A downhole isolation tool adapted to be converted in the field into any one of a bridge plug, ball drop plug, or a caged ball plug. A storage element 10, a nonlinear resistance element 20, and an MOS transistor 30 are electrically connected in series. This relationship is used to establish the optical pathlength for samples containing an unknown concentration of analyte but having similar solvent composition. The push slider has a head portion coming into spherical contact with the power piston. Specifically, the circuit uses a semiconductor controlled rectifier or a semiconductor controlled switch to minimize the leakage. Provided is a vitreous silica crucible for pulling a silicon single crystal, the crucible has a wall having, from an inner surface toward an outer surface of the crucible, a synthetic vitreous silica layer, a natural vitreous silica layer, an impurity-containing vitreous silica layer and a natural vitreous silica layer.