Supercritical fluid chromatograph with pneumatically controlled pump

An apparatus for analyzing a process stream via supercritical fluid chromatography comprising:

• a. chromatograph means for analyzing a sample of the process stream diluted in a volume of the supercritical fluid via supercritical fluid chromatography;
• b. pump means for drawing a supercritical fluid substance from a supply means through a supply line, pressurizing said supercritical fluid substance and delivering said pressurized supercritical fluid substance to said chromatograph means, said pump means comprising a pneumatically driven piston type pump having a first large diameter chamber, a second smaller diameter chamber extending axially therefrom, and an axially extending piston rod having a first large diameter head disposed and axially translatable in sealing relationship within the first chamber and a second smaller diameter head disposed and axially translatable in sealing relationship within the second chamber;
• and
• c. control means for controlling the supply low pressure air to the first chamber of said pump means in a controlled manner so as to draw said supercritical fluid substance from said container means into the second chamber when the pressurized air is vented from the first chamber and to compress said substance drawn into the second chamber to maintain said supercritical fluid substance at a desired pressure above the critical pressure for delivery to said chromatograph means.