Preparation of graphite targets from small marine samples for AMS radiocarbon measurements.

Abstract

A vacuum sample processing line was set up and methods were developed for the determination of radiocarbon in small-volume seawater and biota samples. Seawater samples (500 mL per borosilicate glass bottle and poisoned with HgCl (sub 2) ) were acidified with 5 mL concentrated hydrochloric acid. Pure N (sub 2) was used as a carrier gas to strip CO (sub 2) from the samples for 10 min in a circulation mode. After purification through several water traps, the CO (sub 2) was isolated cryogenically. Using Na (sub 2) CO (sub 3) standard solutions, recovery yields were calculated superior to 95+ or -5%. Freeze-dried marine biota samples were thoroughly mixed with Cu(II)O and combusted at 900 degrees C. The CO (sub 2) was purified by passing through Ag wool and Cu granules at 450 degrees C before reduction to graphite. Finally, graphite was synthesized using Zn dust heated to 450 degrees C in the presence of an Fe catalyst at 550 degrees C. Although this method takes about 8 hr (synthesis done overnight), the advantage is that no water vapor by-product is formed to hinder the reaction. The graphite yields, measured both by gravimetric methods and by pressure readings, were 95+ or -5%. Accelerator mass spectrometry (AMS) measurements were carried out at the NSF-Arizona AMS Facility. Results for water samples from the northwest Pacific Ocean are reported which are in agreement with data reported elsewhere.