Development of a Single-Step Microwave-Assisted Digestion Method using Dilute Nitric Acid for Determination of Bismuth in Bismuth-containing Pharmaceuticals by Hydride Generation-Atomic Fluorescence Spectrometry

A single-step microwave assisted digestion (MWAD) procedure employing very dilute solutions of HNO3 was developed for the quantitative determination of bismuth in bismuth-containing pharmaceuticals by hydride generation-atomic fluorescence spectrometry (HG-AFS). Experimental parameters affecting MWAD process such as acid concentration (HNO3), digestion time and temperature were optimized to get quantitative recovery of bismuth. The studies indicated that the method is rapid (within 15 min) including cooling time and recovery > 98% was obtained using 10mL of 5% (v/v) HNO3 as digestion medium with ~0.1g of sample. The optimum microwave digestion parameters obtained were: temperature – 180oC, pressure – 25 bar and hold time - 10 min. A clear solution with negligible residue was obtained after microwave digestion. The digested sample solution was appropriately diluted with 2% (v/v) HCl for subsequent analysis by HG-AFS. The reproducibility, expressed as % RSD was lower than 2% for the allopathic medicine. Under optimal conditions, the limit of detection (LOD) for Bi was calculated to be 0.024mg/kg. The methodology was optimized using a bismuth-containing pharmaceutical – Pylobis, purchased from a local pharmacy. The optimized MWAD approach was further applied to few other bismuth-containing pharmaceutical products. The developed method has significant advantages when compared to the conventional hot-plate digestion methods reported for Bi-containing pharmaceuticals, employing large volumes of concentrated acids. These investigations revealed that the proposed MWAD method in combination with HG-AFS can be utilized for the rapid determination of Bi in pharmaceutical products on regular basis.