Atmospheric Pressure Ionization Mass Spectrometry and Its Application in the Analysis of Ultra High Purity Gases: A Review

Ultra high purity (UHP) gases are very important in the manufacturing industry. As an example, in the semiconductor industry, the quality of electron gas directly affects the performance of semiconductor devices; a few parts per million of trace impurities in gases can reduce component store information in integrated circuits. Thus, challenges to the sensitivity of the analytical instruments and the analysis methods arise for detection of ultra-trace impurities. Atmospheric Pressure Ionization Mass Spectrometry (APIMS), which could ionize impurities under the atmospheric pressure conditions with high efficiency of ionization and sensitivity, becomes the most important instrument for the purity analysis of the UHP gases. APIMS is especially suitable for the analysis of trace impurities with a concentration below 10^-9 mol/mol or even 10^-12 mol/mol. Two ionization sources based on corona discharge and ⁶³Ni have been developed for APIMS, where the former is more commonly equipped. Quadruple rod is usually used as the mass filter. A sampling and dilution system, in which the components in standard gas can be diluted to far lower concentrations for calibration, is utilized in APIMS for analysis of UHP gases. The detection sensitivity of impurities of APIMS is quite suitable to the ionization modes. One of the main modes is charge transfer reaction, which is applicable to matrix and impurities with large different ionization energy. For example, most impurities in Ar and N₂ can be determined by this mode due to the high ionization energy of Ar and N₂. And by adding H₂, APIMS is also used for detecting N₂ in Ar. The third mode is the reaction of forming cluster ions, for which few applications have been reported. The mode could enhance the detection sensitivity of H₂O in matrix O₂ by detecting the ion of O₂⁺·H₂O. Based on these ionization modes, various detecting methods for different impurities can be designed to effectively detect trace or ultra-trace impurities. The sensitivity of APIMS is usually 10⁴-10⁶ times higher than Electron Impact-Mass Spectrometry. As a result, APIMS is still irreplaceable in UHP gas analysis however, methods such as those required for the analysis of corrosive electronic special gases, should see an improvement in the sensitivity.