Synchronizing droplet microfluidics and ion mobility spectrometry via the electrospray current recorded with a tailored low-noise, high-gain current amplifier

In the process of coupling ion mobility spectrometry (IMS) and droplet microfluidics, the sample to be analyzed is ionized by electrospray ionization (ESI), thereby transferring the liquid samples into the gas phase with concurrent ionization. In contradistinction to conventional ESI, in droplet microfluidics the sample to be analyzed is present in individual droplets, which are separated from each other by an oil phase resulting in the process of ESI becoming discontinuous due to alternating sample droplets and oil phase. Typically, averaging is necessary in IMS to suppress noise and thus improve detection limits. However, in combination with droplet microfluidics, continuous averaging leads to reduced signal-to-noise ratio, because the spectra of the oil phase not containing any information are included into averaging. To avoid this effect, it is imperative to synchronize the ESI-IMS with the droplet microfluidics using a suited trigger signal. Therefore, we present a fast, low-noise, high-gain current amplifier that can operate at high electrical potential and allows for monitoring the ESI current in both polarities. The design of this amplifier is predicated on the current change of the ESI current during the volume transition between oil and droplet inside the emitter. This transition has been mathematically modelled and incorporated as a volume-dependent term within an existing equation for the ESI current. Finally, we present experimental data to support the idea of increasing signal-to-noise ratio by triggering the ESI-IMS via the electrospray current. Furthermore, triggering the IMS facilitates droplet-wise storage of data.