The advantages of miniaturization are obvious. Conserving precious enzymes and substrates, reducing reagent cost, and increasing throughput continue to drive researchers toward lower reaction volumes.
However, dealing with nanoliter-range samples without proper liquid-handling equipment has consequences. Nonspecific binding of proteins to pins or nozzles during transfer alters concentration. Insufficient cleaning after any contact liquid transfer can cause cross contamination between samples. Shear forces from air-displacement liquid handling can injure cells. Regardless of the liquid-transfer technique used, poor transfer precision and low accuracy lead to inconsistent results.
To solve these bottleneck issues, non-contact acoustic droplet ejection (ADE) moves nanoliter and picoliter volumes of liquid with sound. ADE has been widely applied to DMSO-based solutions and has been demonstrated to improve screening results in drug discovery. The Labcyte® (www.labcyte.com) Echo® 555 liquid-handling system uses acoustic energy to transfer samples. Without touching the samples, the Echo system ejects 2.5-nL droplets from the sample surface upward onto inverted targets such as assay microplates or slides (Figure 1).
Fifty nanoliters of a wide range of aqueous solutions commonly used in biochemical applications were transferred to test the performance of the Echo system. The precision and accuracy were measured by fluorescence as described by Harris and Mutz in a 2006 JALA article. TRIS buffer and PBS were transferred with less than 2% error (average volume transferred was 49.8 nL for 10-mM TRIS and 49.3 nL for PBS; n=384 for each solution) and less than 2% CV. Cell culture medium DMEM was transferred with 2.78% error (average volume 51.4 nL) and 2.27% CV. Glycerol—a viscous solution that improves spot morphology in protein arrays and enhances low-temperature storage of enzymes and bacterial cell cultures—was tested at 50% concentration. We measured the error to be 1.98% and the precision to be 1.54% CV. Potassium phosphate, frequently used in biochemical assays, was transferred with 1.51% CV at 50 nL. The results indicate that the Echo liquid handler can transfer various solutions without losing precision or accuracy.
TRIS is used in many nucleic acid applications such as PCR and DNA sequencing. We used TRIS as a test solution to check the linearity of acoustic liquid transfers. Four different volumes (5, 10, 25, and 50 nL) of 10-mM TRIS were transferred with an Echo 555 liquid handler. Measured volume was linear with expected volume with a correlation of 0.9999. As shown in the Table, the precision of the transfer of 10-mM TRIS was <2.5% at all measured transfer volumes (n=384 for each volume). Deviation from expected was <2% in all cases. The transfer performance of the Echo is independent to solution volume transferred, as we did not observe decreasing precision or accuracy with different transfer volumes.