Characteristics of Picoliter Droplet Dried Residues as Standards for Direct Analysis Techniques

The characteristics of dried residues of picodroplets of single-, two-, and three-element aqueous solutions, which qualify these as reference materials in the direct analysis of single particles, single cells, and other microscopic objects using, e.g., laser ablation inductively coupled plasma time-of-flight mass spectrometry (LA-ICP-TOF-MS) and micro-X-ray fluorescence (MXRF), were evaluated. Different single-, two-, and three-element solutions (0.01−1 g/L) were prepared in picoliter volume (around 130 pL) with a thermal inkjet printing technique. An achievable dosing precision of 4−15% was calculated by total reflection X-ray fluorescence (TXRF) determination of the transferred elemental mass of an array of 100 droplets. The size of the dried residues was determined by optical microscopy to be 5−20 μm in diameter depending on the concentration and the surface material. The elemental distribution of the dried residues was determined with synchrotron micro-X-ray fluorescence (SR-MXRF) analyses. The MXRF results show high uniformity for element deposition of every single droplet with an RSTD of 4−6% depending on the concentration of spotted solution. The shape and height profile of dried residues from picoliter droplets were studied using atomic force microscopy (AFM). It was found that these dry to give symmetrical spherical segments with maximum heights of 1.7 μm. The potential of this technique for direct LA-ICP-TOF-MS analysis is shown.