We have investigated the emission properties of gas field ionization emitters built up from electrochemically de-etched W[111] and W[100] single crystal wires. A standard thermal field method was used to confine the ion emission to a single spot at the apex of the emitter. The angular current density obtained for these kinds of tips with a small end radius (100-200 nm) is in the range of 5 mu A/sr. For H-2 and Ne, maximum ion current was observed in the temperature range between 18 and 21 K. We have measured the ion current density for different gases (H-2, He, and Ne) as a function of extraction voltage and gas pressure. The linear dependence of the angular current density on the gas pressure shows that current densities of 10 mu A/sr and above can be expected with this type of emitter.