Mass Spectrometry of Acoustically Levitated Droplets

Abstract

Containerless sample handling techniques such as acoustic levitation offer potential advantages for mass spectrometry, by eliminating surfaces where undesired adsorption/desorption processes can occur. In addition, they provide a unique opportunity to study fundamental aspects of the ionization process as well as phenomena occurring at the air-droplet interface. Realizing these advantages is contingent, however, upon being able to effectively interface levitated droplets with a mass spectrometer, a challenging task that is addressed in this report. We have employed a newly developed charge and matrix-assisted laser desorption/ionization (CALDI) technique to obtain mass spectra from a 5-μL acoustically levitated droplet containing peptides and an ionic matrix. A four-ring electrostatic lens is used in conjunction with a corona needle to produce bursts of corona ions and to direct those ions toward the droplet, resulting in droplet charging. Analyte ions are produced from the droplet by a 337-nm laser pulse and detected by an atmospheric sampling mass spectrometer. The ion generation and extraction cycle is repeated at 20 Hz, the maximum operating frequency of the laser employed. It is shown in delayed ion extraction experiments that both positive and negative ions are produced, behavior similar to that observed for atmospheric pressure matrix-assisted laser absorption/ionization. No ion signal is observed in the absence of droplet charging. It is likely, although not yet proven, that the role of the droplet charging is to increase the strength of the electric field at the surface of the droplet, reducing chargere combination after ion desorption.Progress in the biological, medical, and pharmaceutical sciences has placed ever-increasing demands on technologies for handling and analyzing smaller and smaller sample volumes. Chip-based assays involving microfluidics have begun to address many of these demands. 1 In chip-based microfluidics, the surface-to-volume ratio increases as the fluid channel sizes decrease. As a result, nonspecific binding or adsorption of the analyte to the channel surfaces can become problematic, limiting the sensitivity of the desired assay. 2,3 One method of avoiding this issue is through containerless processing of samples using droplet levitation. Numerous containerless trapping techniques are becoming recognized for their ability to handle small volumes, serving as micromanipulators and microreactors. 4-6 Among these trapping techniques, single-axis acoustic levitation has gained wide acceptance because there are relatively few restrictions on the properties of the sample. 7 One of the most famous experiments involving droplet levitation was the Millikan oil drop experiment of 1909. 8 Since this early application to measure the electronic charge, e, droplet levitation has been used in various studies pertaining to the following: Brownian motion, photoemission, droplet evaporation, thermophoresis, 8 diffusion, 9 magnetization of superconducting particles, 10 Raman spectroscopy of...

Electrodynamic balance–mass spectrometry of single particles as a new platform for atmospheric chemistry research

“…Modification of the ionization scheme may also be necessary to obtain sufficient signal when working with smaller particles or analyte compounds present in smaller quantities. One approach may be to vaporize the particle not with a heated platform, but with an infrared laser, possibly followed by a separate gas-phase ion source (as in Warschat et al, 2015;Westphall et al, 2008;or Zelenyuk et al, 2009). Alternately, an electrospray-type scheme may be used, such as delivering the particle onto a paper spray source (as in Jacobs et al, 2017), ionizing via an interaction between the particle and a spray of ions (e.g., as in Doezema et al, 2012;Gallimore and Kalberer, 2013;or Horan et al, 2012), or by producing a spray directly from the particle when dropped onto a charged needle tip (as in Tracey et al, 2014).…”

“…Additionally, in a different laboratory a quadrupole ion trap mass spectrometer was modified to levitate individual micron-sized droplets, followed by reducing the trap pressure over 20 min to ∼ 0.1 Pa, ablating the particle with a pulsed laser (532 nm), and collecting a mass spectrum using the same ion trap (Yang et al, 1995). Previous work has also reported measuring mass spectra of aqueous droplets suspended in acoustic traps (Crawford et al, 2016;Stindt et al, 2013;Warschat et al, 2015;Westphall et al, 2008). The aqueous droplets suspended in acoustic traps tend to have a diameter on the order of a millimeter, much larger than the micron or submicron diameter of atmospheric aerosol particles.…”

Charge assisted laser desorption/ionization mass spectrometry of droplets

“…This simplifies the instrumentation, especially in situations where the electrodes cannot be placed in close proximity to the sample to generate a high electric field. One example is recent studies from our laboratory on generating gas-phase ions from acoustically levitated droplets, where objects placed in close proximity to the droplet would disturb the levitation field and destabilize the droplet levitation [32]. The results presented in the current report clearly demonstrate that droplet charging plays an important role when corona ions are used to improve the ionization efficiency of laser desorption/ionization techniques, and that the gas-phase ion-neutral reactions which are central to the mechanism of APCI may not be the dominant ionization mechanism in LD-APCI.…”

New mass spectrometry techniques for studying physical chemistry of atmospheric heterogeneous processes

“…Westphall et al [200] took advantage of CALDI's lower electric field requirements in what appears to be the first MS study of acoustically levitated microdroplets. Acoustic levitation is ideally suited for CALDI because this technique depends only on the density of the levitated particle, not on its size or charge.…”