Samples and monitors were step heated using a Staudacher-type resistance furnance (using a coiled tungsten filament, a tantalum crucible with no liner and tungsten-rhenium thermocouple) and gas was purified continuously during extraction by two SAES ST-172 porous getters and analyzed on a MAP 516 mass spectrometer fitted with a Baur-Signer source and a Johnston MM1 multiplier with a sensitivity of 2.0 X 10^-14 molV^-1.