We describe three gravimetric methods for accurately measuring the mass of picoliter-sized fluid droplets generated by a piezoelectric drop-on-demand (DOD) dispenser. Optical methods are also described for co-measurement of droplet diameters. Droplets are dispensed, either continuously at a known frequency, or as a burst or series of bursts, into a weighing vessel positioned on a submicrogram balance. Mass measurements are acquired by computer at precisely-specified time intervals. Mass depositions are corrected for evaporative losses, which are minimized by pre-saturating the weighing vessel and balance chamber with vapors from the fluid used. The capabilities and limitations of the methods are demonstrated through DOD ejection and weighing of isobutyl alcohol droplets. For ejection rates greater than 100 Hz, the repeatability of droplet mass measurements was 0.2 %, while the relative combined standard uncertainty (uc) was 0.9 %. For lower rates of ejection, the limit of quantitation (due to evaporative variance) was reached at 4 Hz, where uc was 1.4 %. When dispensing a burst of droplets, the limit of quantitation (due to microbalance hysteresis) was 72 µg (here, 1490 droplets) with uc = 1.0 %. The applicability of these methods to reflect individual droplet mass was evaluated by high-speed videography and dimensional imaging. Droplet diameters were consistent from the tenth droplet ejected to beyond the 8800th droplet. Within this range, analysis of variance signified that the mass of each individual droplet was best estimated by the average droplet mass with a combined uncertainty of about 1 %. The diameters of the first several droplets displayed differences from the average, but their contribution could be accounted for when dispensing droplet bursts. Above the limits of quantitation, the gravimetric methods provided statistically equivalent droplet masses, whereas the less precise optical method overestimated droplet mass due to systematic errors in arbitrarily-defined boundary delineations. We anticipate that the gravimetric methods described will permit detailed study of operational and environmental factors that influence droplet mass during dispensing, and facilitate calibration of other high-sensitivity sensors such as cantilevers, and allow the development of reliable standard materials using nano-dispense technologies.
Citation: Analytical Chemistry
Pub Type: Journals
calibration, droplet, drop-on-demand, evaporation, gravimetry, inkjet, fluidic head, mass, microbalance, microdrop, picoliter, piezoelectric, pressure head, static head