Halogens are a group of reactive elements, which are capable of making stable bonds with metals and nonmetals. Due to their properties, they play an important biochemical role in living organisms; some, such as F, Cl and I, are essential for humans, whereas others, such as Br, are non-essential and can cause key toxicological effects. This is probably the main reason why this group of elements has been extensively studied in food and biological materials, especially in recent years. The determination of halogens at trace levels requires suitable instrumentation, and plasma-based techniques, such as inductively coupled plasma optical emission spectrometry (ICP-OES) and especially inductively coupled plasma mass spectrometry (ICP-MS), are particularly well suited for this purpose. Although they are very versatile for performing multielemental determination, halogens present high ionization potentials and low wavelength emission lines, and they are prone to interferences due to matrix effects. Such drawbacks are difficult to overcome without changing instrumental features, such as mass or optical resolution. On the other hand, some interferences due to the presence of dissolved carbon in solution, enhancement or suppression of ionization, and related matrix effects can be minimized by applying an appropriate sample preparation step. In this sense, the present review covers the main sample preparation methods applied for halogen determination in food and biological materials by using ICP-OES and ICP-MS. Methods based on dilution, solubilization, extraction or those involving matrix decomposition (e.g., acid digestion and combustion) are discussed based on key applications selected in the last 20 years.

The efficiency of diluted nitric acid solutions under oxygen pressure for decomposition of bovine liver was evaluated using microwave-assisted wet digestion. Calcium, Cu, Fe, Mg, Mn and Zn were determined by inductively coupled plasma optical emission spectrometry (ICP OES). Efficiency was evaluated by determining the residual carbon content (RCC) by ICP OES and residual acidity in digests. Samples (up to 500 mg) were digested using nitric acid solutions (0.1, 0.5, 1, 2, 3, 7, and 14 mol L⁻¹ HNO₃) and the effect of oxygen pressure was evaluated using pressures of 0.5, 1, 1.5 and 2 MPa. It was demonstrated that 2 and 0.5 mol L⁻¹ nitric acid solutions may be used for efficient digestion of 500 and 100 mg of bovine liver, respectively, with oxygen pressures ranging from 0.5 to 2 MPa. Using these conditions, less than 0.86 and 0.21 mL of concentrated nitric acid were necessary to digest 500 and 100 mg of sample, respectively. Similar digestion efficiencies for both conditions were obtained under pressures of O₂ ranging from 0.5 to 2 MPa. The residual acidities in final digests were lower than 24% when compared to the initial amount of acid used for digestion. The accuracy of the proposed procedure was evaluated using certified reference materials of bovine liver and bovine muscle. Using a solution of 2 mol L⁻¹ with oxygen pressure of 0.5 MPa for 500 mg of sample, the agreement with certified values ranged from 96 to 105% (n = 5). Using the proposed procedure with diluted nitric acid it was possible to obtain RCC values lower than 15% that is important for minimizing the generation of laboratory residues and improving limits of detection.