The gas mixture in the cartridge is also quite important as there are cold weather mixtures now available using a mixture of isobutane and propane. The MRS Isopro Fuel cartridges can be used about 10C colder than standard 70/30 butane/propane cartridges because the isobutane gas has a boiling point temperature at -12C rather than the butane boiling point of -0.5C

http://www.msrcorp.com/stoves/fuel_blend.asp

If liquid gas feed i.e inverted gas cartridge techniques are not suitable then using the MSR Isopro Fuel together with the heat pad technique would probably be just as effective as using just pure propane in very cold temperatures.

http://store.oceanriver.com/store/product.php?productid=16706&cat=0&page=3