Gravimetric and optical methods were developed to assure quantitative deposition of compounds from nano-dispense devices onto the surfaces of materials. This metrological development has enabled the reliable production of a new type of reference material that utilizes the precise spatial deposition abilities and speed of inkjet printers. Now, NIST reference materials may be produced that closely simulate samples screened at airports (and other critical locations) for the presence of trace explosives, narcotics, and chemical weapons. These reference materials are critically needed to test the performance of trace contraband detectors that are widely deployed for the purpose, and support the continual improvement of detection technologies as threat priorities evolve.
A commercial inkjet device with XYZ motion stages (JetLab IV, MicroFab Technologies, Plano, TX) was adapted to enable precise spatial and quantitative deposition of trace materials onto a variety of surfaces. As part of this adaptation, we have developed gravimetric and optical methods for accurately characterizing the mass of the microdroplets that are ejected from the piezoelectric transducer (PZT) during the production of NIST reference materials. For the gravimetry, the microdroplets were ejected into an ultra-light weighing vessel (Figure 1) placed inside an automated sub-microgram balance that streamed data into a computer spreadsheet. Evaporation effects were monitored and controlled by saturating the sealed weighing chamber with the fluid being ejected. The air-vapor buoyancy correction to the measured differential masses was determined to be insignificant. Repeatability of measured droplet mass (about 50 ng/drop) across 6 hours of continuous ejection was 0.43 % (relative standard uncertainty). Droplet mass was also determined by ejecting known numbers of drops as bursts into the weighing vessel; the results were statistically equivalent to the continuous gravimetric method. By changing PZT operational variables (piezoelectric waveform and ejection frequency, fluid backpressure, and hydrostatic column between the fluid reservoir and PZT tip) we could induce reproducible changes in droplet mass greater than 100 %. At nominally constant operating conditions, day-to-day reproducibility of droplet mass was typically ranged within 4 %, which was attributed to subtle changes in operational and environmental factors and the wettability condition of the PZT tip. This uncertainty was improved to less than 2 % by monitoring droplet velocity, which was performed using a microscope with strobed illumination and a dimensionally calibrated field of view below the PZT tip. We determined that droplet mass was highly correlated with droplet velocity at nominally constant PZT operating conditions, so by monitoring velocity during production of reference materials, droplet mass could be determined and deposition accuracy of 2 % or better could be assured.
Development of high-accuracy gravimetric methods for determining masses of semivolatile droplets ejected from PZTs (about 50 ng per droplet)
Linking droplet velocity with droplet mass to assure 2 % uncertainty in deposited mass during production of NIST reference materials by inkjet printers
Start Date:October 3, 2008
Lead Organizational Unit:cstl
Advanced Measurement Laboratory
JetLab precision microdeposition system
Mettler-Toledo UT2 microbalance
Related Programs and Projects:
Trace explosives and narcotics research / standards development