Example 3

Probe Materials

A number of porous materials were tested to generate charged droplets for mass spectrometry. The materials were shaped into triangles having sharp tips and sample solution was then applied to the constructed probes. Data herein show that any hydrophilic and porous substrate could be used successfully, including cotton swab, textile, plant tissues as well as different papers. The porous network or microchannels of these materials offered enough space to hold liquid and the hydrophilic environment made it possible for liquid transport by capillary action. Hydrophobic and porous substrates could also be used successfully with properly selected hydrophobic solvents.

For further investigation, six kinds of commercialized papers were selected and qualitatively tested to evaluate their capabilities in analyte detection. Filter papers and chromatography paper were made from cellulose, while glass microfiber filter paper was made from glass microfiber. FIG. 19 shows the mass spectra of cocaine detection on those papers. The spectrum of glass fiber paper (FIG. 19E) was unique because the intensity of background was two orders of magnitude lower than other papers and the cocaine peak (m/z, 304) could not be identified.

It was hypothesized that the glass fiber paper was working on mode II and prohibiting efficient droplet generation, due to the relative large thickness (˜2 mm). This hypothesis was proved by using a thin layer peeled from glass fiber paper for cocaine detection. In that case, the intensity of the background increased and a cocaine peak was observed. All filter papers worked well for cocaine detection, (FIGS. 19A-19D). Chromatography paper showed the cleanest spectrum and relative high intensity of cocaine (FIG. 19F).

Probe Shape and Tip Angle

Many different probe shapes were investigated with respect to generating droplets. A preferred shape of the porous material included at least one tip. It was observed that the tip allowed ready formation of a Taylor cone. A probe shape of a triangle was used most often. As shown in FIGS. 25A-25C, the sharpness of the tip, the angle of the tip (FIGS. 27A-25B), and the thickness of the paper substrate could effect the spray characteristics. The device of a tube shape with multiple tips (FIG. 25-D) is expected to act as a multiple-tip sprayer, which should have improved spray efficiency. An array of micro sprayers can also be fabricated on a DBS card using sharp needles to puncture the surface (FIG. 25E).

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