Pulsed Acceleration Charge Detection Mass Spectrometry:  Application to Weighing Electrosprayed Droplets

Abstract

We describe a new approach to measuring the masses of individual macroions. The method employs a pulsed acceleration tube located between two sensitive image charge detectors. The charge and velocity of the macroion are recorded with the first image charge detector. The ion is pulse accelerated through a known voltage drop, and then the charge and velocity are remeasured using the second image charge detector. The mass of the ion is deduced from its charge and its initial and final velocities. The approach has been used to measure masses in the 10(10)-10(14) Da range with z = 10(3)-10(6) and m/z = 10(6)-10(9). It should be extendable to masses of <10(6) Da. We have used the method to determine the size and charge of water droplets transmitted through a capillary interface and an aperture interface. The droplets detected from the aperture interface are approximately 1 order of magnitude smaller in mass than those detected from the capillary interface. The droplets from both interfaces have relatively low charges, particularly with the capillary interface where they are only charged to a small fraction of the Rayleigh limit. These results suggest that the aerodynamic breakup of the droplets plays a significant role in the mechanism of electrospray ionization.

Universal scaling laws for the disintegration of electrified drops

“…The close agreement between the measurements of ref. 24 and our simulations lends further credence to the accuracy of the scaling laws developed in this paper.…”

“…We may infer from this that an aerosol resulting from EHD tip-streaming should be roughly as monodisperse as the droplets formed from capillary pinching of tip-streaming jetswhich are quite monodisperse indeed. It also is noteworthy that in a recent experimental study, Mabbett et al (24) measured the charge on electrospray droplets and showed that the distribution of charge on the droplets peaks at ≈ 0:40 Q R . The close agreement between the measurements of ref.…”

“…A droplet with charge exceeding this limit is coulombically unstable and will explode in a manner analogous to the disintegration of an uncharged drop in a strong electric field (i.e., by cone-jetting) (23). Previous estimates of the charge of droplets produced from EHD tip-streaming range from close to Q R to a tiny fraction thereof (24). If droplets formed from EHD tip-streaming are unstable, or nearly so, the resulting aerosol will not be optimally monodisperse or of uniform charge distribution, which has important ramifications in applications.…”

“…Finally, using a geometric mean of the two previously introduced flow rate scales ðF μ F e Þ 1=2 as a characteristic scale, we recover the scaling laws r* ∝ β Although the sizes of droplets immediately after they are created and their size distribution anywhere in an aerosol produced in electrospraying can be measured readily, experimentally determining the charge of such droplets at the instance of their formation is a formidable challenge. Indeed, the droplets may (i) deform, oscillate, and break as a result of aerodynamic and other forces (2, 13, 32); (ii) interact, collide, and possibly coalesce (33) with one another and with solid surfaces; and (iii) evaporate and shrink before their charge can be measured (24). Thus, there has been considerable variation in the experimentally reported values of droplet charge.…”

Non-destructive characterization and alignment of aerodynamically focused particle beams using single particle charge detection

“…Previous single particle charge detection studies have focused on using the signals for mass-to-charge ratio measurements of individual particles (Benner, 1997;Fuerstenau & Benner, 1995;Mabbett, Zilch, Maze, Smith, & Jarrold, 2007;Schultz, Hack, & Benner, 1998). Here we use these signals for measuring the rate at which particles are injected into the vacuum chamber and apply the rates to particle beam characterization.…”