Desenvolvimento de métodos alternativos para a determinação de metais em suplementos dietéticos por cromatografia de íons com detecção condutimétrica direta

Abstract

In this work, microwave-induced combustion (MIC) was evaluated for sample preparation of dietary supplements (hypercaloric, multivitamin and/or mineral and botanical supplements) for further alkali, alkaline-earth and transition metals determination by ion chromatography with direct conductivity detection (IC-DC). For this, it was required the chromatographic parameters optimization in order to provide the suitable separation of these species from a cation exchange column with carboxyl groups supported on silica gel. The most suitable results were obtained using 2.7 mmol L-1 oxalic acid in 15% (v/v) acetonitrile as mobile phase and using 0.7 mL min-1 as flow rate. Separation of 8 species (Ca2+, Co2+, K+ , Li+ , Mg2+, Mn2+, Na+ and Zn2+) was possible in less than 15 minutes. The maximum residual acidity that allows a suitable determination of all analytes by IC-DC was also evaluated, and it was possible to observe that final digests with residual acidity above 15 mmol L-1 can make impossible the determination of the analytes by IC-DC. Thus, as an alternative to the use of concentrated acids on the sample preparation step, the MIC was evaluated and some parameters such as sample mass, oxygen pressure, absorbing solution and reflux time were studied. In short, the MIC enabled the adequate preparation of high sample masses (up to 1 g) and the use of diluted acids (HNO3: HCl 1: 1 mol L-1 ), allowing to obtain digests fully compatible with IC-DC determination. Accuracy was evaluated by recovery tests using standard solution, and recoveries ranged from 91% to 107% for all analytes in the three matrices evaluated. A reference method was also performed for comparison of the results with those obtained using proposed methods, and statistical differences were not found. Proposed methods presented suitable limits of detection (from 20 to 154 mg kg-1 ) and quantification (from 38 to 250 mg kg-1 ), as well as suitable precision (relative standard deviations ≤ 7%). It is important to highlight that the use of diluted acids on the sample preparation promoted the reduction of the handling of concentrated reagents, as well as the generation and hazardousness of waste. In addition, it was possible to carry out the analytes determination using a technique that presents relatively low acquisition and maintenance costs, and sequential multielemental determination capability. Regarding to applicability of the proposed methods, stands out the discrepancies observed between the determined values and the informed values by manufactures, which are possibly related to the use of unsuitable analytical methods for quality control of these products.