An oscillator that provides a quadrature output and has a cross-coupled configuration is disclosed. These probes are useful for characterization and measurement of unbound metabolites in a fluid sample, particularly characterization and measurement of levels of unbound free fatty acids. The insertion of the don’t care gates creates a first intermediate circuit and a second intermediate circuit. The airknife further is adapted for low pressure direction and application of air laden particles, such as preatomized liquid particles.