Single droplet generation by dripping-mode electrospray for ICP-MS measurement

Abstract

The feasibility of single droplet generation by dripping-mode electrospray for inductively coupled plasma-mass spectrometry (ICP-MS) measurement has been demonstrated. A novel electrospray droplet generator has been developed for stable droplet generation. High electrical potential is applied simultaneously to the sample capillary and a ring electrode that is placed concentrically to the sample capillary and 1 mm in recess of the tip of the capillary. The working range of the applied potential increases from 2–3 kV to 3–6 kV and the maximum droplet generation rate increases from 8 to >〖140 s〗^(-1) s-1. The capability of high droplet generation rate is required to produce small droplets because the drop size is inversely proportional to the drop generation rate.

The minimum diameter of the droplets generated by dripping-mode electrospray equals to the outer diameter of the sample capillary. Electrospray sample capillary of diameter of 50 to 100 m were fabricated by drawing standard capillary electrophoresis (CE) glass capillary (outer diameter = 320 m) in a micro-butane flame. A novel setup using two microsyringe pumps to provide consistent tension for capillary drawing has been developed. Operating conditions of heating time and capillary drawing speed and duration were optimized.

The droplet generation rate and the initial velocity of the droplets were determined using laser light scattering and digital photography. The forward scattered light was detected using a photodiode. The initial velocity of the droplet was estimated to be 1–2 m/s from the half-width of the current pulse. The electrically charged droplets have acquired sufficient kinetic energy to break into fine droplets for transportation to the ICP.

The electrospray-produced droplets are carried by a stream of oxygen-argon mixed gas (6.7% of oxygen, 1.0 L/min) through a heating chamber to reduce the drop size. The oxygen-argon stream is combined with a make-up argon flow to form the carrier gas (5.6% of oxygen, 1.25 L/min) for transport of the sample droplets to the ICP. The gas composition was optimized for stable electrospray operation and minimum disturbance of the ICP excitation conditions. The carrier gas flow rate was optimized for high transport efficiency of the generated droplets. Detection rate of the generated droplets was >85%.

Gold nanoparticles (nominal diameter 100 nm) were used as model particles for the characterization of the single droplet generation-ICP-MS setup. Gold nanoparticle suspension was pumped through the sample capillary using a microsyringe pump to produce droplets that contain known amount of gold nanoparticles. The nanoparticles were detected as ICP-MS intensity spikes. Each droplet produces a cluster of ICP-MS intensity spikes within a 2-s envelop. ICP-MS spikes corresponding to single particles as well as multiple particles were observed. The overall particle transport efficiency was low (<1%). Possible mechanisms for the relatively high droplet detection rate and low particle transport efficiency are discussed.