Measurement of plasma phenytoin by EMIT in the Monarch centrifugal analyzer: studies on the elimination of within-rotor drift.

We investigated the measurement of phenytoin in plasma by the EMIT method, in the Monarch centrifugal analyzer. When we used the standard protocol supplied by Instrumentation Laboratory, significant drift was observed across a full rotor loaded with 31 replicates of a single specimen. Although the cause remains obscure, the drift was eliminated by using a load-spin-reload-spin procedure. This makes it possible to analyze large batches of samples without excessive use of standards and reagents.