An improved selected ion recording system for precise isotope ratio determination.

An improved analog four-channel selected ion recording system is described, in which major modifications permit a decrease in the dwell time to 33 ms per channel, thus minimizing the mass cycling error. Synchronization with a sinusoidal sweep voltage superimposed on the normal accelerating voltage (8 kV) enables two channels to be monitored simultaneously in real time and each mass to be recorded continuously. These improvements allow measurement of ion current ratios with a precision of 0.2% over a wide dynamic range, permitting accurate determination of isotopic enrichment in biomedical assays even when this enrichment is derived from a single label. Use of the system is illustrated by the analysis of palmitate turnover in dogs (using [1--13C] palmitic acid) with an average standard deviation corresponding to the detection of 0.04% excess of [1--13C] palmitate.