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Sub-picoliter Traceability of Microdroplet Gravimetry and Microscopy



Lindsay C. C. Elliott, Adam L. Pintar, Craig R. Copeland, Thomas Brian Renegar, Ronald G. Dixson, Robert Ilic, R. Michael Verkouteren, Samuel M. Stavis


Volumetric analysis of single microdroplets is difficult to perform by ensemble gravimetry, whereas optical microscopy is often inaccurate beyond the resolution limit. To address the latter issue, we advance and integrate these complementary methods, introducing simultaneous measurements of the same microdroplets, comprehensive calibrations that are inde-pendently traceable to the International System of Units (SI), and Monte-Carlo evaluations of uncertainty. We achieve ex-cellent agreement in measurements of microdroplets in flight with volumes of approximately 70 pL, with gravimetry and microscopy both yielding 95% coverage intervals of ± 0.6 pL, or relative uncertainties of ± 0.9%, and root-mean-square de-viations of mean values between the two methods of 0.2 pL, or 0.3%. These uncertainties match previous gravimetry results and improve upon previous microscopy results by an order of magnitude. Gravimetry precision depends on the continuity of droplet formation, whereas microscopy accuracy requires that optical diffraction from an edge reference resembles that from a microdroplet. Applying our microscopy method, we jet and image water microdroplets containing submicrometer plastic particles, count single particles after deposition, and transfer volumetric traceability to suspension concentration. We expect that our methods will impact diverse fields requiring dimensional metrology and volumetric analysis of single micro-droplets, including inkjet microfabrication, droplet epidemiology, and industrial sprays.
Analytical Chemistry


single microdroplet volume, edge diffraction, optical edge, fraunhofer approximation, edge threshold, edge reference, gravimetry, imaging, shadowgraphy, transmission, metrology, monte carlo uncertainty, SI traceability, inkjet printing, in-flight, particle concentration, water contaminants


Elliott, L. , Pintar, A. , Copeland, C. , Renegar, T. , Dixson, R. , Ilic, R. , Verkouteren, R. and Stavis, S. (2021), Sub-picoliter Traceability of Microdroplet Gravimetry and Microscopy, Analytical Chemistry, [online],, (Accessed April 18, 2024)
Created December 20, 2021, Updated November 29, 2022