Introduction

Crustaceans are arthropods that are characterized mainly by the presence of rigid exoskeleton, respire with, and possess two pairs of antennae. This group of animals is highly appreciated in cooking due to their flavor and differentiated characteristic, being widely consumed by the world population [1]. The consumption of crustaceans is considered beneficial to human health, as they contain high protein content and essential vitamins, minerals, and amino acids in their composition [2, 3]. However, these animals can contain high levels of inorganic elements, which can present a risk to human health when consumed in high amounts [4, 5].

Sample preparation is an extremely important step in chemical analysis and, in some cases, can involve high costs and time-consuming processes. Most procedures found in literature for elemental analysis by spectrometric techniques are based on decomposition procedures using concentrated acids, which, under controlled temperature and pressure conditions, are able to convert analytes into simple inorganic species [6,7,8,9].

In contrast to traditional acid decomposition methods, studies on the use of alternative reagents in sample preparation have grown in recent years, including the use of formic acid. The application of formic acid in the solubilization of different samples has been promising, since it significantly reduces the sample preparation time, in addition to providing greater simplicity and speed in steps prior to analyses [6, 10, 11].

Few studies based on the use of formic acid to extract inorganic elements are reported in literature and most of them require a heating step for solubilization. Lopes et al. [6] proposed the use of formic acid for the extraction of 14 elements in Brazil nuts and babassu by inductively coupled plasma mass spectrometry (ICP-MS), and the results obtained were compared with values ​​found for decomposed samples with nitric acid and presented similar concentrations for most elements under study. A procedure based on the solubilization of meat samples using formic acid was proposed by Silva et al. [10]. In this study, other procedures were also proposed using tetramethylammonium hydroxide (TMAH) and a mixture of HNO3 and H2O2 in order to compare results. The authors obtained close results for the three procedures. Bianchi et al. [11] used diluted formic acid to determine selenium in bovine semen by ICP-MS and found Se concentrations in the procedure with formic acid close to values ​​obtained for digestion with nitric acid and hydrogen peroxide, indicating that the procedure was effective.

Several analytical techniques are being used in the determination of inorganic constituents in crustaceans, mainly atomic absorption spectrometry (AAS) and optical emission spectrometry with inductively coupled plasma (ICP OES). However, the use of microwave-induced plasma optical emission spectrometry (MIP OES) for elemental determination in crustaceans is relatively recent and has shown to be a promising technique mainly due to the lower operating costs when compared to instruments that require argon or other gases [12, 13].

There are few studies in literature on the use of diluted formic acid in the preparation of food samples. In addition, the use of MIP OES in the determination of inorganic elements whose samples were prepared with formic acid is recent, and there are no studies with this approach in literature. Given the above, this study aims to evaluate the use of diluted formic acid in the preparation of shrimp (Macrobrachium amazonicum) and crab (Ucides cordatus) samples for the determination of Al, Cr, K, Mg, Mn, and Zn by MIP OES as an alternative to conventional acid decomposition procedures using microwave oven.

Materials and Methods

Instrumentation

A microwave-induced plasma optical emission spectrometer (MIP OES) (model 4100, Agilent Technologies, Melbourne, Australia) equipped with nitrogen generator (model 4107, Agilent Technologies) was used for the determination of Al, Cr, K, Mg, Mn, and Zn in crustacean samples. The optimization of the nebulizer pressure and viewing position was automatically performed using the MP EXPERT software (Agilent Technologies), as well as the automatic background correction system. The MIP OES operational conditions are shown in Table 1.

Table 1 Operating conditions for MIP OES determinations
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A lyophilizer (model L101, Liotop, São Carlos, SP, Brazil), cryogenic mill (model 6770, SPEX, SamplePrep, Metuchen, NJ, USA), Dubnoff bath with stirring and heating (model Q226M2, Quimis, São Paulo), and centrifuge (model Q222T2, Quimis, São Paulo) were used in sample preparation.

In order to compare with the proposed procedure, acid decomposition was performed in microwave oven with cavity (model Start E, Milestone, Sorisole, Italy).

Reagents, Solutions, and Samples

Polyethylene flasks and glassware used in this study were kept in bath containing 10% nitric acid (v v−1) for 24 h and washed with deionized water prior to use. All reagents used were of analytical grade. Solutions were prepared with ultrapure water obtained from a Sinergy-UV purification system (Millipore, Bedford, USA) with resistivity of 18.2 MΩ cm. Formic acid 85% (v v−1) (Neon, São Paulo, Brazil) was used for the treatment of samples. A certified fish protein reference material (DORM-4) was used to assess the accuracy of the MIP OES analysis procedure.

Analytical reference solutions were prepared from the appropriate dilution of the stock solution containing 1000 mg L−1 of each analyte (Sigma-Aldrich, MO, USA) in medium of 5% formic acid (v v−1). Analytical curves were constructed using 0.5–2.0 mg L−1 for Cr; 5–20 mg L−1 for Al, Mn, and Zn; and 10–20 mg L−1 for K and Mg.

A total of eleven samples was studied. The shrimps (selected pool of 20 individuals per sample) and crabs (one individual per sample) were directly acquired by local fisherman in several municipalities in the State of Pará, in the Northern region of Brazil (Barcarena, Monte Alegre, Abaetetuba, Soure, and São Caetano de Odivelas). After purchased, samples were taken to the laboratory for removal of muscles and subsequent lyophilization for 72 h (Liotop, L101, São Carlos, Brazil) and spraying in cryogenic mill (SPEX SamplePrep, 6770, Metuchen, USA). The grinding program consisted of two steps: step I (pre-freezing) for 10 min and step II (grinding) for 2 min interspersed with freezing cycles of 2 min. Then, samples were transferred to polyethylene flasks and stored in desiccator.

Sample Preparation

The solubilization procedure with formic acid consisted of adding 5 mL of 50% formic acid (v v−1) to 0.25 g of each crustacean sample. Flasks were taken to a thermostatic bath with heating and stirring for 1 h at temperature of 90 °C. After the experimental procedure, after reaching room temperature, samples were diluted to 50 ml with ultrapure water in order to obtain final acidity of 5%. Then, solubilized samples were centrifuged for 10 min at 9,000 rpm and filtered on quantitative filter paper. Analytical blanks were prepared using the same procedure, with no sample added. The concentrations of elements were determined by MIP OES.

A crustacean sample (S4) was submitted to acid decomposition in microwave oven with cavity using a mixture of 4 ml of nitric acid, 4 ml of hydrogen peroxide, and 4 ml of water [13], in order to compare with the proposed procedure using diluted formic acid. Decomposed samples were diluted in order to obtain final acidity of 5%. The contents of elements were determined by MIP OES.

The accuracy of the MIP OES analysis procedure was assessed using certified fish protein reference material (DORM-4) prepared by the same procedure used on the samples. On the other hand, the efficiency of the proposed procedure using diluted formic acid was evaluated by comparing it with a microwave-assisted decomposition procedure using nitric acid. All procedures were performed in triplicate (n = 3).

Statistical Analysis

The results obtained were submitted to basic statistical analysis using the resources of the Origin 6.1 software (Microcal Software, Northampton, MA, USA) for Windows. In this study, Student’s t test was used to determine the significant differences in the results found in this study with 95% confidence level.

Results and Discussion

Optimization of MIP OES

The choice of wavelengths for the elements under study was made by selecting the spectral lines free from interferences, with the aid of the MP EXPERT software. The optimization of the nebulizer pressure (kPa) and viewing position (mm) was automatically performed by the software, using calibration solution containing all analytes under study.

Although some authors have reported that the injection of air into the plasma of the MIP OES through EGCM (External Gas Control Module) accessory can prevent carbon deposition on the torch, and in this study, there was no injection of air into the nitrogen plasma [14]. However, no disadvantages were observed such as carbon deposition on the torch and pre-optical part of the equipment. The use of diluted reagent, low reagent volume, and adequate dilution of standards and samples probably contributed to obtain adequate merit parameters for all analytes.

Analytical Performance

Limits of detection and limits of quantification were calculated according to previous studies [15] and are shown in the following equations: LOD = 0.03 × RSDblank × CSR / (ISR / Iblank); LOQ = 0.10 × RSDblank × CSR / (ISR / Iblank), where RSD represents the relative standard deviation for 10 measurements of the analytical blank; CSR represents the concentration of the analyte present in the multi-element reference solution; and ISR and Iblank are the emission intensities of analytes in the reference solution and in the blank, respectively. The values ​​of the merit parameters obtained for procedure with formic acid and for procedure with nitric acid are shown in Table 2.

Table 2 Figures of merit for the two sample preparation procedures using the MIP OES
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Limits of detection were higher for the formic acid procedure for most of the elements under study. This fact was also observed in other studies using diluted formic acid and may be associated with the fact that the formic acid solution used was not ultrapure or sub-boiled [11, 16]. Oliveira et al. [16] obtained high limits of detection when compared to values ​​obtained in this study using formic acid for Mg (2.6 mg kg−1), Mn (0.12 mg kg−1), and Zn (1.1 µg g−1) in foods by ICP OES. This study showed lower LDs compared to those obtained by Scriver et al. [17] in a study with marine tissues for K (200 µg kg−1), Mg (2.0 µg kg−1), and Mn (1.0 µg kg−1) using ICP-AES. On the other hand, the limits of detection for Cr and Mn were lower for procedure using diluted formic acid, indicating greater sensitivity for these elements. Lopes et al. [6] obtained low LDs in the preparation of samples with formic acid compared to conventional acid digestion by ICP-MS.

The Brazilian legislation [18] establishes acceptable values ​​only for Cr and Zn, among the studied elements which are 50 mg kg−1 and 0.1 mg kg−1, respectively. In this study, the detection limits for these elements meet the maximum levels allowed by the legislation, indicating that the technique used can be successfully applied to also evaluate the tolerable limits of the current legislation.

Calibration curves showed good linearity (R2 > 0.99) for both sample preparation procedures.

The accuracy of the MIP OES analysis procedure was assessed by determining the concentrations of elements in the certified fish protein reference material (DORM-4). Recovery rates ranged from 91% (K) to 117% (Mn). Student’s t test showed that the results obtained for most elements were close to certified values, with 95% confidence interval (tcritical = 4.30). It was possible to observe good repeatability with accuracy of less than 10%. Table 3 presents certified values and obtained values (mean ± SD), recovery rates calculated for accuracy, RSD% values, and the t test values.

Table 3 Determination of Al, Cr, K, Mg, Mn, and Zn in the certified reference material DORM-4 (mean ± SD, n = 3), RSD%, and t test values for solubilization with formic acid
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A comparative study on a crustacean sample submitted to decomposition with diluted nitric acid and diluted formic acid was carried out in order to confirm the accuracy of the proposed procedure. A paired t test with 95% confidence level was applied in order to compare the two procedures. Al, Cr, K, Mg, Mn, and Zn concentrations determined by MIP OES for both procedures are shown in Table 4.

Table 4 Analytes concentrations (mg kg−1) in a crustacean sample (S4) after digestion with diluted nitric acid and solubilization with formic acid diluted by MIP OES and t test values between two procedures
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It could be observed that the values obtained for all analytes under study by the nitric acid procedure and the formic acid procedure are statistically equal at 95% confidence level (tcritical = 4.30) in the studied sample, which indicates that the procedure proposed in this study using diluted formic acid can be used in sample preparation, as an alternative method to traditional decomposition methods.

The RSD values for formic acid were higher than those found for sample digested with nitric acid, but they were adequate for precision, with RSD values ranging from 1 to 8%. This fact has also been reported by previous studies in literature and may be associated with temperature control in the bath heating and the use of microwave, as well as the complexity of the matrix under study [16].

Regarding the analytical frequency, this study showed that procedure with formic acid had analytical frequency about 2.5 times higher compared to microwave-assisted digestion. The use of the thermostatic bath with shaking allows the solubilization of a larger number of samples in a certain period of time, when compared to decomposition using microwave oven. In addition, the use of a simpler instrument allows implementing the proposed procedure for routine laboratory analyses.

Among the advantages of using the formic acid procedure in relation to the nitric acid decomposition procedure carried out in this study are the use of less volume of reagents, less time in sample preparation, simplicity in preparation steps, reducing the chances of contamination, and allowing greater speed in determining analytes.

Elemental Determination in Crustaceans

Table 5 presents the levels obtained for Al, K, Mg, Mn, and Zn in shrimp and crab samples by MIP OES, and the results are presented in mg kg−1 dry weight. Potassium and Mg were the most abundant elements in the crustacean species studied.

Table 5 Analytes concentrations (mg kg−1) in crustacean samples after solubilization with formic acid (mean ± standard deviation; n = 3)
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Cr is both an essential and toxic trace element and can be under different forms of oxidation. The European Food Safety Authority has proposed a tolerable daily intake (TDI) equal to 0.3 mg kg−1 of Cr(III) [19]. In this study, Cr concentrations were below the limit of detection (< 0.1 mg kg−1). In other studies with different shrimp species, Cr values ranged from 2.6 to 10 mg kg−1 [20] in Malaysia and from 0.13 to 1.05 mg kg−1 [21] and 8 to 18 mg kg−1 [22] in Brazil. Virga et al. [4] reported high Cr values (0.4 to 7.0 mg kg−1 dry weight) in blue crab samples when compared to levels found in this study.

Aluminum can be neurotoxic in high concentrations, and is related to the development of neurodegenerative diseases [23]. Aluminum had average concentration of 15 mg kg−1 for shrimp and 18 mg kg−1 for crab. These values are below those found by Silva et al. [24], which ranged from 65 to 138 mg kg−1 in shrimp samples from the state of Bahia, Brazil.

The presence of Al in the evaluated species may have an entropy and natural since Al is part of local geochemistry and is one of the elements most abundant in nature. There is no maximum level of aluminum for crustacean established in Brazilian legislation. According to Sparling and Lowe [25], diets with amounts of aluminum lower than 1000 mg kg−1 are not considered harmful.

Potassium is an important essential element, acting an important role in cellular metabolism [26]. Potassium was the element that showed the highest concentrations in crustaceans, with average levels of 11,937 mg kg−1 in shrimp and 17,089 mg kg−1 in crab. These values were close to those reported by Mancinelli et al. [27] whose K levels ranged from 15,010 to 16,770 mg kg−1 in seafood samples. Lu et al. [28] and Lu and Wang [29] also reported K levels similar to those obtained in this study for oyster and mussel samples, with averages of 9,630 and 5,770 mg kg−1, respectively.

Magnesium is essential element acting function as an enzyme cofactor and plays a vital role in the synthesis of adenosine triphosphate, the major source of energy in cells [30]. Mg concentrations ranged from 1,319 to 1,851 mg kg−1 in shrimp and from 3,840 to 5,376 mg kg−1 in crabs. High Mg concentrations were obtained in two studies with seafood in China, which ranged from 6,076 to 10,239 mg kg−1 [31] and from 1,070 to 13,300 mg kg−1 [28], respectively. Mancinelli et al. [27] reported levels similar to those found in this study, with averages between 1,630 and 1,830 mg kg−1 in crayfish species in Italy.

Mn is an essential trace element, and it is found in all tissues. This element has immune function and acts regulate blood sugar and cellular reproduction, digestion, energy, and bone growth [32]. In this study, Mn levels ranged from < 0.1 to 10 mg kg−1 in shrimp and < 0.1 to 16 mg kg−1 in crab. Anandkumar et al. [20] and Migues et al. [22] found Mn levels above those obtained in this study in shrimp species from Malaysia and Brazil, ranging from 7.3 to 51.4 mg kg−1 and from 10.5 to 23.5 mg kg−1, respectively. Other shrimp species have been studied in northeastern Brazil, with Mn levels ranging from 0.36 to 3.17 mg kg−1[24].

Zinc is an essential element acting vital roles mainly in immunity and growth [33]. Zinc showed average concentration of 52 mg kg−1 in shrimp and 261 mg kg−1 in crab. George et al. [34] reported Zn contents ranging from 44.8 to 88.7 mg kg−1 in six marine shrimp species in southwestern India. Anandkumar et al. [20] and Hossain and Khan [35] presented Zn levels in literature ranging from 62 to 203 mg kg−1 and from 49 to 102 mg kg−1 in shrimp in Malaysia and India, respectively. Higher Zn levels were obtained by Annabi et al. [36], with average value of 590 mg kg−1 in crab muscles in Tunisia.

Daily Recommended Intake

The contribution of Mg, Mn, and Zn levels found in crustacean samples to the recommended daily intake (RDI) of minerals established by Brazilian legislation [18] is shown in Table 6.

Table 6 Contribution of crustaceans to recommended daily intake (RDI – adults) for Mg, Mn and Zn (mg kg−1, ww)
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The values used to calculate the RDI were obtained on a wet weight basis according Lemos et al. [13]. The average daily intake was determined using the average levels listed in Table 5, considering 50 g portions of crustacean samples.

The Brazilian legislation does not establish RDI values for K [18]. The results obtained showed that Zn (37% RDI) and Mg (18% RDI) in crabs contributed substantially to the daily intake values. All samples contributed to the recommended intake of Mg, Mn, and Zn can be considered a good source of these minerals.

Conclusion

The proposed procedure using diluted formic acid showed adequate efficiency, simplicity, accuracy, and precision for the preparation of shrimp and crab samples. MIP OES is an interesting and suitable alternative for elemental analysis in samples prepared with formic acid. The limits of detection for the formic acid procedure were higher than those found in the decomposed sample but were adequate for MIP OES determinations. The contents of elements under study showed that crustaceans can be good sources of these constituents for human diet, mainly Zn and Mg. Thus, the procedure using diluted formic acid can be considered a viable alternative for determining Al, Cr, K, Mg, Mn, and Zn concentrations in crustaceans using MIP OES and can be successfully applied in routine analysis.