Complex sentence: Chemical ionization (CI) is another method that requires samples to be in the gas phase so that the sample can react with a reagent gas to form an ion that can be analyzed by the mass spectrometer.
Split into simpler sentences: Chemical ionization (CI) is another method that requires samples to be in the gas phase. This is so that the sample can react with a reagent gas to form an ion that can be analyzed by the mass spectrometer.