In a cylinder, a zeotropic blend will have a different vapor composition sitting above the bulk of the liquid. If you remove this vapor, you will: 1) take the wrong composition refrigerant out of the cylinder, and 2) leave behind the wrong composition refrigerant for future use. Liquid must be removed from the cylinder in order to avoid this fractionation effect. Somewhere between the cylinder and the compressor the liquid refrigerant should be flashed to vapor to avoid slugging. This can be done, for example, by just cracking open the valve on the gauge set while charging.