determination? determination of what?

Context:

Microdialysis and voltammetry have dominated the modern era of neurochemical monitoring in vivo. With exquisite sensitivity and selectivity by virtue of removing brain analytes for ex vivo determination,
microdialysis is better suited for measuring basal neurotransmitter levels. By using electrochemistry at the probe tip for in situ detection, the superior temporal resolution of voltammetry is more appropriate
for capturing faster chemical signals.