When must liquids be vaporized in a chromatograph process?

In the context of chromatographic processes, vaporization of the liquid sample is essential during the measurement phase when the chromatograph is actually being used. This is because chromatography, particularly gas chromatography, requires that the sample be in a gaseous state to effectively separate and analyze the components.

When a chromatograph operates, it relies on the movement of vaporized samples through a column where different components will separate based on their interaction with the stationary phase. If the liquid was not vaporized before entering the chromatograph, it would not transition into an appropriate state for analysis and would essentially render the chromatographic process ineffective.

The other options reflect different stages of handling a sample but do not accurately capture the requirement for vaporization at the point of use. For example, the sample preparation stage may involve various processes, but the critical transformation to vapor happens specifically when the chromatograph is engaged to initiate analysis.