Needle Tip Electrode

If there were only one such tip electrode, the yield of ions would be very limited (small surface area and small numbers of ions formed per unit time). To increase ion yield, it is better to use 

In (a), a molecule alights onto a positive electrode surface, its electrons being attracted to the surface and its nuclei repelled. In (b), an electron has tunneled through a barrier onto the electrode, leaving a positive ion that is repelled by and moves away from the positive electrode. 

A positive ion formed at a positive electrode tip is repelled and travels toward the negative counter electrode, which has a slit in it so that the ion can pass into the mass spectrometer. 

These thin wires are supported on a special carrier that can be inserted into the ion source of the mass spectrometer after first growing the whiskers in a separate apparatus. Although the wires are very fragile, they last for some time and are easily renewed. They are often referred to as emitter electrodes (ion emitters). 

Practical Considerations of Field lonization/Field Desorption 

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The electrical reverse of the above arrangement produces negative ions. Thus, a negative needle tip places an electron on the molecule (M) to give a negative ion (M -), which is repelled toward a positive counter electrode. 

ESI nebulization is the result of charging a liquid at a needle tip by applying a high potential (ca. 3 kV), between the needle and a nearby counter electrode. The formation of the aerosol depends on the competition between coulomb repulsion and surface tension. Stable nebulization strongly depends on experimental parameters such as the potential difference applied, the inner and outer diameter as well as the shape of the needle, and the composition of the liquid sprayed. 

Figure 6.8 shows the equipotential field lines caused by a CC needle electrode having the form of a long, thin ellipsoid. The potential drop is very significant near the needle tip. 

EA and according to Section 6.1, the total electrode impedance may be high and dominated by electrode polarization impedance. A needle tip may look bright and shining, but the magnified surfaces shown in Figure 7.35 shows fractal surface increase and debris from the manufacture. This may lead to unstable polarization impedance on the tip. 

Figure 7.37 shows the monopolar impedanee spectrum measured with a thin insulated needle inserted in porcine living tissue. The 90% sensitivity zone is roughly a sphere of about 3 mm centered at the exposed needle tip. Above = 10 kHz the data are dominated by tissue properties, below 1 kFlz by electrode polarization impedance. 

Fig. 8. Labeling the endoderm of 7.0-d embryo by electroporation. A The embryo is first bathed in plasmid DNA solution before transfer to the electroporation drop on the culture dish. To electroporate the distal tip, the embryo is held at the extraembryonic ectoderm by suction using the holding pipet (hp) and positioned between the platinum plate (cathode) and tungsten needle (anode). The electrodes are connected to an Electro Square Porator and the electroporation is focused at the tissue closest to the needle tip. B Fluorescent cells in the distal region of the embryo following electroporation with a P-actin-eGFP construct and cultured for 3h.

Early studies by Meulemans et al. used differential pulse voltammetry (see Chapter 11) to measure the concentration of electroactive drugs in the cytoplasm of cholinergic neurons of Aplysia californica and to determine rates of cellular uptake and clearance (108). Another report by Meulemans et al. described a needle tipped glass insulated platinum electrode for detection of intracellular serotonin in metacerebral neurons from Aplysia (109). Again differential pulse voltammetry was used to measure intracellular concentration of serotonin after cell stimulation, after injection of serotonin into the cell, and after extracellular treatment with tryptophan, a serotonin precursor. Serotonin concentration in resting untreated cells was determined to be 0.94 0.61 mM. Chen et al. have also used platinum electrodes with multiple pulse voltammetry to measure dopamine levels in the giant dopamine neuron of Planorbis comeus (110). 

Once the needle tip is in the right place, preferably assessed in all three planes (Fig. 5.1.4b), the tines are deployed to a 2-cm array and the probe is connected to the generator. One should be alert to the risk of push back, in that instead of forwarding the electrodes from the device s trocar and into the tumor, the trocar is pushed backwards exposing the electrodes proximal to the tumor and not penetrating the tumor, as intended. 

Fig. 9.2. a Osteoid osteoma of the femoral shaft in a 32-year-old male patient with typical nocturnal dominant pain attacks over the last 12 months. There was moderate to minor pain relief after salicylate intake. Note the reactive sclerosis completely surrounding the soft-tissue dense nidus, b The dense sclerosis surrounding the nidus could not be passed directly therefore, an access route from the opposite site was chosen. Note that the tip of the electromechanical drill is shown slightly beside the nidus arrow). c The radiofrequency electrode was positioned through the drilled cortical hole eccentrically next the nidus. Due to the penetration depth of about 10 mm of the radiofrequency around the needle tip the nidus is still within the range of the ablation zone. This osteoid osteoma could be successfully ablated with 7 W over 9 min... 

Recent experimental investigations demonstrate the possibility of external perturbation and control of spiral waves in the BZ system [54-60]. For this purpose, Perez-Munuzuri et al. [57] applied a dc current to a shallow layer of the medium with a cylindrical condensator. A small (needle-shaped) electrode was placed over the spiral tip which increased the core size by slowly attracting the inhibitory bromide ions. Thus, the period of spiral rotation was increased with the applied voltage. 

Lencioni R, Goletti O, Armillotta N, et al (1998) Radiofrequency thermal ablation of liver metastases with a cooled-tip electrode needle results of a pilot clinical trial. Eur Radiol 8 1205-1211... 

Lehnert T, Otto G, Herfarth C (1995) Therapeutic modalities and prognostic factors for primary and secondary liver tumors. World J Surg 19 252-263 Lehnert T, Knaebel HP, Diick M, et al (1999) Sequential hepatic and pulmonary resections for metastatic colorectal cancer. Br J Surg 86 241-243 Lencioni R, Vignali C, Caramella D, et al (1994) Transcatheter arterial embolization followed by percutaneous ethanol injection in the treatment of hepatocellular carcinoma. Cardiovasc Intervent Radiol 17 70-75 Lencioni R, Goletti O, Armillotta N, et al (1998) Radiofrequency thermal ablation of liver metastases with a cooled-tip electrode needle results of a pilot clinical trial. Eur Radiol 8 1205-1211... 

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