Determination of Hexavalent Chromium Fractions in Plastics Using Laboratory-Based, High-Resolution X-ray Emission Spectroscopy.

Cr(VI) is a well-known human carcinogen with many water-soluble moieties. Its presence in both natural and man-made substances poses a risk to public health, especially when contamination of groundwater is possible. This has led the European Union and other jurisdictions to include Cr(VI) in restriction of hazardous substances regulations. However, for several important industrial and commercial purposes, analytical capability to characterize Cr(VI) is known to be insufficient for regulatory purposes. For example, advanced X-ray spectroscopies, particularly synchrotron-based X-ray absorption fine structure (XAFS) studies, have shown that species interconversion and under-extraction can be difficult to prevent in many existing liquid extraction protocols when applied to plastics, mining ores and tailings, and paint sludges. Here, we report that wavelength dispersive X-ray fluorescence spectroscopy taken at energy resolutions close to the theoretical limit imposed by the core-hole lifetime, generally called X-ray emission spectroscopy (XES) in the synchrotron community, can be used in the laboratory setting for noninvasive, analytical characterization of the Cr(VI)/Cr ratio in plastics. The selected samples have been part of ongoing efforts by standards development organizations to create improved Cr(VI) testing protocols, and the present work provides a direct proof-of-principle for the use of such extremely high-resolution laboratory WDXRF as an alternative to liquid extraction methods for regulatory compliance testing of Cr(VI) content. As a practical application of this work, we report a value for the Cr(VI) mass fraction of the new NIST Standard Reference Material 2859 Restricted Elements in Polyvinyl Chloride.