Determination of Inorganic Elements in Paper Food Packaging Using Conventional Techniques and in Various Matrices Using Microwave Plasma Atomic Emission Spectrometry (MP-AES): A Review

Abstract

As one of the world’s most widely used packaging materials, paper obtains its properties from its major component: wood. Variations in the species of wood result in variations in the paper’s mechanical properties. The pulp and paper production industry is known to be a polluting industry and a consumer of a large amount of energy but remains an essential heavy industry globally. Paper production, based largely on the kraft process, is mainly intended for the food packaging sector and, thus, is associated with contamination risks. The lack of standardized regulations and the different analytical techniques used make information on the subject complex, particularly for inorganic elements where little information is available in the literature. Most research in this field is based on sample preparation using mineralization via acid digestion to obtain a liquid and homogeneous matrix, mainly with a HNO₃/H₂O₂ mixture. The most commonly used techniques are Atomic Absorption Spectrometry (AAS), Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES), and Inductively Coupled Plasma Mass Spectrometry (ICP-MS), each with its advantages and disadvantages, which complicates the use of these tech-niques for routine analyses on an industrial site. In the same field of inorganic compound analysis, Microwave Plasma Atomic Emission Spectrometry (MP-AES) has become a real alternative to techniques such as AAS or ICP-AES. This technique has been used in several studies in the food and environmental fields. This publication aims to examine, for the first time, the state of the art regarding the analysis of inorganic elements in food packaging and different matrices using MP-AES. The entire manufacturing process is studied to identify possible sources of inorganic contaminants. Various analytical techniques used in the field are also presented, as well as research conducted with MP-AES to highlight the potential benefits of this technique in the field.

1. Introduction

Paper intended for food contact is mainly made from wood, a non-homogeneous material whose chemical composition varies according to the species [1]. The two most commonly used types of wood are softwood and hardwood. Softwood has longer fibers, which increase the paper’s strength; hence, it is more commonly used for paper production [2]. This category includes maritime pine, which is abundant in southwest France [3]. Prior to entering in the techniques that can be used for inorganic elements analysis, it is important to understand the process of paper production in order to see where these elements are coming from.

1.1. From Wood to Kraft Pulp Production

• Chemical composition of wood

Wood is composed of three main compounds: cellulose, hemicellulose, and lignin. The proportions of these three compounds vary according to the origin and nature of the wood: approximately 40–50% cellulose, 20–30% hemicellulose, and 15–30% lignin. The main chemical compound, cellulose, is the most widespread polymer on the Earth’s surface (chemical formula: C₆H₁₀O₅)_n) [4], mainly derived from cotton and wood. These cellulose fibers are responsible for the mechanical properties of wood and, therefore, paper. This compound is extracted for pulp production. Hemicelluloses, another major compound, are branched polysaccharides that polymerize less than cellulose. Although their formulas are complex and vary according to the species studied, wood hemicelluloses comprise specific monomers [5]. The last major compound, lignin, is also a complex, amorphous polymer whose structure reacts in response to light, causing wood and paper to yellow. This compound «glues» cellulose and hemicelluloses together [6] in the cell structure. The pulp-making process seeks to eliminate this compound. The structure of wood can be represented schematically, with hemicelluloses being the most frequently found compound in this structure (Figure 1) [7].

Finally, the minor part of wood (less than 5%) consists of extractives, parts of which are eliminated during the pulping process, while the rest remain in the final paper. This category includes several classes of compounds, such as alkanes, fatty acids, terpenes, and phenolic compounds [8]. Although they represent less than 1%, inorganic elements are still present in wood and can be classified into three categories: macroelements (Calcium (Ca), Potassium (K), Phosphorus (P) and Sulfur (S)), microelements (Manganese (Mn), Iron (Fe),Copper (Cu), and Zinc (Zn)) and toxic elements (Arsenic (As), Chromium (Cr), Nickel (Ni) and Lead (Pb)), whose concentrations also vary depending on human activities [9].

• Kraft Process

Pulp and paper production is known as a major consumer of natural resources (wood and water, which already represent a possible primary source of contamination in inorganic elements) and electricity. It is mainly based on two chemical pulping processes: kraft and sulfite pulping. Developed in 1884 by Carl F. Dahl [10], the kraft process is the most widely used and now accounts for 90% of pulp production because it offers better pulp mechanical properties and is less resource-intensive [11].

The wood used is mechanically reduced into chips that are as uniform as possible before starting the cooking stage, aiming to extract the cellulose fibers from the other compounds. First, the chips are introduced into large tanks called digesters with sodium hydroxide (NaOH) and sodium sulfide (Na₂S), a mixture known as white liquor. The reaction occurs under controlled conditions at approximately 170 °C and 7 bars [12]. As the white liquor loads up with lignin, it becomes a washing juice known as black liquor (sodium sulfate (Na₂SO₄)). The reaction also forms sodium carbonate (Na₂CO₃) and lignin residues. At this stage, the black liquor obtained is described as weak, as it is mainly composed of water. Meanwhile, the pulp is stored and passed through classifiers and filters to be graded, washed, and thickened to remove any residues before being used in papermaking.

A major advantage of the kraft process is the possibility of regenerating the chemical additives used. Black liquor can be concentrated by evaporating the water to obtain a more concentrated liquor for burning in a controlled atmosphere. During combustion, Na₂SO₄ reacts to form Na₂S (initially used for pulp cooking), while Na₂CO₃ remains in its original form.

Na₂S and Na₂CO₃ thus accumulate as a molten liquid before being discharged and dissolved with water to form the product known as green liquor. Slaked lime (Ca(OH)₂) is added to this liquor to regenerate NaOH. This reaction occurs in a causticizing workshop. At the end of the reaction, the CaCO₃ remains intact and is separated via decantation to be further processed to regenerate the other chemical additives used. After this stage, the originally used white liquor is regenerated (NaOH + Na₂S) and can be used again for pulp production.

All these chemical regeneration steps are essential to make the process profitable, wherein the kraft process derives its major advantage. However, there is also a risk of inorganic contaminants being present, even in trace amounts, in the chemicals used: having a closed process loop means that even the smallest amount of contaminant introduced at any point could have an impact on the entire Kraft loop, including the paper produced at the end of the process.

1.2. Paper-Manufacturing Process

The pulp undergoes a refining and purification process following the cooking stage: the fibers are cut, fibrillated, and hydrated to improve the final paper’s homogeneity and strength. The pulp containing less than 1% fiber and 99% water is then sent to a paper machine. The paper machine (Figure 2) is a device for continuously forming, dewatering, pressing, and drying a network of paper fibers to form a sheet [13].

The entrance to the paper machine is called the headbox, where the chemical additives are introduced with the pulp to start the paper sheet’s [14] formation. This introduction of additives may also be a potential source of inorganic contaminants. The pulp mixed with production additives is then injected at a uniform rate onto a forming table to start forming the fibrous mat. For this purpose, suction boxes are used under the forming wire to extract some of the water and, thus, increase the concentration to approximately 20%. The formed paper sheet then passes through the press section, consisting of a series of steel cylinders. The sheet is then crushed, surrounded by a felt that soaks up water as it passes between the cylinders, resulting in approximately 35–40% dry matter and a paper sheet that can stand independently. The final drying stage occurs in the dryer section, where the sheet is brought into contact with drying rollers in which steam is circulated at a controlled temperature and pressure (approximately 12 bars at 190 °C). Two phenomena occur: the wet sheet of paper becomes hot even with over 50% humidity (heat transfer), and the water evaporation dries the sheet of paper (mass transfer) [14]. The paper can also be calendered (providing a smoother appearance) or rubbed (providing a glossier look) before, finally, being rolled into reels and shaped as required. The paper produced can also be coated, where a layer of additional additives is deposited on the paper’s surface to provide certain properties (e.g., for oven application or heat-sealing). Only approximately 5% of the water remains in the sheet of paper produced at the end of production. The water extracted throughout the manufacturing process is recovered before being partially reused in the process and then sent for treatment at a wastewater treatment plant (WWTP) before being discharged into the natural environment.

2. Inorganic Elements in Paper Packaging Regulations and Analytical Techniques

Paper is a major source of food packaging worldwide because of its lightness, low production cost, and ease of integration into recycling channels. There is growing concern about the possible transfer of substances present in paper to package food (and, consequently, to the consumer); this phenomenon is known as migration [15]. The migration of substances from paper to food depends on the type of packaged food (dry, moist, or fatty) [16], time [17], and contact temperature [16].

Inorganic elements can be found in foods [18]. Some of these inorganic elements, as well as being non-essential for humans [19], represent a major source of environmental pollution [20]. Some of these are known as major essential elements: Ca, K, P, sodium (Na) and magnesium (Mg) or trace essential: Fe, Zn, Mn, Cu and selenium (Se) [21]. Furthermore, the toxicity of some of other elements, such as As, Pb, and mercury (Hg), can cause numerous illnesses, even at low concentrations [21,22]. Additionally, Cr, Cu, Zn, and Ni all have harmful effects on the environment and human health [23]. Although the mechanism of toxicity of aluminum (Al) is not fully understood, it has the potential to affect cells and neurons, playing a role in several diseases such as cancer and Alzheimer’s [24]. Although essential for humans, Fe has also been associated with the development of several diseases (e.g., diabetes, heart disease, immune system dysfunction, and hormonal abnormalities) when present in excess [25].

2.1. Inorganic Elements in the Various Regulations Governing the Paper Industry

The food contact material (FCM) field is regulated worldwide, but there is no harmonization at the European level [26], unlike in the plastic packaging [27] field. As a result, the chemical safety of FCM paper and cardboard is difficult to regulate, with different thresholds for different target elements depending on the country and region. The only harmonization at the European level is provided by regulation (EC) No. 1935/2004, which states that packaging must not harm human health, bring about an unacceptable change in the composition of foodstuffs, or impair their organoleptic characteristics [28], and directive (EC) 94/62 on packaging and packaging waste, which limits the amount of Pb, Ca, Cr (VI), Cd, and Hg to 100 mg·kg⁻¹ [29].

No harmonized threshold exists for all inorganic elements presenting a risk to humans for paper intended for contact with food. For example, Pb and Hg have thresholds set at 0.10 mg·kg⁻¹ food and 0.003 mg·kg⁻¹ food in France, respectively [30]; meanwhile, Al (1 mg·kg⁻¹ food), Cd (5 µg·L⁻¹ water extract), and Pb (10 µg·L⁻¹ water extract) are regulated [31] in Germany. In addition to the lack of regulatory thresholds or harmonization, these regulations are constantly evolving. In Germany, for example, Al has only been regulated since 2019, and the regulatory threshold has evolved since this introduction.

All these regulations and their constant evolution over the last few years make interpreting the data obtained very complex, with many different analytical methods. Furthermore, the literature contains little information on the determination of inorganic elements in recycled fiber-based materials and paper-based food packaging as a whole [32].

2.2. Commonly Used Techniques for the Analysis of Inorganic Elements

In recent years, elemental analysis methods have developed considerably, generally dividing the analysis into two stages: a sample preparation stage to homogenize the sample and eliminate organic matter, and a second stage to quantify the targeted elements [33] (Figure 3).

2.2.1. Sample Preparation Step

Mineralization is the most widely used sample preparation technique for elemental analysis due to higher homogeneity, stability, and ease of use [34]. The different techniques used include wet digestion in an open system (heating block) and micro-wave-assisted digestion at high pressure and temperature. Various acids can be used, the most common being nitric acid (HNO₃), which does not cause major analytical issues even at high concentrations. Hydrogen peroxide (H₂O₂) is often combined with nitric acid in this preparation step [33], increasing nitric acid’s oxidizing power and enabling a better attack of the organic matter in the sample [35]. Hydrofluoric acid (HF), boric acid (H₃BO₃), or tetrafluoroboric acid (HBF₄) can also be used in certain cases, notably to dissolve silicates in certain samples [36].

2.2.2. Element Analysis

The detection and quantification of chemical elements, particularly inorganic elements known for their toxicity, is a key area of environmental and health issues [37,38].

• Atomic Absorption Spectrometry

Atomic absorption spectrometry (AAS) is one of the most widely used techniques in the food, environmental, and pharmaceutical [38] fields. This technique can be used with different atomization methods: flame atomic absorption spectrometry (FAAS), electro-thermal atomization atomic absorption spectrometry (ETAAS), and chemical vapor generation atomic absorption spectrometry (CVG-AAS), depending on analyzed element, statistical parameters, limits of detection (LOD) and quantification (LOQ). FAAS is the easiest technique to implement, which makes it the most widely used. However, one of its biggest drawbacks is that, typically, it is not a multi-element technique in conventional operation and cannot use internal standardization techniques [39].

Elements are transformed into atomic vapor with AAS by drawing an aerosol of the sample solution into an open flame. Exposure to a radiation source excites most of the atoms released. The radiation absorbed by the unexcited atoms can then be determined as a function of the concentration in the sample [40]. Although this technique has a low investment cost, it does not allow detection limits as low as those of other elemental analysis techniques (detection limits are limited in the parts per million (ppm) range [41]).

• Inductively Coupled Plasma Atomic Emission Spectrometry

Among the traditional methods for elemental analysis, inductively coupled plasma (ICP) methods are known for their excellent accuracy and precision and low detection limits compared with other techniques [38]. Inductively coupled plasma atomic emission spectrometry (ICP-AES) has become a preferred method, particularly due to its capacity for rapid multi-element analysis in the same sample. This technique uses argon plasma as the radiation source [21]. The sample is injected as an aqueous solution and transformed into an aerosol by passing through a nebulizer before being introduced into the ICP [42]. The energy applied to the sample excites the atoms to higher energy levels. Measurements are then carried out on the wavelength emission characteristics of each element as the atoms’ electrons return to their ground state or a lower energy state [43]. However, this technique suffers from complex spectral interference problems and a lack of precision at the ultra-trace scale [44]. The detection limits for most elements are in the parts per billion (ppb) range [41].

• Inductively Coupled Plasma Mass Spectrometry

Inductively coupled plasma mass spectrometry (ICP-MS) has rapidly established itself as the leader in elemental analysis because it overcomes the limitations of other techniques, such as AAS and ICP-AES. ICP-MS enables multi-element analysis with greater sensitivity and linearity over a wide measurement range for most elements. ICP-MS can also distinguish between different isotopes of the same element by separating elements in ionized form according to their mass-to-charge ratio (m/z), which is not possible with other techniques [45].

Once introduced into the plasma (usually via pneumatic nebulization), samples undergo desolvation, vaporization, atomization, and ionization before entering the mass analyzer. Therein, they are usually detected with an electron multiplier, with detection limits in the ppb–parts per trillion (ppt) range [44]. ICP-MS can analyze all the elements in a single sample in just a few minutes (usually between 2 and 6 min); meanwhile, ICP-AES can analyze several elements (between 5 and 30) per minute, and FAAS can only analyze 1 element in approximately 15 s [41].

• Microwave Plasma Atomic Emission Spectrometry

Microwave plasma atomic emission spectrometry (MP-AES) was introduced simultaneously with ICP-AES and ICP-MS but experienced later success due to the other two techniques being preferred for their better accuracy and detection limits. MP-AES is considered highly promising for routine applications thanks to its smaller footprint, multi-element capability, low cost (investment, operation, and maintenance), and good detection capabilities. It has become a cost-effective analytical instrument compared with ICP-AES or ICP-MS and provides more possibilities than AAS [46]. MP-AES uses nitrogen, a much less expensive gas than the argon used in ICP techniques. Moreover, nitrogen can be obtained directly from ambient air with a N₂ generator coupled to the measuring device [46]. All these points would make this technique advantageous for a paper manufacturing site, enabling routine monitoring of all paper products thus preventing any contamination.

MP-AES’s principle is similar to ICP-AES’s: The sample introduction consists of a standard torch, a spray chamber, and a glass nebulizer. The sample solution is converted into an aerosol and directed into the plasma where atomization and ionization occur [47]. A major difference between the two techniques is the plasma temperature: 5000 K for MP-AES versus 6000–8000 K for ICP. As a result, the ionization of certain elements may be incomplete with MP-AES technique [48]. Its application to different types of samples will be presented thereafter.

MP-AES allows detection limits to be achieved that are of particular interest in the field of food paper packaging. This is particularly true for certain elements with a range from 0.05 pbb (Ca) to 2.1 ppb (Pb) compared to conventional techniques such as ICP-AES (0.02 ppb for Ca to 10 ppb for Pb) or ICP-MS (0.04 ppb for Pb to 6.1 ppb for Cr) with similar capabilities for other elements such as Cu, with a detection limit of 0.6 ppb by MP-AES compared to 1 ppb by ICP-AES and 0.4 ppb by ICP-MS, or Al, where a detection limit similar to that obtained by ICP-MS is possible (0.6 ppb by MP-AES compared to 0.9 ppb by ICP-MS) [46].

3. Analysis of Inorganic Elements in Food Packaging: State of the Art

Paper and paperboard are the most studied matrices in the field [20,49,50,51], alongside recycled papers [52], which can be a source of high concentrations of inorganic elements. However, no studies have been carried out on virgin papers (i.e., those without elements such as printing inks, glues, etc.). It is, therefore, impossible to determine whether the concentrations measured come from the virgin paper or the additives used to form the final packaging (inks, glues, varnishes, etc.).

Two different approaches are used in the sample preparation step, depending on whether the aim is to determine the total quantity of elements in the package or the quantity of elements released by the package. In the first case the most commonly used is mineralization with concentrated acids. Several mixtures can be used in varying proportions, such as HNO₃ alone [49,53], HNO₃ and H₂O₂ [50,51], HCl [20,52], and HClO₄ as a complement [54]. HNO₃ is used in concentrated form in all sample preparations presented, except for the study of food packaging paper, paperboard and corrugated board, where it is used at only 5% [51]. Quantity of HNO₃ used varies from study to study, depending on the matrix studied: for example, HNO₃ is used in quantities of less than 10 mL for paper matrices [49,50,51] and up to 20 mL for the analysis of recycled plastic [54], which is more difficult to dissolve., using HNO₃ is commonplace and seems indispensable for this type of sample preparation.

In addition to HNO₃, other reagents are also used in concentrated form, notable H₂O₂, which is the most commonly additional reagent used at 30% concentration. However, when used additionally, the quantities of H₂O₂, HCl or HClO₄ are lower than quantity of HNO₃ used [20,50,51,52,53]. These quantities are also heterogeneous and depend on the matrices studied.

Same heterogeneity applies to digestion conditions, whether hot-plate or microwave: there is no defined time/temperature pair. These parameters vary according to reagents used and matrices studied. However, the maximum temperature used seems to be around 170–180 °C for fairly short digestion times: less than 15 min [20,49] or less than 1 h [50,51] when assisted by microwave. Temperatures involved are lower for hot-plate assisted digestion, but the times are longer [52]. For more complex matrices, more extreme conditions are involved, up to 500 °C for plastics samples [54].

The mineralization of the matrix studied enables the direct measurement of the concentrations of inorganic elements in the sample (the results are expressed in mg·kg⁻¹).

For the second objective, food simulants can be used to simulate migration [52,53,55]. The packaging under study is put into a solution according to the contact to be simulated. The most commonly used simulants include water for contact with aqueous foods, a 3% acetic acid (CH₃COOH) solution for acidic foods (pH < 4.5), vegetable oil for fatty foods, and poly-(2,6-diphenyl-p-phenylene oxide) (also known as MPPO or Tenax) for dry foods [27]. Studies are frequently carried out with CH₃COOH under extraction conditions for 24 h at 40 °C [52,55], while for plastic matrices several conditions can be tested to simulate multiple food contacts under different conditions [53]. Once extracted, this simulant is analyzed to determine the concentration of each element that has migrated from the packaging into the food simulant.

The analytical techniques presented above have enabled research to be carried out on various types of food packaging (

Table 1. Summary of analytical techniques (sample preparations and detection techniques) used for analyzing inorganic elements in food packaging.

Reference	Sample Type	Targeted Elements	Sample Preparation	Digestion Conditions	Analysis	Measured Values
Bandara and Indunil [52]	Recycled papers for food packaging	Cd, Cr, Cu, Mn, Pb, and Zn	Hot-plate digestion with conc. HNO3 (12 mL), conc.H2O2 (4 mL) and conc. HCl (2 mL)	60 min/50 °C 24 h/Room Temperature (RT)	AAS	Cd [0.0095; 1.94] mg·kg−1 Cr [0.0264; 0.1963] mg·kg−1 Cu [0.0095; 1.940] mg·kg−1 Mn [0.0134; 0.0156] mg·kg−1 Pb [0.1255; 0.2618] mg·kg−1 Zn [0.4520; 0.4573] mg·kg−1
Elmas and Cinar [51]	Paper, paperboard, and corrugated board packaging	Al, Cd, Cr, Cu, Hg, Ni, Pb, and Zn	Microwave digestion with 5% HNO3 (5 mL) and 30% H2O2 (2 mL)	2 min/120 °C 5 min/140 °C 15 min/170 °C 1 min/50 °C 1 min/50 °C	ICP-AES ICP-MS	Al [1.268; 3909] mg·kg−1 Cd [0.02; 0.18] mg·kg−1 Cr [0.51; 6.16] mg·kg−1 Cu [0.52; 166.6] mg·kg−1 Hg [0.01; 3.80] mg·kg−1 Ni [0.92; 4.93] mg·kg−1 Pb [1.39; 12.94] mg·kg−1 Zn [1.36; 61.30] mg·kg−1
Han et al. [49]	Take-out food paper containers	Cd, Co, Cr, Mn, Ni, Pb, and Sb	Microwave digestion with analytical grade HNO3 (8 mL)	5 min/up to 175 °C 4.5 min/175 °C	ICP-MS	Cd [0.004; 0.18] mg·kg−1 Co [0.02; 5.63] mg·kg−1 Cr [0.65; 6.58] mg·kg−1 Mn [1.61; 89.8] mg·kg−1 Ni [0.37; 3.76] mg·kg−1 Pb [0.004; 5.58] mg·kg−1 Sb [0.002; 1.19] mg·kg−1
Skrzydlewska et al. [50]	Papers and paperboards for food packaging	Cd, Cr, and Pb	Microwave digestion with 65% HNO3 (6 mL) and 30% H2O2 (2 mL)	15 min/up to 100 °C 10 min/up to 180 °C 15 min/180 °C 30 min/up to RT	ICP-TOF-MS	Cd [0.09; 0.12] µg·kg−1 Cr [0.25; 0.64] µg·kg−1 Pb [0.28; 0.99] µg·kg−1
Sood and Sharma [18]	Papers and paperboards for food packaging	Al, As, B, Ba, Co, Cr, Cu, Fe, Mn, Ni, Pb, Te, Ti, and V	Microwave digestion with conc. HNO3 (10 mL) and conc. HCl (3 mL)	5.5 min/up to 175 °C 4.5 min/175 °C	ICP-AES	Al [11.763; 102.722] mg·kg−1 As [0.008; 0.300] mg·kg−1 B [0.008; 0.035] mg·kg−1 Ba [0.265; 1.118] mg·kg−1 Co [0.024; 0.053] mg·kg−1 Cr [0.026; 2.174] mg·kg−1 Cu [0.045; 0.832] mg·kg−1 Fe [0.418; 37.209] mg·kg−1 Mn [0.208; 547] mg·kg−1 Ni [0.008; 0.196] mg·kg−1 Pb [0.051; 388] mg·kg−1 Te [0.004; 0.012] mg·kg−1 Ti [0.007; 0.153] mg·kg−1 V [0.032; 0.547] mg·kg−1
Whitt et al. [54]	Recycled plastic food packaging	Cd, Cr, Ni, Pb, and Sb	Hot-plate digestion in two steps: (1) 67% HNO3 (20 mL) and trace metal gradeHClO4 (3 mL) (2) 67% HNO3 (1 mL) and 37% HCl (1 mL)	reduction up to 1 mL/500 °C cooled 5 min until boiling after add. of new reagent/500 °C cooled 5 min	ICP-AES	Cd [2.02; 22.61] mg·kg−1 Cr [1.71; 16.67] mg·kg−1 Ni [2.20; 23.59] mg·kg−1 Pb [0.02; 0.36] mg·kg−1 Sb [0.05; 11.38] mg·kg−1
Bandara and Indunil [52]	Recycled papers for food packaging	Cd, Cr, Cu, Mn, Pb, and Zn	Migration of sample in 3% CH3COOH food simulant	24 h/40 °C	AAS	Cd [0.0012; 0.011] µg·L−1 Cr [0.0069; 0.206] µg·L−1 Cu [0.262; 0.98] µg·L−1 Mn [0.001; 0.056] µg·L−1 Pb [0.0072; 0.126] µg·L−1 Zn [0.024; 0.45] µg·L−1
Duran and Soylak [55]	Different food packaging materials	Co, Cd, Cr, Cu, Fe, Mn, Ni, and Pb	Migration of sample in 3% CH3COOH food simulant	24 h/40 °C	FAAS	Cd [0.03; 0.64] µg·dm2 Co [0.40; 5.03] µg·dm2 Cr [0.30; 2.45] µg·dm2 Cu [0.04; 29.8] µg·dm2 Fe [0.15; 67.8] µg·dm2 Mn [0.006; 36.8] µg·dm2 Ni [0.02; 9.43] µg·dm2 Pb [0.41; 11.8] µg·dm2
Khan and Khan [53]	Plastic food containers	Cu, Mn, Ni, Pb, and Zn	Migration of sample in aqueous food simulants (water, 3% CH3COOH, 8% EtOH, 0.9% NaCl + 5% Na2CO3) and then digestion of solution with conc. HNO3 (1 mL)	3 different conditions: 72 h/4 °C 2 h/60 °C 24 h/25 °C	AAS	Cu [1.003; 1.61] mg·L−1 Mn [1.002; 1.01] mg·L−1 Ni [1.01; 1.31] mg·L−1 Pb [1.002; 1.9] mg·L−1 Zn [1.001; 1.02] mg·L−1

). ICP techniques are the most widely used multi-elemental analysis methods, including ICP-AES [20], ICP-MS [49,50], and it is also possible to use both techniques in the same study [51]. AAS is also widely used [52,53,55] but offers less accurate results than those obtained using ICP. Moreover, some elements, such as Al and Hg, have only been studied using ICP techniques.

The elements measured in the paper and paperboard packaging studied, such as As, B, Ba, Co, Cd, Cr, Cu, Hg, Ni, Sb, Te, Ti, and V, are mostly found in concentrations of less than 10 ppm. However, other elements are measured at higher concentrations, such as Al, which can be measured at up to 102.722 mg·kg⁻¹ [19] or more than 3909 mg·kg⁻¹ [51], Pb at 388 mg·kg⁻¹ [20], or Fe at 37.209 mg·kg⁻¹ [20]. Significant variations in measurements have also been noted, probably depending on the composition of the sample studied and analytical parameters, e.g., Pb has been measured at over 300 mg·kg⁻¹ versus measurements below 15 mg·kg⁻¹ in other studies [49,50,51,52] and Mn at up to 547 mg·kg⁻¹ versus 89.8 mg·kg⁻¹ [49] or greater than or equal to 1 mg·kg⁻¹ [52,53].

4. MP-AES Elemental Analysis in Different Research Fields: State of the Art

MP-AES is being established as an alternative to conventional elemental analysis techniques. Despite spectral interferences and lower sensitivities than ICP, it also offers numerous advantages, such as the possibility of multi-elemental analysis (unlike AAS) and using nitrogen instead of argon (ICP techniques) [46].

Numerous research projects have recently been carried out using this technique on various matrices (

Table 2. Summary of studies carried out using MP-AES (specifying the digestion protocols used) on various matrices.

Reference	Sample Type	Targeted Elements	Sample Digestion Protocol	Digestion Conditions	Detection Limits	Quantification Limits
Geisenblosen et al. [64]	Rock	Al, Ba, Ca, Fe, K, Mg, Mn, Na, Sr, and Ti	Microwave digestion with (1) 65% HNO3 (3 mL), 37% HCl (1 mL), and 48% HF (3 mL) (2) 5% H3BO3 (18 mL)	5 min ramp + 60 min hold time at 1400 W 5 min ramp + 15 min hold time at 1400 W	100 µg·g−1 (Al) 1.0 µg·g−1 (Ba) 4.0 µg·g−1 (Ca) 70 µg·g−1 (Fe) 140 µg·g−1 (K) 1.0 µg·g−1 (Mg) 1.0 µg·g−1 (Mn) 110 µg·g−1 (Na) 0.3 µg·g−1 (Sr) 6.0 µg·g−1 (Ti)	300 µg·g−1 (Al) 4.0 µg·g−1 (Ba) 13 µg·g−1 (Ca) 210 µg·g−1 (Fe) 400 µg·g−1 (K) 2.0 µg·g−1 (Mg) 4.0 µg·g−1 (Mn) 320 µg·g−1 (Na) 0.8 µg·g−1 (Sr) 19 µg·g−1 (Ti)
Jung et al. [48]	Wine	Mn	Hot-plate digestion with 30–32% H2O2 (15 mL) 70% HNO3 (1 mL)	reduction up to 20 mL/80 °C reduction up to 5 mL/80 °C	0.67 µg·L−1	2.22 µg·L−1
Oliveira et al. [56]	Chocolate bar	Ba, Ca, Cu, Cr, Fe, K, Mg, Mn, Na, Ni, P, and Zn	Microwave digestion with analytical grade HNO3 (2.3 mL) and analytical grade H2O2 (1 mL)	4 min/up to 120 °C 2 min/120 °C 4 min/up to 190 °C 20 min/190 °C 10 min/ventilation	0.002 mg·kg−1 (Ba) 21 mg·kg−1 (Ca) 0.05 mg·kg−1 (Cu) 0.2 mg·kg−1 (Cr) 0.5 mg·kg−1 (Fe) 73 mg·kg−1 (K) 1.4 mg·kg−1 (Mg) 0.01 mg·kg−1 (Mn) 3.8 mg·kg−1 (Na) 0.1 mg·kg−1 (Ni) 11 mg·kg−1 (P) 0.2 mg·kg−1 (Zn)	0.007 mg·kg−1 (Ba) 69 mg·kg−1 (Ca) 0.16 mg·kg−1 (Cu) 0.6 mg·kg−1 (Cr) 1.6 mg·kg−1 (Fe) 241 mg·kg−1 (K) 4.6 mg·kg−1 (Mg) 0.03 mg·kg−1 (Mn) 12.5 mg·kg−1 (Na) 0.3 mg·kg−1 (Ni) 36 mg·kg−1 (P) 0.6 mg·kg−1 (Zn)
Ozbek and Akman [59]	Bread	Ca, Cu, Fe, K, Mg, Mn, and Zn	Hot-plate digestion with 65% HNO3 (3 mL) and 35% H2O2 (1 mL)	2 h/100 °C	13.1 mg·kg−1 (Ca) 0.28 mg·kg−1 (Cu) 4.47 mg·kg−1 (Fe) 118 mg·kg−1 (K) 1.10 mg·kg−1 (Mg) 0.41 mg·kg−1 (Mn) 3.00 mg·kg−1 (Zn)	43.8 mg·kg−1 (Ca) 0.93 mg·kg−1 (Cu) 14.9 mg·kg−1 (Fe) 393 mg·kg−1 (K) 3.66 mg·kg−1 (Mg) 1.38 mg·kg−1 (Mn) 10.1 mg·kg−1 (Zn)
Ozbek and Akman [60]	Cheese	Ca, K, and Mg	Hot-plate digestion with 65% HNO3 (3 mL) and 35% H2O2 (1 mL)	4–5 h/100 °C	0.036 mg·kg−1 (Ca) 0.19 mg·kg−1 (K) 0.012 mg·kg−1 (Mg)	0.118 mg·kg−1 (Ca) 0.63 mg·kg−1 (K) 0.038 mg·kg−1 (Mg)
Rodriguez-Solana et al. [61]	Beverage	Ca, Cu, Cd, Fe, K, Mg, Mn, Na, P, Pb, and Zn	Hot-plate digestion with: Method 1: 65% HNO3 (6 mL) and 60% HClO4 (4 mL) Method 2: 65% HNO3 (2 mL) and 60% HClO4 (8 mL) Method 3: 65% HNO3 (4 mL) and 36% HCl (1 mL) Method 4: 65% HNO3 (1 mL) and 36% HCl (1 mL) Muffle furnace digestion with 65% HNO3 (10 mL)	Method 1: 20 min/60 °C and 45 min/90 °C Method 2: 20 min/60 °C and 45 min/90 °C Method 3: 20 min/RT, 30 min/80 °C, 30 min/100 °C and 30 min/110 °C Method 4: 20 min/60 °C and 45 min/90 °C Muffle furnace method: 30 min/80 °C, 120 min/150 °C and 240 min/450 °C	0.52 mg·L−1 (Ca) 0.05 mg·L−1 (Cu) 0.11 mg·L−1 (Cd) 0.10 mg·L−1 (Fe) 0.07 mg·L−1 (K) 0.05 mg·L−1 (Mg) 0.08 mg·L−1 (Mn) 0.15 mg·L−1 (Na) 0.14 mg·L−1 (P) 0.13 mg·L−1 (Pb) 0.10 mg·L−1 (Zn)	1.74 mg·L−1 (Ca) 0.17 mg·L−1 (Cu) 0.38 mg·L−1 (Cd) 0.33 mg·L−1 (Fe) 0.07 mg·L−1 (K) 0.17 mg·L−1 (Mg) 0.25 mg·L−1 (Mn) 0.49 mg·L−1 (Na) 0.46 mg·L−1 (P) 0.42 mg·L−1 (Pb) 0.32 mg·L−1 (Zn)
Sajtos et al. [57]	Honey	Al, B, Ba, Bi, Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Na, Ni, Pb, Sr, and Zn	Hot-plate digestion with 65% HNO3 (4 mL) and 30% H2O2 (1 mL)	-	0.2315 mg·kg−1 (Al) 0.2372 mg·kg−1 (B) 0.0777 mg·kg−1 (Ba) 1.2260 mg·kg−1 (Bi) 2.0610 mg·kg−1 (Ca) 0.3325 mg·kg−1 (Cd) 0.0720 mg·kg−1 (Co) 0.0044 mg·kg−1 (Cr) 0.0442 mg·kg−1 (Cu) 0.3694 mg·kg−1 (Fe) 0.1852 mg·kg−1 (K) 0.0138 mg·kg−1 (Li) 0.0802 mg·kg−1 (Mg) 0.0080 mg·kg−1 (Mn) 0.2126 mg·kg−1 (Na) 0.0162 mg·kg−1 (Ni) 0.0165 mg·kg−1 (Pb) 0.2878 mg·kg−1 (Sr) 0.1899 mg·kg−1 (Zn)	0.7718 mg·kg−1 (Al) 0.7905 mg·kg−1 (B) 0.2590 mg·kg−1 (Ba) 4.0865 mg·kg−1 (Bi) 6.8701 mg·kg−1 (Ca) 1.1084 mg·kg−1 (Cd) 0.2399 mg·kg−1 (Co) 0.0148 mg·kg−1 (Cr) 0.1473 mg·kg−1 (Cu) 1.2312 mg·kg−1 (Fe) 0.6172 mg·kg−1 (K) 0.0459 mg·kg−1 (Li) 0.2673 mg·kg−1 (Mg) 0.0266 mg·kg−1 (Mn) 0.7085 mg·kg−1 (Na) 0.0540 mg·kg−1 (Ni) 0.0549 mg·kg−1 (Pb) 0.9594 mg·kg−1 (Sr) 0.6331 mg·kg−1 (Zn)
Sao Bernardo Carvalho et al. [58]	Instant soup	Cu, K, Mg, Mn, P, and Zn	Microwave digestion with 65% HNO3 (1 mL) and 30% H2O2 (1 mL)	10 min/up to 120 °C 3 min/120 °C 13 min/up to 200 °C 14 min/200 °C	0.09 mg·kg−1 (Cu) 4.9 mg·kg−1 (K) 1.0 mg·kg−1 (Mg) 0.04 mg·kg−1 (Mn) 5.4 mg·kg−1 (P) 0.88 mg·kg−1 (Zn)	0.31 mg·kg−1 (Cu) 16 mg·kg−1 (K) 3.4 mg·kg−1 (Mg) 0.12 mg·kg−1 (Mn) 18 mg·kg−1 (P) 2.9 mg·kg−1 (Zn)
Sungur and Gülmez [63]	Textile	Al, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Tl, and Zn	Hot-plate digestion with 1:5 30% H2O2/70% HNO3 (10 mL)	55 min/110 °C	0.123 mg·L−1 (Al) 0.415 mg·L−1 (Cd) 0.202 mg·L−1 (Co) 0.101 mg·L−1 (Cr) 0.256 mg·L−1 (Cu) 0.343 mg·L−1 (Fe) 0.059 mg·L−1 (Mn) 0.118 mg·L−1 (Ni) 0.088 mg·L−1 (Pb) 0.285 mg·L−1 (Tl) 0.705 mg·L−1 (Zn)	0.409 mg·L−1 (Al) 1.382 mg·L−1 (Cd) 0.673 mg·L−1 (Co) 0.336 mg·L−1 (Cr) 0.852 mg·L−1 (Cu) 1.142 mg·L−1 (Fe) 0.196 mg·L−1 (Mn) 0.393 mg·L−1 (Ni) 0.293 mg·L−1 (Pb) 0.949 mg·L−1 (Tl) 2.348 mg·L−1 (Zn)
Vella and Attard [65]	Toothpaste	Ag, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Sn, and Zn	Hot-plate digestion with 5% HNO3 (5 mL) and 34.5% H2O2 (2 mL) Then calcination in a muffle furnace with 5% HNO3 (5 mL)	80–90 °C after each addition of reagent 6 h/500 °C in muffle furnace	0.0549 mg·kg−1 (Ag) 0.0067 mg·kg−1 (Cd) 0.0005 mg·kg−1 (Cr) 0.0007 mg·kg−1 (Cu) 0.0037 mg·kg−1 (Fe) 0.0789 mg·kg−1 (Hg) 0.0042 mg·kg−1 (Mn) 0.0056 mg·kg−1 (Ni) 0.0169 mg·kg−1 (Pb) 0.0375 mg·kg−1 (Sn) 0.0301 mg·kg−1 (Zn)	0.1665 mg·kg−1 (Ag) 0.0204 mg·kg−1 (Cd) 0.0014 mg·kg−1 (Cr) 0.0022 mg·kg−1 (Cu) 0.0113 mg·kg−1 (Fe) 0.2391 mg·kg−1 (Hg) 0.0127 mg·kg−1 (Mn) 0.0169 mg·kg−1 (Ni) 0.0511 mg·kg−1 (Pb) 0.1137 mg·kg−1 (Sn) 0.0912 mg·kg−1 (Zn)
Zhao et al. [62]	Leather and fur	Cd, Co, Cr, Cu, Hg, Ni, and Pb	Microwave digestion with 65% HNO3 (4 mL) and 30% H2O2 (1 mL)	up to 130 °C and hold for 5 min up to 180 °C and hold for 10 min up to 220 °C and hold for 20 min	1.3 mg·kg−1 (Cd) 1.9 mg·kg−1 (Co) 0.9 mg·kg−1 (Cr) 1.5 mg·kg−1 (Cu) 2.0 mg·kg−1 (Hg) 0.9 mg·kg−1 (Ni) 1.2 mg·kg−1 (Pb)	Not determined

), such as chocolate bars [56], honey [57], instant soup [58], bread [59], and cheese [60], as well as liquid matrices, such as wine [48] or liqueurs [61], and more complex matrices, such as leather and fur [62], various textiles [63], and stone [64].

All these studies used the same technique to dissolve the matrices: mineralization in an acidic solution, either with an assisted microwave or a hot plate. When using these two devices, digestion conditions (temperature and time) vary according to sample type. In the case of hot-plate assisted digestion, temperatures do not exceed 110 °C, but longer digestions are required, ranging from around 1 h [61,63,64] to several hours of reaction [59,60,65]. Microwave digestion, on the other hand, enables higher temperatures to be reached for matrices that are more difficult to digest [62], and also allows shorter reaction time of around 30 min [56,58,62] and sometimes exceeding 1 h [64]. Microwave assisted digestion also enables a gradual increase in temperature, with stages (up to 3 stages in the works studied) during digestion, which is used in several studies [56,58,62,64]. Only the work on liqueurs [60] tried mineralization with a muffle furnace in comparison with hot-plate or microwave assisted digestion. Muffle furnace can also be used as a complement to conventional hot-plate digestion, enabling higher conditions to be reached [65].

Different reagents are also used in varying proportions. HNO₃ is the only reagent used in all the work presented, being concentrated to at least 65% except for study on toothpaste samples [65]. In addition to HNO₃ during digestion H₂O₂ (concentrated to a minimum of 30%) is the most commonly used reagent, although in smaller quantities [48,56,57,58,62,63,65]. HNO₃ can also be combined with HCl (36% concentrate) or HClO₄ (60% concentrate) [61]. HNO₃ can also be associated with several acids for matrices that are more complex to digest [64]. Consequently, HNO₃/H₂O₂ mixture appears to be the preferred choice for MP-AES studies if matrix studied allows its use.

MP-AES enables very low LODs and LOQs to be achieved for most elements, below ppm in most cases for each element. However, it may be difficult to lower the limits for some elements, such as Ca, with an LOD of up to 21 mg·kg⁻¹ [56]; K, with an LOD of up to 393 mg·kg⁻¹ [59]; and Na (measured in chocolate bars), with an LOD of 12.5 mg·kg⁻¹ [56]. Nevertheless, these studies confirm that using MP-AES for multi-elemental analysis is effective.

However, no study based on MP-AES analysis has been carried out on food pack-aging (paper, paperboard, or plastic).

5. Challenges and Perspectives

The multi-elemental analysis of inorganic elements in food packaging has been studied in recent years with the most commonly used methods: AAS, ICP-AES, and ICP-MS. Most studies have focused on the concentrations of elements present directly in the packaging studied, giving a precise idea of the concentration of each. This contrasts with the use of aqueous simulants (used to represent different food contacts) which, while providing an estimate of the concentration of each element likely to migrate into the food, provides no information on the concentrations present in the packaging. Furthermore, no studies have been carried out on virgin paper before the addition of various glues, inks, etc. Therefore, there is no information on the concentrations of inorganic compounds in paper leaving a paper mill. Such data would be of interest to elucidate the impact of the manufacturing process on the final elemental concentrations in packaging intended for food contact. In addition, the techniques used would be difficult to implement in an industrial site, particularly ICP techniques, due to the high investment costs and facilities required (i.e., the argon to be consumed continuously when running the apparatus).

Most analytical protocols developed for sample preparation use a mineralization step via acid digestion to convert the studied solid matrix into a liquid matrix that can be directly injected into an analytical device. According to the studies carried out on food packaging, this sample preparation technique seems effective on a paper matrix. In parallel, digestion via mineralization in an acidic solution is used for MP-AES analyses on matrices of variable compositions, particularly with a HNO₃/H₂O₂ mixture in variable proportions, except for the most complex matrices, such as stone, requiring other acids.

The results obtained indicate this technique to be a good alternative to the usual techniques, with similar detection limits despite having a lower precision than ICP techniques and can also become a good alternative to FAAS and ICP-AES as a routine multi-element analysis technique. In addition, the low investment cost and the ease of installation and current use would allow laboratory manufacturers to perform routine multi-element analyses without investing in more expensive ICP techniques [45]. However, no study has been carried out on food packaging or paper packaging. Prior preparation of the sample by acid mineralization would enable this analysis technique to be used on a paper matrix, replacing conventional detectors that are less sensitive or more costly, and easily implemented on an industrial site. This technique could prove to be an interesting way of quantifying inorganic elements of regulatory interest, directly in the food contact paper matrix with analytical limits adapted to regulations. This would also enable more frequent monitoring of paper packaging directly on production site pro-duction.

6. Conclusions

This article compiled various research works on the quantification of inorganic elements in food packaging. Studying the composition of food packaging can prevent risks to consumers’ health in the event of the migration of elements in the packaging to the packaged food.

The techniques commonly used in this field are AAS, ICP-AES, and ICP-MS. Acidic mineralization is the sample preparation technique most often used to obtain a liquid matrix from a solid matrix to obtain a liquid and homogeneous sample that can be injected into an analytical device. Various mixtures are used, although the HNO₃/H₂O₂ mixture is the most common. Another sample preparation technique is migration in aqueous simulants, depending on the contact with the food to be simulated. The major disadvantage of this technique is lacking a real indication of the concentrations of the elements in the tested packaging.

MP-AES has emerged in recent years as an alternative to conventional techniques and has been used in numerous research projects, enabling the multi-elemental analysis of various matrices with similar results to FAAS and ICP-AES analyses and evidencing the interest of using this method for routine multi-elemental analyses.

The development of an analysis method using an MP-AES detector in a paper packaging matrix is therefore feasible and would make routine control easier and less costly directly on production sites.

This review offers a valuable opportunity to compare state-of-the-art techniques for quantifying inorganic elements in food contact paper packaging with studies employing Microwave Plasma–Atomic Emission Spectroscopy (MP-AES) for inorganic element determination across various food matrices.

Globally, the regulation and monitoring of inorganic elements vary significantly between countries and are often conducted too infrequently. Given its potential advantages particularly in terms of cost-effectiveness, sensitivity, and operational simplicity—MP-AES warrants further investigation for application in the food packaging sector. Its integration at an industrial scale could enable more frequent and reliable monitoring of inorganic elements of concern, thereby enhancing food safety and regulatory compliance.