Apparatus and methods for supplying a portion of a fluid stream and, alternately, a fluid of known composition and concentration to an analyzer are provided. The fluid stream is directed through a series of connected chambers formed in an integral housing. A sampling needle has an inlet in one of the chambers and an outlet in fluid communication with the analyzer. When desired, a tube or vial containing a known fluid may be inserted into a chamber containing the sampling needle, so that the known fluid will be supplied to the analyzer. A second needle provides ventilation to the vial to prevent the formation of a vacuum as the known fluid is drained from the vial.