Estudo da eficiência de um sistema de preparo de amostras utilizando radiação infravermelha e frascos fechados

Abstract

Trace elements are those that are found in a concentration lower than 100 g g-1 and comprise a group of essential micronutrients, but at the same time they can be harmful to human health. Therefore, the detection and measurement of these elements is extremely important. However, trace element analysis is not a simple task. To introduce the sample into the analyzing equipment, it must be digested and in the form of a solution. In this work, an infrared radiation heating system was developed for sample digestion, using pressurized flasks. 250 mg of mass of rice samples, fish tissue and bovine liver of reference material, 2 mL of HNO3 and 1 mL of H2O2 were used. As temperature is an important variable for sample digestion and as the system was still under development and evaluation, temperature tests were carried out using a PT-100 type thermometer installed in the equipment and a digital infrared thermometer with laser sight fixed to the bottom of the cabinet directed to the bottom of the digestion flasks to compare temperatures. Factorial planning was applied in order to find the best operating conditions for the system. For bovine liver and fish tissue samples, 250 mg of sample, 2 mL of HNO3 and 1 mL of H2O2, as a response, carbon was measured in the (total organic content) TOC analyzer. The heating time was set at 8 min, with a ramp of 15 min to reach a temperature of 200ºC. Subsequently, it was verified the need to carry out tests in the microwave under the same conditions in which the experiments were carried out on the proposed system. As a comparative method, microwave digestion of the afore mentioned samples was used, 500 mg of sample, 7 mL of HNO3 and 2 mL of H2O2, according to the methodology established for food in the equipment. The heating time was set at 15 min, with a 15 min ramp to reach a temperature of 200ºC. The temperature tests demonstrated that from 190ºC the digital thermometer and the thermocouple presented very close temperatures, with no major fluctuations from this point. The carbon concentration of the samples in the proposed system was 383.8 mgL-1 in the first tests carried out and the carbon concentration in the samples digested in the microwave was 16.93 mgL-1. After temperature tests and optimization of time/temperature variables, the value was 58.65 mgL-1 and in the microwave, it was 33.53 mgL-1still considered much lower than those permitted for introduction into carbon detection equipment. elements such as the inductively coupled plasma optical emission spectrometer (ICP-OES), values below 2000 mg/L of organic carbon. The system proved to be attractive for several aspects, the total time in which the sample was subjected to heating is fast; the cost associated with creating the system and replacing parts is low; the operation of the equipment and its maintenance are simple, and the fact that closed bottles and a small volume of reagents are used can minimize losses and reduce waste generated.