Unique single and multi-electrode atmospheric pressure glow discharge (APGD) ionization source designs and configurations for mass spectrometry use are disclosed. These inventions build and expand on intellectual property described in existing US patents (attached), owned by Clemson University, Pacific Northwest National Laboratory, and Lawrence Berkeley National Laboratory. These inventions were conceived and brought to practice by the CU and PNNL teams. Conventional/existing APGD ion sources consist of two electrodes, one composed of an electrolytic liquid, typically containing a solution or sample to be analyzed, and the second a metal body through which a potential can be applied and current can pass. Within the literature, the apparatus is called the liquid sampling-atmospheric pressure glow discharge (LS-APGD). As described in the existing patents above, the liquid or solution electrode is held at ground potential, and a positive potential is applied to the metallic counter electrode. The solution electrode is typically mounted in-line with the ion sampling orifice, and the metallic counter electrode perpendicular to that direction (see Fig. 1).The plasma existing between the electrodes in placed in proximity to the ion sampling aperture of the mass spectrometer instrument. No specific potential is placed on the orifice, with plasma ions introduced to the mass spectrometer via aerodynamic forces caused by a directed flow of cooling sheath gas and the vacuum existing within the instrument. The plasma operating between the two electrodes is initially ignited by placing the electrodes in contact, applying a high voltage to generate a current flow, and then retreating them away from each other to form the plasma. Incorporated herein is a new ignition strategy that eliminates this process, allow for the electrodes to remain in a fixed position and thus improving the day-to-day precision of the ion source operation. We refer to this as 'auto-ignition". Described herein are novel, new, and alternative LS-APGD electrode designs and apparatus: In the first case, the need for a distinct metallic counter electrode is alleviated by using the metallic ion sampling aperture of the mass spectrometer as the counter electrode. This aperature can, for example, be hole in a plate, a cone with an aperature, or a capillary inlet. The solution electrode is mounted as before (Fig. 2).The plasma is established by the potential difference existing between solution surface and the aperture. As such, the largest fraction of the discharge sustaining voltage is applied to the electrolytic solution. In moving to this approach, geometric and positioning limitations of the original designed are alleviated (i.e., complexities related to the the proximity of the two electrodes and the proximity of the microplasma to the sampling orifice are greatly reduced). As such, the design and operation of the ionization source and the coincident ion sampling are both simplified, and the operational time and costs are reduced. Likewise, there remains only one sampling parameter (the distance between the solution electrode and the orifice) to be controlled. This single integrated (with mass spectrometer) invention was primarily conceived of and demonstrated by the Clemson University team, and is decribed in more detail in attached document entitled Single Integrated Electrode Experiments and Results. In the second case, multiple (2 or more) electrodes are employed as the counter electrodes, with the solution electrode mounted as typical in-line with the sampling orifice and the counter electrodes mounted collinearly or in other arrangements with each other, but perpendicular to the solution flow axis(Figs. 3,4,5). The plasma is this case is formed by the potential difference between the solution electrode, and the metallic counter electrodes held at the same potential. In doing so, the microplasma can operate at total discharge currents which are 2x or more that normally applied. In practice, the aerosolized solution species pass between the two counter electrodes, resulting in greater degrees of analyte ionization along with lower amounts of ions having solvent adducts (M+H2O, +O, etc.). Thus more sensitive responses are seen, with simpler spectral composition, than the original LS-APGD design. Additionally, a physically larger plasma is obtained, providing for more efficient sampling positioning with lower levels of extraneous and undesired atmospheric ions being sampled. This invention was primarily conceived of (March 2018) and demonstrated (March - May 2019) by the PNNL team, and is described in more detail in attached document entitled Multi-Electrode Experiments and Results. Inventive features, compared to existing designs, thus include: - use of single or multi-electrode designs/systems, either a single solution electrode with an interface element serving as the counter electrode (a single integrated electrode), or configurations with a single solution electrode and multiple counter electrodes. Both inventions are unique in the glow discharge literature. - use of a dual or multi-modal power supply that can automatically ignite and completely control operation of the glow discharge plasma. No such integrated system exists to our knowledge, commericially or not. - ability to use the devices/designs in single or multielectrode (multimodal) modes, depending on application and need, allowing - higher sensitivity - greater reproducibility - lower detection limits - improved linearity and calibration ease - greater freedom from carryover (interference or memory from prior samples or use) - greater freedom from extraneous and undesired atmospheric constituent interferences - improved ion source interfacing and coupling options, and more freedom and flexibility in integrating the ion source with different mass spectrometer systems or for specialized applications or tandem system use (ie with laser ablation, chromatography, etc) In addition, these features and improvements are embodied in a new, easily manufactured, robust cartridge mount that enables facile, electrical, gas, and sample inter-connections, together with a modular source mounting assembly that together provides for improved operational performance and maintenance/replacement.