What type of impurities are you refering to? If there are excessive tar or heavy ends accumlated in the regulator and mixer you might want to install an inline thermostat in the coolant return line between the regulator and the suction side of the water pump. This keeps the water temp down to about 140 deg F in the regulator and slows down the refining process of the LPG in the regulator. Filter assemblies generally catch scale and debris from the tank like iron filings.