I think that you might be referring to the Constant Phoenix WC-135W and WC-135C. The former was modified from the C-135B an airlifter and later modified to a WC-35B, which was a weather sampler and the latter from the EC-135C, an airborne command post. It started as the KC-135B. The company, Boeing Aircraft company was formed when it was split from United Aircraft and Transport Corporation in 1934, but started as The Pacific Aero Products Company in 1916, became the Boeing Airplane Company and later the United Aircraft and Transport Corporation.
I am not aware of any news of the Constant Phoenix aircraft doing sampling missions of the Icelandic volcano eruption, but would not be surprised.
If I am correct, as an aside, I worked on WC-135B 667, now WC-135W, during the brief time that it was returned from TAC, where it had been used as a bounce aircraft for the AWACs Wing at Tinker. It was later taken away by SAC, I believe. It was a non standard WC-135B at that time as it still had the empty racks for the AMQ-25 Automated Weather Sampling System in the rear cargo deck.
Other sampling aircraft used to sample the ash from the volcano include the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) Dassault Falcon 20E, D-CMET, NERC's Airborne Research & Survey Facility's Dornier 228 and BAE 146, and the Finnish Air Force's BAE Hawk modified to carry sampling probes.
I might be wrong about the WC-135s, though.
Best wishes,
Grant