Metal microneedles

Kim, K., et al. 2004. A tapered hollow metallic microneedle array using backside exposure of SU-8. J Micromech Microeng 14 597. 

Most practitioners use carbon microneedle emitters, and a few others use silicon emitters. Silicon emitters can be manufactured more rapidly, but carbon emitters are more rugged and can be heated to much higher temperatures (for flash cleaning). Other types of emitters (bare wire, metal microneedles, metal tip, razor blade, or volcano) are only sparingly used. Users either make their own emitters (with commercial or home-built devices) or else purchase them (from mass spectrometer manufacturers or independent vendors). The choice of making or buying emitters usually comes down to cost and time. 

The experimental technique for the trace analysis of metals simply involves the production of an emitter of acceptable quality. In general, 10 /im tungsten wires are activated at high temperature with benzonitrile in a multiple activation device. As the result of such an activation process, the tungsten wire is covered with dendrites of partially ordered pyrocarbon. Due to the small radii of curvature of the tips of the microneedles, the field strength is enhanced to a. level suitable for FDMS. These emitters are mechanically stable, which is important for repeated use they can also be chemically and thermally strained. This property is a prerequisite for the pyrolysis of the organic matrix and desorption of the metal cations, and last not least, the surface area of the emitter is sufficient for sample application. 

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