Which ionization method can produce either positive or negative ions by ionizing a molecular gas?

Chemical ionization is a method that can create either positive or negative ions by ionizing a molecular gas. This technique involves the introduction of a reagent gas that interacts with the analyte molecules. In a typical process, the reagent gas is ionized first, often by electron bombardment, and then these ions interact with the analyte molecules, resulting in the formation of either positive ions (by proton transfer) or negative ions (by capturing an electron from the neutral analyte) depending on the conditions used and the choice of the reagent gas.

This dual capability to ionize molecules in both polarity directions makes chemical ionization particularly versatile for mass spectrometry applications. It allows for enhanced signal detection and can facilitate the identification of various compounds depending on the ionization needs of the specific analysis being conducted.

Other ionization methods have specific characteristics that generally favor the production of either positive or negative ions exclusively, such as electron ionization primarily producing positive ions, whereas field ionization typically favors positive ion formation. Laser ionization can be useful in producing positive ions and is sometimes adjusted for generating negative ions, but it is not as direct and consistent in yielding both as chemical ionization.