Chemical ionization source, the reagent gas is present in large excess compared to the analyte. Electrons entering the source with energy around 200-500 eV will preferentially ionize the reagent gas. Then, the ion/molecule reactions produces more stable reagent ions and the resultant collisions with other reagent gas molecules will create an ionization plasma. Positive and negative ions of the analyte are formed by reactions with this plasma. The following reactions are possible with methane as the reagent gas. If ammonia is the reagent gas, For isobutane as the reagent gas, Self chemical ionization is possible if the reagent ion is an ionized
Positive and negative sets Positive and negative sets In measure theory, given a measurable space ("X",Σ) and a signed measure μ on it, a set "A" ∈ Σ is called a positive set for μ if every Σ-measurable subset of "A" has nonnegative measure; that is, for every "E" ⊆ "A" that satisfies "E" ∈ Σ, one has μ("E") ≥ 0. Similarly, a set "A" ∈ Σ is called a negative set for μ if for every subset "E" of "A" satisfying "E" ∈ Σ, one has μ("E") ≤ 0. Intuitively, a measurable set "A" is positive (resp. negative) for μ if μ is