Purified trichlorosilane is the principal precursor to ultrapure silicon in the semiconductor industry. Some of the first ultrapure samples of gallium arsenide were zone refined in these crucibles. This method is used to produce ultrapure alkali metals. Dialysis uses highly purified (also known as "ultrapure") water. A typical application is monitoring of ultrapure water in semiconductor fabrication facilities. This method is used commercially, yielding ultrapure barium. Barium is a soft, silvery-white metal, with a slight golden shade when ultrapure. Ultrapure water is treated through multiple steps to meet the quality standards for different users. Ultrapure water is often used as a critical utility for cleaning applications (as required). Ultrapure water (UPW) is used extensively in the fabrication steps of making computer microchips. Ultrapure nonactin, practically free of other nactins, is used for ammonium-specific electrodes.