Copper particles that degraded when subjected to CO2 ER exhibit an EELS signature for oxidized copper at the surface (Figure ) . This contrasts with as-plated and re-precipitated copper particles, which exhibit an EELS signature for metallic copper, even at the surface (Figure ). These results suggest that CO2 may be playing either a direct or indirect role in copper degradation process. Therefore, an important question to resolve was whether degradation was induced by CO2 itself, CO2 ER products or a combination of both.