Single-reaction-chamber (SRC) microwave digestion was used for the digestion of pharmaceutical samples before inductively
coupled plasma–mass spectrometry (ICP-MS) analysis according to draft USP chapters <232> and <233>. Following the optimization of digestion methodology (vial type, digestion matrix, and temperature
program), dietary supplements were digested and analyzed for the "big four" toxic elements. Good quality control data demonstrates
the suitability of SRC microwave digestion for this application.
New USP chapters <232> and <233> for the measurement of inorganic contaminants in pharmaceutical samples are scheduled to be implemented
in early 2014. While samples that are soluble in aqueous and organic solvents may be analyzed directly, a large proportion
of samples will require digestion; in fact, digestion may be preferred for inductively coupled plasma–mass spectrometry (ICP-MS)
analysis even if the sample is soluble in organic solvent. Closed-vessel digestion is stipulated by USP and it is expected that microwave digestion will be the predominant digestion technique used; its high pressure and temperature
capability offers greater digestion power than hot-plate closed-vessel digestion.
Single-reaction-chamber (SRC) microwave digestion is a relatively new type of closed-vessel digestion that differs significantly
from traditional closed-vessel digestion. A commercially available benchtop SRC digestion system can digest as many as 15
samples simultaneously, at high temperature and pressure. This high temperature and pressure capability enables the complete
digestion of virtually every pharmaceutical sample type, producing digest solutions with a very low total organic carbon (TOC)
content, which is beneficial for ICP-MS analysis.
Two sample types, St. John's wort and fish oil, typical of finished product pharmaceuticals, were digested using an SRC digestion
system and analyzed for the four toxic USP elements using collision-cell ICP-MS to evaluate the effectiveness of SRC digestion for this application. Because all samples
are digested together in a single chamber with SRC, duplicates and spike recoveries were performed to confirm the retention
of volatile elements and the absence of cross contamination.