Successive digestions for pre-concentration and ultra-trace determination of Br and I by plasma-based atomic spectrometry and ion chromatography

This study demonstrates the feasibility of successive digestions as a strategy for Br and I pre-concentration and ultra-trace determination by ion chromatography and plasma-based spectrometric techniques. Digestion and pre-concentration steps were performed by using microwave-induced combustion (MIC), and milk powder samples for adult and infant nutrition were used as an example of application. Samples were pressed as pellets (up to 0.7 g) and combusted in closed quartz vessels pressurized with oxygen. Up to 5 combustion cycles were sequentially performed using the same vessel without changing the absorbing solution. In each cycle, a new filter paper, igniter, and sample pellet (0.7 g) were used allowing digestion and pre-concentration. Ammonium hydroxide solutions (100 to 300 mmol L−1) were evaluated for analytes absorption. Using 300 mmol L−1 NH4OH as absorbing solution, up to 5 digestion cycles (0.7 g each cycle, total mass of 3.5 g) can be applied while keeping a suitable relative standard deviation (<5%). The accuracy of the proposed MIC method was evaluated by using a whole milk powder reference material (RM NIST 8435) and results were in agreement with reference values (better than 95%). The digestion efficiency was demonstrated by the low residual carbon in digests (<25 mg L−1), allowing Br and I determination virtually free of interferences caused by the presence of dissolved organic matter. The limits of detection (LODs) for Br and I were 1.25 and 0.15 ng g−1, respectively, which are 5 times lower than the LOD obtained using the conventional MIC method. Taking into account the very low LODs obtained, the proposed sequential digestion and pre-concentration MIC method can be recommended for further Br and I determination in ultra-trace levels by inductively coupled plasma mass spectrometry or even by less sensitive techniques, such as inductively coupled plasma optical emission spectrometry or ion chromatography.