is neutron activation analysis (NAA) in combination with γ-ray spectrometry (GRS) [20], and another one is X-ray fluorescence (XRF) [21,22]. Over the past two decades, progress has been made in the XRF method, which has evolved from laboratory stand-alone units to portable and lightweight instruments. These portable instruments, called portable XRF (pXRF) spectrometers, have allowed agricultural scientists to investigate materials (soil, plant material, and fertilizers) with greater flexibility than ever before [23]. The method is fast, precise, and non-destructive, while sample preparation is usually not complex or time-consuming [24].
Knowledge of metal content in mineral nitrogen fertilizers can contribute to various risk assessment studies, as well help to support and promote the application of soil management measures, which will maintain and preserve many important soil functions. The aim of this research was to determine the metal content (As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Si, Sr, Th, U, Zn, Zr, and Y) in thirteen commercial mineral nitrogen fertilizers by means of the pXRF method.

2. Materials and Methods

2.1. Fertilizer Samples

This research was based on analyses of thirteen solid commercial mineral nitrogen fertilizers (Table 1) produced in Croatia. The samples differed according to their production technology (prilled and granulated) and nitrogen formulation (single-component or straight fertilizers (ammonium nitrate, ammonium sulphate nitrate, limestone ammonium nitrate, and urea) and complex fertilizers (various NPK formulations)). In addition to ammonium nitrate (with 33.5% of nitrogen), which was prilled, all twelve of the other listed fertilizers were granulated, although they are not marked as such in Table 1.

2.2. Laboratory Measurement

Fertilizers were ground (Tube Mill 100 control, IKA; 15,000 rpm/300 s) and dried (70 °C) to a constant mass. Homogenized samples (~10 grams of sample, particle size 50 μm) were spread out on a film material (Proline Thin film, Chemplex Industries Inc, Palm City, FL, USA) and pucked in plastic sample cups (30.7 mm O.D. × 22.9 mm High, Chemplex Industries Inc, Palm City, FL, USA). Measurement was conducted according to the loose powder method [25,26] and the “point and shoot” technique using a VantaTM handheld (portable) XRF analyzer C Series (Olympus, Waltham, MA, USA, 2019). Particle sizes lower than 60 μm are recommended for a suitable pXRF analysis [27], and due to the fact that the repeatability of the results obtained by the loose powder method is weak to moderate (surface effects were prone to be more severe) [28], each fertilizer sample was added gradually in a sample cup, and each layer of fertilizer up to the very top of the measuring cup was manually compressed using a suitable tool.
A Vanta C analyzer contains a silicon drift detector and excitation source (Rh anode X-ray tube, 50 keV). Measurements were carried out by a method for geochemical analyses (GeoChem—two beam). Two beams were calibrated at 40 keV (the excitations of As, Cd, Co, Cr, Cu, Fe, Mo, Ni, Pb, Sr, Th, U, Zn, Zr, and Y) and 10 keV (the excitations of Mn and Si). GeoChem uses a fundamental parameter algorithm, which automatically corrects for the inter-element effect. This method enables one to correct a specific set of factors on particular elements of interest in order to correct matrix effects. However, for this research, this was not performed. The limits of detection (LOD) for the Vanta C pXRF analyzer varied from 1 mg/kg for As, Sr, U, Zr, and Y up to 325 mg/kg for the Si content (Table 2). All measurements were performed in accordance with the standards in [29,30]. Each sample was measured in four repetitions, and each measurement time was 120 s. The results represent the total concentrations of quantified metals.

2.3. Data Quality Control

Data quality control was performed via an analysis of reference fertilizer samples and certified and reference soil samples. The quantified metal content in those samples, along with the assigned and certified metal contents in the reference and certified samples, enabled the calculation of the precision and accuracy of the measurement. The precision of the measurement was expressed as the relative standard deviation (RSD). The accuracy of measurement was estimated as the relative percent difference (RPD) between the assigned values in the reference fertilizer sample (RF value) and the values (amounts) determined by pXRF (pXRF value). This was achieved with the following equation:
[(CpXRF − CRF)/CRF] × 100
The same calculation was performed to determine the precision and accuracy of the analysis of the certified and reference soil samples. According to the international standards that describe measurement by means of the pXRF technique, the permissible deviation in the measurements’ precision and accuracy is up to ±20% [29] or up to ±25% [30].
Five reference fertilizer samples (“Mineral fertilizer—MF” (6-3424-0046), “NPK + S” (4-1924-0082), “Diammonium phosphate—DAP” (2-4124-0008), “Complex nitrogen fertilizer—CNF” (13-3024-0049), and “NPK + SO3” (6-1924-0045)) were collected as part of the participation conducted by the analytical laboratory of the Department of Plant Nutrition (University of Zagreb Faculty of Agriculture) in the international program of inter-laboratory comparison, i.e., the BIPEA program. The results of the available assigned values of the metal content in the samples (Table 3) were taken from the official reports of the international laboratory comparison and were determined by standard analytical techniques (acid digestion and AAS/ICP-MS detection) [31].
The precision of the measurement was calculated for all quantified amounts of metals in the reference fertilizer samples, while accuracy was determined only for several elements (As, Cr, Cu, Fe, Mn, Ni, and Zn) due to the available assigned mean values of the metals in the reference fertilizer samples (Table 3) (this was also the case due to the limits of detection of the pXRF analyzer (Table 2)). The contents of Co, Pb, Mo, and Cd were not detected, and they were below the LODs. In the five reference soil samples (Table 4), a total of 53 metal contents were quantified (10 metals in MF and NPK + SO3 fertilizer, 11 metals in the NPK + S, DAP, and CNF fertilizer), and all of them were measured with a satisfactory allowed precision (RSD < 25) [29]. The quantified amounts of As found in MF, NPK + S, DAP, and CNF; Cr in NPK + S; Cu in NPK + S, DAP, CNF, and NPK + SO3; Ni in NPK + SO3; and Fe, Mn, and Zn in all five reference samples were compared with the assigned mean values of Table 3. Thus, the relative percent difference (RPD, accuracy) was calculated. The results indicated that, from all 24 compared results, only As in DAP, Fe in MF, Cr in NPK + S, and Cu in CHF fertilizer samples were not quantified with actable accuracy; all other RPD values indicated that the Vanta C pXRF analyzer is capable of detecting and quantifying As, Cu, Fe, Mn, Ni, and Zn with prescribed and permitted accuracy.
Due to the fact that the available analyzed reference fertilizer samples did not give an insight into the content of other metals of interest (Si, Sr, Th, U, Zr, and Y), analyses of the certified reference material (SRM 2711a, Montana II Soil; National Institute of Standards & Technology, USA [32]) and reference soil material (ISE 989, WEPAL, 2015) were used to control the measurement accuracy of those metals. Along with the accuracy, the precision of the measurement was also calculated (Table 5). ISE 989 is a soil sample collected as part of the participation of the Analytical Laboratory of the Department of General Agronomy (University of Zagreb Faculty of Agriculture) in the worldwide Wageningen Evaluating Programme for Analytical Laboratories (WEPAL) within the International Soil-Analytical Exchange (ISE) program conducted by Wageningen University (Netherlands). The assigned values were taken from the WEAPL official report from 2015, and they represent the real totals determined by several techniques (XRF, NAA, ICP-AES, and ICP MS with acid digestion—HF and different final medium) [33]. It was noticeable that the results of the precision of the measurement indicated that only the content of Co and Th were not quantified with the prescribed precision in the SRM 2711a soil sample; meanwhile, from the thirteen quantified metal contents in the SRM 2711a sample, only three (Cr, Ni, and Pb) were detected with a lower accuracy (RPD = 32.7—53.2%). From the seventeen quantified metal contents in the ISE 989 sample, only the Si content was detected beyond the permitted accuracy (RPD = 27.2%).

2.4. Statistical Analyses and Calculation

The obtained data of the metal content in the thirteen solid commercial mineral nitrogen fertilizers were processed at the level of descriptive statistics (the mean of each individual element in the investigated sample and standard deviation). The precision of the measurement was also expressed as the relative standard deviation (RSD).

3. Results

Metal Content in Commercial Fertilizers

The variability in the metal content of the commercial fertilizers from Croatia was detected and quantified by means of the pXRF method presented in Table 6. It is necessary to point out that the contents of Co, Pb, and Mo were not detected in any investigated fertilizer sample and were below 3 mg/kg (Co and Pb) or 2 mg/kg (Mo).
The results indicated that the straight fertilizers (ammonium nitrate (AN), ammonium sulphate nitrate (ASN), limestone ammonium nitrate (LAN), urea) contained lower amounts of certain elements (As, Cr, Cu, Ni, Sr, Zr, and Y) when compared to the complex fertilizers (NPK and NP formulations). The quantified average amounts of As, Cr, Cu, Ni, Zr, and Y in the complex fertilizers were, respectively, 4.9 mg/kg, 91.0 mg/kg, 17.1 mg/kg, 16.8 mg/kg, 12.8 mg/kg, and 48.4 mg/kg; meanwhile, in the AN, ASN, and LAN formulations, the content of the adduced elements was lower than 1 mg/kg (As, Zr, and Y), 4 mg/kg (Cu), 6 mg/kg (Ni), or 15 mg/kg (Cr). Also, it was noticeable that the Cd content was quantified in all of the investigated fertilizers in the range of 11.5 mg/kg (NPK 15-15-15 and NPK (S) 15-15-15) to 31.3 mg/kg (urea), and the average was 57.4% higher in the straight fertilizers (mean 23.3 mg Cd/kg) when compared to the complex fertilizer formulations (mean 14.8 mg Cd/kg). In terms of Fe content, the highest amount was recorded in the NP 20-20 fertilizer (2209 mg/kg), and the quantified content of this metal can be divided into three groups in the investigated fertilizers. The first group determined in the AN and ASN fertilizers was detected with an average amount of 49.1 mg Fe/kg, the second group (LAN) with 319.4 mg Fe/kg, and the third fertilizer group with an average amount of 1300 mg Fe/kg. These same contents were determined in the NPK formulations with nitrogen in the range of 7% to 15%. In urea, the Fe content was below 15 mg/kg. The highest amount of Mn was detected in ammonium nitrate (5290 mg/kg), followed by urea with 4707 mg/kg. Compared to the single-component fertilizer formulations, the Mn content was lower in all of the investigated complex fertilizers with an average of 145.7 mg/kg. The Si content in the complex fertilizers (mean 3009 mg/kg) was 105.4% higher than the average Si amount in the straight fertilizer formulations (mean 1465 mg/kg), and the highest individual value of this element was recorded in the NPK 15-15-15 fertilizer (4581 mg/kg). In terms of the quantified Sr content results, it was revealed that urea and ammonium sulfate nitrate fertilizers contained less than 1 mg/kg. Meanwhile, the ammonium-nitrate-composed fertilizers contained 3 mg/kg on average, and the limestone ammonium nitrate fertilizers contained an average of 21.2 mg/kg. The Sr complex fertilizer contained relatively higher amounts of Sr (mean 259.7 mg/kg), whereby a particularly high amount, 469.8 mg/kg, was determined in the NPK 13-10-12 fertilizer. In addition to the detected Cd and Mn contents, the Th content was also relatively higher in single-component fertilizers (especially in terms of ammonium nitrate and urea) when compared to the complex fertilizers. From the fourteen quantified metals and the already described variability of As, Cr, Fe, Ni, Si, Sr, Zr, and Y, U and Zn were also contained in a higher amount in the complex fertilizers compared to the straight ones. On average, the multi-element compound fertilizers (NPK formulations) had averages of 47.6 mg/kg of U and 112.9 mg/kg of Zn, while the single-component nitrogen fertilizers (AN, ASN, LAN, and urea) contained 2.82 mg/kg of U and 4.28 mg/kg of Zn.
Regarding the precision of the detected metal content, Table 7 shows the relative standard deviation (RSD) as the indicator of precision. The results from Table 7 disclose that, in the analyzed fertilizer samples, Fe (RSD 0.88–4.48%), Si (RSD = 1.96–17.7%), Sr (RSD = 0.00–4.50%), Zr (RSD = 5.02–12.2%), and Y (RSD = 0.74–7.82%) contents were determined with satisfactory and acceptable precision for all content ranges. In contrast to that result, the precision of the measured As content was acceptable in six of the eight quantified As amounts (RSD 0–16.7%), while in the NPK 13-10-12 and NP 20-20 fertilizers, there was a lower detected As content (2 mg/kg), as well as a slightly lower measurement precision (RSD 33.3–45.7%). An acceptable precision was not recorded in terms of the Ni content in the NPK 20-10-10 fertilizer (RSD = 39.2%). Cd content was determined with a poor precision in the NPK 13-10-12, NPK 15-15-15, and NPK (S) 15-15-15 fertilizers (RSD = 26.0–47.7%). Furthermore, regarding the remaining four elements, the determined content of Cu in the NPK 13-10-12 fertilizer was the only one above the permissible precision value (RSD = 53.1%). The content of Mn in the NP 20-20 fertilizer (RSD = 85.0%) and U in ammonium nitrate (RSD = 70.7%), however, was not acceptable. Meanwhile, the Th content determined in the NPK 13-10-12 and NPK 15-15-15 fertilizers was above the acceptable precision (RSD = 43.3% and RSD = 30.3%). Also, all fourteen elements were detected and quantified with satisfactory precision in the NPK 7-14-21 fertilizer. The results from Table 6 indicate that the majority of the quantified metals were determined with satisfactory precision.

4. Discussion

Metal Content in Commercial Fertilizers, and the Precision and Accuracy of the pXRF Technique

In many studies, the quality of fertilizers has been mainly monitored from the perspective of the presence of potentially toxic elements (Cd, Cr, Cu, Hg, Ni, Pb, and Zn) and other trace elements (As, Co, and Mo) [8,9,10,11]. Certain historical investigations and monitoring of environments has indicated particularly significant productions of these elements in different ecosystems, especially in soils. In these, it was found that anthropogenic inputs have overwhelmed the natural biogeochemical cycles of the trace elements on Earth [34]. This is partly due to the application of mineral and organic fertilizers, compost, and liming materials that contain certain amounts of metals [35,36,37,38,39]. In this research, the contents of Co, Pb, and Mo were not detected due to the higher detection limits (LOD) of the instruments (3 mg/kg for Co and Pb, and 2 mg/kg for Mo). Generally, the amounts of these elements in nitrogen mineral fertilizers are low. For example, in [10], it was reported that several formulations of NPK fertilizer contained lead at averages of 0.4 mg/kg to 4.6 mg/kg, while the single-component fertilizers calcium nitrate and ammonium nitrate contained 0.3 mg Pb/kg and 0.2 mg Pb/kg, respectively; this is in addition to Mo, which was lower than 0.1 mg/kg. These findings suggest that the pXRF method is not appropriate for the determination of these elements in mineral fertilizers. One of the main limitations of this method is the higher LOD values [40]. For this reason, it is particularly important to have a certain degree of knowledge of the investigated sample, especially from the perspective of the approximate sample composition and the range of the expected element concentrations; furthermore, it must be noted that detection limits can differ, especially due to instrument type and technical specifications [41]. Although complex fertilizers (such as in the present study) contain sufficient quantities of As, Cr, Cu, Ni, Zr, and Y (which were detected with good precision), ammonium nitrate, ammonium sulphate nitrate, limestone ammonium nitrate formulations, and urea contain these elements below the LOD required, and which the single-component fertilizers from this study possessed, i.e., < 1 mg/kg of As, Zr, and Y; <4 mg of Cu/kg, <6 mg of Ni/kg, and <15 mg of Cr/kg. In support of this notion, it should be noted that single-component nitrogen fertilizers usually contain lower amounts of those elements; for example, urea contains 0.68 mg of Cr/kg, 0.38 mg of Cu/kg, and 0.48 mg of Ni/kg, whereas ammonium nitrate contains 1.33 mg of Cr/kg and 0.30 mg of Ni/kg [8]. This again suggests that the pXRF method is particularly suitable for measurement and metal detection in complex nitrogen mineral fertilizers with higher amounts of metals; however, it is not suitable for the detection and quantification of lower amounts of certain metals in single-component nitrogen fertilizers. The pXRF method can theoretically determine many elements, but the excitation of low numbers of atomic elements (e.g., <Mg) is often problematic [23]. It is well known that there are higher amounts of Cd in phosphorus fertilizers when compared to the Cd levels in other mineral fertilizers, and this is conditioned by cadmium’s natural presence in phosphate rock. However, the results from this research disclosed that, on average, a 57.4% higher amount of Cd was detected in single-component nitrogen fertilizers compared to the Cd amounts in complex fertilizer formulations (which contained 10–20% of phosphorus in their formulation). For the latter, Cd was detected in the range of 17.5 mg/kg (LAN) to 31.3 mg/kg (urea), which is more in accordance with the Cd concentrations determined in the 20 phosphorus fertilizer samples (DAP, MAP, and TSP) from Saudi Arabia, where Cd varied from 22.7 mg/kg to 36.8 mg/kg [9]. The results from this research also show that, along with Cd, the contents of Mn and Th were also relatively higher in single-component nitrogen fertilizers when compared to complex fertilizer formulations. However, the amounts of all other eleven detected elements (As, Cr, Cu, Fe, Ni, Si, Sr, U, Zn, Y, and Zr) were higher in the NPK formulations than in the single-component nitrogen fertilizers. This variability, along with the metals’ origin in the investigated fertilizers, can be explained by several factors: impurities from the raw material and parent rock; the corrosion of equipment; the catalysts and reagents used in their manufacture; and the materials added to commercial preparations like coatings, fillers, and conditioners (e.g., gypsum, kaolin, and limestone) [34]. Ref. [10] also determined higher amounts of Zn, Ni, and Cr in complex fertilizers (NPK) compared to single-component nitrogen fertilizers, which is in accordance with the findings in this research. Even though most of the research includes investigations of only the potentially toxic and trace elements, there are no available data for the degree of Sr, Th, U, Zr, and Y content in fertilizers. There is one exception [10], however, where Sr content was detected in various fertilizers and ranged, in the NPK formulations, from 32 mg/kg (NPK 12-16-16) to 321 mg/kg (NPK 0-20-25). In the triple superphosphate, it was 538 mg/kg, in calcium nitrate 3340 mg/kg, and in urea and ammonium nitrate less than 1 mg/kg. The highest amount of Sr was detected in NPK 13-10-12 fertilizer (469.8 mg/kg) in this research. The findings of this study regarding the quantified metal content should also be briefly discussed from the point of view of the precision and accuracy of the utilized measuring technique (the pXRF technique).
Previous research has shown that pXRF analyzers are capable of accurately measuring Fe, Pb, Zn, Mn, Ca, and K but only provide relative results for elements such as Cr, Ni, and As. This is primarily due to the lack of knowledge of the interfering effects of the matrix and moisture content in the samples [42]. In order to obtained accurate, precise, and relevant results, certain rules should become mandatory measures during such analyses, such as the following: analysis of the certified reference materials (CRM), the creation of internal standards, and the calibration of instruments with specific factors that are derived from laboratory analyses [43,44]. Factors that have an influence on the measurement results achieved by this method include (besides moisture and the interference of the matrix), the heterogeneity and geometry of the sample, the thickness of the sample, and the detector resolution [45]. A quantitative XRF/pXRF analysis of the fertilizers is not a common practice as is the case for soil analysis; this is because fertilizers are a “challenge” in XRF analysis, especially due to the different density of fertilizers. The densities of individual fertilizers can range from 0.61 g/cm3 for ammonium chloride to 3.78 g/cm3 for sodium molybdate. Thus, this significantly affects the penetration depth of the emitted X-rays, which consequently overpower the secondary XRF signal and affect the result [46]. Also, correct sample preparation and homogenization, along with direct contact between the instrument and the sample, could contribute to greater accuracy and precision [47]. Sample drying will also assure more accurate results because the presence of moisture in the sample can negatively affect the absorption of primary emitted X-rays and reduce the scattering of the secondary emitted X-rays from the analyte [42]. In order to achieve greater accuracy and precision, a longer measurement time should also be applied (up to 300 s) [30]. The results of this research indicate that a good sample preparation, homogenization, drying, and 120 s analysis time contribute to a good accuracy and precision in the results, but there is the possibility to improve the accuracy of the achieved results via the calibration of the instrument used with specific factors for this type of analyzed matrix.

5. Conclusions

Based on the determined precision (RSD < 25%) and accuracy (RPD < 25%) of the reference fertilizer samples, the results indicate that a pXRF analyzer is capable of accurately and precisely determining several metals in a certain range in commercial fertilizers (As (2.0–6.5 mg/kg), Cu (12.5–26.3 mg/kg), Fe (16.5–1442 mg/kg), Mn (20.3–5290 mg/kg), Ni 6.2–27.8 mg/kg), and Zn (1.4–166.0 mg/kg)). Also, the results revealed that the determined contents of Cd, Mn, and Th were relatively higher in the single-component fertilizers, while the contents of As, Cr, Fe, Ni, Si, Sr, Zn, Zr, Y, and U were relatively higher in the complex fertilizers. One of the main limitations of the pXRF method is its higher LOD values, which was particularly evident in the impossibility of quantifying Co, Pb, and Mo in all of the investigated fertilizer samples, as well as the impossibility of detecting and quantifying the contents of As, Cr, Cu, Ni, Zr, and Y in the single-component nitrogen fertilizers.

Author Contributions

Conceptualization, A.P. and T.K.; methodology, A.P.; formal analysis, A.P. and N.Ž.; investigation, A.P. and N.Ž.; resources, I.K.; data curation, A.P. and I.Š.; writing—original draft preparation, A.P.; writing—review and editing, Ž.Z., I.Š., T.K. and I.K.; visualization, A.P. All authors have read and agreed to the published version of the manuscript.

Funding

This publication was supported by the Open Access Publication Fund of the University of Zagreb Faculty of Agriculture.

Data Availability Statement

The data presented in this study are available on request from the corresponding authors.

Conflicts of Interest

The authors declare no conflict of interest.

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Table 1. List of investigated fertilizer samples.
Table 1. List of investigated fertilizer samples.
FertilizerNutrient Amount (%)Mark
Ammonium nitrate—prilledN (33.5)AN_Prilled
Ammonium nitrateN (34.8)AN
Ammonium sulphate nitrateN (26) + S (15)ASN
Limestone ammonium nitrateN (28)LAN_28N
Limestone ammonium nitrateN (27)LAN_27N
Complex fertilizer (nitrogen + phosphorus + potassium)N (7) + P (14) + K (21)NPK 7-14-21
Complex fertilizer (nitrogen + phosphorus + potassium)N (7) + P (20) + K (30)NPK 7-20-30
Complex fertilizer (nitrogen + phosphorus + potassium)N (13) + P (10) + K (12)NPK 13-10-12
Complex fertilizer (nitrogen + phosphorus + potassium)N (15) + P (15) + K (15)NPK 15-15-15
Complex fertilizer (nitrogen + phosphorus + potassium)N (15) + P (15) + K (15) + SNPK (S) 15-15-15
Complex fertilizer (nitrogen + phosphorus + potassium)N (20) + P (10) + K (10)NPK 20-10-10
Complex fertilizer (nitrogen + phosphorus)N (20) + P (20)NP 20-20
UreaN (46)Urea
Table 2. Limits of detection (LOD) for the Vanta C pXRF analyzer.
Table 2. Limits of detection (LOD) for the Vanta C pXRF analyzer.
AsCdCoCrCuFeMnMoNiPbSiSrThUZrZnY
mg/kg
LOD143154159263325121121
Table 3. Assigned values of the metal content in the reference fertilizer samples.
Table 3. Assigned values of the metal content in the reference fertilizer samples.
MFNPK + SDAPCNFNPK + SO3
ῡ ± u (mg/kg)
As1.80 ± 0.13.20 ± 0.26.6 ± 1.01.2 ± 0.10.4 ± 0.1
Cd0.96 ± 0.042.87 ± 0.090.11 ± 0.030.77 ± 0.02-
Co0.91 ± 0.08----
Cr12.7 ± 0.929.0 ± 0.95.7 ± 0.79.3 ± 0.7-
Cu3.71 ± 0.299.35 ± 0.4713.9 ± 0.646.3 ± 0.317.0 ± 0.65
Fe880 ± 181213 ± 22-2053 ± 376219 ± 297
Mn79.3 ± 2.3970.2 ± 2.9340 ± 865.0 ± 0.3123.1 ± 3.9
Mo0.41 ± 0.09----
Ni3.60 ± 0.38.80 ± 0.53.9 ± 0.33.6 ± 0.226.3 ± 2.8
Pb0.40 ± 0.10.50 ± 0.11.2 ± 0.32.1 ± 0.21.1 ± 0.2
Zn255.5 ± 4.871.6 ± 1.8615.7 ± 1.05465 ± 9.173.0 ± 2.04
ῡ—assigned mean value of the mass fraction of the element in samples; u—standard uncertainty of the assigned value. Values were assigned from [31]. MF—mineral fertilizer, DAP—diammonium phosphate, CNF—complex nitrogen fertilizer, and NPK + S and NPK + SO3—complex fertilizer with sulfur.
Table 4. Measurement precision and accuracy based on the analyses of the reference fertilizer samples.
Table 4. Measurement precision and accuracy based on the analyses of the reference fertilizer samples.
As Cr Cu Fe Mn Ni Si SrTh U Y Zn Zr
MFῡo ± SD (mg/kg)2.00 ± 0.00<15<41133 ± 15.391.0 ± 9.0<62251 ± 77.2141.8 ± 1.895.00 ± 0.009.75 ± 0.509.75 ± 0.96290.2 ± 5.625.50 ± 0.58
RSD (%)0.00--1.359.6-3.431.340.005.139.821.9410.5
RPD (%)11.1--28.814.8------13.6-
NPK + Sῡo ± SD (mg/kg)3.50 ± 0.5817.0 ± 0.0010.5 ± 2.381379 ± 12.784.0 ± 7.53<63561 ± 180168.5 ± 1.29<216.5 ± 1.2911.5 ± 1.9183.7 ± 2.069.50 ± 1.00
RSD (%)16.50.0022.70.928.96-5.060.77-4.8716.62.4610.5
RPD (%)9.4−41.412.313.719.7------16.9-
DAPῡo ± SD (mg/kg)2.33 ± 0.58<1513.0 ± 1.153938 ± 2.16346.0 ± 14.3<62061 ± 18.9218.0 ± 0.0013.0 ± 1.004.00 ± 0.00146.2 ± 0.9616.5 ± 1.2939.5 ± 0.58
RSD (%)24.7-8.880.054.15-0.920.007.690.000.007.821.46
RPD (%)−64.7-−6.5-1.8------5.1-
CNFῡo ± SD (mg/kg)1.00 ± 0.00<158.00 ± 1.832376 ± 8.8974.5 ± 9.11<68332 ± 125307.0 ± 1.414.00 ± 0.007.75 ± 0.966.00 ± 0.82513.5± 4.939.75 ± 0.96
RSD (%)0.00-22.80.3712.2-1.500.460.0012.313.60.969.82
RPD (%)−16.7-27.015.714.6- -- - - - 10.4-
NPK + SO3ῡo ± SD (mg/kg)<1<1519.8 ± 1.507661 ± 9.46120.5 ± 14.227.5 ± 2.3810,824 ± 318.7133.7 ± 0.96<23.00 ± 0.004.00 ± 0.8288.7 ± 3.2026.0 ± 0.82
RSD (%)--7.590.1211.88.662.940.72-0.0020.43.613.14
RPD (%)--16.523.2−2.14.6-----21.5-
ῡo—mean obtained value of the mass fraction of the element in samples; SD—standard deviation; RSD—relative standard deviation (precision); RPD—relative percent difference (accuracy); MF—mineral fertilizer, DAP—diammonium phosphate, CNF—complex nitrogen fertilizer, NPK + S and NPK + SO3—complex fertilizer with sulfur; and N = 4.
Table 5. Measurement precision and accuracy based on the certified and reference soil samples.
Table 5. Measurement precision and accuracy based on the certified and reference soil samples.
As Cd Co Cr Cu Fe Mn Mo Ni Pb Si Sr Th U Zn Zr Y
SRM 2711a ῡc ± U (mg/kg)107 ± 554.1 ± 0.59.89 ± 0.1852.3 ± 2.9140 ± 228,200 ± 0.04675 ± 18-21.7 ± 0.71040 ± 0.001314,000 ± 0.7242 ± 10-3.0 ± 0.12414 ± 11--
ῡo ± SD (mg/kg)91.3 ± 1.557.0 ± 3.610.1 ± 13.576.5 ± 9.2148.7 ± 3.728,331 ± 131316 ± 202.3 ± 0.1827.3 ± 3.11380 ± 9.3259,398 ± 577232 ± 2.612 ± 3.62.9 ± 0.17427.7 ± 4.2296.7 ± 2.537.7 ± 1.2
RSD (%)1.646.32133.612.02.490.4663.37.8311.40.670.221.1230.05.860.890.843.18
RPD (%)−14.75.42.146.36.20.5−53.2-25.832.7−17.4−4.1-−3.33.3--
ISE 989 ῡA ± U (mg/kg)45.9 ± 3.568.75 ± 0.6521.7 ± 1.40274.9 ± 25.7156.7 ± 8.339,930 ± 17901112 ± 771.827 ± 0.58462.1 ± 3.71301.7 ± 15.4246,000 ± 7900187.4 ± 13.010.73 ± 0.733.00 ± 0.401047 ± 74243.1 ± 15.926.7 ± 3.11
ῡA ± SD (mg/kg)51.0 ± 0.588.7 ± 1.1536.0 ± 0.71288.3 ± 18.0182.3 ± 3.2140,555 ± 1741044 ± 20.12.0 ± 0.4662.8 ± 0.35288.3 ± 2.08179,096 ± 817186.0 ± 0.4011.0 ± 4.043.2 ± 0.291123 ± 6.24223.7 ± 1.5327.0 ± 2.08
RSD (%)1.1413.21.976.241.760.431.9323.00.560.720.460.2236.79.060.560.687.70
RPD (%)11.1−0.5765.94.8716.31.57−6.129.472.61−4.44−27.2−0.752.526.677.26−7.981.12
ῡc—mean certified value of the mass fraction of the element in samples; U—expanded uncertainty; ῡo—mean obtained value of the mass fraction of the element in samples; SD—standard deviation; ῡA—mean assigned value of the mass fraction of the element in samples; RSD—relative standard deviation (precision); RPD—relative percent difference (accuracy); SRM 2711a—certified soil referent material 2711a SRM 2711a, Montana II Soil; ISE 989—International Soil-Analytical Exchange program—referent soil sample 989 (2015). N = 4. The certified and assigned values were obtained from [32,33].
Table 6. Metal content in the commercial fertilizers from Croatia.
Table 6. Metal content in the commercial fertilizers from Croatia.
As Cd Cr Cu Fe Mn Ni Si Sr Th U Zn Zr Y
ῡ ± SD (mg/kg)
AN_Prilled<126.5 ± 2.5<15<457.1 ± 3.71527 ± 16.1<61247 ± 1064.0 ± 0.014.3 ± 2.5<14.3 ± 0.5<1<1
AN<125.0 ± 2.9<15<416.5 ± 1.05290 ± 490<61733 ± 34.12.0 ± 0.025.0 ± 1.42.0 ± 0.05.0 ± 0.0<1<1
ASN<120.5 ± 4.3<15<473.8 ± 3.3224.7 ± 1.0<6<325<13.0 ± 0.02.0 ± 0.05.8 ± 0.9<1<1
LAN_28N<119.0 ± 4.8<15<4298.8 ± 2.620.3 ± 4.93<61362 ± 91.721.0 ± 0.03.0 ± 0.02.0 ± 0.01.4 ± 0.5<1<1
LAN_27N<117.5 ± 1.0<15<4340 ± 6.522.7 ± 3.8<61156 ± 17021.3 ± 0.93.0 ± 0.02.0 ± 0.05.7 ± 1.3<1<1
NPK 7-14-215.7 ± 0.912.5 ± 3.0107.7 ± 7.9 20.0 ± 3.51314 ± 25.5113.0 ± 0.022.3 ± 3.11959 ± 158216 ± 2.75.0 ± 0.065.7 ± 2.5143.0 ± 2.814.8 ± 1.535.2 ± 1.7
NPK 7-20-308.0 ± 2.113.0 ± 0.0118.5 ± 15.326.3 ± 4.61442 ± 18.9148.5 ± 0.627.8 ± 4.13694 ± 26040.3 ± 0.513.3 ± 1.082.8 ± 1.7166.0 ± 5.215.3 ± 1.016.5 ± 1.3
NPK 13-10-123.0 ± 1.014.0 ± 6.795.3 ± 7.27.8 ± 4.11069 ± 18.9142.3 ± 0.56.2 ± 0.53749 ± 247469.8 ± 1.38.0 ± 3.528.2 ± 1.369.5 ± 3.515.0 ± 1.854.5 ± 0.6
NPK 15-15-154.7 ± 0.511.5 ± 3.0110.5 ± 16.514.3 ± 1.71352 ± 17.2155.2 ± 1.017.0 ± 1.24581 ± 250276.3 ± 0.98.8 ± 2.638.0 ± 1.8104.7 ± 1.911.5 ± 0.675.7 ± 1.3
NPK (S) 15-15-15 6.5 ± 0.611.5 ± 3.0104.8 ± 19.421.7 ± 3.11325 ± 20.9152 ± 0.817.3 ± 1.72998 ± 229262.5 ± 3.15.0 ± 0.048.3 ± 1.7143.8 ± 0.911.3 ± 1.235.5 ± 1.3
NPK 20-10-103.7 ± 0.519.0 ± 2.929.8 ± 16.112.5 ± 3.1789 ± 8.1195 ± 1.010.0 ± 3.92349 ± 172141.0 ± 0.54.0 ± 0.041.0 ± 0.880.7 ± 1.99.7 ± 1.322.8 ± 0.5
NP 20-202.8 ± 1.322.3 ± 1.570.5 ± 8.6<42209 ± 19.7114 ± 97<61736 ± 105409.3 ± 1.710.3 ± 1.1529.0 ± 1.484.0 ± 2.612 ± 0.8128 ± 1.0
Urea2.0 ± 0.031.3 ± 2.5<15<4<154707 ± 295<61830 ± 51<135.3 ± 1.36.0 ± 0.03.5 ± 0.6<1<1
ῡ—mean value of the mass fraction of the element in the samples; SD—standard deviation; and N = 4.
Table 7. Measurement precision.
Table 7. Measurement precision.
As Cd Cr Cu Fe Mn Ni Si Sr Th U Zn Zr Y
RSD (%)
AN_Prilled-9.49--6.561.05-8.490.0017.5-11.7--
AN-11.8--6.069.26-1.960.005.6570.70.00--
ASN-21.5--4.480.22---0.000.0016.6--
LAN_28N-25.0--0.8824.3-6.730.000.000.0011.7--
LAN_27N-5.71--1.9016.6-17.74.500.000.0021.9--
NPK 7-14-2116.724.07.4017.31.940.0013.78.101.230.003.821.9710.24.84
NPK 7-20-3027.00.0012.917.41.300.3914.87.031.247.222.063.116.277.82
NPK 13-10-1233.347.77.5453.11.770.358.006.580.2643.34.455.0512.21.05
NPK 15-15-1510.526.114.911.91.270.626.795.470.3430.14.801.805.021.66
NPK (S) 15-15-15 8.8826.018.514.21.570.539.907.651.180.003.530.6611.23.63
NPK 20-10-1013.315.554.224.91.030.5139.27.330.350.001.992.3412.92.19
NP 20-2045.76.7412.2-0.8985.0-6.050.4111.24.873.076.800.74
Urea0.008.00---6.27-2.78-3.560.0016.5--
Range 0.00–45.70.00–47.77.40–54.211.9–53.10.88–6.560.00–85.06.79–39.21.96–17.70.00–4.500.00–43.30.00–70.70.00–21.95.02–12.20.74–7.82
RSD—relative standard deviation (precision).
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Perčin, A.; Zgorelec, Ž.; Karažija, T.; Kisić, I.; Župan, N.; Šestak, I. Metals Contained in Various Formulations of Mineral Nitrogen Fertilizers Determined Using Portable X-ray Fluorescence. Agronomy 2023, 13, 2282. https://doi.org/10.3390/agronomy13092282

AMA Style

Perčin A, Zgorelec Ž, Karažija T, Kisić I, Župan N, Šestak I. Metals Contained in Various Formulations of Mineral Nitrogen Fertilizers Determined Using Portable X-ray Fluorescence. Agronomy. 2023; 13(9):2282. https://doi.org/10.3390/agronomy13092282

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Perčin, Aleksandra, Željka Zgorelec, Tomislav Karažija, Ivica Kisić, Nikolina Župan, and Ivana Šestak. 2023. "Metals Contained in Various Formulations of Mineral Nitrogen Fertilizers Determined Using Portable X-ray Fluorescence" Agronomy 13, no. 9: 2282. https://doi.org/10.3390/agronomy13092282

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