This paper presents an improved conditioned glow discharge ionization method, based on Penning ionization by metastable argon atoms at fore-vacuum gas pressures. The afterglow flow passes through a capillary to remove charged species (so called “conditioning”) and then the analyte molecules are mixed with metastable atoms within the throat of a supersonic jet. The method provides exclusively M+. molecular ions accompanied by limited fragmentation, even for alkanes, phthalates, and nitro compounds. When in-source fragmentation is induced, spectra appear similar to NIST ones. The signal is a linear function of concentration within a dynamic range of at least four orders of magnitude and is notably independent on the presence of a chemical matrix. Sensitivity is uniform within a wide range of analyte compounds, including alkanes and fatty acids. A combination of soft ionization, NIST identification of fragment spectra, and quantitative analysis presents a novel opportunity in analytical chemistry. The method may be particularly suitable for the analysis of crude oils and for quantitative metabolomic measurements.