A Better Balance for Tiny Liquid Masses

Scientists from the National Institute of Standards and Technology (NIST) are developing a mass balance to measure miniscule amounts of liquid with much higher accuracy than ever before.

As part of the True Becquerel Project, the balance will support efforts to create an improved method for measuring radioactive material – specifically measurements of a unit of radioactivity called the becquerel. This work could provide benefits in the fields of medicine, energy, and safety.

But potential applications of the balance go far beyond measurements of radioactivity. On its own, it could someday be used in medicine, helping pharmacists measure out small, precise amounts of non-radioactive liquid, such as life-saving medications. It could also be used for national security and public health, helping scientists test the performance of detectors used to measure trace amounts of explosives or narcotics.

Measuring Liquid Mass with an Electrostatic Force Balance

NIST’s Gordon Shaw, who leads the team working on the mass measurement portion of the project, has developed similar instruments (called electrostatic force balances, or EFBs) in the past. But whereas previous EFBs have measured solid masses in a vacuum environment, the new EFB will be used to measure liquid masses that cannot be exposed to a vacuum because they would quickly evaporate.  

This would be the first high-accuracy measurement of such a small mass of liquid with an electrostatic force balance, and the first high-accuracy measurement of a small mass of liquid dispensed directly from the balance itself.

The researchers have not yet formally tested their balance’s ability to measure liquid masses. But when they do, those droplets will be ultra-small – around a microliter, about 5,000 times smaller than the liquid contained in a teaspoon. A small, sealed vial of liquid will be attached near the bottom of the balance. Droplets will be dispensed one at a time using a mechanism similar to an inkjet printer.  

Liquid Challenges

Measuring the mass of a liquid is tricky because, as soon as it’s dispensed, the droplet begins evaporating and its mass begins to change. To get around this problem, the researchers will measure the amount of mass in the vial before and immediately after a droplet is ejected.  

“That droplet can evaporate all it wants after it leaves the balance,” Shaw said. “We just need to know what it weighed before it left.”

Shaw and his colleagues report that, so far, their scale can measure the mass of tiny, milligram-sized solid masses with an accuracy better than 0.1 percent. That already matches the accuracy of the most sensitive methods for determining solid mass, but in the coming months the scientists hope to further improve it. The researchers report their most recent results in a paper accepted to Measurement Science and Technology.

Mass Redefined

Making this balance is only possible because the kilogram, the fundamental unit of mass, was redefined in 2019 in terms of the fundamental constants of nature.

Before 2019, mass measurements were based on a golf ball-sized cylinder of platinum iridium that was, by definition, exactly 1 kilogram (a little over 2 pounds) in mass. If scientists wanted to assess the mass of something much smaller, they had to conduct an extended series of comparisons with smaller and smaller masses, adding uncertainties at every stage.

But since the redefinition, the kilogram is now defined according to fixed constants of nature including the Planck constant. That means that with the right type of balance, such as the one used for this project, researchers can get a direct, SI-traceable measurement of mass, even from something very small, and without needing to calibrate the balance with another mass.

“Because of the SI redefinition, we have the SI-traceable realization built into the measurements,” Shaw said. “Our balance is effectively an electronic milligram.”

--Reported and written by Jennifer Lauren Lee

Paper: S. Schulze, K. Arumugam, S. Schlamminger, R. Fitzgerald, R. M. Verkouteren, R. Theska, and G. Shaw. Development of a High Precision Electrostatic Force Balance for Measuring Quantity of Dispensed Fluid as a New Calibration Standard for the Becquerel. Measurement Science and Technology. In press. DOI: 10.1088/1361-6501/ad3a06