Analysis of Liquid Particles in Aerosols via Charge-Induction Amperometry (ALPACA) for Rapid Electrospray Droplet Charge Analysis.

The rapid and on-line study of aerosols and their properties is technically demanding due to their small size (<10 µm diameter) and the resultant required scale of any such measurements. Most such techniques require the use of lasers (e.g., phase Doppler anemometry), condensation growth, or other complex hardware. To this end we introduce analysis of liquid particles in aerosols via charge-induction amperometry (ALPACA), an extremely simple potentiostat-based technique capable of on-line, rapid measurement of the aggregate charge of aerosol particles. This technique demonstrates high signal-to-noise responses, is not subject to chemical noise, and has the potential for significant future miniaturization. This technique is applied in this work for the detection of charges on electrosprayed droplets. The mechanism of detection of the technique is discussed using both amperometry and open circuit potential (OCP) to measure droplet charge properties. ALPACA represents a significant advancement toward simple, inexpensive aerosol charge detection.

Investigating Electrosprayed Droplets Using Particle-into-Liquid Sampling for Nanoliter Electrochemical Reactions.

Electrospray ionization (ESI) is a powerful ionization technique that can generate charged solvent droplets and bare analyte ions from sample solutions. Despite seeing extensive use in mass spectrometry due in part to the low internal energy deposited into the ions formed during ionization, some unknowns persist regarding the exact dynamics of droplet breakup and molecule behavior during spray, and research is still underway regarding how various types of molecules acquire charge during the ESI process. Previously, the authors introduced a novel aerosol measurement technique, particle-into-liquid sampling for nanoliter electrochemical reactions (PILSNER). The current work introduces a new method utilizing PILSNER for the examination of the particles generated during ESI using simple analysis techniques with a commercially available potentiostat. This technique is applied in this work for the detection of charges on electrosprayed droplets, including the estimation of the number of charges on individual ESI droplets using a fluorescent proxy. This technique provides an additional tool for the exploration of the complex process of droplet generation and ion liberation during ESI.

Aerosol Electroanalysis by PILSNER: Particle-into-Liquid Sampling for Nanoliter Electrochemical Reactions.

Particle-into-liquid sampling (PILS) has enabled robust quantification of analytes of interest in aerosol particles. In PILS, the limit of detection is limited by the factor of particle dilution into the liquid sampling volume. Thus, much lower limits of detection can be achieved by decreasing the sampling volume and increasing the surface area-to-volume ratio of the collection substrate. Unfortunately, few analytical techniques can realize this miniaturization. Here, we use an ultramicroelectrode in a microliter or smaller sampling volume to detect redox active species in aerosols to develop the technique of Particle-into-Liquid Sampling for Nanoliter Electrochemical Reactions (PILSNER). As a proof-of-concept to validate this technique, we demonstrate the detection of K4Fe(CN)6 in aerosol particles (diameter ∼0.1-2 µm) and quantify the electrochemical response. To further explore the utility of the method to detect environmentally relevant redox molecules, we show PILSNER can detect 1 ng/m3 airborne Pb in aerosols. We also demonstrate the feasibility of detecting perfluorooctanesulfonate (PFOS), a persistent environmental contaminant, using this technique. PILSNER is shown to represent a significant advancement toward simple and effective detection of a variety of emerging contaminants with an easily miniaturizable and tunable electroanalytical platform.