Picoliter droplet deposition using a prototype picoliter pipette: control parameters and application in micro X-ray fluorescence.

In this study, we introduce a Hewlett-Packard prototype picoliter pipette, the "thermal inkjet picofluidic system" (TIPS), for analytical purposes. In contrast to the use of actual inkjet printers, this instrument allows for control of all energy and time settings. We are able to show that in contrast to techniques delivering microliter and nanoliter volumes, evaporation has a major influence on the deposition of picoliter volumes and has to be treated seriously when picoliter depositions are applied in the laboratory for calibration purposes. We developed a strategy to reduce evaporation by varying different parameters, thereby achieving a precision of less than 10% for elemental depositions ranging from 1 to 300 picoliters and 1 to 2000 pg elemental deposits. Additionally, we determined the performance of the micro X-ray fluorescence (MXRF) instruments in terms of limit of detection (LODs), focal spot size, sensitivity, and precision and evaluated the TIPS deposits as reference materials for MXRF using single and multielement solutions. A linear response was observed with correlation coefficients from 0.991 to 0.999 for elemental deposits on AP1 film, and there was a standard deviation from 1 to 40%, depending on the element and the mass deposited. LOD's for Ni deposits on AP1 films were found to range from low picogram levels to subpicogram levels. The dried deposits were characterized for size and shape using light microscopy and atomic force microscopy (AFM) to estimate matrix effects and the area covered with sample material for the MXRF analysis. Diameters from 14 to 39 microm and thicknesses from 200 nm to 2 microm were measured. The accuracy of the dried spot approach was demonstrated by comparing multielement deposits from the TIPS with the NIST SRMs 1833 and 1832 thin film standards for MXRF analysis. The deviation from the SRMs was found to be better than 10%.