A calibration gas standard is a homogeneous mixture of components that have the same proportions of its components throughout a given sample.
A good analogy is like a cup of tea. If you were to add sugar to a cup of tea and not stir it, the sugar would sink to the bottom. As you drink the tea, it would be bitter at the beginning and get continually sweeter as you get to the bottom. In this case, the cup of tea would not be a homogeneous mixture.
The same situation can occur with a calibration gas mixture. If not mixed correctly, the lighter components of a natural gas mixture (Methane, Ethane etc.) would be at the top and exit first when drawing a sample. The heavier components would remain at the bottom therefore not providing a clear representation of the sample.
When a calibration gas is prepared, the cylinder is rolled for a period of time after mixing to ensure it is homogeneous. These proportions can be altered when exposed to low temperatures during transport and storage.
If a hydrocarbon component calibration gas is exposed to temperatures below the minimum usage temperature the heavier hydrocarbons can begin to move into the liquid phase, leaving the gas mixture as a two phase, non-homogeneous gas standard. If used for calibration the instrument will be biased for higher hydrocarbons and a higher heating value.
For this reason, it is important that the calibration gas is supplied with a phase diagram, outlining the hydrocarbon dew point. This way we can identify when the standard has been exposed to low temperatures and, also, the minimum usage temperature. If indeed the cylinder has been exposed to a low temperature it is important that the standard is warmed and sufficiently rolled to ensure the mixture is homogeneous again.