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

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 HgCl₂) were acidified with 5 mL concentrated hydrochloric acid. Pure N₂ was used as a carrier gas to strip CO₂ from the samples for 10 min in a circulation mode. After purification through several water traps, the CO₂ was isolated cryogenically. Using Na ₂CO₃ standard solutions, recovery yields were calculated superior to 95 ± 5%. Freeze-dried marine biota samples were thoroughly mixed with Cu(II)O and combusted at 900 °C. The CO₂ was purified by passing through Ag wool and Cu granules at 450 °C before reduction to graphite. Finally, graphite was synthesized using Zn dust heated to 450 °C in the presence of an Fe catalyst at 550 °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 ± 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.