Screening of Geological Rocks for Metal Composition Using Three Different Sample Preparation Methods for Atomic Spectroscopy

This study used three distinct sample preparation techniques to examine the metal content of geological rocks. Aqua regia, alkali fusion, and microwave digestion were among the techniques compared. Inductively coupled plasma optical emission spectroscopy (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) assessed the effectiveness of each method for determining and evaluating metal content. Based on the preparation technique used, the results indicated notable variations in metal recovery rates. Aqua regia has been demonstrated to be especially beneficial for refractory elements, even if alkaline fusion has shown to be highly effective for the majority of metals.Although microwave digestion is less effective for some metals, it provides a quick and non-destructive alternative for sample preparation. The results demonstrated the importance of using the appropriate sample preparation method to ensure accurate and thorough analysis of geological materials. Principal component analyses (PCAs) that were produced as a result can help researchers and geologists refine their methods for screening metal composition in different geological contexts.

It is essential to identify the structural and technical properties of rocks for applications that utilize rock structures. The rocks are made of a variety of mineral grains with various characteristics and mixtures. It is known that rocks on Earth are formed up of 98.6% of the eight elements (O, Si, Al, Fe, Mg, Ca, K, and Na) and 1.4% of the other 98 elements (1). The numerous applications of rocks make them significant. Rocks can be used as building materials, in manufacturing and in scientific research. It is crucial to understand the chemical composition of the rocks for this purpose.

The geochemical, mineralogical, and elemental composition of rock samples can be determined using analytical techniques, in addition to other geological aspects; this process, therefore, is crucial in geological investigations. The X-ray fluorescence analysis (XRF) (2,3), flame atomic absorption spectrometry (AAS) (4), and inductively coupled plasma optic emission spectrometry (ICP-OES) (5,6) are among the methods now in use. The measurements of concentrations of certain elements and components, as well as all analyses of reference rock and mineral samples as required by instruments and other samples, continued to be performed using modified classical methods and several fast procedures.

Sample preparation is of the most important steps in the ICP analytical process (7). There are several stages of sample preparation between the analytical technique and sampling. Sample preparation often included the objective of producing a homogeneous subsample that is typical of the material. Getting relevant analytical results require careful planning. Depending on the kind and size of the sample, unique sample preparation techniques will be chosen. The procedure for preparing the sample for analysis followed after sampling. Most element determination take place in solutions, which makes it necessary to dissolve them using techniques like fusion, high-pressure explosives, microwave digestion, or inorganic acids in the open air. Some refractory minerals (such as zircon and chromium, for example) are difficult to dissolve using microwave digestion, particularly in geologic samples. The sample is prepared in accordance with the solvent and digestion procedure chosen to avoid affecting the results of the subsequent procedures. The fact that no chemicals are wasted during these processes and that the acids, bases, and salts utilized are ultra pure were further important aspects. All rocks are composed of the same fundamental components; however, their ratios vary. The most notable variation in content is in silica (SiO₂). Silicates are known as rock-forming minerals. Only a few of the hundreds of silicate minerals are extremely common in rocks; most of them are found as secondary minerals. The analytical chemistry of rocks and minerals includes a wide range of sample preparation techniques (digestion) that make use of several physical phenomena. Samples are typically digested using three fundamental techniques: i) aqua regia, ii) microwave digestion, and iii) alkali fusion. Despite the fact that several of these techniques seem to have little in common, each of them shares some similar analytical principles. The targeted aim was to find the best digesting technique for the main, minor, trace, and rare earth elements in relation to the concentration of elemental constituents in rock samples.

It is possible to make reliable predictions about the evaluation of mineral processes as long as the kind and nature of the mineral are studied and examined. Refractory minerals must be dissolved under high pressure and with extremely corrosive solutions. Based on this knowledge, three certified geological rocks were subjected to inductively coupled plasma spectroscopy measurements to determine the main, minor, trace, and rare earth elements. The results from various digesting techniques applied to sample preparation were compared. In addition, principal component analysis (PCA) of many elements detected in rock samples were made, and the similarities and differences between the sample preparation methods were explained statistically.

Material and Methods

Materials

Three different certified rock samples (Columbia River basalt [BCR-2], Guano Valley andesite [AGV-2], and granodiorite [GSP-2] were obtained from the U.S. Geological Survey (USGS). The concentrated acids, consisted of 65% (w/v) HNO₃, 37% (w/v) HCl, HF, and 30% (w/v) H₂O_2, were purchased from Merck. The calibration standard solutions were prepared by using Perkin Elmer (1000 ppm) standard solutions with a purity of 99.8%.

Methods

Aqua Regia

Aqua regia, a 3:1 volume ratio of hydrochloric (37% HCl, Merck) and nitric acids (65% HNO₃, Merck), was used to partially decompose rocks (8,9). 250 mg of the sample, weighed with an accuracy of 10 mg, was added to 9 mL of aqua regia in a balloon flask, and that mixture was refluxed at 180 °C for 4 h with stirring. After reflux, the mixture was cooled for 30 min and filtered, with the remaining clear solution used in analysis.

Microwave Digestion

0.1 g of the ground dried sample was taken into the microwave digestion container, and 5.0 mL of H₃PO₄, 3.0 mL of HCl, 0.5 ml of HF, and 1.0 mL HNO₃, respectively, were added.The container was carefully mixed, and then left to stand for at least 10 min. The microwave device was heated and operated with program presented in Table I:

Alkali Fusion

For alkali fusion of samples, a mixture of Na₂CO₃ and K₂CO₃ was reacted. The flowchart of alkali fusion is presented in Figure 1.

Figure 1: The flowchart of the alkali fusion process.

Analysis

This work successfully used ICP-OES and inductively coupled plasma mass spectrometry (ICP-MS) to identify elements at major, minor, and trace levels in three rock samples with acceptable accuracy and precision. An ICP-OES method (PerkinElmer Optima 7000) method was used to determine the major and minor elements, and a ICP-MS method (PerkinElmer Nexion 2000P) was used to determine the trace elements in rock samples.

Multivariate Analysis

The relationship between digesting processes was found using principal component analysis (PCA).Statistical multivariate analysis software was used to perform PCA on elemental composition data (XLSTAT Trial version, 2020).

Results and Discussion

In the study, three different rock samples were prepared with different digestion methods and their element compositions were determined. Reference values for these rock samples were shown in Table II. The values presented in Table II was obtained from the U.S. methods were investigated by comparing these values with the values obtained from different sample preparation methods. Additionally, comparisons were made using statistical methods.

Aqua Regia Digestion

One technique that researchers often use is the aqua regia method. In this procedure, the rock was broken down, and the relevant components were released by dissolving a sample in an acidic solution. This was a reasonably simple and inexpensive technique, although it can take some time, and not all minerals will dissolve completely. Table II presents the fundamental composition of the rock samples after their digestion in aqua regia. Compared to the elemental composition of reference samples (Table II), the recovery of almost all elements was quite low (Table III). Silicon, which was found in high amounts in rocks, could only be recovered at a rate of 50% with this method. Aqua regia was not widely used for elemental analysis of rocks because of its remarkable capacity to dissolve precious metals (10,11). Alternative techniques for elemental analysis of rocks were more appropriate to being utilized when determining the mineralogical and chemical components of samples.

When the minor element analysis carried out using the aqua regia method was evaluated, the majority of minor elements had recovery rates of over 90%. However, a few trace elements were also identified that were not present in the rock samples.Notably, elements Ni and Pb that were not present in the BCR-2 sample were observed, along with Mo that was not present in the GSP-2 and AGV-2 samples (Figure 2).This brought to mind the possibility of contamination or interference during the sample preparation or analysis process.

Figure 2: Minor elemental composition of rock samples via aqua regia digestion.

Microwave Digestion

The results obtained from microwave digestion indicated element recovery under 50% for Ti and Ca in rock samples. The recovery percentage for Si was in the range 76–81% (Figure 3a). However, it was shown that the recovery rate of trace elements ranged from 91 to 100% (Figure 3b).

Figure 3: The composition of rock samples via microwave digestion for (a) major, and (b) trace elements.

Alkali Fusion Digestion

It was possible to decompose many inorganic substances by preparing fusion. Fusion was required to dissolve some mineral alloys as rocks, silicates, and a few ferrous alloys, because these materials are slowly affected by liquid reagents. Alkaline fusion was used to examine the elements and matrix that compose up the rocks in order to prepare them for analysis.

Analysis revealed that the recovery of major and trace elements in the rock samples produced by the alkali fusion process was 100% and 95%, respectively, in contrast to standard values (Figure 4). These results indicate that the alkali fusion decomposes refractory minerals better than the aqua regia method.

Figure 4: The composition of rock samples via alkali fusion for (a) major, and (b) trace elements.

Statistical Analyses

Statistical analysis was frequently performed using principal components analysis (PCA), which provides a fast visual way to identify relationships or patterns while optimizing and simplifying large data sets (12). In this study, data on element composition and quantities in rock samples were processed and analyzed using PCA utilizing various digestion methods (Figure 5 and Figure 6).

Figure 5: PCA results of major elements of rock samples (a) AGV-2, (b) BRC-2, (c) GSP-2

Figure 6: PCA results of trace elements of rock samples (a) AGV-2, (b) BRC-2, and (c) GSP-2.

In order to determine which digestion methods were more suitable for one group than another and which were suitable for both major and trace elements, we performed a PCA analysis of the data. As shown in Figure 4, the first two PCA ordination axes, F1 and F2, were used to evaluate the major elements found in reference rock samples. These axes together included 98.20%, 95.98, and 98.85 of the total variance for AGV-2, BCR-2, and GSP-2, respectively.The alkali fusion method was observed better than other methods in the element composition of reference rock samples among sample preparation methods. For the AGV-2 reference rock sample, almost all major elements were obtained very close to each other in the alkaline fusion method (Figure 5a). The following method for preparing a sample of this rock was microwave digestion. The reference rock samples BCR-2 and GSP-2 were in identical conditions. (Figure 5b and Figure 5c). Data indicated that the alkaline fusion approach was a good option for further examination of these components, since it successfully extracted and preserved the relative proportions of significant elements in reference rock samples.

A total of 16 trace elements were identified as the result of the data analysis performed on the ICP-OES results. The quantity of elements was used as a variable in the principal component analysis (Figure 6).The relationship between reference rock samples and element composition as determined by different sample preparation methods was presented with the biplot of principal components. The components explained 93.56%, 92.56%, 92.58%, and 95.62% of the total variance for AGV-2, BCR-2, and GSP-2, respectively. Based on the findings of all the experiments, each method achieved the highest analyte recovery for trace elements in reference rock samples.The methods used to measure the trace element digesting capability did not differ significantly from one another. The reference rock samples AGV-2 and GSP-2 had no trace of molybdenum (Mo). Pb and Ni elements were not present in the BCR-2 rock sample. However, the analysis results showed that molybdenum was observed after alkali fusion and aqua regia sample preparation methods. (Figure 6a). The solution that was produced after microwave digestion did not include Mo. Figure 6c revealed the presence of the elements Ni and Pb, which were not present in the BCR-2 rock sample. While Ni was observed in all three methods, it was observed that the most contamination was in the alkaline fusion method. Only the agua regia method revealed the presence of Pb. High analyte recoveries were obtained for most of trace elements in the reference rock samples using all three sample methods of preparation (alkali fusion, aqua regia, and microwave digestion). There were differences in terms of completeness and contamination potential for some elements, even though each method shows its advantages and disadvantages.

Conclusion

An accurate elemental analysis depends greatly on the sample preparation method selected. The type of rock, the target elements, the required detection limits, and other considerations all play a role in selecting the best method. Therefore, although all methods were effective for most elements, choosing the most appropriate method depends on the specific elements of interest and the need to avoid potential contamination. The best possible sample preparation approach to ensure a comprehensive and accurate representation of major element composition seemed to be the alkaline fusion method, which was based on the PCA of major element data acquired from reference rock samples. While aqua regia appears to be the most selective method for trace elements, microwave digestion offered the advantage of avoiding Mo contamination. Alkali fusion, despite its high recovery rate, raises concerns about potential contamination of some elements such as Ni.

References

(1) Rudnick, R. L.; Go, S. Composition of the Continental Crust. Treatise Geochem. 2003,3, 1-64. DOI: 10.1016/B0-08-043751-6/03016-4

(2) Gardner, L. R. Geochemical Analysis of Silicate Rocks and Soils by XRF Using Pressed Powders and a Two-Stage Calibration Procedure. Chem. Geol. 1990, 88 (1-2), 169–182. DOI: 10.1016/0009-2541(90)90111-J

(3) Ramsey, M. H.; Potts, P. J.; Webb, P. C.; Watkins, P.; Watson, J. S.; Coles, B. J. An Objective Assessment of Analytical Method Precision: Comparison of ICP-AES and XRF for the Analysis of Silicate Rocks. Chem. Geol. 1995, 124 (1-2), 1–19. DOI: 10.1016/0009-2541(95)00020-M

(4) Macedo, S. M.; de Jesus, R. M.; Garcia, K. S.; Hatje, V.; Queiroz, A. F. D. S.; Ferreira, S. L. Determination of Total Arsenic and Arsenic (III) in Phosphate Fertilizers and Phosphate Rocks by HG-AAS after Multivariate Optimization Based on Box-Behnken Design. Talanta 2009, 80 (2), 974–979. DOI: 10.1016/j.talanta.2009.08.025

(5) Cardoso, G. S. A.; Freitas, T.; do Rosário, F. F.; Cajaiba, J. Enhancing the Permeability of a Carbonate Rock Core by Dissolution/Precipitation Treatment with Organophosphorus Additives Evaluated by SEM/EDS and ICP-OES. J. Pet. Eng. 2020, 193, 107341. DOI: 10.1016/j.petrol.2020.107341

(6) Xu, J.; Guo, Y.; Yang, S.; Hohl, S. V.; Zhang, W. Reliable Determination of SiO₂ Concentrations in Sediments via Sequential Leaching and ICP-OES/MS Analysis. J. Geochem. Explor. 2022, 242, 107090. DOI: 10.1016/j.gexplo.2022.107090

(7) Pinto, F. G.; Junior, R. E.; Saint'Pierre, T. D. Sample Preparation for Determination of Rare Earth Elements in Geological Samples by ICP-MS: A Critical Review. Anal. Lett. 2012, 45 (12), 1537–1556. DOI: 10.1080/00032719.2012.677778

(8) Church, S. E.; Mosier, E. L.; Motooka, J. M. Mineralogical Basis for the Interpretation of Multielement (ICP-AES), Oxalic-Acid, and Aqua Regia Partial Digestions of Stream Sediments for Reconnaissance Exploration Geochemistry. J. Geochem. Explor. 1997, 29, 207–233. DOI: 10.1016/0375-6742(87)90078-1

(9) Chen, M.; Ma, L. Q. Comparison of Three Aqua Regia Digestion Methods for Twenty Florida Soils. SSSAJ 2001, 65, 491–499. DOI: 10.2136/sssaj2001.652491x

(10) Gang, S.; Xingguo, C.; Yunkun, Z.; Mancang, L.; Zhide, H. Determination of Pd (II) by Application of an On-Line Microwave Oven and Artificial Neural Networks in Flow Injection Analysis. Anal. Chim. Acta. 2000, 420 (1), 123–131. DOI: 10.1016/S0003-2670(00)00977-6

(11) Tilch, J.; Schuster, M.; Schwarzer, M. Determination of Palladium in Airborne Particulate Matter in a German City. Fresenius J. Anal. Chem. 2000, 367, 450–453. DOI: 10.1007/s002160000380

(12) Li, S. L.; Ma, Y. Z.; Gomez, E. Importance of Modeling Heterogeneities and Correlation in Reservoir Properties in Unconventional Formations: Examples of Tight Gas Reservoirs. J. Earth Sci. 2021, 32 (4), 809–817. DOI: 10.1007/s12583-021-1430-2

Ugur Caglayan and Bahar Meryemoglu are with Cukurova University Central Research Laboratory, in Adana, Turkey. Direct correspondence to: meryemoglubahar@gmail.com (B. Meryemoglu).